Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments

October 7, 2016, 11:27 am

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Let us have a comparison between two major type of electrical measuring instruments they are moving coil (MC) and moving iron (MI) instruments.In this article all major differences between moving iron and moving coil instruments are discussed.When you go viva or interview most often question on electrical measurements is what is the difference between mi and mc type instruments?.In this article you will get answer to all differences between MI & MC typ instruments.

Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments

M.C Instruments	M.I Instruments
1. MC type instruments are more accurate.	1. MI type are less accurate than MC type.
2. Manufacturing cost is high.	2. Cheap in cost.
3. Reading scale is uniformly distributed.	3. Non-uniform scale (scale cramped at beginning and finishing)
4. Very sensitive in construction & for input.	4. Robust in construction.
5. Low power consumption	5. Slightly high power consumption.
6. Eddy current damping is used.	6. Air friction damping is used.
7. Can be used only for D.C measurements.	7. Can be used for A.C as well as for D.C measurements.
8. Controlling torqure is provided by spring.	8. Controlling torque is provided by gravity or spring
9. Deflection proportional to current.( θ α I ).	9. Deflection proportional to square of current. ( θ α I² ).
10. Errors are set due to aging of control springs,permanent magnet (i.e. No Hysteresis loss)	10. Errors are set due to hysteresis and stray fields. (i.e. hysteresis loss takes place).

In this comparison between MC and MI Instruments we shared top points.

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3 Point Starter Working & Construction

February 23, 2016, 9:41 am

≫ Next: Electrical Braking of Induction Motor

≪ Previous: Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments

3 Point Starter | Working & Construction of Three Point Starter

A starter is a device which helps in smooth starting and running of a dc motor(mostly dc shunt,compound motors).More technically,an electrical arrangement which limits the starting current supplied to the motor.

Before going to see the working of a 3 point starter let's see why we need a 3 point starter?

We know that in a dc motor to get back emf the armature has to rotate in a magnetic field,due to generator action in the armature core back emf is produced. This back emf opposes the supply voltage which helps in governing the motor. But initially armature doesn't rotate so back emf will be zero so high amount of current will be drawn.Which can damage the motor as we know resistance of armature is low.So by using a 3 point starter we can start dc shunt or dc compound motor without damage.

Let,

E=Supply voltage

Eb=Back emf

Ia=Armature current

Ra= Armature resistance

In running conditions of dc motor

Ia=(V-Eb)/Ra

but initially Eb=0 then Ia=V/Ra

So Ia value will be several times higher than rated current as Ra value is small.

To limit the high initial current we use this 3 point starter.

Construction of Three Point Starter

It's well known that a resistor will limit current flow. Exactly what we are using in this is series of resistors which are divided into sections. Here other parts like no volt coil, over load release are the other two main parts which come into picture during the operation of a 3 point starter.

In the above diagram of three point starter you can see 3 points.They are,

L- Line terminal and it is connected to the positive terminal of the supply.

F- Connected to the field winding of motor.

A- Connected to the armature of motor.

Here we can clearly see that this L point is further connected to over load release which is nothing but an electromagnet. This other end of over load release is attached to the starter handle which is fixed at one end and at the other end it is free to move and it moves against the force of spring. You can see the other connection from stud 1 to the no volt coil. And the other end of this NVC (no volt coil) is connected to point F. Now last stud is connected in series with armature as shown figure of three point starter.

3 Point Starter:Working of Three Point Starter

Switch on the supply first. Initially the handle will not be in contact with the resistance. Now after switching on the supply move this handle to stud 1 manually. Now field gets supply through the NVC (no volt coil) as this movement of handle on to the stud 1 will form a closed path. As mentioned earlier armature is in series with the resistances the current through armature will decrease. Now slowly move this handle over the studs 2,3,4 so resistance will be cut down slowly. We need to cut the resistance after initial operation because after the rotation of armature back emf will be produced so there will not be any necessity with this resistances. The handle will be always maintained in run position during operation.

Handle Operation In 3 Point Starter

To know this we should know what does a no volt coil do. While supply is on the field is supplied through NVC. NVC is also an electromagnet it magnetizes when handle comes into contact with stud 1 . To this handle a soft iron piece is attached when all the resistances are cut off this handle is attracted to NVC due to soft iron and remains in this position against the spring force during working condition. In three point starter whenever there is failure in supply or damage in field winding this NVC demagnetizes and handle comes to its initial position due to spring force. As the resistances are in series with armature the high current passage will not be there and the motor is protected if we try to start the motor during any damage.

Working of No Voltage Coil of 3 Point Starter ( or )Over Load Release In Three Point Starter

Motor gets its suppy through OLR. This is also a electromagnet which magnetizes when supply is on. Under this there is an arm and its fixed to fulcrum.In the 3 point starter under over load it magnetizes and attracts the arm due to this arm moves upwards. There is a triangle shaped piece which gets attached to the two ends of NVC when arm is attracted upwards which shorts the NVC. So now voltage across NVC is zero and hence NVC demagnetizes so handle comes to original position in this way motor is protected automatically. During normal condition the force of attraction gets balanced with gravitational force( that is till full load current is reached).So NVC and OLR acts as protecting devices.

Disadvantages Of Three Point Starter

Here the NVC and field are in series. To get more than rated speed for a motor we need to add extra resistances which makes current through NVC less and there is a chance that NVC doesn't magnetize appropriately and thus soft iron piece doesn't attract as a result handle will move to initial position thus stopping motor.

To avoid this we use 4 point starter because in this NVC and field are connected in parallel.In the next article we discuss about advantages & disadvantages of Three point starter,Four pointer starter.

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Electrical Braking of Induction Motor

February 24, 2016, 12:52 am

≫ Next: 4 Point Starter Working & Construction

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Induction Motor Braking Regenerative Plugging Dynamic Braking of Induction Motor

Induction motors are most widely using motors in present days for domestic and industrial purpose.But controlling of AC motor is little bit difficult than DC motor due to alternating nature of voltage.Speed control of induction motors is difficult than dc motors.That’s why induction motor use was restricted.But efficiency wise,usage wise induction motors has lots of advantages.So some speed control,braking methods of induction motor are invented.

Braking is the most important part of speed control.Braking is two types.They are,mechanical and electrical.In mechanical braking we have disadvantages like heat dissipation,mechanical brakes also depends on the skill of the operator.Electric braking is easy and reliable,it can stop the induction motors very quickly.So we go for electrical braking on induction motor.Though the motor is brought to rest electrically, to maintain its state of rest a mechanical brake is must.

Types Of Electrical Braking Methods On Induction Motor

Dynamic or Rheostatic Braking On Induction Motor

In this braking a high resistance is inserted in the rotor circuit of induction motor, with the help of rheostat.In the below diagram braking on 3 phase induction motor is shown. Whenever we need braking one of the supply line out of R, Y or B is disconnected from the supply.Depending upon the condition of this disconnected line,we classified two types they are,

1. Two lead connections : In this method, the line disconnected, kept open. This is shown in the Fig(a) and is called two lead connections of rheostatic braking.

2. Three lead connections : In this method, After disconnection of the line,it is connected directly to the other line of the machine. This is shown in the Fig(b).

Note : This method is effective only for slip ring or wound rotor induction motors.

As one of the motor terminal is disconnected from the supply, the motor continues to run as single phase motor. In this case the, breakdown torque i.e. maximum toque decreases to 40% of its original value and motor develops no starting torque at all. And due to high rotor resistance, the net torque produced becomes negative and the braking operation is obtained.

Which is preferred in rheostat braking on induction motor?

The braking torque is small in the case of two lead connections,high in the case of three lead connections.The braking torque is high at high speeds. But in three lead connections there is possibility of inequality between the contact resistances in connections of two paralleled lines. This might reduce the braking torque and even may produce the motoring torque again.So we prefer two lead connection over three lead connection.Even there is low braking torque in two lead system.

The torque-slip characteristics for motoring and braking operation of 3 phase induction motor is shown here,

Uses: We use dynamic or rheostat braking in cranes.

Plugging On Induction Motor

Plugging induction motor braking is applied by just reversing the supply phase sequence.by interchanging connections of any two phases of stator we can attain plugging braking of induction motor.Due to the reversal of phase sequence, the direction of rotating magnetic field gets reversed. This produces a torque in the reverse direction and the motor tries to rotate in opposite direction. This opposite flux acts as brake and it slows down the motor.During plugging the slip is (2 - s), if the original slip of the running motor is s.

Disadvantages of plugging induction motor braking :

1. During the plugging operation very high I²R losses occur in the form of heat.This heat is more than produced when rotor is normally locked.

2. So we can't apply plugging frequently as due to high heat produced rotor which can damage or melt the rotor bars and even may over heat the stator as well.

Advantages of plugging induction motor braking :
1. It the quickest way.

D.C.Braking On Induction Motor

This is also similar to dynamic braking of induction motor,two connections of stator are disconnected from the supply and connected to a dc source.When d.c. is supplied to the stator, stationary poles N, S are produced in stator. As rotor is rotating, rotor cuts the flux produced by the stationary poles. Thus the a.c. voltage gets induced in the rotor. This voltage produces an a.c. current in the rotor. The motor works as a generator and the R losses are dissipated at the expenditure of kinetic energy stored in the rotating parts. Thus D.C dynamic braking is achieved. This is the quick way to stop induction motor along with high load.

Advantages of d.c. dynamic braking of induction motor are,

1. Quick,Less heat produced as compared to the plugging method.

2. The energy dissipated in the rotor is not dependent on the magnitude of the d.c. current.

3. The braking torque is proportional to the square of the d.c. current.

4. Can be used for wound rotor,squirrel cage rotor induction motors.

Regenerative Braking On Induction Motors

The input power to a three phase induction motor is given by, Pin = 3 Vph Iph cos Φ Where, Φ is the phase angle between stator phase voltage Vph and the stator phase current Iph.In motoring action This Φ is less than 90° for the motoring action.Φ > 90° for generating action.

When the induction motor runs above the synchronous speed, relative speed between the conductors and air gap rotating field reverses, as a result the phase angle because greater than 90° rotor produces torque in opposite direction to achieve the braking thus regenerative braking takes place.

Regenerative braking of induction motor can be applied only in case of constant frequency and speed must be above the synchronous speed.In generating action we get some power,we can use this to run other machine.

The torque-slip characteristics for motoring and generating action are shown in the above figure.

Tags:Induction motor braking methods wikipedia,ppt,advantages of electric braking over mechanical braking,regenerative braking of dc motor pdf

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4 Point Starter Working & Construction

February 24, 2016, 1:07 am

≫ Next: Differences Between Instrument and Power Transformers

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4 Point Starter | Working & Construction of Three Point Starter

In the previous post we have seenhow does a 3 point starter works. There is only one difference in construction between 3 point starter and four point starter. Here the no volt coil and field are not connected in series as in 3 point starter but the NVC is connected to the supply independently through a point called N which can be seen clearly in the below figure.

Operational difference between 3 point starter and 4 point starter

In 4 point starter NVC gets supply directly from the mains it doesn't depend on field which can be seen clearly from the diagram. So this NVC will produce force which helps to keep the handle in RUN position all the time and also the current through NVC can be adjusted through R(series with NVC).

What is the disadvatage of this?

