e m 1 lab manual

A LABORATORY MANUAL IN

ELECTRICAL MACHINES – II

LIST OF EXPERIMENTS

Regulation of three phase alternator by EMF and MMF methods

Regulation of three phase alternator by ZPF and ASA methods Regulation of three phase salient pole alternator by slip test Measurements of negative sequence and zero sequence

impedance of alternator V and inverted V curves of three phase synchronous motor Load test on three phase induction motor No load and Blocked rotor test on three phase induction motor Separation of No load losses of three phase induction motor Load test on single phase induction motor No load and Blocked rotor test on single phase induction motor

S.NO CONTENTSName of the Experiment

1.

2.

3.

4.

5.

1.

2.

3.

4.

5.

SYNCHRONOUS MACHINES

Regulation of three phase alternator by EMF and MMF methods

Regulation of three phase alternator by ZPF and ASA methods

Regulation of three phase salient pole alternator by slip test

Measurements of negative sequence and zero sequence impedance of alternator

V and Inverted V curves of three phase synchronous motor

INDUCTION MOTOR

Load test on three phase induction motor

No load and Blocked rotor test on three phase induction motor

Separation of no load losses of three phase induction motor Load test on single phase induction motor

No load and Blocked rotor test on single phase induction motor

2

REGULATION OF THREE PHASE ALTERNATOR BY EMF AND MMF METHOD

AIM:

The aim of the experiment is to predetermine the regulation of three phase alternator by EMF and MMF methods.

NAME PLATE DETAILS:

3Ф alternator Dc shunt motor Volts:

Amps:

KVA:

RPM:

FUSE RATING:

125% of rated current (full load current)

For Dc shunt motor:

For Alternator:

APPARATUS REQUIRED:

S.NO NAME OF THE APPARATUS

RANGE TYPE QUANTITY

1.2.3.4.5.6.7.8.9.10.

AmmeterAmmeterAmmeterAmmeterVoltmeterVoltmeterRheostatRheostat

TachometerConnecting wires

(0-2)A(0-10)A(0-10)A(0-10)A(0-600)V(0-150)V

370Ω,1.5A270Ω,1.5A

--

MCMCMCMIMIMI

Wire woundWire wound

Digital-

111111111

As required

3

FORMULA USED:

EMF METHOD:

1. Armature resistance Ra = 1.6*RdcRdc = Resistance in dc supply

2. Synchronous impedance Zs = Open circuit voltage(E1(ph))/ Short circuit current(Isc) (From the graph)

3. Synchronous impedance Xs = √(Zs2 – Ra2) 4. Open Circuit voltage Eo = √(Vrated CosФ + IaRa) 2+ (Vrated sinФ + IaXs)2(For

lagging power factor)5. Open circuit voltage Eo = √Vrated + IaRa)2 + (IaXs)2 (For unity power factor)6. Percentage regulation = Eo-Vrated / Vrated *100

(For both EMF and MMF method)

THEORY:

EMF METHOD:

i) Open circuit characteristics are plotted from the data obtained from the open circuit test.

ii) Short circuit characteristics are drawn from the data given by the short circuit test. It is a straight line passing through the origin.

Consider a field current If. The open circuit voltage corresponding to this field current is Eo. When winding short circuited the terminal voltage is zero. Hence it may be assumed that the whole of this voltage Eo is being used to circulate the armature short circuit current Ia against the synchronous impedance Zs,

Eo= IaZs

iii) Knowing Ra and Xs, Vector diagram can be drawn for any load and any power factor.

MMF METHOD:

This method also utilizes open and short circuit data and it is the converse of the emf method in the sense, that armature leakage reactance is treated as an additional armature reaction.

Now field ampere turn (AT) required to produce a voltage of V on full load is the vector sum of the following.

4

i) Field AT required to produce rated voltage V on no load. This can be found from open circuit curve.

ii) Field AT required to overcome the demagnetizing effect of armature reaction on full load. This value is found from short circuit test. The field AT required to produce full load current on short circuit balances the armature reaction and the impedance drop.

The impedance drop can be neglected because Ra is usually very small and Xs is also small. Under short circuit power factor is almost zero lagging and the field AT are used entirely to overcome the armature reaction which is wholly demagnetizing. AT on full load are equal and opposite to the field AT required to produce full load current on short circuit.

PRECAUTIONS:

i) The motor field rheostat should be kept in minimum resistance position.ii) The alternator field rheostat should be in maximum position.iii) Initially all switches are in open position.

PROCEDURE:

i) Note down the name plate details of motor and alternator.ii) Connections are made as per the circuit diagram.iii) Give the supply by closing the DPST switch.iv) Using 3 point starter, start the motor to run at the synchronous speed by varying

the motor field rheostat.v) Conduct an open circuit test by varying the rheostat of alternator for various

values of field current and tabulate the corresponding open circuit voltage readings.

vi) Conduct a short circuit test by closing the TPST switch and adjust the rheostat of alternator to set the rated armature current , tabulate the corresponding field current.

vii) Conduct a stator resistance test by giving connections as per the circuit diagram and tabulate the voltage and current readings for various resistive loads.

HOW TO DRAW THE GRAPH FOR EMF METHOD:

i) Draw the open circuit characteristics curve (V vs. If).ii) Draw the short circuit characteristics curve (short circuit current Vs If).iii) From the graph find the open circuit voltage per phase ( E1(ph)) for the rated short

circuit current ( Isc).iv) By using respective formulae find the Zs, Xs, Eo and percentage regulation.

5

HOW TO DRAW THE GRAPH FOR MMF METHOD:

i) Draw the open circuit characteristics curve (V vs. If).ii) Draw the short circuit characteristics curve (short circuit current Vs If).iii) Draw the line OL to represent If, which gives the rated generated voltage (V).iv) Draw the line LA at an angle (90 +or –Ф) to represent If which gives the rated full

load current (Isc) on short circuit. (90+Ф) for lagging power factor and (90-Ф) for leading power factor.

v) Join the points O and A and find the field current (if) by measuring the distance OA that gives the open circuit voltage (Eo) from the open circuit characteristics.

vi) Find the percentage regulation by using suitable formula.

RESULT:

6

TABULAR COLUMN:

OPEN CIRCUIT TEST:

S.NO Field Current (If)

(amps)

Open Circuit Voltage(VOL)

(volts)

Open Circuit Phase Voltage

(Voph) (volts)

SHORT CIRCUIT TEST:

S.NO Field Current (If) (amps) Short Circuit Current (120 to 150%of rated current)(Isc) (amps)

ARMATURE RESISTANCE Ra:

S.NO Armature Current

(I) (amps)

Armature Voltage(V)

(volts)

Armature Resistance

Ra= V/I ohm

7

VIVA QUESTIONS:

1) What are synchronous machines?

The machines generating ac emf are called alternating or synchronous generators. While the machine accepting input from ac supply to produce mechanical output are called synchronous motors. Both these machines work at a specific constant speed called synchronous speed and hence in general called synchronous machines.

2) Define voltage regulation. Name two methods used to determine voltage regulation of alternators. %Reg = E-Vrated / Vrated *100 Where E = No load voltage Vrated = Rated voltageTwo methods to determine voltage regulation:i) EMF methodii) MMF method

3) What are the two types of alternators?i) Non salient pole alternatorii) Salient pole alternator.

4) State the principle of alternator. When the rotor is rotated by the prime mover, the stator windings or conductors are cut by the magnetic flux hence an emf is induced in the stator conductors. (Faraday’s law of electromagnetic induction).

5) Is EMF method an accurate method? No, it is not an accurate method because the value of synchronous impedance found is always more than the original value.

6) Write the emf equation of an alternator. E = 4.44f ФTKcKd volts Where, f = frequency in hertz Ф = flux per pole T = Number of turns in stator windings

Kc = Pitch factor

Kd = Distribution factor

7) What is known as Armature reaction? The effect of armature flux on main flux is called as armature reaction.

