e.m-ii lab manuals jntuh

ELECTRICAL MACHINES LAB-II


DEPARTMENT OFELECTRICAL AND ELECTRONICS ENGINEERING

ACADEMIC YEAR-2013III B.Tech EEE I-SEMESTER

PRAGNA BARATH INSTITUTE OF TECHNOLOGYRanga Reddy, Andhra Pradesh

Affiliated To :Jawaharlal Nehru Technological University, Hyderabad

Established In :2008

Electrical & Electronics Engineering Department- PBIT


EXPERIMENT NO: DATE:

V CURVES OF SYNCHRONOUS MOTOR

AIM : (a) To study the effect of variation of field current upon the stator current an power factor with

synchronous motor running at no load, hence to drew V and inverted V curves of the motor.

(b) To repeat the above, with synchronous motor loaded to half the full load and 3/4 the full load.

(c) To drew the unity power factor curve on the above characteristics.

(d) To compare the above characteristics critically and discuss the effect of loading the motor on the nature of theses characteristics.

INSTRUMENTS

S.No. Name Type Range Quantity

1.

2.

3.

4.

5.

6.

Ammeter

Ammeter

Wattmeter

Voltmeter

Ammeter

Voltmeter

MI

MC

Dynamometer

MI

MC

MC

0-10/20 A

0-5/10 A

10/20 A, 200/400 V

0-300/600 V

0-10/20 A

0-300 V

1

1

2

1

1

1

SPECIFICATION : The above ranges of the instruments are based on the specification of the synchronous motor, given below.

400 V, 3-, 50Hz, 11 A, 1500 rpm, delta connected.PROCEDURE

1. Connect the circuit as per fig (10.11).

2. Switch-on the ac supply feeding to 3-phase synchronous motor and start the motor, using the starter.

3. Observe the direction of rotation of the motor, in case, it is rotating in opposite direction, stop the motor and reverse the phase sequence. Start the motor again, using the starter. Ensure that the motor is running on no load.

4. In this case, field winding is excited automatically with the help of exciter, provided on the shaft of the main motor.



5. Set the rheostat in the field circuit of the motor to the position of normal excitation. Under this condition, armature will drew minimum current from the mains. Note down the readings of all the meters connected in the circuit.

6. Reduce the excitation in steps and note down the corresponding armature current and reading of both the wattmeters. Excitation may be reduced, till the current in the armature winding is of rated value. Under this condition, armature current should increase on reducing the excitation .

7. Again, adjust the rheostat in the field circuit to normal excitation. Now increase the excitation in steps and note-down the readings of all the meters at each setting of increased excitation. Excitation may be increased, till the behaviour of the motor is normal. At large excitation, the motor will try to fall out of step.

8. Adjust the voltage of the dc generator coupled to synchronous motor to rated value by varying the field current of the generator.

9. Load the dc generator to half the full load and maintain it constant throughout the next part of the experiment.

10. Repeat step 5, 6, and 7 sequentially under this condition of loading.

11. Increase the load on the generator to 3/4th of full load, keeping its voltage constant. Maintain this load constant throughout the next part of the experiment.

12. Repeat step 5, 6, and 7 sequentially for this increased load on the motor.

13. Remove the load on the dc generator gradually.

14. Switch-off the supply to the motor to stop it.

OBSERVATION : May be tabulated as follows.

S. No. V If Ia W1 W2 Vdc Idc Cos

Results:

QUESTIONS

1. What are the basic differences between a 3-phase synchronous motor and 3-phase induction motor ?

2. What is the magnitude of starting torque in 3-phase synchronous motor ?

3. what are the various methods of starting a 3-phase synchronous motor ?

4. what is the power factor of the motor at normal excitation ?

5. What is the nature of power factor, when a synchronous motor is operated (i) under excited (ii) over excited ?

6. Draw on the same graph, approximate V curves, corresponding to 25% and 100 percent load.

7. Is it possible to operate a synchronous motor on any other speed than the synchronous spped ?




REGULATION BY ZPF METHOD

AIM : (a) Perform open circuit and short circuit test on a 3 phase alternator.

(b) Perform load test on 3 phase alternator with highly lagging load, (Approximately zero power factor) and at rated voltage with rated current flowing in the stator winding.

(c) Draw open circuit and zero power factor saturation characteristics of the alternator on the same graph.(d) Calculate the regulation of alternator by drawing the proper phasor diagrams, connected with the above

characteristics.