As NVC is independent of field NVC will be in RUN position all the time. Now if the field gets opened while motor is in running condition due to the presence of residual flux motor rotates with dangerously high speed (since N is proportional to 1/ᴓ) this happens because still the handle will be in run position only. But in 3 point starter as soon as the field fails handle comes to off position so motor stops. But in 4 point starter as this is not the case the handle will remain in RUN condition all the time it is not suitable to protect the motor from high speeds.

Over load release is similar as in the 3 point starter. When the current increases more than full load current the OLR which is an electromagnet pulls the arm upwards and the triangular piece shorts the two ends of NVC and it demagnetizes the NVC and motor shuts down automatically. But till the full load current the force of attraction gets balanced with the gravitational force.

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Differences Between Instrument and Power Transformers

October 24, 2016, 10:41 pm

≫ Next: What Is Chopper ? Classification Of DC To DC Power Converters/Choppers

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In this article differences between power transformer and instrument transformer are discussed.Current transformer and Potential transformers come under instrument transformers.

Differences Between Instrument and Power Transformers

Power Transformers	Instrument Transformers
1. Mainly used to change voltage levels in a power system.	1. Mainly used to extend the ranges of the instruments while measuring parameters like voltage,current,power etc.
2. They are required to transform huge amount of power to the load.	2. They are required to transform very small power as their loads are generally delicate moving elements of the instruments.
3. They can be used to step up or step down the voltage.	3. They are basically step down transformers and used along with devices such as protective relays,indicators etc.
4. The exciting current is a small fraction of the secondary winding of load current.	4. As the load it self is small, the exciting current is of the order of the secondary winding.
5. The cost is main consideration in the design while efficiency and regulations are secondary considerations.	5. Accuracy is the main consideration while designing to keep ratio and phase angle errors minimum. Cost is the second consideration.
6. As they handle large power, the heat dissipation is the major consideration and cooling method is necessary.	6. The power output is very small as loads are light. Hence heating is not severe.
7. The limitation on the load is due to temperature issue.	7. Accuracy is the main load limitation factor and not the temperature rise.
8. Example : Distribution transformer used in transmission.	8. Example : Current transformer and Potential transformer

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What Is Chopper ? Classification Of DC To DC Power Converters/Choppers

October 29, 2016, 7:46 am

≫ Next: Principle & Working of Buck Converter ( Step-Down Chopper )

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What Is Called A DC Chopper ?

A d.c. chopper is a static device (switch) used to obtain variable d.c. voltage from a source of constant d.c. voltage.

In the right below figure you can see circuit diagram of a chopper. Therefore, chopper may be thought of as d.c equivalent of an ac. transformer since they behave in an identical manner. Besides, the saving in power, the dc.chopper offers greater efficiency, faster response, lower maintenance, small size, smooth control, and, for many applications, lower cost, than motor-generator sets or gas tubes approaches.

Solid-state choppers due to various advantages are widely used in trolley cars, battery-operated vehicles, traction-motor control, control of a large number of d.c. motors from a common d.c. bus with a considerable improvement of power factor, control of induction motors, marine hoists, forklift trucks and mine haulers. The objective of this chapter is to discuss the basic principles of chopper operation and more common types of chopper configuration circuits.

Classification Of Choppers or DC To DC Power Converters

DC choppers can be classified as:

(A) According to the Input/Output Voltage Levels

(i) Step-down chopper: The output voltage is less than the input

voltage.Click here to read full article on step-down chopper

(ii) Step-up chopper: The output voltage is greater than the input

voltage.

(B) According to the Directions of Output Voltage and Current

(i) Class A (type A) chopper

(ii) Class B (type B) chopper

(iii) Class C (type C) chopper

(iv) Class D (type D) chopper

(v) Class E (type B) chopper

The voltage and current directions for above classes are shown in below figure.

(C) According to Circuit Operation

(i) First-quadrant chopper: The output voltage and both must be positive.(Type A).

(ii) Two-quadrant chopper: The output voltage is positive and current can be positive or negative (class-C) or the output current is positive and the voltage can be positive or negative (class-D).

(iii) Four-quadrant chopper: The output voltage and current both can be positive or negative (class-E).

(D) According to Commutation Method

(i) Voltage-commutated choppers

(ii) Current-cornmutated choppers

(iii) Load-commutated choppers

(iv) Impulse-commutated choppers

↧

Principle & Working of Buck Converter ( Step-Down Chopper )

October 29, 2016, 11:36 am

≫ Next: Unijunction Transistor (UJT) With Operation & Applications

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In the previous articles we have seen basic operation and principle of a chopper.In this tutorial we will see Principle of Step-Down Chopper (Buck-Converter) with a neat sketch of circuit diagram

Don't Know What Is Chopper ? Click Here To Read It Now..

Buck-Converter ( Step-Down Chopper ) Principle & Working

In general, d.c. chopper consists of power semiconductor devices (SCR, BJT, power MOSFET, IGBT, GTO, MCT, etc., which works as a switch), input d.c. power supply, elements (R, L, C, etc.) and output load. (Refer below figure). The average output voltage across the load is controlled by varying on-period and off-period (or duty cycle) of the switch.

Buck Converter ( Step-Down Chopper ) Circuit Diagram

Buck-Converter ( Step-Down Chopper ) Operation

A commutation circuitry is required for SCR based chopper circuit. Therefore, in general, gate commutation devices based choppers have replaced the SCR based choppers. However, for high voltage and high-current applications, SCR based choppers are used. The variations in on and off periods of the switch provides an output voltage with an adjustable average value. The power-diode (DP) operates in freewheeling mode to provide a path to load-current when switch (S) is OFF. The smoothing inductor filters out the ripples in the load current. Switch S is kept conducting for period Ton and is blocked for period Toff. The chopped load voltage waveform is shown in figure.

Buck Converter ( Step-Down Chopper ) Output

During the period Ton, when the chopper is on, the supply terminals are connected to the load, terminals. During the interval Toff, when the chopper is off, load current flows through the freewheeling diode DF.As a result, load terminals are short circuited by DFand load voltage is therefore, zero during Toff. In this manner, a chopped dc. voltage is produced at the load terminals.

From output of buck converter figure, the average load-voltage E0 is given by

From Eq 1, it is obvious that the output voltage varies linearly with the duty-cycle. It is therefore possible to control the output voltage in the range zero to Edc.

If the switch S is a transistor, the base-current will control the ON and OFF period of the transistor switch. If the switch is GTO thyristor, a positive gate pulse will tum-it ON and a negative gate pulse will turn it OFF. If the switch is an SCR, a commutation circuit is required to turn it OFF.

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Unijunction Transistor (UJT) With Operation & Applications

October 30, 2016, 1:49 am

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What Is Unijunction Transistor (UJT) ?

The unijunction transistor, abbreviated UJT, is a three-terminal, single-junction device. The basic UJT and its variations are essentially latching switches whose operation is similar to the four-layer diode, the most significant difference being that the UJT’s switching voltage can be easily varied by the circuit designer. Like the four-layer diode, the UJT is always operated as a switch and finds most frequent applications in oscillators, timing circuits and SCR/TRIAC trigger circuits.

Unijunction Transistor (UJT) Operation

A typical UJT structure, pictured in figure, consists of a lightly doped, N-type silicon bar provided with ohmic contacts at each end. The two end connections are called base-l , designated B, and base-2, B2. A small, heavily doped P-region is alloyed into one side of the bar closer to 82. This P-region is the UJT emitter E, and forms a P-N junction with the bar.

An interbase resistance, RBB, exists between B1 and B2. It is typically between 4 kΩ and 10kΩ, and can easily be measured with an ohmeter with the emitter open. RBB is essentially the resistance of the N-type bar. This interbase resistance can be broken up into two resistances, the resistance from B1 to emitter, called RB1 and resistance from B2 to emitter, called RB2. Since the emitter is closer to B2, the value of RB1 is greater than RB2 (typically 4.2 kΩ versus 2.8 kΩ).

The operation of the UJT can better be explained with the aid of an equivalent circuit.The UJT’s circuit symbol and its equivalent circuit are shown in below. The diode represents the P-N junction between the emitter and the base-bar (point x). The arrow through RB1, indicates that it is variable since during nonnal operation it may typically range from 4 kΩ down to 10 Ω.

The essence of UJT operation can be stated as follows:

(a) When the emitter diode is reverse biased, only a very small emitter current flows. Under this condition, RB1 is at its normal high-value (typically 4 kΩ). This is the UJT’s “off" state.

(b) When the emitter diode becomes forward biased, RB1 drops to a very low value (reason to be explained later) so that the total resistance between E and BI becomes very low, allowing emitter current to flow readily. This is the “on” state.

Circuit Operation of Uni junction Transistor (UJT)

The UJT is normally operated with both B2 and E biased positive relative to B1 as shown in below figure. B1 is always the UJT reference terminal and all voltages are measured relative to B1. The VBB source is generally fixed and provides a constant voltage from B2 to B1. The VEE source is generally a variable voltage and is considered the input to the circuit. Very often, VEE is not a source but a voltage across a capacitor.

We will analyze the UJT circuit operation with the aid of the UJT equivalent circuit, shown inside the dotted lines in Fig.(a). We will also utilize the UJT emitter-base-1 VE-IE curve shown in Fig.(b). The curve represents the variation of emitter current IE, with emitter-base-1 voltage, VE, at a constant B2-B1 voltage. The important points on the curve are labelled, and typical values are given in parentheses.

The “Off ” state If we neglect the diode fora moment, we can see in Fig.(a) that RB1 and RB2 form a voltage divider that produces a voltage Vx, from point x relative to ground.

Where η (the greek letter “eta") is the internal UJT voltage divider ratio RB1/RBB and is called the intrinsic stand of ratio.

Values of η typically range from 0.5 to 0.8 but are relatively constant for a given UJT.

The voltage at point x is the voltage on the N-side of the P-N junction. The VEE source is applied to the emitter which is the P-side. Thus, the emitter diode will be reverse-biased as long as VEE is less than Vx This is the “off” state and is shown on the VE-IE curve as being a very low current region. In the “off" state, then, we can say that the UJT has a very high resistance between E and B1, and IE is usually a negligible reverse leakage current. With no IE, the drop across RE is zero and the emitter voltage, VE, equals the source-voltage.

The UJT "off " state, as shown on the VE-IE curve, actually extends to the point where the emitter voltage exceeds Vx by the diode threshold voltage, VD, which is needed to produce forward current through the diode. The emitter voltage and this point, P, is called the peak-point voltage, VP, and is given by

VP= Vx + VD= ηVBB+ VD

where VD is typically 0.5 V. For example, if η = 0.65 and VBB= 20V, then VP=13.5 V. Clearly, VP will vary as VBB varies.

The “On " state As VEE increases, the UJT stays “off” until VE approaches the peak-point value VP, then things begin to happen. As VE approaches Vp, the P-N junction becomes forward biased and begins to conduct in the opposite direction.