8

8) What is meant by synchronous reactance?

Synchronous reactance Xs = Xl + Xa

Xl = leakage reactance

Xa = Armature reactance

9) Can a DC generator be converted into an alternator? How? Yes, by providing two collector rings on end of the armature and connecting these two rings to two points in the armature windings 180 degree apart.

10) What is the other name for EMF and MMF method? The other name for EMF method is called as Synchronous impedance method and MMF method is called as Ampere turn method. EMF method – Pessimistic method MMF method – Optimistic method

REGULATION OF THREE PHASE ALTERNATOR BY ZPF AND ASA METHOD

9

AIM:

The aim of the experiment is to predetermine the regulation of three phase alternator by ZPF and ASA methods.

NAME PLATE DETAILS:


Amps:

KVA:

RPM:

FUSE RATING:


For Dc shunt motor:

For Alternator:

APPARATUS REQUIRED:


RANGE TYPE QUANTITY

1.2.3.4.5.6.7.8.9.10.11.12.

AmmeterAmmeterAmmeterAmmeterVoltmeterVoltmeterRheostatRheostatRheostat

Inductive loadTachometer

Connecting wires

(0-2)A(0-10)A(0-10)A(0-10)A(0-600)V(0-75)V

370Ω,1.5A270Ω,1.5A1000Ω,1A

3 phase--

MCMCMCMIMIMC

Wire woundWire woundWire wound

-Digital

-

11111111111

As required

FORMULA USED:

1. Percentage regulation = Eo – Vrated / Vrated *100

(For both ZPF and ASA method)

10

THEORY: ZERO POWER FACTOR METHOD:

ZPF method is based on the separation of armature leakage reactance and armature reaction effects. To determine armature leakage reactance and armature reaction mmf separately, two tests are performed on the alternator. The two tests are

1. Open circuit test

2. Short circuit test

3. Zero power factor tests

ASA METHOD:

ASA means American standard Association method. This is a modification of mmf method. In these methods, the magnetic circuit is assumed to be unsaturated. If we consider the saturated magnetic circuit, the resultant excitation is not If, but it is If’. This additional excitation can be obtained by ASA method.

PRECAUTIONS:

i) The motor field rheostat should be kept in minimum resistance position.ii) The alternator field rheostat should be in maximum position.iii) Initially all switches are in open position.

PROCEDURE:

i) Note down the name plate details of motor and alternator.ii) Connections are made as per the circuit diagram.iii) Give the supply by closing the DPST switch.iv) Using 3 point starter, start the motor to run at the synchronous speed by varying

the motor field rheostat.v) Conduct an open circuit test by varying the rheostat of alternator for various

values of field current and tabulate the corresponding open circuit voltage readings.

vi) Conduct a short circuit test by closing the TPST switch and adjust the potential divider to set the rated armature current, tabulate the corresponding field current.

vii) Conduct a ZPF test by adjusting the potential divider for full load current passing through either inductive load or capacitive load with zero power and tabulate the readings.

viii) Conduct a stator resistance test by giving connection as per the circuit diagram and tabulate the voltage and current readings for various resistive loads.

PROCEDURE TO DRAW THE POTIER TRIANGLE: (ZPF METHOD)

1. Draw the open circuit characteristics curve (Generated voltage per phase Vs field current).

11

2. Mark the point A at X axis, which is obtained from short circuit test with full load armature current.

3. From the ZPF test, mark the point B for the field current to the corresponding rated armature current and the rated voltage.

4. Draw the ZPF curve which is passing through the point A and B in such a way parallel to the open circuit characteristic curve.

5. Draw the tangent for the OCC from the origin (Air gap line).6. Draw the line BC from B towards Y axis which is parallel and equal to OA.7. Draw the parallel line for the tangent from c to the OCC curve.8. Join the point B and D also draws a perpendicular line DE to BC.

DE = Armature leakage reactance drop

BC=Armature reaction excitation

PROCEDURE TO DRAW THE POTIER TRIANGLE: (ASA METHOD):

1. Draw the open circuit characteristics curve (Generated voltage per phase Vs field current).

2. Mark the point A at X axis, which is obtained from short circuit test with full load armature current.

3. From the ZPF test, mark the point B for the field current to the corresponding rated armature current and the rated voltage.

4. Draw the ZPF curve which is passing through the point A and B in such a way parallel to the open circuit characteristic curve.

5. Draw the tangent for the OCC from the origin (Air gap line).6. Draw the line BC from B towards Y axis which is parallel and equal to OA.7. Draw the parallel line for the tangent from c to the OCC curve.8. Join the point B and D also draws a perpendicular line DE to BC.

DE = Armature leakage reactance drop

BC=Armature reaction excitation

9. Extend the line BC towards the Y axis up to the point O. The same line intersects the air gap line at point G.

10. Mark the point I in Y axis with the magnitude of Eair and draw the line from I towards OCC curve which should be parallel to X axis. Let this line cut the air gap line at point H and the OCC curve at point F.

11. Measure the length of O’G, HF and OA

RESULT:

TABULAR COLUMN:

OPEN CIRCUIT TEST:

12


(amps)

Open Circuit Voltage(VOL)

(volts)

Open Circuit Phase Voltage

(Voph) (volts)

SHORT CIRCUIT TEST:


(amps)

Short Circuit Current (120 to 150%of rated current)

(Isc) (amps)

ZERO POWER FACTOR TEST:

S.NO Field current (If)

(amps)

Rated armature

current (Ia) (amps)

Rated armature Voltage (V)

(volts)

W1

Obs Act

(watts)

W2

Obs Act

(watts)

Total power (W1+W2) (watts)


S.NO Armature Current Armature Voltage(V) Armature Resistance

13

(I) (amps) (volts) Ra= V/I ohm

VIVA OUESTIONS:

1. Why is the field system of an alternator made as a rotor?

14

The field system of an alternator is made rotating to avoid interaction of mechanical and electrical stress. So with rotating field system, it is easier to collect currents at very high voltages from stationary member. The insulation required is less; the problem of sparking is avoided.

2. Define the terms distribution factor and pitch factor. The factor by which there is a reduction in the emf due to distribution of coils is called distribution factor, denoted by Kd. The factor by which induced emf gets reduced due to short pitching of coil is called pitch factor, denoted by Kc.

3. Potier method is also called Potier reactance method. Why? It is based on the separation of armature leakage reactance and armature reaction effects. The armature leakage reactance XL is called Potier reactance in this method, hence this method is also called as Potier reactance method.

4. What are the experimental data required for Potier method?i) Data’s obtained from open circuit test to draw the OCC curveii) Field current required to obtain the zero armature voltage and the field

current to obtain the rated armature current. These two field currents are required to draw the ZPF curve.

5. What is ASA method? This is American Standard Association method. Both the EMF and MMF method is capable of giving the reliable values of the voltage regulation, the error is present. So to rectify that a modification of MMF method is introduced called as ASA method. It includes the additional excitation.

6. Whether the results obtained by ASA method is reliable for salient pole machines. Yes, it is reliable for salient pole and Non salient pole machines.

7. Compare EMF method and Potier method.

S.no EMF METHOD POTIER METHOD1.

2.

Regulation of alternator of any load condition and power factor condition can be determined.

It is also called as pessimistic method.

Separate curves foe every load condition are to be plotted if regulation at various load condition is required. It is also called as Potier reactance method.

8. Compare MMF method and Potier method.

S.no MMF METHOD POTIER METHOD

15

1.

2.

This method which gives regulation lower than actual, hence called optimistic method.

Energy is not wasted.

The results obtained are nearer to reality.

Energy is wasted.

9. What are the different methods available to determine the voltage regulation of an alternator?

1. Direct loading method2. Synchronous Impedance method or EMF method3. Ampere Turn method or MMF method4. Zero Power Factor method or Potier method5. ASA method6. Two reaction theory

10. State the reason of errors in EMF and MMF method1. The magnetic circuit is assumed to be unsaturated. This assumption is

unrealistic.2. In salient pole alternators, it is not correct to combine field ampere turns and

armature ampere turns .This is because the field winding is always concentrated on a pole core while the armature winding is always distributed.