INSTRUMENTSS. No. Name Type Range Quantity

1.2.3.4.5.6.7.

AmmeterAmmeterVoltmeterRheostatRheostat3 φ, Highly inductive loadTechometer

MIMCMISingle tubeSingle tubeLaggingdigital

0-10/20 A0-2.5/5 A0-300/600 V272 Ω, 1.7A300 Ω, 2.2 A15 A, 400 V0-2000 rpm

1111111

PROCEDURE

(a) For zero power factor test :

1. Connect the circuit as per fig (10.8).2. Set the rheostat, R1, so that the field current of the motor is maximum possible at the instant of starting the motor (fully

out of field circuit).3. Set the rheostat, R2, so that the field current of alternator is minimum.4. Ensure that the alternator is on no-load.5. Switch on the dc supply and start the dc motor with the help of starter.6. Vary the field current of the motor by rheostat, R1, so as to obtain rated speed.7. Switch on the dc supply to the field of the alternator.8. Vary the field current of the alternator by rheostat, R1, so as to obtain rated voltage.9. Load the alternator gradually in steps and adjust the rated terminal voltage across the load at each step by increasing

the field current, till full load of the alternator. Record, field current of the alternator, corresponding to rated full load current and rated terminal voltage of the alternator. The complete zero power factor characteristic can be plotted, based on one reading only.

10. Decrease the load on the alternator gradually and side by side, reduce the field current of the alternator.11. Switch off the dc supply to the field of the alternator and dc motor.12. Measure the dc resistance of the stator phase by voltmeter-ammeter method and convert it to ac resistance by

multiplying the same with a factor 1.3, which takes into account the skin effect.

(b) Open circuit and short circuit test :

Procedure for conducting open circuit and short circuit tests on alternator has already been described in art 10.5, which may be referred for performing these tests.



CIRCUIT DIAGRAM

OBSERVATION : May be tabulated as follows :

(a) For ZPF Test

S. No. Stator current Term. Voltage Field current

(b) For open circuit and short circuit tests.

Open circuit test Short circuit testS. No. Field current E.M.F. S. No. Field current Short circuit current

Results:




REGULATION BY SYNCHRONOUS IMPEDANCE METHOD

AIM : (a) Perform no load and short circuit tests on a 3-phase alternator.

(b) Measure the resistance of the stator winding of alternator.(c) Find out regulation of alternator at full load and at (i) unity p.f. (ii) 0.85 p.f. lagging (iii) 0.85 p.f leading, using

synchronous impedance method.

INSTRUMENTSS.No. Name Type Range Quantity1.

2.

3.

4.

5.

Ammeter

Ammeter

Voltmeter

Rheostat

Techometer

Mc

MI

MI

Single tube

digital

0-1/2 A

0-10/20 A

0-300/600 V

272 Ω, 1.7 A

0-200 rpm

1

1

1

2

1

PROCEDURE

1. Connect the circuit as per fig (10.4)

2. Adjust the position of rheostat, R1 for maximum possible current in the field circuit of dc motor, to Ensure (i) low starting speed (ii) high starting torque.



3. Set the position of rheostat, R2 for minimum current in the field circuit of alternator, to ensure low value of generated emf at starting.

4. Switch on the dc mains, feeding the dc motor and the field circuit of alternator.5. Start the dc motor, using the starter properly. Various resistance steps of the starter should be cur out slowly, so that

the motor does not draw high current during starting.6. Set the speed of the motor and hence the alternator at its rated value by varying rheostat, R 1, provided in the field

circuit of the motor.7. Note-down the open circuit voltage of the alternator and the field current.8. Repeat step 7 for various value of field current (can be obtained by varying the rheostat, R 2 provided in the field

circuit of alternator). Observation should be continued, till the open circuit voltage is 25 to 30 percent higher than its rated value.

9. Set the position of rheostat, R2 again for minimum possible current in the field circuit of alternator.10. Short-circuit the stator winding of the alternator, by closing the switch, provided for this purpose in the circuit

diagram.11. Note-down the short circuit current and the field current.12. Repeat step 11, for various values of field current, till the short circuit current becomes equal to the full load current

of alternator.13. Readjust the setting of rheostats R1 and R2 to their initial positions and then switch-off the dc supply to stop the dc

motor.14. Measure the dc resistance of the stator winding by usual voltmeter-ammeter method. To obtain ac resistance, skin

effect must be taken into account. As such, ac resistance may be taken approximately 1.3 times the dc resistance measured.