Note on the VE-IE curve that IE becomes positive near the peak point P. When VE exactly equals VP, the emitter current equals IP, the peak-point current. At this point, holes from the heavily doped emitter are injected into the N-type bar, specially into the B1 region. The bar, which is lightly doped, offers very little chance for these holes to recombine. As such, the lower half of the bar becomes replete with additional current carriers (holes) and its resistance RB1, is drastically reduced. The decrease in RB1 causes Vx to drop. This drop in turn causes the diode to become more forward biased, and IE increases even further. The larger IE injects more holes into B1 further reducing RB1, and so on. When this regenerative or snowballing process ends, RB1. has dropped to a very small value (2-25 Ω) and IE can become very large, limited mainly by external resistance RE.

The UJT operation has switched to the low-voltage. high-current region of its VE- IE curve. The slope of this “on” region is very steep, indicating a low resistance. In this region, the emitter voltage VE, will be relatively small, typically 2V, and remains fairly constant as IE is increased up to its maximum rated value,IE(sat). Thus, once the UJT is “on,” increasing VEE will serve to increase IE while VE remains around 2V.

Turning “Off” the UJT Once it is “on,” the UJT‘s emitter current depends mainly on VEE and RE. As VEE decreases, IE will decrease along the “on” portion of the VE - IE curve. When IE decreases to point V, the valley point, the emitter current is equal to IV, the valley current, which is essentially the holding current needed to keep the UJT “on”. When 15 is decreased below IV, the UJT turns “off" and its operation rapidly switches back to the “off” region of its VE - IE curve,where IE = 0 and VE - VEE. The valley current is the counterpart of the holding current in PNPN devices, and generally ranges between 1 and 10 mA.

Applications of UJT:

Unijunction transistors are used extensively in oscillator, pulse and voltage sensing circuits. Some of the important applications of UJT are discussed below :

(i) UJT relaxation oscillator.

(ii) Overvoltage detector.

↧

A Text Book Of Electrical Technology Vol.1+2+3+4 By BL Theraja PDF Free Download

November 13, 2016, 7:38 pm

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A Text Book Of Electrical Technology By BL Theraja and AK Theraja

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Power Systems Book by C L Wadhwa PDF Free Download

December 26, 2016, 4:15 am

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No Load Test on Induction Motor / Procedure & Application

December 30, 2016, 7:49 am

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No Load Test on Induction Motor

There are three types of tests to be performed for checking the performance of an induction motor

1.No load test or open circuit test.

2.Blocked rotor test or short circuit test or locked rotor test.

3.Stator winding resistance test.

Now let's see each test in detail:

No load test:

Main objective of conducting the no load test is:

1. To find the constant losses of the induction machine.

2. To divide constant losses into iron losses and mechanical losses.

3. To find shunt branch elements of the equivalent circuit.

4. To find no load power factor of the induction machine.

Condition to conduct no load test:

1.To conduct the no load test we need to apply rated voltage and rated frequency to stator under no load condition and also the air gap flux is to be maintained constant.

2. If the power supply given to the induction machine is constant then one watt meter is enough in the circuit if not two watt meter's are required.

Circuit diagram to conduct no load test:

Procedure to conduct no load test:

1. Connections are to be done as per the circuit diagram given above.

2. load connected to the motor is to be removed.

3. Now we need to supply rated voltage at rated frequency to the stator and also maintain constant air gap flux.

4. Input power can be measured with the help of two watt meters connected in the circuit.

5. Ammeter and voltmeter connected in the circuit gives no load voltage and current.

6. As said there is no load given to the motor the total input power given to the motor is equal to the sum of constant iron loss, mechanical losses like friction and windage losses and copper loss but it is in small amount.

7. Copper losses at rotor side can be neglected in no load test because the slip is small at no load.

Experiment values table:

P₁	P₂	I_LINE	V_LINE

Equivalent circuit:

at no load as slip is very less we can take slip approximately equal to zero.

R₂'( 1/s -1) ≈ ∞ since no load I_2r' = 0

so the equivalent circuit now gets modified as follows:

And the current flowing through x_mis I_uand R₀is I₂.

Calculations:

V = Line to line voltage.

I = Average value of line currents flowing throw ammeters.

W_NL = P₁ + P₂which is input power or three phase power.

All the readings we get from the circuit are line values.

With star connected stator winding:

V_NLperphase= V/√3 here v is the voltmeter reading in the circuit.

I_{0 perphase}= I here I is the ammeter reading in the circuit.

cosᴓ₀= W_NL/ 3×V_NLperphase×I₀_perphase

I_1perphase = I_0perphase × cosᴓ₀.

I_uperphase = I_0perphase× sinᴓ₀.

R₀= V_NLperphase / I_2perphase.

X_m= V_NLperphase/ I_{u perphase}.

With delta connected stator winding:

V_NL= V = vlotlmeter reading of the circuit.

I_{0 perphase}= I/√3 I is the ammeter reading.

cosᴓ₀= W_NL/ 3×V_NLperphase×I₀_perphase.

I_1perphase = I_0perphase × cosᴓ₀.

I_uperphase = I_0perphase× sinᴓ₀.

R₀= V_NLperphase / I_2perphase.

X_m= V_NLperphase/ I_{u perphase.}

Power factor:

Based on watt meter readings we can find power factor approximately

If P₁= P₂ then power factor (lagging) = Unity.

if P₁= 2P₂then power factor(lagging) = 0.866.

if P₂= 0 , P₁ = total 3 phase power then power factor(lagging) = 0.5

if P₁= - P₂ then power factor(lagging) = Zero.

Finding out constant losses:

At no load input power supplied W_NL= iron losses of stator + mechanical losses + stator copper losses.

1.Here the core losses of rotor are not considered as there is no load losses at rotor side are negligible.

2.And as there is no load mechanical power output is zero.

3.For induction machine mechanical losses are almost constant irrespective of load because of less speed variations.

Therefore iron loss of stator + mechanical losses = Constant losses.

W_NL= Constant losses + stator copper loss at no load .

constant losses = W_NL - stator copper losses at no load.

constant losses = W_NL- 3I₀²R₁.

where R₁is the per phase resistance of stator winding.

R₁can be measured by using bridges.

Dividing constant losses into Iron losses and Mechanical losses:

1. We need to conduct the no lo load test using variable voltage at constant frequency.

2. By changing the applied voltage note down the readings of watt meter.

3. Now find constant losses using, constant losses = W_NL- 3I₀²R_1.

Calculating iron loss:

As frequency is constant iron losses α V₁².

V₁is the varying voltage.

Calculating mechanical loss:

1.As we said that speed is almost constant in induction motor at constant frequency the mechanical losses are constant.

2. By adding load to the induction motor and decreasing the voltage at constant frequency the speed of induction motor falls to maintain load torque constant so mechanical losses will decrease. If applied voltage becomes zero iron losses will become zero.

Now draw the graph and from graph we can calculate the values of mechanical loss and iron loss.

Test has to be conducted from V_ratedto V_min.

In this way we can find out Iron losses and mechanical losses.

Now all the objectives of No load test are fulfilled and hence the performance of machine can be known.

↧

All Day Efficiency of Transformer/ Distribution Transformer All Day Efficiency

December 30, 2016, 10:56 am

≫ Next: Voltage Regulation of Transformer

≪ Previous: No Load Test on Induction Motor / Procedure & Application

All Day Efficiency of Transformer/ Distribution Transformer All Day Efficiency

In our previous articles we have discussed transformer working,construction etc.Today we discuss one of the important parameter of distribution transformer I.e, "all day efficiency of a distribution transformer".We know for a transformer, the efficiency is defined as the ratio of output power to input power.

Transformer Efficiency= Output Power /Input Power

The above equation is efficiency of any transformer. But for some special types of transformers such as distribution transformers power efficiency is not the true measure of the performance.For that purpose distribution transformer we calculate all day efficiency.Distribution transformer serve residential and commercial loads.

The load on distribution transformers vary considerably during the period of the day. For most period of the day these transformers are working at 30 to 40 % of full load only or even less than that. But the primary of such transformers is energised at its rated voltage for 24 hours, to provide continuous supply to the consumer.

The core loss which depends on voltage, takes place continuously for all the loads. But copper loss depends on the load condition. For no load, copper loss is negligibly small while on full load it is at its rated value. Hence power efficiency can not give the measure of true efficiency of distribution transformers. in such transformers, the energy output is calculated in kilo watt hour (kWh). Then ratio of total energy output to total energy input (output + losses) is calculated. Such ratio is called energy efficiency or All Day Efficiency of a transformer.

Based on this efficiency, the performance of various distribution transformers is compared. All day efficiency is defined as,

While calculating energies, all energies can be expressed in watt hour (Wh) instead of kilo watt hour (kWh). Such distribution transformers are designed to have very low core losses. This is achieved by limiting the core flux density to lower value by using a relative higher core cross-section i.e. larger iron to copper weight ratio.

The maximum efficiency in distribution transformers occurs at about 60-70 % of the full load. So by proper designing, high energy efficiencies can be achieved for distribution transformers. Numerical problems on all day efficiency of a transformer will be posted soon.

Related:

maximum efficiency of distribution

transformer all day efficiency of transformer pdf

efficiency of transformer formula

all day efficiency of transformer ppt

all day efficiency of transformer examples

distribution transformer efficiency in 24hrs

↧

Voltage Regulation of Transformer

January 1, 2017, 9:31 am

≫ Next: Voltage Regulation Of Synchronous Machines [Alternator] By Direct Loading Test

≪ Previous: All Day Efficiency of Transformer/ Distribution Transformer All Day Efficiency

Voltage Regulation Of Transformer

Hello everyone,
In this post we are going to discuss about Voltage regulation of a transformer.

Read here : Differences Between Core And Shell Type Transformers

What is meant my Voltage regulation of transformer?

Voltage regulation of a transformer may be defined as the difference between no load voltage of the secondary terminal of a transformer and full load voltage of the secondary terminal of that transformer at a certain power factor. Voltage regulation of a transformer is expressed in percentage of either no load secondary terminal voltage or full load secondary terminal voltage.

Read here : EMF Equation of Transformer & Voltage Transformation Ratio

Objective to calculate voltage regulation of transformer:

Calculating voltage regulation of transformer gives how much efficiently the transformer is resisting the voltage changes from no load to full load. If there is no change in value of secondary voltage from no load to full load then the transformer is ideal and has voltage regulation 0%. So the lower the value of voltage regulation the higher is the performance of the transformer.

Procedure to calculate voltage regulation of transformer:

Consider a transformer which is at no load which means the secondary of the transformer is open circuited. In this case the secondary voltage of the transformer and induced emf are same let it be E₂ . Now full load is connected to the secondary of a transformer. In this case current I₂ passes in the secondary which will lead to voltage drop and is given by I₂Z₂. Where Z₂ is called secondary impedance of transformer. During this situation primary winding will draw equivalent full load current. Because of the voltage drop the secondary voltage cannot be E₂ anymore so secondary induced emf will be V₂.

Equivalent circuit for calculating voltage regulation of transformer:

Equation for calculating voltage regulation of a transformer:

Voltage regulation of transformer in percentage can be represented as:

Voltage regulation % = (E₂-V₂/V₂)×100%. This is called regulation down. Power factor is specific.

Calculating voltage regulation of transformer for lagging power factor:

Now lets derive the expression for calculating voltage regulation of transformer for lagging power factor.

Phasor diagram:

here cos𝚹₂ is lagging power factor

From diagram,

OC = OA + AB + BC

OA = V₂

AB = AEcos𝚹₂= I₂R₂cos𝚹₂

BC=DEsin𝚹₂=I₂R₂sin𝚹₂

Angle between OC and OD is very less so OC is approximately equal to OD.