REGULATION OF THREE PHASE SALIENT POLE ALTERNATOR BY SLIP TEST

16

AIM:

The aim of the experiment is to predetermine the regulation of three phase salient pole alternator by conducting the slip test

NAME PLATE DETAILS:


Amps:

KVA:

RPM:

FUSE RATING:


For Dc shunt motor:

For Alternator:

APPARATUS REQUIRED:


RANGE TYPE QUANTITY

1.2.3.4.5.6.7.8.

AmmeterAmmeterVoltmeterVoltmeterRheostat

3 Ф Auto transformerTachometer

Connecting wires

(0-5) A(0-5) A

(0-150) V(0-150)V

300Ω,1.5A

MIMCMIMC

Wire wound

1111111

As required

FORMULA USED:

1. Armature Resistance Ra =1.6 *Rdc

17

2. Direct impedance per phase (Zd) = Vmin / Imax in Ω3. Quadrature axis impedance per phase (Zq) = Vmax / Imin in Ω4. Direct axis reactance per phase (Xd) = √Zd2 –Ra2 in Ω5. Quadrature axis reactance per phase (Xq) = √Zq2 –Ra2 in Ω6. Percentage Regulation = Eo – Vrated / Vrated *1007. Eo = Vtcosδ- IqRa –IdXd (Motoring)8. Eo = Vtcosδ+ IqRa+IdXd (Generating)9. δ = Ψ-Ф (Generator)10. δ = Ф – Ψ (Motor)11. Ψ = tan -1 (VtsinФ+- IaXq / VtcosФ +- IaRa)

+ For generating mode- For Motoring mode

THEORY: In non salient pole alternators air gap length is constant and reactance is also constant. Due to this the mmfs of armature and field act upon the same magnetic circuit all the time hence can be added vector ally. But in salient pole alternators the length of the air gap varies and reluctance also varies. Hence the armature flux and field flux cannot vary sinusoid ally in the air gap. So the reluctance of the magnetic circuit on which mmf act is different in case of salient pole alternators. This can be explained by two reaction theory.PRECAUTIONS:

1. The motor field rheostat should be kept in minimum resistance position.2. The alternator field should be kept open throughout the experiment.3. The direction of rotation due to prime mover and due to the alternator run as the

motor should be same.4. Initially all the switches are kept open.

PROCEDURE:1. Note down the name plate details of motor and alternator.2. Connections are given as per the circuit diagram.3. Give the supply by closing the DPST switch.4. Using the three point starter start the motor to run at the synchronous speed by

varying the motor field rheostat at the same time check whether the alternator field has opened or not.

5. Apply 20% to 30% of the rated voltage to the armature of the alternator by adjusting the autotransformer.

6. To obtain the slip and maximum oscillations of pointers, the speed is reduced slightly lesser than the synchronous speed.

7. Maximum current, minimum current, maximum voltage and Minimum voltage are noted.

8. Find out the direct and quadrature axis impedance (Zd,Zq).

RESULT:TABULAR COLUMN:

TO FIND OUT THE DIRECT AXIS IMPEDANCE (Zd) :

18

Speed of the alternator:Minimum Voltage applied to the stator:(Nearly 20% to 30% of rated voltage)

S.NO Minimum current per phase (Imin) (amps)

Maximum Voltage per phase (Vmax) (volts)

Direct axis impedance per phase (Zd) (Ohms)

Direct axis Reactance per phase (Xd) (Ohms)

TO FIND OUT THE QUADRATURE AXIS IMPEDANCE (Zq) :

S.NO Maximum current per phase (Imax) (amps)

MinimumVoltage per phase (Vmin) (volts)

Quadrature axisimpedance per phase (Zd) (Ohms)

Quadrature axis Reactance per phase (Xd) (Ohms)

VIVA QUESTIONS:

19

1. What is meant by salient pole type rotor? The rotor poles projecting out from the rotor core of large diameter but small length. This is used in low and medium speed (engine driven alternator)

2. What is the necessity of damper winding? Most of the alternators have the pole shoes slotted for receiving copper bars of a grid or damper winding. They are useful in preventing the hunting in generators and are needed in synchronous motors to provide the starting torque.

3. What is meant by Two Reaction theory? The method of analysis of the distributing effects caused by salient pole construction is called Two Reaction theory.

4. State Two Reaction theory. The armature mmf can be divided into two components as 1. Component acting along the pole axis called direct axis (d)2. Component acting along at right angles to the pole axis called quadrature axis

(q).

5. What is d axis and q axis? The reluctance offered to the mmf wave is lowest when it is aligned with the field pole axis. This axis is called direct axis of pole. The reluctance offered is highest when the mmf wave is oriented at 90 to the field pole axis which is called quadrature axis.

6. What is meant by magnetizing and cross magnetizing component? The component along direct axis can be magnetizing and the component acting along the quadrature axis is called cross magnetizing component.

7. What is called slip test? The method used to determine Xd and Xq, the direct and quadrature axis reactances is called slip test.

8. What is meant by power angle? The angle δ between Eo and V is called the power angle.

9. Compare salient pole and Non salient pole rotor.

20

S.no SALIENT POLE ROTOR NON SALIENT POLE ROTOR1.

2.

Large diameter and small axial length.

It is employed with hydraulic turbines or diesel engines.

The diameter of the rotor is reduce, to reduce the peripheral velocity of rotor

Used in high speed turbine like steam turbines.

10. What is meant by Auto – Transformer?

It is a transformer with one winding only, part of this being common to both primary and secondary. In this transformer the primary and secondary are not electrically isolated from each other.

21

V AND INVERTED V CURVES OF THREE PHASE SYNCHRONOUS MOTOR

AIM:

The aim of the experiment is to draw the V and inverted V curves of three phase synchronous motor.

NAME PLATE DETAILS:

3Ф synchronous motor Dc ExcitationVolts:

Amps:

KVA:

RPM:

FUSE RATING:


For Dc excitation:

For Synchronous motor:

APPARATUS REQUIRED:


RANGE TYPE QUANTITY

1.2.3.4.5.6.7.8.

AmmeterAmmeterVoltmeterWattmeterRheostat

Tachometer3 Ф Auto transformer

Connecting wires

(0-10) A(0-2) A

(0-600) V500V,10 A500Ω,1.2 A

415/(0-470)V

MIMCMI

Double elementWire wound

Digital

1111211

As required

22

THEORY:

Synchronous motor is constant speed motor which are not self starting in nature, so that we have to start this motor by any one of the following starting methods,

1. Pony motor method starting

2. Auto induction starting

3. DC exciter starting

4. Damper winding method of starting

By construction there is no difference between synchronous generator and synchronous motor. It is capable of being operated under wide range of power factor, hence it can be used for power factor correction.

The value of excitation for which back emf is equal to applied voltage is known as 1005 excitation. The other two possible excitations are over excitations and under excitation if the back emf is more or less to the applied voltage respectively.

The variations of armature current with field current are in the form of V curves and the variation of power factor with field current are in the form of Inverted V curves.

PRECAUTIONS:

1. The potential divider should be in the maximum position.

2. The motor should be started without any load.

3. Initially TPST switch is in open position.

PROCEDURE:

1. Note down the name plate details of motor.

2. Connections are given as per the circuit diagram.

3. Close the TPST switch.

4. By adjusting the auto transformer from minimum position to maximum position the rated supply is given to the motor. The motor starts as an induction motor.

5. In order to give the excitation to the field foe making it to run as the synchronous motor close the DPST switch.

6. By varying the field rheostat note down the excitation current, armature current and the power factor for various values of excitation.