OBSERVATION : May be tabulated as follows sOpen Circuit Test Short Circuit Test

S. No. If E If Isc

Model graph:

Results:




OPEN-CIRCUIT AND SHORT-CIRCUIT TEST

AIM : (a) To perform open circuit test on single phase transformer

(b) To perform short circuit test on the same transformer

(c) Calculate the complete parameters of the equivalent circuit of this transformer

(d) Calculate the efficiency at 1/4, 1/2, 3/4th, full load and 1.25 times full load and plot the efficiency curve VS load. Take the operating power factor as 0.85 lagging.

(e) Calculate the regulation at full load and at the following power factor (i) 0.85 lagging (ii) unity (iii) 0.85 leading.

INSTRUMENTS

S. No. Name Type Range Quantity1.

2.

3.

4.

5.

6.

7.

Ammeter

Wattmeter

Voltmeter

Single phase variac

Ammeter

Wattmeter

Voltmeter

MI

Dynamometer

MI

MI

Dynamometer

MI

0-2A

2.5A, 200 V

0-300 V

230/0-270 V, 15 A

0-15 A

15 A, 75 V

0-30 V

1

1

1

1

1

1

1

PROCEDURE

(a) Open Circuit Test

Connect the circuit as per fig (6.3 a)

1. Ensure that the setting of the variac is at low output voltage.

2. Switch on the supply and adjust rated voltage across the transformer circuit.

3. Record no load current, voltage applied and no load power, corresponding to the rated voltage of the transformer winding.

4. Switch off the ac supply.

(b) Short Circuit Test

1. Connect the circuit as per fig (6.3 b) for conducting short circuit test.

2. Adjust the setting of the variac, so that the output voltage is zero.

3. Switch on the ac supply to the circuit.

4. Increase the voltage applied slowly, till the current in the windings of the transformer is full load rated value.

5. Record, short circuit current, corresponding applied voltage and power with full load current flowing under short circuit conditions.

CIRCUIT DIAGRAM



OBSERVATION : May be tabulated as follow :S. No. No Load Test Short Circuit Test

V0 I0 W0 Vsc Isc Wsc

CALCULATIONS : May be tabulated as follows,



S. No. Cos φ0 Iw Im R0 Xm Req Xeq Load Regulation

Performance calculations

Complete performance of the transformer can be calculated based on the above observations of open-circuit and short-circuit test following the steps given by,

Efficiency at different loads :

Efficiency at full load :

Total losses at full load, = W0 + Wsc

Let the full load out put power of the transformer in KVA be P0.

P0 X 1000 X cos φThen percentage efficiency at full load, f = X 100

P0 X 1000 X cos φ + W0 Wsc

Efficiency at half the full load :

Iron losses at half the full load = W0 (constant)

1 1

Total copper losses at half the full load = Wsc = Wsc

4

1

Output power at half full load = P0 KVA

2

1/2 P0 X 1000 X cos φThus, percentage efficiency at half the full load, 1/2f =

X 1001/2 P0 X 1000 X cos φ + W0 + 1/4 Wsc

In a similar manner, efficiency at other loads can be found out and the efficiency Vs output power curve can be plotted.OPENCIRCUIT TEST

Hence, total iron losses = W0 (Reading of wattmeter)

From the observations of this test, the parameters R0 and Xm of the parallel branch of the equivalent circuit can also be calculated, following the steps given below :

Power drawn, W0 = V0 I0 cos φ0

W0

Thus, no load power factor, cos φ0 =V0 I0

Core loss component of no load current, Iw = I0 cos φ0

And, magnetizing component of no load current, Im = I0 sin φ0



VEquivalent resistance representing the core loss, R0 =

IW

VMagnetizing reactance representing the magnetizing current, Xm =

Im

RESULTS




NO LOAD AND BLOCKED ROTOR TEST ON SINGLE PHASE INDUCTION MOTOR

AIM : (a) Perform no load and blocked-rotor test on single phase induction motor.

(b) Find out experimentally the hot resistance of the stator winding.

(c) Determine the parameters of the equivalent circuit drawn on the basis of double-revolving field theory.

(d) Calculate the complete performance of the motor at 5 percent slip.

NAME-PLATE SPECIFICATION OF MOTOR

1.5 h.p. 230 V, 50 Hz, 1000 r.p.m. 7 A, single phase, capacitor start, split phase induction motor.