E₂ = OC = OA + AB + BC.

E₂= OC = V₂ + I₂R₂cos𝚹₂ + I₂R₂sin𝚹₂

Now voltage regulation of transformer at lagging power factor is,

Voltage regulation% = (E₂-V₂/V₂) × 100%

Voltage regulation%=( I₂R₂cos𝚹₂+ I₂R₂sin𝚹₂/ V₂ ) ×100%.

Calculating voltage regulation of transformer for leading power factor:

Now lets derive the expression for calculating voltage regulation of transformer for leading power factor.

Phasor diagram:

here cos𝚹₂is leading power factor

From diagram,

OC = OA + AB - BC

OA = V₂

AB = AEcos𝚹₂ = I₂R₂cos𝚹₂

BC = DEsin𝚹₂ = I₂R₂sin𝚹₂

Angle between OC and OD is very less so OC is approximately equal to OD.

E₂ = OC = OA +AB - BC.

E₂= OC = V₂ + I₂R₂cos𝚹₂ - I₂R₂sin𝚹₂

Now voltage regulation of transformer at leading power factor is,

Voltage regulation% = (E₂-V₂/V₂) × 100%

Voltage regulation%=(I₂ R₂cos𝚹₂ - I₂R₂sin𝚹₂ /V₂ ) ×100%.

Thus we have learnt what is voltage regulation of transformer and derived expressions for voltageregulation of transformer for lagging and leading power factors.

You can download this article of Voltage Regulation OF Transformer as a PDF here.

↧

Voltage Regulation Of Synchronous Machines [Alternator] By Direct Loading Test

January 2, 2017, 6:35 am

≫ Next: Voltage Regulation of Synchronous Machine (Alternator) by E.M.F Method or Synchronous Impedance Method.

≪ Previous: Voltage Regulation of Transformer

Voltage Regulation Of Synchronous Machines [Alternator] By Direct Loading Test

Today in this post we are going to learn what is Voltage regulation of synchronous machine and different methods to calculate Voltage regulation of synchronous machine.

Definition for Voltage regulation of a synchronous machine:

Voltage Regulation of synchronous machine is defined as the difference between terminal voltage at no load and terminal voltage at full load and excitation , speed must remain same.Voltage Regulation of synchronous machine is generally calculated in percentage of full load terminal voltage.

Objectives for calculating Voltage regulation of a synchronous machine:

1. Parallel operation of alternators is affected by the voltage regulation. By calculating voltage regulation of synchronous machine we can adjust the parallel operating machines to be in synchronism.

2. Calculating voltage regulation of a synchronous machine determines the type of automatic voltage control equipment required for resisting the voltage changes.

3.When the load is thrown off voltage rise must be known because with the rise in voltage the insulation must be able to withstand this rise.

So calculation of voltage regulation of synchronous machine has a great importance.

General expression for calculating Voltage regulation of synchronous machine:

Now let us derive general expression for calculating voltage regulation of a synchronous machine

Let E be the terminal voltage of the synchronous machine at no load. Now if the synchronous machine is given full load the terminal voltage will no longer be E because of the losses so let the terminal voltage now be V.

So general expression for Voltage regulation of a synchronous machine is given by

Voltage regulation% = (E - V / V) × 100

Methods for calculating voltage regulation of synchronous machine:

There are two types of methods for calculating voltage regulation of synchronous machine.

1. Direct load test method.

2. Indirect Method.

Indirect method of calculating voltage regulation of synchronous machine can be further classified into 3 types:

1.EMF method or Synchronous impedance method.

2. MMF method or Ampere turn method.

3.Zero power factor method or potier method.

Direct load test method for calculating voltage regulation of synchronous machine:

Now let's see how to calculate voltage regulation of synchronous machine by using direct load test method:

Circuit diagram for calculating Voltage regulation of synchronous machine by direct load test:

Circuit connections for calculating voltage regulation of synchronous machine by direct load test:

1.Firstly connections are to be made as given in the circuit diagram:

2. Armature which is star connected is connected to the three phase load with the help of TPST. TPST is a switch and it means triple pole single through.

3. A rheostat is connected in series with the field winding.

4. Field winding is excited by using D.C supply and flux is adjusted by adjusting the rheostat. Flux adjustment is nothing but adjust the current flow through field winding.

Procedure for calculating voltage regulation of synchronous machine by direct load test:

1. Adjust the prime mover such that the alternator rotates at synchronous speed Ns.

we know Eph α 𝞍 from emf equation

2. Now DC supply is given to the field winding and the current flow through field is adjusted so that the flux is adjusted such that the rated voltage is obtained at its terminals which can be seen on the voltmeter connected across the lines.

3. Now load is connected to alternator with the help of TPST switch.

4.The load is then increased such that the ammeter reads rated current. This is full load condition of alternator. Now as load is connected due to armature reaction there is loss in voltage so let the induced voltage be V.

5.Now again adjust the rheostat of the field winding to get rated voltage at alternator terminals.

6.Now remove the load by opening TPST switch and the excitation , speed should not be changed it should be same as before removing the load.

7. As there is no load there is no armature reaction the induced emf is equal to terminal voltage which is E.

Now we can calculate voltage regulation of synchronous machine by

Voltage regulation% =( E - V / V) × 100 at a specific power factor.

Limitations for calculating voltage regulation of synchronous machine by using direct load method:

This method is applicable only for small capacity machines for larger capacity machines it is not economical because that much load cannot be given directly.

In this way we have calculated the voltage regulation of synchronous machine by direct load test method.

For larger capacity machines voltage regulation can be calculated by Indirect method.

In the next post we can see how to calculate voltage regulation of synchronous machine by Indirect method.

You can download PDF form of voltage regulation of synchronous machines here

↧

Voltage Regulation of Synchronous Machine (Alternator) by E.M.F Method or Synchronous Impedance Method.

January 2, 2017, 10:40 am

≫ Next: Voltage regulation of synchronous machine by M.M.F method or ampere turn method

≪ Previous: Voltage Regulation Of Synchronous Machines [Alternator] By Direct Loading Test

Voltage regulation of synchronous machine by EMF method or synchronous impedance method

In this post we are going to learn how to calculate voltage regulation of synchronous machine by EMF method or synchronous impedance method.

In the previous post it has been clearly explained what is meant by voltage regulation of synchronous machine you can refer it in the below provided link.

Requirements for calculating voltage regulation by EMF method or synchronous impedance method:

1. Per phase resistance of armature Ra.

2.Graph of open circuit characteristics which is drawn between open circuit voltage and field current. This can be obtained by conducting open circuit test on the alternator.

3.Graph of short circuit characteristics which is drawn between short circuit voltage and field current. This can be obtained by conducting short circuit test on the alternator.

Open circuit test on synchronous machine:

Let's see how open circuit test on synchronous machine is done.

Circuit diagram for conducting open circuit test on synchronous machine:

Circuit connections for conducting open circuit test on synchronous machine:

1.Firstly connections are to be made as given in the circuit diagram:

2. Armature is connected to TPST switch terminals on one side the terminals of TPST switch on other side are short circuited with the help of ammeter.

3. An alternator is coupled to the prime mover which can drive the alternator at synchronous speed.

4. A voltmeter is connected across the lines to measure the open circuit voltage of alternator.

5. A rheostat is connected in series with the field winding.

6. Field winding is excited by using D.C supply and flux is adjusted by adjusting the rheostat. Flux adjustment is nothing but adjust the current flow through field winding.

Procedure for conducting open circuit test on synchronous machine:

1. By adjusting the prime mover make the synchronous machine to run at synchronous speed.

2.Now rheostat in the field circuit is kept at maximum position and switch on dc supply.

3. Now keep TPST switch in the open position.

4. Now by adjusting the rheostat field current is changed from minimum to maximum and the corresponding values of open circuit voltage is noted down.

Observations table for open circuit test on synchronous machine:

I_f A	V_oc(Line) V	V_oc(phase) = V_oc(line)/

By using above table draw the graph between E0 against If

Graph of open circuit characteristics of synchronous machine:

This is called open circuit characteristics of synchronous machine which is obtained by conducting open circuit test on synchronous machine.

Short circuit test on synchronous machine:

Let's see how short circuit test on synchronous machine is done.

Circuit diagram for conducting short circuit test on synchronous machine:

Circuit connections for conducting short circuit test on synchronous machine:

1.Firstly connections are to be made as given in the circuit diagram:

2. Armature is connected to TPST switch terminals on one side the terminals of TPST switch on other side are short circuited with the help of ammeter.

3. An alternator is coupled to the prime mover which can drive the alternator at synchronous speed.

4. A voltmeter is connected across the lines to measure the open circuit voltage of alternator.

5. A rheostat is connected in series with the field winding.

6. Field winding is excited by using D.C supply and flux is adjusted by adjusting the rheostat. Flux adjustment is nothing but adjust the current flow through field winding.

Procedure for conducting short circuit test on synchronous machine:

1. By adjusting the prime mover make the synchronous machine to run at synchronous speed.

2.Now rheostat in the field circuit is kept at maximum position and switch on dc supply so field current will have minimum value.

3.Now close the TPST switch as the ammeter has negligible resistance armature will be short circuited.

4.Adjust the field excitation until full load current is obtained through the ammeter connected to armature circuit.

5. Note down short circuited armature current value for different values of field current.

Observations table for short circuit test on synchronous machine:

I_f A	I_ascA

By using above table draw the graph between Iasc against If.

Graph of short circuit characteristics of synchronous machine:

The above graph is called short circuit characteristics of synchronous machine and is obtained by conducting short circuit test on synchronous machine. This curve resembles a B-H curve of a magnetic material.

Calculating synchronous impedance Zs from open circuit characteristics and closed circuit characteristics:

Now let's calculate synchronous impedance of synchronous machine.

Requirements for calculating synchronous impedance:

1.To calculate synchronous impedance we require values of open circuit emf and short circuit current

2.From short circuit test on synchronous machine short circuit current can be calculated and from open circuit test on synchronous machine open circuit voltage can be calculated

Short circuit test on synchronous machine equivalent circuit:

Procedure for calculating Short circuit current:

1.External load impedance of short circuit test is zero

2.So short circuit armature current flows through the impedance Zs.

3. voltage responsible for this short circuit current to flow is emf which is induced internally.

Now from the circuit,

Zs = Eph / Iasc.

The value of Iasc can be noted down from the ammeter reading but the voltmeter reading will be zero as it shows voltage across the short circuited terminals. So we need to calculate calculate the voltage which helps Iasc to flow through Zs which can be calculated by conducting open circuit test on synchronous machine

Open circuit test on synchronous machine equivalent circuit:

Procedure for calculating open circuit voltage:

From E.M.F equation we know that

Internally induced emf Eph is directly proportional to flux which means field current

Eph 𝝰 𝞍 𝛂 I_f

1. I_f is kept same as before in the short circuit test.

2. Now terminals of the synchronous machine is removed.

3. As I_f is same internally induced E.M.F will be same but current will be zero.

4. Now ammeter gives zero reading but voltmeter gives the open circuit e.m.f which is equal to internally induced e.m.f.

Now Eph = (Voc)ph since open circuit.