7. The same procedure has to be repeated for loaded condition.

23

8. Later the motor is switched off and the graph is drawn

GRAPH:

The graph is drawn for

1. Armature current Vs Excitation current

2. Power Factor Vs Excitation current

RESULT:

24

TABULAR COLUMN:

Armature voltage:

Without load:

S.no Excitation current (If) (Amps)

Armature current (Ia) (Amps)

Power factor (CosФ)

With load:

S.no Excitation current (If) (Amps)

Armature current (Ia) (Amps)

Power factor (CosФ)

25

VIVA QUESTIONS:

1. Define V and Inverted V curves.

The magnitude of armature current varies with excitation. If graph of armature current drawn by the motor against field current is plotted then we get V curves.

If the power factor is plotted against field current then the shape of the graph looks like an inverted V and are called as Inverted v curves.

2. When Synchronous motor is is said to receive 100% excitation?

The value of excitation for which back emf is equal to the applied voltage is known as 100% excitation or when the power factor of the synchronous motor is unity.

Eb=V or cosФ =1

3. Define critical excitation.

When the excitation is changed, the power factor changes. The excitation for which the power factor of the motor is unity is called critical excitation.

4. What do you mean by under excitation and over excitation?

When the excitation is adjusted in such a way that the magnitude of induced emf is less than the applied voltage the excitation is called under excitation (lagging power factor)

When the excitation is adjusted in such a way that the magnitude of induced emf is greater than the applied voltage the excitation is called over excitation (leading power factor).

5. What is synchronous capacitor?

When synchronous motor is over excited it takes leading power factor current. This characteristic is similar to a normal capacitor which always takes leading power factor current.

Hence over excited synchronous motor operating on no load condition is called as synchronous condenser or synchronous capacitor. This is the property due to which synchronous motor is used as a phase advancer or as a power improvement device.

6. What is hunting?

When synchronous motor is on no load, the stator and rotor pole axis coincide with each other. When motor is loaded, the rotor pole axis falls back with respect to stator.

26

If the load connected to motor is suddenly changed by a large amount , the rotor tries to take its new equilibrium position. But due to inertia of rotor, it cannot achieve equilibrium instantaneously while achieving new position, it passes beyond its final position corresponding to new load. This will produce more torque than demanded. So the load angle is reduced and rotor swings in other direction.

Such oscillations of the rotor about its new equilibrium position, due to sudden application or removal of load is called hunting.

7. Mention some application of synchronous motor.

1. Constant speed load service

2. Reciprocating compressor drives

3. Power factor correction

4. Voltage regulation of transmission lines

8. What could be the reasons if a synchronous motor fails to start?

It is usually due to the following reasons

1. Voltage may be to low

2. Some faulty connections in auxiliary apparatus

3. Too much starting load

4. Open circuit in one phase or short circuit

5. Field excitation may be excessive.

9. A synchronous motor starts as usual but fails to develop its full torque. What could be due to?

1. Exciter voltage may be too low

2. Field spool may be reversal

3. There may be either open or short circuit in the field

10. What are the various methods of starting synchronous motor?

1. Pony motor method starting

2. Auto induction starting

3. DC exciter starting

4. Damper winding method of starting

27

11. What significant characteristic of a synchronous motor is revealed by its V-curves?

The V curves of synchronous motor reveals the fact that its power factor is controllable by means of its excitation.

28

LOAD TEST ON THREE PHASE SQUIRREL CAGE INDUCTION MOTOR

AIM:

The aim of the experiment is to conduct the load test on three phase squirrel cage induction motor and to draw the performance characteristics.

NAME PLATE DETAILS:

3Ф Induction motorVolts:

Amps:

KVA:

RPM:

FUSE RATING:


APPARATUS REQUIRED:


RANGE TYPE QUANTITY

1.2.3.4.5.

AmmeterVoltmeterWattmeter


(0-10) A (0-600) V600V,10 A

MIMI

UPFDigital

1121

As required

FORMULA USED:

1. Torque T =(S1~S2) (R+t/2)*9.81 N-M

Where S1, S2 = Spring balance readings in Kg

R = Radius of the brake drum in m, t = Thickness of the belt in m

2. Input power = (W1+W2 )watts

W1, W2 = wattmeter readings in watts

3. Output Power = 2ЛNT / 60 watts

29

N = Rotor speed in RPM

T = Torque in N-M

4. Percentage efficiency = ( Output power/Input power ) *100

5. Percentage slip = (Ns-N)/Ns *100

Ns = synchronous speed in rpm

N = speed of the motor in rpm

6. Power factor = (W1+W2 )/√3VLIL

THEORY:

Induction motor is a machine which converts AC electrical energy into mechanical energy. In this motor the rotor does not receive electric power by conduction but by induction in exactly as the secondary of 2 – winding transformer receives its power from the secondary. That is why such motors are known as rotating transformer. When you give three phase supply to the three phase stator winding then it is called as three phase induction motor.

The effect of applying load on the speed, slip, stator current, power factor, efficiency and torque are discussed below:

EFFECT ON SPEED:

When the induction motor is on no load the speed is slightly below the synchronous speed. The current due to induced emf in the rotor is responsible for torque production required at no load, as the load is increased the rotor speed is slightly reduced. The emf induced in the rotor and hence the current increases to produce higher torque required until the torque is equal to the torque required by the load on the motor.

EFFECT ON SLIP:

Synchronous speed depends upon of frequency stator supply voltage and number of poles for which that motor winding is made. Therefore if poles and frequency are constant, synchronous speed is constant. Thus with increase in load on the motor, rotor speed decreases, slip will increase.

%slip = (Ns – N /Ns) *100

EFFECT ON STATOR CURRENT:

Current drawn by the stator is determined by two factors. One component is the magnetizing current required to maintain the rotating field. The second component produces a field which is equal and opposites to that formed by the rotor currents. The rotor current increases with loads, the stator current will also therefore increases with load. Power factor of an induction motor on no load is very low because of the high value of

30

magnetizing current. With load the power factor increases because the power component of the current is increased.

EFFECT ON TORQUE:

The torque will increases with increase in loads, with increase in output.\

APPLICATIONS:

1. Squirrel cage induction motor having moderate starting torque and constant speed characteristics preferred for driving fans, blowers, water pumps, grinders, lathe machine, printing machines and drilling machines.

2. Slip ring induction motors can have high starting torque as high as maximum torque. Hence they are preferred for lifts, hoists, elevators, cranes, compressors.

PRECAUTIONS:


2. The supply should not be switched OFF before the motor is unloaded.

3. While running on load, the brake drum is cooled by pouring cold water.

PROCEDURE:


2. The TPSTS is closed and the motor is started using on line starter to run at rated speed.

3. At no load, the speed, current, voltage and power are noted down.

4. By applying the load gradually with the help of spring balance and brake drum arrangement, the speed, current, power and spring balance readings are noted for various loads up to the rated current.

5. The load is released late and the motor is switched off and the graph is drawn.

GRAPH:

1. Output Power Vs Speed

2. Output Power Vs Line current

3. Output Power Vs torque

4. Output Power Vs Power factor

5. Output Power Vs Efficiency

6. Output Power Vs %Slip

31

RESULT:

TABULAR COLUMN:

Radius of the Brake Drum: Multiplication Factor:

Thickness of the belt:

S.no IL VL W1 W2 W1 +W2 N S1 S2 S1~S2 T O/P Power

η S PF

(A) (V) (Watts) (Watts) rpm Kg Kg Kg N-m (Watts) % % CosФ

32

IL = Load current

VL = Load Voltage

W1, W2 = Wattmeter readings

W1 + W2 = Input power

S1, S2 = Spring balance readings

T = Torque

η = Efficiency

S = Slip

PF = Power factor

VIVA OUESTIONS:

1. What is the function of slip rings and brush assembly in three phase induction motor?

Slip rings are used to connect external stationary circuit to the internal rotating circuit. Hence in induction motor, the external resistances can be added with the help of brushes and slip ring arrangement in series with each phase of rotor winding.