1.2.3.4.5.6.7.8.

AmmeterVoltmeterWattmeterAuto transformerAmmeterVoltmeterLamp bank loadTechometer

MIMIDynamometer-MCMCResistivedigital

0-5 A/10 A0-150/300 V5/10 A, 200/400 V8 A, 230/0-270 V0-10 V0-30 V10 A, 250 V0-2000 rpm

11111111

PROCEDURE

(a) No load test

1. Connect the circuit as shown in Fig. (8.17), with the meter ranges suggested above.

2. Switch-on the ac supply to the circuit.

3. Adjust the voltage applied to the circuit to a low value and then increase it to the rated voltage of the motor.

4. Centrifugal switch will get opened automatically at approximately 75 percent of the rated speed, disconnecting the starting winding from the main winding.

5. Record the applied voltage, V0, the no load current, I0 and the no load power input, P0.

6. Reduce the applied voltage in suitable steps and record the no load current and power for various values of applied voltages

7. Switch-off the supply to stop the motor.

(b) Block-rotor test :

1. Connection are basically the same for block-rotor test except that the meters are replaced with proper ranges suggested already and the starting winding is disconnected from the circuit.

2. Adjust the variac in the circuit, such that its output voltage is quite low approximately 5 to 10 percent of the rated voltage of the motor.

3. Switch-on the ac to the circuit.



4. Adjust the applied voltage, such that the current drawn by the motor is full load rated current. Record applied voltage, input current and power.

5. Switch-off the ac main to stop the motor.

(c) Resistance of the main winding :

Measure the dc resistance of the main stator winding, using voltmeter-ammeter method.OBSERVATION : May be tabulated as follows :

(a) No Load Test (b) Block Rotor Test (c) Measurement of resistanceS. No V0 I0 W0 Vsc Isc Wsc Vm Im Rdc

CIRCUIT DIAGRAM

Modal graph

CALCULATION OF PERFORMANCE

Performance of the motor can be calculated from the parameters of the equivalent circuit (Fig. 8.13), following the equations and procedure given below :



Calculate the impedance of forward rotor i.e.

Xm R2` X2` j + j

2 2 S 2Impedance of forward rotor, Zf = …….. (i)

R2` X2` Xm` + j +

2 S 2 2

= Rf + J Xf

Calculate the impedance of the backward rotor i.e.

Xm R2` X2` j + j

2 2(2- S) 2Impedance of forward rotor, Zb = …….. (i)

R2` X2` Xm` + j +

2(2- S) 2 2

= Rb + J Xb

Calculate the total impedance of the equivalent circuit i.e.

Total impedance of the circuit, Zt = (R1 + j X1) + Rf + j Xf ) + (Rb + j Xb)

= Rt + j Xt ……(iii)

Calculate the current drawn by the motor at the above slip i.e. V

Motor current, I1 = ……(iv) Zt

Calculate the operating power factor of the motor i.e. Rt

Power factor, cos = ……(v) Zt

Find out the voltage across the forward rotor i.e.

Voltage across the forward rotor, Ef =I1Zf ……(vi)

Find out the impedance, current and torque developed of forward rotor branch i.e.

R2` 2 X2` 2 1/2

Impedance, Z3 = 2S 2

Rt

Current, I3 =



Zt

R2`

Torque developed by forward rotor, Tf = (I3)2 Synchronous watts. 2 S

Calculate voltage, impedance, current and torque developed of backward rotor i.e.,Voltage across the backward rotor, Eb = It Zb

R2` 2 X2` 2 1/2

Impedance , Z5= 2(2-S) 2

Current, I5 = Eb / Z5

R2`

Torque developed by backward rotor, Tb = I25

2(2-S)

(ix) Find out, net torque, net output, efficiency i.e.

Net torque, Tnet = Tf - Tb

Mech output, Pm = Tnet (1 – S) syn wattsNet output, P0 = Pm – Friction and windage loss

P0

Efficiency, = V I1 cos

RESULTS




NO LOAD AND BLOCK ROTOR TEST ON THREE PHASE INDUCTION MOTOR

AIM : (a) perform no load and block rotor test on 3 phase induction motor.

(b) Using the data obtained above, draw the circle diagram complete in all respect.

(c) Find out, input current, power factor, slip, torque and efficiency, corresponding to full load, using the above circle diagram.