Now we can calculate synchronous impedance as

Zs = phase voltage on open circuit / phase current on short circuit ,at same excitation

Zs = (Voc)ph / (Iasc)ph at same I_f

In this way we can calculate Zs from open circuit characteristics of synchronous machine and short circuit characteristics of synchronous machine.

As Zs is different for different values of If we can calculate it from graph of open circuit characteristics of synchronous machine and short circuit characteristics of synchronous machine.

To calculate synchronous impedance Zs we need to draw open circuit characteristics and short circuit characteristics on a same graph as shown below:

Graph for open circuit characteristics and closed circuit characteristics:

Procedure for calculating synchronous impedance from open circuit characteristics of synchronous machine and short circuit characteristics of synchronous machine:

1. From short circuit characteristics of synchronous machine determine If required to drive full load short circuit current.

2. From the same I_f value draw a line such that it touches both open circuit characteristics of synchronous machine and short circuit characteristics of synchronous machine.

3. Now extend this line on to Y-axis which gives open circuit voltage and short circuit current.

4. Now calculate Zs from the below formula

Zs = (Voc)phase / (Iasc)phase where I_f is constant and I_f is at Isc = Irated.

It can also be calculated for different load conditions the process is same but Isc may not be equal to rated for the corresponding I_f.

Calculation of voltage regulation by E.M.F method or synchronous impedance method:

Now let's calculate voltage regulation of synchronous machine by E.M.F method or synchronous impedance method.

Few requirements are there to calculate voltage regulation of synchronous machine by E.M.F method or synchronous impedance method.

Requirements for calculating voltage regulation of synchronous machine by E.M.F method or synchronous impedance method:

1. Armature resistance per phase. This can be calculated by many methods one of the ways is applying known dc voltage across the two terminals and calculating the value of current. Now

Ra will be

Ra = v / i

2. synchronous impedance Zs which we have calculated in the before steps.

Expression for for calculating voltage regulation of synchronous machine by E.M.F method or synchronous impedance method:

Now let's see derivation for calculating voltage regulation of synchronous machine by E.M.F method or synchronous impedance method

Now,

From this synchronous reactance per phase is determined

Now no load E.M.F per phase Eph can be calculated by the following expression:

For lagging power factor we use positive sign and for leading power factor we use negative sign.

Now voltage regulation of synchronous machine by E.M.F method or synchronous impedance method is given by

Voltage regulation% = (Eph - Vph / Vph) × 100.

Value of Eph is calculated from above expression.

So we have determined voltage regulation of synchronous machine by E.M.F method or synchronous impedance method

Advantage of Calculating voltage regulation by E.M.F method or synchronous impedance method:

1. Zs at any load value can be determined so voltage regulation of alternator at any load condition and load power factor can be calculated

2. Total actual load need not to be connected for determining voltage regulation of synchronous machine by E.M.F method or synchronous impedance method.

Limitations of Calculating voltage regulation by E.M.F method or synchronous impedance method:

Here we have considered drop due to armature reaction as additional leakage reactance this method gives large values of synchronous reactance. This gives large values of percentage voltage regulation than actual value. This method is also called Pessimistic method.

Thus in this post we have calculated voltage regulation of synchronous machine by E.M.F method or synchronous impedance method.

In the next post we see how to calculate voltage regulation of synchronous machine by M.M.F method or ampere turn method.

You can download this post on voltage regulation of synchronous machine by E.M.F method or synchronous impedance method as PDF here.

↧

Voltage regulation of synchronous machine by M.M.F method or ampere turn method

January 3, 2017, 6:04 am

≫ Next: Voltage regulation of synchronous machine by zero power factor method or potier triangle method

≪ Previous: Voltage Regulation of Synchronous Machine (Alternator) by E.M.F Method or Synchronous Impedance Method.

Calculation of voltage regulation of synchronous machine by M.M.F method or ampere turn method

In this post let us see how to calculate voltage regulation of synchronous machine by M.M.F method or ampere turn method.

Requirements for Calculating voltage regulation of synchronous machine by M.M.F method or ampere turn method:

1. Any synchronous machine requires m.m.f to induce rated terminal voltage on open circuit. This m.m.f is denoted by Fo. To calculate this we conduct open circuit test on synchronous machine.

2. In the same way a synchronous machine also requires m.m.f to act opposite to armature reaction such that it helps full load current to flow in the armature.This m.m.f is denoted by Far.To calculate this we conduct short circuit test on synchronous machine.

3. From open circuit test on synchronous machine we obtain open circuit characteristics of synchronous machine and from short circuit test on synchronous machine we obtain short circuit characteristics of synchronous machine.

For details about open circuit test on synchronous machine , open circuit characteristics of synchronous machine and short circuit test on synchronous machine , short circuit characteristics of synchronous machine refer below link.

Graph for calculating voltage regulation of synchronous machine by M.M.F method or ampere turn method:

The graph shown below is the combined graph of open circuit characteristics of synchronousmachine and short circuit characteristics of synchronous machine.

Note: As in many cases we don't know the number of turns though m.m.f is product of current and turns here we express m.m.f in terms of field current.

What is meant by Fo and Far ?

Now let us see in detail about Fo and Far

1.Fo is the field m.m.f required to induce rated terminal voltage when the armature is open circuited. This value can be obtained from open circuit characteristics of synchronous machine byconducting open circuit test on synchronous machine.

2. Synchronous impedance has two components namely synchronous reactance and armature resistance .

3. Synchronous resistance further contains two components namely armature leakage reactance and armature reaction reactance.

4. In short circuit test onsynchronous machine field m.m.f is required to overcome drop across armature resistance, leakage reactance and armature reaction and allow full load current to pass through short circuited armature. But the drop due to armature resistance, leakage reactance is very small and can be neglected. So the m.m.f required to allow full load current to pass through short circuited armature by balancing armature reaction is Far which can be obtained from short circuit characteristics of synchronous machine by conducting short circuit test on synchronous machine.

Calculation of resultant m.m.f Fr for calculating voltage regulation of synchronous machine by M.M.F method or ampere turn method:

When the alternator supplies full load the total field m.m.f Fr is the vector sum of Fo and Far. And this depends on the power factor of load which the synchronous machine is supplying.

Now lets see how Fr is calculated for different load conditions:

Zero lagging power factor load:

1. If the load has zero power factor lagging then the armature reaction is demagnetizing in nature.

2. So resultant m.m.f Fr is algebraic sum of two vectors Fo and Far.

3. So here field m.m.f should be able to provide not only rated terminal voltage but also it should overcome demagnetizing armature reaction.

This can be represented as follows:

OA = Fo

AB = Far

OB = Fr = Fo + Far

This shows total field m.m.f is greater than Fo.

Steps to draw vector diagram for calculating resultant m.m.f Fr for lagging power factor load:

1. load power factor is lagging and it is represented by cos𝞍. So draw phase current Iaph which lags Vph by an angle 𝞍.

2. Fo is at right angle to Vph.

3. Far will be in phase with the Iaph because armature current Iaph decides armature reaction.

4. This Far has to be overcome by resultant m.m.f Fr which is also called field m.m.f so - Far should be added to Fo vertically so that Fr counter balances armature reaction and produce rated voltage.

Phasor diagram for calculating resultant m.m.f Fr for lagging power factor load:

Expression for resultant m.m.f or field m.m.f Fr for lagging power factor load:

From diagram,

OA = Fo

AB = Far

OB = Fr

From right angled triangle OCB

Far can be split into two parts

AC = Far sin𝞍

BC = Far cos𝞍

Hence, Fr can be calculated in this way.

Calculating voltage regulation of synchronous machine by M.M.F method or ampere turn method for lagging power factor load:

To calculate voltage regulation of synchronous machine by m.m.f method or ampere turn method draw the graph of open circuit characteristics of synchronous machine and short circuit characteristics of synchronous machine and indicate values of Fo , Far , Fr as shown below.

Graph for calculating voltage regulation of synchronous machine by M.M.F method or ampere turn method for lagging power factor load:

Steps for calculating voltage regulation of synchronous machine by M.M.F method or ampere turn method for lagging power factor load from graph:

1. Calculate Fo value from open circuit test on synchronous machine and mark it on x - axis. Now extend this point on to open circuit characteristics of synchronous machine curve and extend this point on y - axis which gives the value of Vph of synchronous machine.

2. Calculate Far value from short circuit test on synchronous machine and mark it on x - axis. Now extend this point on to short circuit characteristics of synchronous machine line and extend this point on y - axis which gives the value of rated Isc of synchronous machine.

3. Now calculate Fr value from the equation

and and mark it on x - axis. Now extend this point on to open circuit characteristics of synchronous machine curve and extend this point on y - axis which gives the value of Eph of synchronous machine.

So finally we get voltage regulation of synchronous machine by m.m.f method or ampere turnmethod for lagging power factor load by using below formula

Voltage regulation% = (Eph - Vph / Vph) × 100.

Hence in this way we have calculated voltage regulation of synchronous machine by m.m.f method or ampere turn method for lagging power factor load.

Zero leading power factor:

1. If the load has zero power factor leading then the armature reaction is magnetizing in nature.

2. This will help main flux to induce rated terminal voltage.

3. So net m.m.f is less than that required to produce rated voltage.

4. So net m.m.f is algebraic difference between the two components Fo and Far.

This can be represented as follows:

OA = Fo

AB = Fr

OB = Fr = Fo - FAar

This shows total m.m.f is less than Fo.

Steps to draw vector diagram for calculating resultant m.m.f for lagging power factor load:

1. load power factor is leading and it is represented by cos𝞍. So draw phase current Iaph which leads Vph by an angle 𝞍.

2. Fo is at right angle to Vph.

3. Far will be in phase with the Iaph because armature current Iaph decides armature reaction.

4. Fr is obtained by adding - Far to Fo.

Phasor diagram for calculating resultant m.m.f Fr for leading power factor load:

Expression for resultant m.m.f or field m.m.f Fr for leading power factor load:

From diagram,

AC = Far sin𝞍

BC = Far cos𝞍

OA = Fo

AB = Far

OB = Fr

From right angled triangle OCB

Hence, Fr can be calculated in this way.

Calculating voltage regulation of synchronous machine by M.M.F method or ampere turn method for leading power factor load:

Graph for calculating voltage regulation of synchronous machine by M.M.F method or ampere turn method for leading power factor load:

Steps for calculating voltage regulation of synchronous machine by M.M.F method or ampere turn method for leading power factor load from graph:

3. Now calculate Fr value from the equation

So finally we get voltage regulation of synchronous machine by m.m.f method or ampere turn method for leading power factor load by using below formula.

Voltage regulation% = (Eph - Vph / Vph) × 100.

Hence in this way we have calculated voltage regulation of synchronous machine by m.m.f method or ampere turn method for leading power factor load.

Important point to be noted while calculating voltage regulation of synchronous machine by m.m.f method or ampere turn method :

Fo is the field m.m.f required to give rated Vph when armature resistance is neglected. But if armature resistance Raph is given then Fo calculated from open circuit characteristics of synchronous machine represents excitation required to produce voltage of Vph + Iph Ra cos𝛟

Vph = rated voltage per phase.

Iaph = full load current per phase.

Ra = armature resistance per phase.

cos𝛟 = power factor of load.