2. List the difference between squirrel cage rotor and slip ring rotor

S.no SLIP RING ROTOR SQUIRREL CAGE ROTOR1.

2.

Rotor consists of winding similar to the stator winding.

Resistances can be added externally.

Rotor consists of bars which are short circuited at the ends with the help of end rings.

External resistances cannot be added.

3. Define the term slip

Slip of the induction motor is defined as the difference between the synchronous speed and actual speed of rotor expressed as a fraction of the synchronous speed.

4. How the direction of rotation of three phase induction motor can be reversed?

The direction of rotation of three phase induction motor can be reversed by interchanging any two terminals of the three phase windings while connecting to the three phase supply.

33

5. Under what condition, the slip in an induction motor is zero, one, negative and greater than one.

a. Zero: when N = Ns, S=0 which is not possible for an induction motor.

b. One: when N = 0, S=1. At start motor is at rest and hence its speed N is zero.

c. Negative: When the rotor is running at a speed above synchronous speed, slip is negative.

d. Greater than one: When the motor is rotated in opposite direction to that of rotating field, slip is greater than 1. When slip is greater than one, the machine works in breaking mode.

6. What are the fundamental characteristics of a rotating magnetic field?

a. The resultant of three alternating fluxes separated from each other by 120 degree has constant amplitude of 1.5 Фm, where Фm is maximum amplitude of an individual flux due to any phase.

b. The resultant always keeps on rotating with certain speed in space, and the speed is given by Ns = 120f / P

7. What is induction generator?

When run faster than its synchronous speed, an induction motor runs as a generator called as induction generator. Slip is negative.

8. What is the effect of slip on frequency, induced emf, current, power factor, Reactance, Impedance?

a. Fr = Sf b.E2r = S E2 c.X2r = S X2 d.I2r = E2r / Z2r

e. Cos Ф 2r = R2 / Z2r d.Z2r = √ R2 2 + (S X2) 2

9. What are the types of three phase induction motor?

1. Squirrel cage induction motor.

2. Slip ring induction motor

10. Can N = Ns in three phase induction motor?

When rotor starts rotating it tries to catch the speed of rotating magnetic field.

If it catches the speed of rotating magnetic field, the relative motion between rotor and RMF will vanish. In fact the relative motion is the main cause for the individual emf in the rotor. So induced emf will vanish and hence there cannot be rotor current and rotor flux which is essential to produce the torque on the rotor. Eventually motor will stop. The induction motor never rotates at synchronous speed.

34

NO LOAD AND BLOCKED ROTOR TEST ON THREE PHASE SQUIRREL CAGE INDUCTION MOTOR

Expt.no: Mark:

Date: Remark:

AIM:

The aim of the experiment is to conduct the no load and blocked rotor test on three phase squirrel cage induction motor and to draw the equivalent circuit and also to predetermine the performance using circle diagram.

NAME PLATE DETAILS:


Amps:

KVA:

RPM:

FUSE RATING:

Load: 125% of rated current (full load current)

No Load: 10% of rated current (full load current)

35

APPARATUS REQUIRED:


RANGE TYPE QUANTITY

1.2.3.4.5.6.7.8.9.10.

AmmeterAmmeterVoltmeterVoltmeterVoltmeterWattmeterWattmeter

Tachometer3Ф Auto transformer

Connecting wires

(0-10) A(0-10) A

(0-600) V(0-150) V(0-75) V

600V,10 A150V, 10 A

-415/(0-470) V

MIMCMIMIMCLPFUPF

Digital

111112211

As required

FORMULA USED:

NO LOAD TEST:

1. No load power factor (CosФ0) = W0/ √3V0I0

Where W0 = No load power per phase in watts

V0 = No load voltage per phase in volts.

I0 = No load current per phase in amps

W0 = W1+ W2 (watts)

2. Working component current (Iw) = I0 CosФ0 / √3 amps

3. Magnetizing component current (Im) = I0 SinФ0 / √3 amps

4. No load resistance R0 = V0 / Iw ohm

5. No load reactance X0 = V0 / Im ohm

BLOCKED ROTOR TEST:

6. Motor equivalent impedance referred to stator Zsc = (Vsc / Isc) / √3 ohm

7. Motor equivalent resistance referred to stator Rsc = Zsc CosФsc ohm

= Wsc / Isc2 ohm

8. Power factor CosФsc = Wsc / √3VscIsc

9. Motor equivalent reactance referred to stator Xsc = √Zsc2 – Rsc2 ohm

36

10. Rotor resistance referred to stator R2’ = Rsc – R1 ohm

11. Rotor reactance referred to stator X2’ = Xsc / 2 = X1 ohm

Where R1 = Rac = 1.6 *Rdc

R1 = stator resistance

X1 = stator reactance

12. Equivalent load resistance (RL’ = R2’ (1/S -1) in ohm

Where slip = (Ns-N)/Ns



THEORY:

An induction motor is simply an electric transformer whose magnetic circuit is separated by an air gap into two relatively movable portions, one carrying the primary and the other the secondary winding. Alternating current supplied to the primary winding induces an opposing current in the secondary winding, when later is short circuited or closed through an external impedance. Relative motion between the primary and secondary ie, stator and rotor is produced by the electromagnetic forces corresponding to the power thus transferred across the air gap by induction.

NO LOAD TEST OR OPEN CIRCUIT TEST:

No load test is performed to determine the no load current, no load power factor, wind age and friction losses, no load input and no load resistance and reactance.

Since there is no power output on no load, the power supplied to the stator furnishes its core loss and the friction and wind age losses in the rotor.

BLOCKED ROTOR TEST OR SHORT CIRCUIT TEST:

It is also known as locked rotor or short circuit test. This test is used to find the short circuit current with normal voltage applied to stator, power factor on short circuit, total leakage reactance and resistance of the motor as referred to stator and full load copper loss.

PRECAUTIONS:

1. The auto transformer should be kept at minimum voltage position.

37

PROCEDURE:

1. Connections are made as per the circuit diagram.

2. For no load test by adjusting autotransformer apply rated voltage and note down the ammeter, voltmeter and wattmeter readings. In this test the rotor is free to rotate.

3. For blocked rotor test by adjusting autotransformer apply rated current and note down the ammeter, voltmeter and wattmeter readings. In this test the rotor is blocked.

4. After that make the connections to measure the stator resistance as per the circuit diagram.

5. By adding the load through the loading rheostat note down the ammeter, voltmeter readings for various values of load.

PROCEDURE TO DRAW THE CIRCLE DIAGRAM:

By using the data obtained from the no load test and the blocked test, the circle diagram can be drawn using the following steps.

1. Draw the line by taking the current in X axis, voltage in Y axis.

2. From the no load test find out the current Io and draw the vector OA with the magnitude of Io from the origin by suitable current scale, which lags behind the voltage by an angle Фo

Фo = Cos-1(Wo / √3VoIo)

3. From the current Isc find out ISN (short circuit current corresponding to the normal voltage) through the formula ISN = Isc (V/Vsc). draw the OB vector with the magnitude of ISN from the origin by the same current scale , which lags the voltage V by an angle Фsc where

Фsc = Cos-1(Wsc / √3VscIsc)

4. Join the points B and A to get the output line.

5. Draw the parallel line for the X axis from point A and for the Y axis from point B upto the X axis (point E) let both the lines intersects at point d.

6. Then draw the bisector for the output line and extend it to the line AD, let the point of intersection be C.

7. By keeping the point C as centre draw a semicircle with the radius CA

8. Let EB be the line of total loss (ED (constant loss) +DB(variable loss))

9. In the line DB locate the point G to separate the stator and rotor copper losses

Rotor Cu loss/Stator Cu loss = Wsc-3Isc2 R1/ 3Isc2 R1

Where R1 = stator resistance in ohm

Variable loss = Stator Cu loss + Rotor Cu loss

10. To get the torque lines join the points A and G.

38

TO FIND OUT MAXIMUM QUANTITIES:

11. Draw the tangent to the semicircle in such a way that should be parallel to the output line. Let the point of tangent be H.

12. Join the points H and C that will be perpendicular to the output line then draw a line parallel to the Y axis from H to output line. The point at where the parallel line meets the output line is names as H’.

13. Here the HH’ will be the maximum output power.

14. Draw the tangent to the semicircle in such a way that should be parallel to the torque line. Let the point of tangent be I.

15. Join the points I and C that will be perpendicular to the torque line then draw a line parallel to the Y axis from I to torque line. The point at where the parallel line meets the output line is named as I’.

16. Here the II’ will be the maximum torque line.

17. Draw the tangent to the semicircle in such a way that should be parallel to the X axis. Let the point of tangent be J.

18. Join the points J and C and extend the same up to X axis. The point at where the line meets the input line is named as J'.

19. Here the JJ’ will be the maximum input power.

20. From the circle diagram find maximum input power, maximum torque, maximum output power, rotor Cu loss, stator Cu loss and slip.

To find the total input power on short circuit with normal voltage PSN by following formula,

PSN = Wsc (Vrated / Vsc) 2 and value of line BE equal to PSN, so power for 1 cm is equal to ratio between PSN and length of EB in cm.

TO FIND OUT THE FULL LOAD QUANTITIES:

21. Extend the line EB from B to K such that

BK = Output power (from the name plate details of motor) / Power scale.

22. Draw the parallel line to output line AB, which cuts the semicircle at point L. (near by Y axis)

23. Draw the parallel line for Y axis from point L to X axis (point Q). Then join O and L.

PROCEDURE TO FIND OUT THE FULL LOAD AND MAXIMUM QUANTITIES FROM THE CIRCLE DIAGRAM:

1. Fixed loss = DE*Power Scale (watts)

39

2. Stator Cu loss = DG* Power Scale(watts)

3. Rotor Cu loss = GB * Power Scale (watts)

4. Maximum torque = [HH’*Power Scale] / [2ЛN/60] N-m

5. Maximum Output power = II’ * Power Scale (watts)

6. Maximum Input power = JJ’ * Power Scale (watts)

7. Maximum Efficiency = Maximum Output power / Maximum input power

8. Full load current = OL * Power Scale (amps)

9. Full load Power factor = Cos (angle between OL and Y axis)

10. Full load torque = [LN’ * Power Scale] / [2ЛN/60] N-m

11. Full load output power = LM * Power scale (watts)

12. Full load input power = LQ * power Scale (watts)

13. Full load efficiency = Full load output power / Full load input power

14. Full load Stator Cu loss = NP * Power Scale (watts)

15. Full load rotor Cu loss = MN * Power Scale (watts)

16. Full load rotor input = LN * Power scale (watts)

17. Full load slip = Full load rotor Cu loss / Full load rotor input

18. Full load speed =synchronous speed * (1-Slip)

19. Starting Torque = [BG * Power Scale] / [2ЛN/60] N-m

RESULT:

40

TABULAR COLUMN:

NO LOAD TEST:

Speed of the induction motor:

Multiplication Factor:

S.no No load current (Io)

No Load Voltage (Vo)

No load PowerW1 W2

Total No load Power

W1+W2 (Amps) (Volts) (Watts) (Watts)

BLOCKED ROTOR TEST:



S.no Short circuit current (Isc)

Short circuit Voltage (Vsc)

Short circuit PowerW1 W2

Total PowerW1+W2

(Amps) (Volts) (Watts) (Watts)

41



(I) (amps)

Armature Voltage(V)

(volts)

Armature Resistance

Ra= V/I ohm

42

VIVA QUESTIONS:

1. Enumerate the possible reasons if a three phase motor fails to start?

1. One or more fuses may be blown

2. Voltage may be too low

3. The starting load may be too heavy

4. Worn bearings due to which the armature may be touching field laminae, thus introducing excessive friction.

2. What happens if single phasing occurs when the motor is running? And when it is stationary?

1. If already running and carrying half load or less, the motor will continue running as a single phase motor on the remaining single phase supply, without damage because half loads do not blow normal fuses.

2. If motor is very heavily loaded, then it will stop under single phasing and since it can neither restart nor blow out the remaining fuses, the burn out prompt.

A stationary motor will not start with one line broken. In fact due to heavy standstill current, it is likely to burn out quickly unless immediately disconnected.

3. What is meant by plugging?

Plugging means stopping a motor by instantaneously reversing it till it stops.

4. What are the starters used in three phase induction motor?

1. Primary resistance starter

2. Auto transformer starter

3. Direct on line starter

4. Star Delta starter

5. Rotor resistance starter

5. What are the different methods of speed control of three phase induction motor?

43

1. Control from stator side:

i) By changing the supply voltage

ii) By changing the supply frequency

iii) By changing the number of poles

2. Control from Rotor side:

i) Rotor rheostat control

ii) By operating two motors in cascade or concatenation

iii) By injecting an emf in the rotor circuit

6. What is meant by crawling?

Induction motor particularly the squirrel cage type, sometimes exhibit a tendency to run stably at speeds as low as one seventh of their synchronous speed. This phenomenon is known as crawling.

7. What is meant by cogging or magnetic locking?

The rotor of a squirrel cage induction motor sometimes refuses to start at all, particularly when the voltage is low. This happens when the number of stator teeth is equal to the number of rotor teeth and is due to the magnetic locking between the stator and rotor teeth. That is why this phenomenon is also called as teeth locking.

8. What are the advantages of skewing?

1. It reduces magnetic humming.

2. It helps in reducing the locking tendency of the rotor. ie, the tendency of the rotor teeth to remain under the stator teeth due to the magnetic attraction between the two.

9. What is jogging?

Jogging means inching a motor ie, make it to move a little at a time by constant starting and stopping.

10. What are the indications of winding faults in an induction motor?

1. Excessive and unbalanced starting currents

2. Some peculiar noises and

3. Over heating

44

SEPERATION ON NO LOAD LOSSES IN THREE PHASE SQUIRREL CAGE INDUCTION MOTOR

Expt.no: Mark:

Date: Remark:

AIM:

The aim of the experiment is to separate the no load losses in three phase squirrel cage induction motor as core loss and mechanical loss.

NAME PLATE DETAILS:


Amps:

KVA:

RPM:

FUSE RATING:


APPARATUS REQUIRED:


RANGE TYPE QUANTITY

1.2.3.4.5.

AmmeterAmmeterVoltmeterVoltmeterWattmeter

(0-10) A(0-10) A

(0-600) V (0-75) V

600V,10 A

MIMCMIMCLPF

11112

45

6.7.8.


Connecting wires

-415/(0-470) V

Digital 11

As required

FORMULA USED:

1. Input Power W = (W1 + W2) watts

2. Stator cu loss = 3Io2Ra watts

3. Constant loss / phase Wc = (W - 3Io2Ra )/3 watts

4. Core loss/phase Wi = Constant loss/phase – Mechanical loss

Here the mechanical loss Wm will be the distance from the origin to the point at where the constant loss/phase Vs voltage curve cuts the y axis.

THEORY:

The no load losses are the constant losses which include core loss and friction and windage loss. The separation between the two can be carried out by the no load test conducted from variable voltage, rated frequency supply.

When the voltage is decreased below the rated value, the core loss reduces as nearly square of voltage. The slip does not increase significantly the friction and windage loss almost remains constant.

The voltage is continuously decreased, till the machine slip suddenly begins to increase and the motor tends to stall. At no load this takes place at a sufficiently reduced voltage. The graph showing no load losses versus voltage is extrapolated to V =0 which gives friction and windage loss as iron or core loss is zero at zero voltage.

PRECAUTIONS:


2. The motor should not be loaded throughout the experiment.

PROCEDURE:


46

2. By giving the three phase supply through the autotransformer, start the motor.

3. The auto transformer should be varied till the motor attains its rated speed and no voltage and current.

4. Repeat the same procedure for some more low values of the voltage and tabulate the readings.

5. Find the stator copper loss and constant loss by respective formula.

6. Draw the suitable graph to find the mechanical loss.

7. Obtain the core loss by separating the mechanical loss from the constant loss.

8. To measure the stator resistance, connections are given as per the circuit diagram.


GRAPH:

The graph drawn between constant loss and input voltage.