Calculation of resultant m.m.f Fr by cosine rule:

Resultant m.m.f Fr can be calculated from cosine rule for both lagging and leading power factor loads.

Phasor diagrams:

by using cosine rule from triangle OAB,

In this way we can calculate Fr from cosine rule.

And hence calculate voltage regulation of synchronous machine by m.m.f method or ampere turn method by using

Voltage regulation% = (Eph - Vph / Vph) × 100.

Where Eph and Vph can be calculated from open circuit characteristics of synchronous machine and short circuit characteristics of synchronous machine as seen above.

In this method drop due to leakage reactance is also considered as drop due to armature reaction so we get voltage regulation less than actual regulation. Hence it is called optimistic method.

Today we have learnt how to calculate voltage regulation of synchronous machine by m.m.f method or ampere turn method .

In the next post we are going to learn voltage regulation of synchronous machine by zero power factor method or potier method.

You can download this post on voltage regulation of synchronous machine by m.m.f method or ampere turn method as PDF here.

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Voltage regulation of synchronous machine by zero power factor method or potier triangle method

January 4, 2017, 10:29 am

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Voltage regulation of synchronous machine by zero power factor method or potier method

Hello everyone,

Today in this post I am going to discuss about voltage regulation of synchronous machine by zero power factor method or potier method.

Already in the previous posts we have learnt what is meant by voltage regulation of a synchronous machine. To see this post click on the below provided link.

Advantage of calculating voltage regulation of synchronous machine by zero power factor method or potier method:

1. In the before methods that is voltage regulation of synchronous machine by e.m.f method or synchronous impedance method and voltage regulation of synchronous machine by m.m.f method or ampere turn method drop due to armature reaction is considered as leakage reactance drop and drop due to leakage reactance is considered as armature reaction drop respectively.So these two methods are away from reality and doesn't give correct value of voltage regulation.

To study voltage regulation of synchronous machine by e.m.f method or synchronous impedance method and voltage regulation of synchronous machine by m.m.f method or ampere turn method refer below link:

2.But while calculating voltage regulation of synchronous machine by zero power factor method or potier method we separate armature leakage reactance and armature reaction effects and calculate voltage regulation so we get almost correct value of voltage regulation by zero power factor method or potier method.

Tests required to be performed to calculate voltage regulation of synchronous machine by zero power factor method or potier triangle method:

As discussed above to separate armature reaction m.m.f and armature leakage reactance we perform following tests.They are:

1. Open circuit test.

2. Zero power factor test.

Open circuit test on synchronous machine:

Now let us discuss how to conduct open circuit test on synchronous machine.

Circuit diagram for conducting open circuit test on synchronous machine:

Below diagram shows circuit diagram for conducting open circuit test on synchronous machine.

Circuit connections for conducting open circuit test on synchronous machine:

1. Armature is star connected.

2. A potential divider is connected in series with the dc supply this whole setup is connected in series with the field which helps to adjust the excitation of the field.

3. A group of three parallel connected pure reactors are connected to a TPST switch S.

4. Switch S is kept open.

In this way circuit connections are to be made to conduct open circuit test on synchronous machine.

Procedure to conduct open circuit test on synchronous machine:

1. Make connections as per the circuit diagram.

2. Now with the help of prime mover make the synchronous machine to run at synchronous speed. This speed is to be maintained throughout the experiment.

3.Now switch on dc supply.With the help of potential divider vary the excitation from zero to rated value step wise and get the open circuit e.m.f from the voltmeter . Note down all the values which helps you to draw open circuit characteristics of synchronous machine.

4. open circuit characteristics of synchronous machine is a graph between If and (Voc)ph.

Here,

If = Field current.

(Voc)ph = open circuit voltage per phase.

Observations table for open circuit test on synchronous machine:

I_f A	V_oc(Line) V	V_oc(phase) = V_oc(line)/

Zero power factor test on synchronous machine:

Now let us discuss how to conduct zero power factor test on synchronous machine.

Circuit diagram for conducting zero power factor test on synchronous machine:

Below diagram shows circuit diagram for conducting zero power factor test on synchronous machine.

Circuit connections for conducting zero power factor test on synchronous machine:

1. Armature is star connected.

2. A potential divider is connected in series with the dc supply this whole setup is connected in series with the field which helps to adjust the excitation of the field.

3. A group of three parallel connected pure reactors are connected to a TPST switch S.

4. Switch S is kept closed.

In this way circuit connections are to be made to conduct zero power factor test on synchronous machine.

Procedure to conduct zero power factor test on synchronous machine:

1. Make connections as per the circuit diagram.

2. Now with the help of prime mover make the synchronous machine to run at synchronous speed. This speed is to be maintained throughout the experiment.

tive.

3. As the switch S is closed power is delivered to the purely reactive load by synchronous machine. The power delivered to the load is to be maintained at its rated full load value by adjusting the variable reactance of the reactor(inductor) and also by varying the excitation of field.

4. As the load is purely reactive load alternator will operate at zero power factor lagging.

5. Here the values to be noted are not many only two values are required to plot the graph for zero power factor test.

What are the requirements for plotting the graph to calculate the values of leakage reactance drop and armature reaction drop ?

1. Firstly we need to draw the open circuit characteristics of synchronous machine curve . This can be obtained by plotting the graph between open circuit voltage against field current whose values are obtained by conducting open circuit test on synchronous machine.

2. To obtain zero power curve we require two points they are:

a) At short circuit condition field current required to give full load short circuit armature current.

b) Field current required to give rated terminal voltage while delivering rated full load armature current.

Graph for calculating the values of leakage reactance drop and armature reaction drop:

Worried seeing the graph?

No need to worry readers. Let me explain you how to draw this graph step wise.

Steps for drawing the graph to calculate the value of leakage reactance drop and armature reaction drop:

1. Draw the open circuit characteristics curve. For different values of field current plot its corresponding values of open circuit voltage whose values are already tabulated by conducting the open circuit test on alternator.

2. Now plot the full load zero power factor curve by using two values.

a) Field current at short circuit full load zero power factor armature current which is denoted by A.

b) Field current required to give rated terminal voltage while delivering rated full load armature current which is denoted by P.

3. To the open circuit characteristics curve draw a tangent through the origin. This is called air gap line it is shown in the graph by dotted line OB.

4.Now draw a line PQ which is parallel and equal to OA as seen in the graph.

5.Now draw a line parallel to air gap line from Q such that it intersects the open circuit characteristics curve at R.

6. Now join RQ and RP.

7. The triangle obtained is called potier triangle.

8. Now from point R draw a perpendicular on to QP. It touches QP at point S.

9. The potier triangle obtained is constant for a given armature current.

10. Now draw a line parallel to PR through point A such that it meets the open circuit characteristics curve at B.

11. Now draw a perpendicular to OA from B which intersects OA at point C.

12. Now triangles OAB and PQR are similar triangles.

13. The length of the perpendicular RS gives the voltage drop due to armature leakage reactance i.e. X_Lph.

14. The length of PS gives field current required to overcome demagnetizing effect of armature reaction at full load.

15. Length SQ represents field current required to induce an e.m.f for balancing leakage reactance drop RS.

So armature leakage reactance can be obtained as follows

length (RS) = length( BC) = (Iaph)f.l × Xlph.

Xlph = length (RS) or length( BC) / (Iaph)f.l

Xlph is called potier reactance.

Determination of voltage regulation by zero power factor method or potier method using potier reactance:

In order to determine voltage regulation by zero power power factor method or potier method using potier reactance we need to draw a phasor to get required values i.e Eph and Vph.

Steps to draw phasor for calculating voltage regulation by zero power factor method or potier method using potier reactance:

1. Take the rated terminal voltage Vph as reference vector.

2. Depending upon the power factor cos𝞍 calculate value of 𝞍 and draw current phasor Iph lagging or leading Vph by an angle 𝞍.

3. Draw IphRaph voltage drop to Vph and it should be in phase with Iph.

4. Volatge drop IphXlph is to be drawn perpendicular to IphRaph vector and leading this IphRaph at the extreme point of Vph.

5.Raph is measure by applying known dc voltage to the Raph and calculating the current value Raph can be obtained by Raph = V/I.

6. Here Xlph is potier reactane.

Now we get E1ph from the above calculated values and it is given by,

7.Now from open circuit characteristics graph obtain value of excitation Ff1 corresponding to E1ph vector.

8.This Ff1 gives excitation required to induce e.m.f without considering the effect of armature reaction.

9. Field current Far required to balance armature reaction can be obtained from potier triangle.

10. Far = length ( PS ) = length ( AC).

11. Now the total excitation required is the vwctor sum of Ff1 and Far.

12. The procedure to obtain this is same as the procedure used in calculation of voltage regulation of synchronous machine by m.m.f method or ampere turn method.

You can see this post on voltage regulation by m.m.f method or ampere turn method from the below provided link.

13. Draw a vector Ff1 leading E1ph by 90°.

14. Iph anf Far are in same phase. Now Add -Far and Ff1.

15. Far can be obtained by drawing a vector opposite to Iph.

16. The total excitation to be supplied by field is nothing but Fr which is the resultant m.m.f or field m.m.f.

Phasor diagram for calculating voltage regulation by zero power factor method or potier method using potier reactance:

Steps for calculating voltage regulation of synchronous by zero power factor method or potier method using potier reactance:

1.Total excitation Fr is calculated by adding -Far and Ff1.

2. Now for this Fr corresponding value of e.m.f is calculated from open circuit characteristics graph. To understand refer calculation of voltage regulation of synchronous machine by m.m.f method or ampere turn method this link is already provided above.

3. length CD represents drop due to armature reaction.

4. Now draw perpendiculars from A and B onto current phasor. It intersects current phasor at points G and H respectively.

5. Now we obtain a right angled triangle OHC.

6. Now E1ph can be determined analytically by using pythogerous theorem.

(E1ph)² = (I_ph)²+ (I_phR_ph+ I_{ph Xph})²

7. In the similar manner we obtain Eph by using pythogerous theorem.

(Eph)² = (I_ph)²+ (I_phR_ph+ I_{ph Xph})²+ Armature reaction drop.

Now voltage regulation of synchronous machine by zero power factor method or potier triangle method is calculated by using,

Voltage Regulation% = (Eph - Vph / Vph) × 100

Here we get almost accurate value of voltage regulation as we have considered drops due to leakge reactance and armature reaction separately. Because of few assumptions made we get small deviation of voltage regulation from actual value of voltage regulation.

Assumptions made in calculating voltage regulation of synchronous machine by using zero power factor method or potier method:

1.Armature resistance is neglected in over all calculation of voltage regulation of synchronous machine by zero power factor method or potier triangle method. As this value is there will be no significant error due to this assumption.

2. Perfect reactor(inductor) is not present so practically we don't get zero power factor load.

look at graph that we have considered for calculating potier reactance.

3. In this we have assumed distances RS , R'S' and BC as equal. Which means that leakage reactance drop in power factor test and short circuit test are equal. But this cant be same as the excitation under short circuit condition i.e at point A is OA while for zero power factor test i.e, point P is OA'.Excitation OA' is higher than OC. P corresponds to saturation condition and has larger leakage flux. As this value is assumed to be unchanged we get error due to this.