RESULT:

47

TABULAR COLUMN:

NO LOAD TEST:



S.no (Io) (Vo) W1 W2 W1+W2 Stator Cu loss

Constant loss per phase

Core loss per phase

(Amps) (Volts) (Watts) (Watts) (Watts) (Watts) (Watts)

Io = No load Current

Vo = No load voltage


W1 + W2 = Total no load input power



(I) (amps)

Armature Voltage(V)

(volts)

Armature Resistance

Ra= V/I ohm

48

VIVA QUESTIONS:

1. Mention the losses that occur in induction motor.

1. Stator losses

i) Stator core loss

ii) Stator copper loss

2. Rotor Copper loss

3. Mechanical losses

2. What are the no load losses?

The no load losses are the constant losses which include core loss and friction and windage loss.

3. In which type of induction motor can resistance be introduced in the rotor circuit?

In slip ring induction motor the resistance can be introduced in the rotor circuit.

4. The squirrel cage rotor is also known as short circuited rotor. Why?

In squirrel cage rotor, the copper bars are placed in the slots. These bars are short circuited at each end with the help of conducting copper ring called end ring. The entire rotor resistance is very small. Hence this rotor is also called as short circuited rotor.

5. Why air gap length is minimum in an induction motor?

The air gap length is kept minimum in induction motor

i) To reduce the magnetizing current which is required to set up the flux.

ii) To improve the power factor.

6. Define operating torque, starting torque and breakdown torque. Which of these is the largest?

Operating Torque: Torque from light load to full load

Starting Torque: Torque at start ie, speed = 0 or slip = 1

Breakdown Torque: Maximum torque that motor can develop.

49

If loaded beyond this torque the motor will decelerate and come to standstill. Breakdown torque is the largest among these for normal induction motor.

7. Why do induction motor runs at low power factor when lightly loaded?

At no load an induction motor draws large magnetizing current to create flux in the air gap. This current has very low power factor of the order 0.2 lag. The power is mainly drawn to provide for core loss. As the motor is loaded, it draws a load component of current with much higher power factor. Therefore the power factor of the net current drawn from main increases with load. It rises to 0.8 - 0.85 at full load. At light load the power factor will be slightly more than that at no load.

8. Why an induction motor is not operated under conditions of maximum load?

Maximum power output condition corresponds to matching of the load resistance in the circuit model to the circuit impedance as seen by the load resistance. This condition corresponds to much larger than the normal operating slip and hence larger current than the full load value. As copper loss is proportional to square of motor current, the motor efficiency under condition of maximum power output is well below 50% which is unacceptable for a power device. Acceptable efficiencies are about 85% or higher.

9. Why an induction motor is called asynchronous motor?

Since the induction motor runs always at a speed lesser than the synchronous speed, it is called asynchronous motor.

10. Why is the efficiency of a three phase induction motor less than that of a transformer?

In induction motor, there are mechanical losses due to the rotation of the rotor. Hence the efficiency of an induction motor is less than that of the transformer.

50

LOAD TEST ON SINGLE PHASE INDUCTION MOTOR

Expt.no: Mark:

Date: Remark:

AIM:

The aim of the experiment is to conduct the load test on single phase induction motor and to draw the performance characteristics.

NAME PLATE DETAILS:


Amps:

KVA:

RPM:

FUSE RATING:


APPARATUS REQUIRED:


RANGE TYPE QUANTITY

1.2.3.4.5.

AmmeterVoltmeterWattmeter


(0-10) A (0-300) V300V,10 A

MIMI

UPFDigital

1121

As required

51

FORMULA USED:

1. Torque T =(S1~S2) (R+t/2)*9.81 N-M

Where S1, S2 = Spring balance readings in Kg

R = Radius of the brake drum in m, t = Thickness of the belt in m

2. Input power = (W) watts

3. Output Power = 2ЛNT / 60 watts

N = Rotor speed in RPM

T = Torque in N-M

4. Percentage efficiency = ( Output power/Input power ) *100

5. Percentage slip = (Ns-N)/Ns *100



6. Power factor = (W)/√3VLIL

THEORY:

Single phase motors are manufactured in fractional kilowatt range to be operated an single phase supply and for use in numerous applications like ceiling fans, refrigerators, food mixers, hair driers, portable drills, vaccum cleaners, washing machines etc.

A single phase induction motor has two main parts namely stator and rotor. A single phase supply is connected to the stator winding. This produces an alternating current to the stator winding. This current produces an alternating flux in the air gap between the stator and the rotor. The alternating flux passes over the rotor conductors and induces an emf into it due to transformer action. Due to this induced voltage, current starts flowing through the rotor conductor. This current will then produces its own flux called the rotor flux. The main flux produced by the stator winding interacts with the rotor flux to produce the torque. The rotor flux gets produced due to the principle of induction hence the name of the motor is induction motor. Single phase induction motors are not self starting like the dc motors.

Depending upon the method of starting, they are classified into,

i) Split phase induction motor

ii) Capacitor start induction motor

iii) Capacitor run induction motor

52

iv) Capacitor start capacitor run induction motor

v) Shaded pole induction motor

PRECAUTIONS:


2. The supply should not be switched OFF before the motor is unloaded.

3. While running on load, the brake drum is cooled by pouring cold water.

PROCEDURE:


2. The TPSTS is closed and the motor is started using on line starter to run at rated speed.

3. At no load, the speed, current, voltage and power are noted down.

4. By applying the load gradually with the help of spring balance and brake drum arrangement, the speed, current, power and spring balance readings are noted for various loads up to the rated current.

5. The load is released late and the motor is switched off and the graph is drawn.

GRAPH:

1. Output Power Vs Speed

2. Output Power Vs Line current

3. Output Power Vs torque

4. Output Power Vs Power factor

5. Output Power Vs Efficiency

6. Output Power Vs %Slip

RESULT:

53

TABULAR COLUMN:

Radius of the Brake Drum: Multiplication Factor:

Thickness of the belt:

S.no IL VL W1 W2 W1 +W2 N S1 S2 S1~S2 T O/P Power

η S PF

(A) (V) (Watts) (Watts) rpm Kg Kg Kg N-m (Watts) % % CosФ

IL = Load current

VL = Load Voltage


W1 + W2 = Input power

S1, S2 = Spring balance readings

T = Torque

η = Efficiency

S = Slip

PF = Power factor

VIVA QUESTIONS:

1. A single phase induction motor is not self starting. Why?

When a single phase supply is connected across a single phase winding, a pulsating magnetic field is produced. The force experienced by the upper conductors of the rotor will be downward and the force experienced by the lower conductors will

54

be directed upward. The two sets of force will cancel and the rotor will experience no torque. Hence the rotor will not rotate.

2. What is the rating of single phase machines? State its applications.

The rating of the single phase machine is from 1/8 to ¾ hp range. They are widely used for fans, washing machines, refrigerators, blowers, centrifugal pumps etc.

3. How will you change the direction of rotation of a split phase induction motor?

The direction of rotation of a split phase induction motor can be changed by changing the direction of current either in the starting winding or in the running winding.

4. What type of motor is used for ceiling fans?

Permanent magnet capacitor motor is used for ceiling fans.

5. Why single phase induction motor has low power factor?

The current through the running winding lags behind the supply voltage by a very large angle. Hence the power factor is low in single phase induction motor.

6. What is the use of shading coil in the shaded pole motor?

The copper shading coil is used to produce rotating magnetic field moving from the unshaded to shaded portion of the pole. Hence the motor produces a starting torque.

7. Why are centrifugal switches provided on many single phase inductions motor?

The centrifugal switches are provided on many single phase induction motors, because when the motor is running at 75% of the synchronous speed, the centrifugal switch connected in the auxiliary winding operates and disconnect the auxiliary winding from the supply.

8. What could be the reasons if a split phase motor runs too slow?

1. Wrong supply voltage and frequency

2. Overload

3. grounded starting and running windings

4. Short circuited or open winding in field circuit

9. What could be the reasons if a split phase motor fails to start and hums loudly?

It could be due to the starting windings being grounded or burnt out.

10. What is universal motor?

55

Universal motor is a series motor of rating less than 1Kw which is designed to operated on both dc and ac supply. They are widely used for food mixers, vaccum cleaners, hair driers, electric shavers, portable drills, sewing machines etc.

NO LOAD AND BLOCKED ROTOR TEST ON SINGLE PHASE INDUCTION MOTOR

Expt.no: Mark:

Date: Remark:

56

AIM:

The aim of the experiment is to conduct the no load and blocked rotor test on single phase induction motor and to draw the equivalent circuit.

NAME PLATE DETAILS:


Amps:

KVA:

RPM:

FUSE RATING:


No Load: 10% of rated current (full load current)

APPARATUS REQUIRED:


RANGE TYPE QUANTITY

1.2.3.4.5.6.7.8.9.10.

AmmeterAmmeterAmmeterVoltmeterVoltmeterVoltmeterWattmeterWattmeter


Connecting wires

(0-10) A(0-10) A(0-10) A

(0-300) V(0-150) V(0-75) V

300V,10 A150V, 10 A

-230/(0-270) V

MIMCMIMIMIMCLPFUPF

Digital

1111111111

As required

FORMULA USED:

NO LOAD TEST:

1. No load power factor (CosФ0) = W0/ V0I0

57

Where W0 = No load power in watts

V0 = No load voltage in volts.

I0 = No load current in amps

2. Working component current (Iw) = I0 CosФ0 amps

3. Magnetizing component current (Im) = I0 SinФ0 amps

4. No load resistance R0 = V0 / Iw ohm

5. No load reactance X0 = V0 / Im ohm

BLOCKED ROTOR TEST:

6. Motor equivalent impedance referred to stator Zsc = (Vsc / Isc) ohm

7. Motor equivalent resistance referred to stator Rsc = Zsc CosФsc ohm

= Wsc / Isc2 ohm

8. Power factor CosФsc = Wsc / VscIsc

9. Motor equivalent reactance referred to stator Xsc = √Zsc2 – Rsc2 ohm

10. Rotor resistance referred to stator R2’ = Rsc – R1 ohm

11. Rotor reactance referred to stator X2’ = Xsc / 2 = X1 ohm

Where R1 = Rac = 1.6 *Rdc

R1 = stator resistance

X1 = stator reactance

12. Magnetizing reactance Xm = 2(Xo –X1 – X2’/2)

13. Slip = (Ns-N)/Ns



THEORY:

The equivalent circuit of a single phase induction motor can be developed by using double field revolving theory. By using the equivalent circuit the performance of the single phase induction motor can be obtained.

58

The single phase induction motor can be visualized to be made of single stator winding and two imaginary rotors. The developing torques of the induction motor is forward torque and backward torque.

When the single phase induction motor is running in the direction of forward revolving field at a slip S, then the rotor currents induced by the forward field has frequency sf. The rotor mmf rotates at slip speed with respect to the rotor but at synchronous speed with respect to the stator. The resultant forward stator flux and the rotor flux produce a forward air gap flux. This flux induces the voltage in rotor. Thus due to the forward flux, the rotor circuit referred to stator has an impedance of R2’ /2s + jX2’/2.

The backward flux induces a current in the rotor at a frequency (2-s)f. the corresponding rotor mmf rotates in the air gap at synchronous speed in the backward direction. The resultant backward stator flux and the rotor flux produce a backward air gap flux. This flux induces the voltage in rotor. Thus due to backward flux the rotor circuit reffered to stator has an impedance of R2’/2(2-s )+ jX2’/2

NO LOAD TEST OR OPEN CIRCUIT TEST:

No load test is performed to determine the no load current, no load power factor, wind age and friction losses, no load input and no load resistance and reactance.

Since there is no power output on no load, the power supplied to the stator furnishes its core loss and the friction and wind age losses in the rotor.

BLOCKED ROTOR TEST OR SHORT CIRCUIT TEST:

It is also known as locked rotor or short circuit test. This test is used to find the short circuit current with normal voltage applied to stator, power factor on short circuit, total leakage reactance and resistance of the motor as referred to stator and full load copper loss.

PRECAUTIONS:


PROCEDURE:

1. Connections are made as per the circuit diagram.

2. For no load test by adjusting autotransformer apply rated voltage and note down the ammeter, voltmeter and wattmeter readings. In this test the rotor is free to rotate.

3. For blocked rotor test by adjusting autotransformer apply rated current and note down the ammeter, voltmeter and wattmeter readings. In this test the rotor is blocked.

4. After that make the connections to measure the stator resistance as per the circuit diagram.

59


RESULT:

TABULAR COLUMN:

NO LOAD TEST:



60

S.no No load current (Io)

No Load Voltage (Vo)

No load PowerW1 W2

Total No load Power

W1+W2 (Amps) (Volts) (Watts) (Watts)

BLOCKED ROTOR TEST:



S.no Short circuit current (Isc)

Short circuit Voltage (Vsc)

Short circuit PowerW1 W2

Total PowerW1+W2

(Amps) (Volts) (Watts) (Watts)



(I) (amps)

Armature Voltage(V)

(volts)

Armature Resistance

Ra= V/I ohm

61

VIVA QUESTIONS:

1. What is the function of capacitor in a single phase induction motor?

Capacitor is used to improve the power factor of the motor. Due to the capacitor connected in series with the auxiliary winding, the capacitive circuit draws a leading current which increases the split phase angle α between the two current I m and Ist.

2. Define double field revolving theory.

According to this theory, any alternating quantity can be resolved into two rotating components which rotates in opposite directions and each having magnitude as half of the maximum magnitude of the alternating quantity.

3. What are the classifications of single phase induction motor based on the method of starting?

1. Split phase motor

2. Capacitor start motor

62

3. Capacitor run motor

4. Capacitor start Capacitor run motor

5. Shaded pole motor

4. What design features are incorporated in a split phase motor to make it starting?

The split phase motor is provided with windings, main winding and auxiliary winding. These two windings are excited from the same voltage. The currents in the two windings can be made out of phase by adjustment of the impedance of the auxiliary winding in relation to the main winding. As a result the mmf of main winding and mmf of auxiliary winding constitute an unbalanced field set with 900 electrical space phase relationship. The two symmetrical components now being unequal the forward rotating field is made stronger than the backward rotating field, which results in the net production of starting torque. Thus the two windings with phase difference make the split phase motor self starting.

5. What is the advantage of a capacitor start motor over a resistance split phase motor?

In case of capacitor start, it is possible to have the phase angle between the two currents. Therefore this type of motor has high starting torque as compared to resistance split phase motor and used for heavy loads such as compressors, conveyors, pumps, certain machine tools, refrigerators and air conditioning equipment.

6. In which direction does a shaded pole motor runs?

It runs from the unshaded to the shaded part.

7. Give the function performed by induction motor starter.

1. To improve the starting torque

2. To limit the initial in rush of current during starting conditions, which would otherwise produce larger line voltage drop affecting equipments connected to the same line.

8. What do you mean by synchronous condenser?

A single machine which is available to convert ac to dc is known as synchronous converter or rotary converter. A synchronous converter combines the function of a synchronous motor and a dc generator.

9. What type of motor is used in computer drives and wet grinders?

For computer drives – Permanent magnet dc motors

Wet grinders – Universal motor

10. What is the difference between the dc motors and single phase induction motor?

63

An important difference between the two is that the dc motors are self starting while single phase induction motors are not self starting.

64

e m 1 lab manual

Documents