In this way we have calculated voltage regulation of synchronous machine by zero power factor method or potier method.

You can download this post on voltage regulation of synchronous machine zero power factor method or potier method as PDF here.

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Speed Control Methods Of DC Motor

January 7, 2017, 1:47 am

≫ Next: Electrostatic type instruments working principle, construction, torque equation and extending range

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Speed Control Methods Of DC Motor

DC motors brought us revolutionary changes in industrial and domestic applications.This all because of an unique feature of DC motors i.e, speed controlling of DC motors .Compared to Synchronous motor , Induction Motor controlling the speed of DC series or DC shunt motor is very easy and efficient. Now let's get into the detailed explanation on how we can control speed of DC motor ?

What is speed control?

Speed controlling is nothing but changing our DC motor speed according to our requirements.We have different methods to control speed.DC motors are majorly categorized into DC series motors and DC shunt motors. We have separate methods of speed control for dc series motors and dc shunt motors.

Factors on which speed control of dc motors lie:

In DC motor an EMF induced in armature conductors due to the rotation of armature in magnetic field this is called back EMF (Eb). The magnitude of the E_b can be given by the EMF equation of a DC generator.

Eb = ^PØNZ/_60A

(where, P = no. of poles, Ø = flux/pole, N = speed in rpm, Z = no. ofarmature conductors, A = parallel paths)

E_bcan also be given as,
E_b = V- I_aR_a
thus, from the above equations
N = ^E_b 60A/PØ
but, for a DC motor A, P and Z are constants
Therefore, N ∝ K ^E_b/_Ø (where, K=constant)

This shows the speed of a dc motor is directly proportional to the back emf and inversely proportional to the flux per pole.

Speed control methods of dc motor:

As said above we have separate speed control methods for both dc shunt motors and dc series motors.

Let see each of them in detail.

Speed control of dc shunt motor:

There are three methods by which speed control of dc shunt motor is done.

1.Flux Control Method.

2.Armature Control Method.

3.Voltage Control Method

Voltage control method is further divided into two:

a) Multiple voltage control.

b) Ward-Leonard System.

Speed control of dc shunt motor by flux control method:

As we said speed of dc motor is inversely proportional to flux per pole. So by decreasing the flux per pole the speed of the dc shunt motor can be increased. To decrease the flux per pole we need to add a rheostat in series with the shunt field winding as shown in the following circuit.

By increasing the resistance current If decreases so flux decreases. As flux decreases speed of dc shunt motor increases as speed is inversely proportional to flux. This method is efficient because If2Rf value is small because If is small. It has limitation though it can gain maximum speed. Limitation arises because weakening of flux beyond the limit will adversely affect the commutation.

Speed control of dc shunt motor by armature control method:

As discussed speed of a dc motor is directly proportional to back e.m.f Eb where Eb = V - IaRa. By keeping V and Ra constant back e.m.f Eb depends on Ia.Add a resistance in series to the dc shunt motor armature as shown in the following circut.

Now by increasing the resistance current decreases.As current decreases back e.m.f Eb decreases. As back e.mf Eb decreases speed of the dc shunt motor decreases.

Speed control of dc shunt motor by voltage control method:

1. Multiple voltage control:

In this method the armature is supplied with variable voltage with the help of a suitable switch gear.The voltage across the shunt field is maintained constant. As the speed of dc shunt motor is directly proportional to voltage across the armature the speed of dc shunt motor can be controlled accordingly.

2. Ward - leonard system:

To understand speed control of dc shunt motor by this method see the circuit diagram given below.

Here motor M1 drives the generator G. Motor M1 has constant speed. M2 is the motor whose speed is required to be controlled.Generator G is coupled to M2.Here the voltage from the generator G is supplied to armature of motor M2. The voltage supplied by the generator can be varied smoothly with the help of field regulator . So by Ward - leonard system smooth speed control of dc shunt motor is obtained.

Speed control of dc series motor:

There are three methods for controlling speed of dc series motor.

1. Flux Control Method

This is further divided into four types. They are

a) Field divertor.

b)Armature divertor.

c)Tapped field control.

d)Paralleling field coils.

2. Armature - resistance control.

3. Series - parallel control.

Speed control of dc series motor by flux control:

Let us discuss speed control of dc series motors by different flux control methods.

Speed control of dc series motor by flux control by using field divertor:

Here we connect a variable resistance in parallel with the series field of dc series motor as shown in the following circuit.

As we have connected a variable resistance in parallel with the series field some of the current that has to flow through series field is diverted to variable resistance( this is the reason why it is called divertor). So the current flowing through the series field decreases as current decreases flux decreases. We know speed of dc motor is inversely proportional to flux as flux decreases speed of dc series motor increases so we can achieve high speeds with this method of speed control of dc motor.

Speed control of dc series motor by flux control by using armature divertor:

In this method we connect a variable resistance parallel to armature of dc series motor as shown in the following circuit.

As we have connected variable resistance in parallel to armature the armature current Ia decreases.We know torque equation for a dc series motor is Ta ∝ ØIa. So for a constant load torque if Ia decreases flux Ø increases to maintain load torque constant. So the dc series motor draws more current from the supply so flux Ø increases as flux increases speed decreases as speed of dc motor is inversely proportional to flux. In this way the speed of dc series motor can be achieved by this method.

Speed control of dc series motor by flux control using tapped field control:

Here the series field winding has tapping as shown in the following circuit.

By selecting the suitable tap the number of turns connected in the circuit can be changed. We know speed of dc motor is inversely proportional to flux by changing the number of turns of series field connected in circuit the flux can be changed. If more number of are turns are connected to the circuit then current decreases so flux decreases. As flux decreases speed increases. If number of turns connected in the circuit are less current will be high. As current is high flux will be high so speed of dc series motor will be less. So according to the required speed we can choose the required number of turns and control speed of dc series motor.

Speed control of dc series motor by flux control using paralleling field coils:

Here the series field coils are regrouped and are connected in parallel as shown in the following circuit diagram.

As the series field winding is regrouped and connected in parallel the flux will decrease since current decreases.As speed of dc motor is inversely proportional to flux the speed of dc series motor will be high. According to the required speed i.e, high or less the coils are regrouped.

Speed control of dc series motor by armature - resistance control:

In this method we connect a variable resistance in series with the series field winding as shown in the following circuit.

By adding the resistance in series with the series field the voltage across the armature is reduced. As the speed of dc series motor is directly proportional to voltage across the armature the speed of dc series motor is reduced accordingly. This is the most common method for speed control of dc series motor.

Speed control of dc series motor by series - parallel control:

This method is generally applicable for electrical traction. For two or more dc series motors which are coupled this method is possible. In this method to gain less speeds the two series motors are connected in series. In series current is high(as current division is not there) we get high current as current is high flux is high. As speed of dc motor is inversely proportional to flux we get low speed. When the two dc series motors are connected in parallel the current is less( since current s divided between two series motors)as current is less flux is less so high speed is obtained. So according to the required speed we connect the dc series motors either in series or parallel. The circuit diagram for speed control of dc series motor by this method is shown below.

In this way speed control of dc series motor and dc shunt motor can be done.

In this post we have learnt speed control of dc motors. To download this post on speed control of dc motors as PDF click here.

↧

Electrostatic type instruments working principle, construction, torque equation and extending range

January 11, 2017, 5:02 am

≫ Next: Synchronization of alternator and methods of synchronization of alternator

≪ Previous: Speed Control Methods Of DC Motor

Working principle, construction , torque equation and extending range of electrostatic type instruments

Hello readers,

In this post we are going to discuss about construction principle and torque equation of electrostatic type instruments.

Working principle of electrostatic type instruments:

Working principle of electrostatic type instruments is electrostatic effect.

What is meant by electrostatic induction??

To understand clearly about electrostatic effect see the below circuit.

1.Here the two plates are being charged by a high voltage battery.

2. Due to this one of the plate gets positive charge and the other plate gets negative charge.

3. Here the deflecting torque is produced by this static electrical field due to attraction present between these opposite charges.

4. The plates move because of the electrostatic force(attraction between plates) that has been produced because of this induced charges.

This effect is called electrostatic effect.

Construction of electrostatic type instruments:

1. Linear type electrostatic instruments.

2. Rotatory type electrostatic instruments.

Linear type electrostatic instruments:

1. Here one of the plates is fixed and the other plate is movable and these plates are charged as shown in the above circuit. So one of the plate gets positive charge and the other plates gets negative charge. Due to this there will be force of attraction between the plates so the movable plate moves towards the fixed plate until movable plate gains maximum amount of electrostatic energy. Fixing pointer to the movable plate we can measure the voltage. These are called linear type electrostatic instruments.

Rotatory type electrostatic instruments:

2. Here we have a rotatory plate. Due this movement of rotatory plate there may be force of attraction or repulsion between the plates. These are called rotatory type electrostatic instruments.

Torque equation of electrostatic type instruments:

Now let us see torque equation of both linear type electrostatic instruments and rotatory type electrostatic instruments.

Torque equation of linear type electrostatic instruments:

Let us see in detail about torque equation of liner type electrostatic instruments.

Observe the following diagram.

1.Here plate A is fixed and it is positively charged and plate B is movable and it is negatively charged.

2. As the forces are opposite we have attraction between plates. So there will be linear motion between these plates.

3. As there is force between these plates at equilibrium electrostatic force will be equal to spring force.

4.Now electrostatic energy stored in the plate is given by,

5.Now let us increase the voltage by a small amount let it be dv due to this there will be displacement of plate let the displacement be dx. So work done against the spring force due to displacement of plate B be F.dx. Relation between current and applied voltage is given by,

6. Now the input energy from this value of electric current is given by,

7. Now the change in this stored energy is given by,

8. Now apply principle of energy conservation by neglecting the higher order terms in the expression.

Input energy to the system = increase in the stored energy of the system + mechanical work done by the system.

By substituting all the values we get,

Now the equation of force from the above equation is given by,

Torque equation of rotatory type electrostatic instruments:

Let us see in detail about torque equation of rotatory type electrostatic instruments.

Observe the following diagram.

1. By replacing F, dx in equation (1) by Td , dA respectively we get deflecting torque of rotary type electrostatic instruments.

2. So the deflecting torque is given by,

3.At steady state we have controlling torque is given by, Tc = K × A. Where A is the deflection and it is given by,

As the deflection is directly proportional to square of voltage we have non- uniform scale.

Hence we have derived torque equation of electrostatic type instruments i.e for liner type electrostatic instruments and rotatory type electrostatic instruments.

Generally electrostatic type instruments are used for measuring high voltages.

The main advantage of using electrostatic type instruments as voltmeters is we can extend the range of voltage that is to be measured.

Methods to extend the range of voltage to be measured for electrostatic instruments:

1. Resistance potential dividers.

2.Capacitor multiplier technique.

Resistance potential dividers to extend the range of voltage to be measured for electrostatic instruments:

Now let us see how to extend the range of voltage to be measured by using resistance potential dividers.

To understand it see the below circuit.

Circuit diagram of resistance potential dividers to extend the range of voltage to be measured for electrostatic instruments:

The following diagram shows the circuit to extend the range of voltage to be measured by

electrostatic instrumentsusing resistance potential dividers.

Procedure to extend the range of voltage to be measured by electrostatic instruments using resistance potential dividers:

1. Across r which is total resistance apply the voltage which is to be measured.

2. Across R which is a part of total resistance r connect an electrostatic capacitor.

3.Make one assumption that the capacitor which is connected is having infinite leakage resistance in case if we apply dc voltage. Here the multiplying factor is ratio of resistances i.e, r/R. Multiplying factor in ac case is same as dc case.

Capacitor multiplier technique to extend the range of voltage to be measured for electrostatic instruments:

Now let us see how to extend the range of voltage to be measured by electrostatic instruments

using capacitor multiplier technique.

To understand it see the below circuit.

Circuit diagram of capacitor multiplier technique to extend the range of voltage to be measured for electrostatic instruments:

The following diagram shows the circuit to extend the range of voltage to be measured by electrostatic instruments using capacitor multiplier technique.

Procedure to extend the range of voltage to be measured by electrostatic instruments using capacitor multiplier technique:

Let us calculate the multiplying factor.

1. From diagram we have series combination of capacitors. The equivalent capacitance is given by

2. Voltmeter impedance is given by Z1 = 1/jωC1 . Now total impedance is given by,

3.Multiplying factor is given by,

Z/Z1 = 1 + C2 / C1.

In this way we can extend the range of voltage to be measured by electrostatic instruments withthe help of resistance potential dividers and capacitor multiplier technique.

Advantages of electrostatic type instruments:

1. As the deflection torque is directly proportional to square of voltage we can measure both a.c and d.c voltages by using electrostatic type instruments.

2.High values of voltage can be measured using electrostatic type instruments.

3. Current drawn by electrostatic type instruments is low so power consumption of electrostatic type instruments is low.

Disadvantages of electrostatic type instruments:

1.Electrostatic type instruments have non uniform scale.

2.Electrostatic type instruments are larger in size.

3.Electrostatic type instruments are costlier compared to other type of instruments.

4.Various operating forces present in electrostatic type instruments are small in magnitude.

Today we have learnt working principle, construction , torque equation and extending range of electrostatic type instruments.

You can download this article about working principle, construction , torque equation and extending range of electrostatic type instruments as PDF here.

↧

Synchronization of alternator and methods of synchronization of alternator

January 11, 2017, 5:03 am

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Synchronization of alternator and methods of synchronization of alternator

What is meant by synchronization of alternator?

Connecting a group of alternators parallel to a bus bar and the alternators should have same voltage and frequency as that of bus-bar. This is called synchronization of alternator. There are some conditions to be satisfied by the alternators which are to be connected in parallel to bus-bar to be in synchronization.

Conditions for synchronization of alternators:

1. The terminal voltage of incoming alternator must be equal to the bus bar voltage.

2. The frequency of voltage generated by incoming alternator must be equal to busbar frequency.

3.The phase sequence of the three phases of the incoming alternator must be same as phase sequence of bus-bars.

4. The phase angle between the voltage generated by incoming alternator and voltage of bus-bar must be zero.

5. Always connect running alternator to bus-bar. If a stationary alternator is connected to bus-bar it will result in short circuit of stator winding.

The above conditions are to be satisfied by alternators to satisfy synchronization.

Why synchronization of alternators is necessary?

1.An alternator cannot deliver power to electric power system until its voltage,frequency,phase sequence and other parameters matches with the network to which the the alternator is connected.

2. The case of synchronization arises because we are connecting many alternators in parallel to supply the demanded load. So we need to match all the parameters of connected alternators with bus-bar to deliver power to load.

3. By synchronization we can match all the parameters of one alternator with the other alternator and also with the bus-bar and deliver the required power to load.

4. Synchronization of alternator is also called as paralleling of alternators.

Advantages of paralleling of alternators:

We get a common doubt why we need to supply the load by paralleling small units of alternator rather than using a single larger unit? This is because we have many advantages by doing so. They are:

Continuity of service:

In case of any damage to one of the alternators it can be removed.Supply to load is not interrupted because other alternators can supply the required load. But if u use a larger single unit even a small damage causes the interruption of supply.

Requirement of load:

As the load demanded is not same all the time, during light load periods we can run two or three alternators in parallel. When the demand is high we can add the required amount of alternators in parallel to meet the load demanded.

Reliability:

Several single units connected in parallel is more reliable than single larger unit because if a single unit gets damaged it can be removed and its work is compensated by other units which are running.

High efficiency:

An alternator runs efficiently when it is loaded at their rated value. By using required number of alternators for required demand i.e, light load or peak load we can load an alternator efficiently.

So because of above advantages we use paralleling of alternators.

Steps to connect alternators in parallel or synchronization of alternators:

1.Consider an alternator-1. It is supplying power to bus bar at rated voltage and frequency.

2. Now we need to connect another alternator let it be alternator-2 in parallel with the alternator-1. In order to match the frequency of alternator-2 with the frequency of bus-bar or alternator-1 (since alternator-1 and bus-bar are already in synchronism) we need to adjust the speed of alternator-2. Now the voltage of alternator-2 is to be matched with the voltage of bus-bar or voltage of alternator-1 (since alternator-1 and bus-bar are already in synchronism). For this purpose we need to vary the field rheostat until the voltage matches.

3. The three phase voltages generated by alternator must be same as the three phase voltages of bus-bar or alternator-1(since alternator-1 and bus-bar are already in synchronism).This can be achieved by matching the phase sequence and frequency of alternator-2 with bus bar or alternator-1(since alternator-1 and bus-bar are already in synchronism) phase sequence and frequency.

By following these steps synchronization of alternators is possible.

Methods for synchronization of alternators:

There are three methods for synchronization of alternators. These methods check whether the above mentioned conditions for synchronization of alternators is satisfied or not.The three methods are.

1. Three dark lamps method.

2. Two bright, One dark method.

3. Synchroscope method.

Three dark lamps method for synchronization of alternators:

Let us study synchronization of alternators using three dark lamps method in detail.

Circuit diagram for synchronization of alternators using three lamp method:

Procedure:

1. Consider alternator-1 is supplying power to load at rated voltage and rated frequency which means alternator-1 is already in synchronism with bus-bar.

2. Now we need to connect alternator-2 in parallel with alternator-1.

3. Across the 3 switches of alternator-2 three lamps are connected as shown in the figure.

4. To match the frequency of alternator-2 with the bus-bar frequency we need to run the prime mover of alternator-2 at nearly synchronous speed which is decided by the frequency of bus-bar and number poles present in alternator-2.

5. To match the terminal voltage of alternator-2 with bus-bar voltage we need to adjust the field current of alternator-2 until terminal voltage of alternator-2 matches with the bus-bar voltage. The required value of voltage can be seen in the voltmeter connected to bus-bar.

6.To know whether the phase sequence of alternator -2 matches with the bus-bar phase sequence we have a condition. If all the three bulbs ON and OFF concurrently then we say the phase sequence of alternator-2 matches with the phase sequence of bus-bar. If the bulbs ON and OFF one after the other then the phase sequence is mismatching.

7. To change the connections of any two leads during the mismatch of phase sequence first off the alternator and change the connections.

8. ON and OFF rate of bulbs depends upon frequency difference of alternator-2 voltage and bus-bar voltage. Rate of flickering of bulbs is reduced when we match the frequency of alternator-2 with bus-bar voltage by adjusting the speed of prime mover of alternator-2

9. If all the conditions required for synchronization are satisfied then the lamps will become dark.

10. Now close the switches of alternator -2 to synchronize with alternator-1.

11. Now the alternators are in synchronism.

Disadvantage of three dark lamps method for synchronization of alternators:

Flickering only says difference between frequency of voltages of alternator and bus bar but correct value of frequency of voltage of alternator cannot be found.

For example, if the bus bar frequency of voltage is 50 HZ and difference in frequency of voltage of bus-bar and alternator is 1 HZ the alternator frequency of voltage can be either 49 HZ or 51 HZ.

Two bright and one dark lamp method for synchronization of alternators:

Let us discuss synchronization of alternator using two bright and one dark lamp method.

Circuit diagram for synchronization of alternators using two bright and one lamp method:

Procedure:

1. Consider alternator-1 is supplying power to load at rated voltage and rated frequency which means alternator-1 is already in synchronism with bus-bar.

2. Now we need to connect alternator-2 in parallel with alternator-1.

3. Here lamp L-2 is connected similar to the three dark lamp method.

4. Lamps L-1 and and L-3 are connected in different manner. One end of lamp L-1 is connected to one of the phases other that the phase to which lamp L-2 is connected and the other end of lamp L-1 is connected to the phase to which lamp L-3 is connected.

5.Similarly one end of lamp L-3 is connected to a phase other than the phase to which lamp L-2 is connected and other end of lamp L-3 is connected to the phase to which lamp L-1 is connected as shown in the following circuit.

6. To match the terminal voltage of alternator-2 with bus-bar voltage we need to adjust the field current of alternator-2 until terminal voltage of alternator-2 matches with the bus-bar voltage. The required value of voltage can be seen in the voltmeter connected to bus-bar.

7. Depending upon the sequence of lamps L1,L2, L3 becoming dark and bright we can decide whether the alternator-2 frequency of voltage is higher or lower than bus-bar frequency.

8. If the sequence of bright and dark of lamps is L1-L2-L3 then the frequency of voltage of alternator-2 is higher than the bus-bar voltage. Now until the flickering reduces to a low value decreases the speed of prime mover of alternator-2.

9. If the sequence of bright and dark of lamps is L1-L3-L2 then the frequency of voltage of alternator-2 is less than the bus-bar voltage. Now until the flickering reduces to a low value increase the speed of prime mover of alternator-2.

10. When the L1 and L3 are equally bright and lamp L2 is dark then close the switches.

11. Now the alternators are in synchronism.

Disadvantage of two bright and one dark lamp method for synchronization of alternators:

Phase sequence of the alternator cannot be checked by this method.

Synchroscope method for synchronization of alternators:

Let us discuss synchronization of alternator using synchroscope method.

Circuit diagram for synchronization of alternators using synchroscope method:

Procedure:

1. A synchroscope is used to achieve synchronization accurately.

2. It is similar to two bright and one dark lamp method and tells whether the frequency of incoming alternator is whether higher or lower than bus bar frequency.

3. This contains two terminals they are a) existing terminal b) incoming terminal.

4. Existing terminals are to be connected to bus-bar or existing alternator here in the diagram it is alternator-1 and incoming terminals are connected to incoming alternator which is alternator-2 according to the diagram which we have considered.

5. Synchroscope has a circular dial inside which a pointer is present and it can move both in clockwise and anti clockwise direction.

7. Depending upon the rate at which the pointer is rotating the difference of frequency of voltage between incoming alternator and bus-bar can be known.

8. And also if the pointer moves anti clockwise then the incoming alternator is running slower and has frequency less than the bus bar or existing alternator frequency and if the pointer moves clock-wise then the incoming alternator is running faster and has frequency greater than bus-bar or existing alternator frequency. So by adjusting the speed of prime mover of incoming alternator we can match the frequency with bus bar or existing alternator frequency. Frequency matches when the pointer is straight up-wards. At this point close the switch.

9. Now both the alternators are in synchronism.

So by these three methods synchronization of alternators is checked.

Today in this post we have learnt what is meant by synchronization of alternator and methods of synchronization of alternator.

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