(d) Compute (i) Max power (ii) Max torque (iii) Starting torque and best power factor, utilizing the above circle diagram.


1.

2.

3.

4.

Ammeter

Voltmeter

Wattmeter

3 phase variac

MI

MI

Dynamometer

Fully variable

0-10/20 A

1-300/600 V

10/20 A, 200/400 V

15 A, 400/0-400 V

1

1

2

1

CIRCUIT DIAGRAM

PROCEDURE

No Load Test1. Connect the circuit as per fig. (8.7).

2. Ensure that the motor is unloaded and the variac is set at zero position.

3. Switch-on the supply and increase the voltage gradually, till the rated voltage of the motor. Thus the motor runs at rated speed under no load.

4. Record the reading of all the meters connected in the circuit.

5. Switch-off the ac supply to stop the motor.

Block Rotor Test1. Readjust the variac at zero position

2. Change the range of all the instruments for block rotor test as suggest in the discussion on circuit diagram.

3. Block the rotor either by tightening the belt firmly or by hand.

4. Switch-on the ac supply and apply reduced voltage, so that the input current drawn by the motor under blocked rotor condition is equal to the full load current of the motor.



5. Record the readings of all the meters, connected in the circuit.]

6. Switch-off the ac supply fed to the motor.

7. Measure the resistance per phase of the stator winding, following ohm;s law concept.

OBSERVATIONS : May be tabulated as follows.

No load test Block rotor testS. No. V0 I0 W01 W02 VSX ISC WSC1 WSC2

CALCULATIONNO LOAD TEST

To obtain no load current and its power factor angle, φ0, no load test is performed at rated voltage and frequency. Let the readings of ammeter, voltmeter, and two wattmeters connected in the circuit be, I 0, V0, W01 and W02 respectively during no load test. Then,

W01 – W02

tan φ0 = 3W01 + W02

Hence, no load power factor angle, φ0, can be calculated from the readings of two wattmeters. No load current, I0 has been directly measured by the ammeter.

BLOCK ROTOR TEST

To obtain short circuit current and its power factor angle, block rotor test is performed on the motor. In this test, rotor is not allowed to move (blocked either by tightening the belt, in case provided or by hand) and reduced voltage (25 to 30 percent of the rated voltage) of rated frequency is applied to the stator winding. This test is performed with rated current flowing in the stator winding. Let the readings of ammeter, voltmeter and two wattmeters be, I sc, VSC, WSC1 and WSC2 respectively under block rotor condition. Then,

Wsc1 – Wsc2

tan φsc = 3Wsc1 + WSC 2

Thus, short circuit power factor angle, φsc can be calculated from the above equation.

Short circuit current, Isx observed during the block-rotor test corresponds to reduced applied voltage, VSC, which should be converted to rated voltage of the motor for plotting the circle diagram. The relation between the short circuit current and the applied voltage is approximately a straight line. Thus, short circuit current, IS C ` corresponding to rated voltage, V of the motor is given by,

VShort circuit current, I sc` = X Isc

VSC

It may be remembered, that the power factor of the power factor of the motor is quite low at no load as well as under blocked rotor condition. Thus, one of the wattmeter connected in the circuit will give negative reading in both the test, which may be recorded by reversing the terminals of the pressure coil or the current coil.

Results


LOAD TEST ON INDUCTION MOTOR ON THREE PHASE INDUCTION MOTOR



AIM: (a) Perform load test on 3-phase induction motor

(b) Compute Torque, output power, input power, efficiency, input power factor and slip for every load Setting.

(c) Plot the following performance curves(i) Efficiency Vs output power(ii) Torque Vs output power(iii) Line current Vs output power(iv) Power factor Vs output power(v) Slip Vs output power(vi) Torque Vs speed.

INSTRUMENTS

S. No. Name Type Range Quantity1.

2.

3.

4.

5.

Ammeter

Voltmeter

Wattmeter

3-phase variac

Techometer

MI

MI

Dynamometer

Fully variable

Digital

0-10/20 A

0-300/600 V

10/20 A, 200/400 V

15 A, 400/0-400 V

0-2000 rpm

1

1

2

1

1

PROCEDURE

1. Connect the circuit as per fig. (8.6)

2. Ensure that the motor is unloaded and the variac is set at zero output voltage.

3. Switch-on 3 phase ac mains and start the motor at reduced applied voltage. Increase the applied voltage, till its rated value.

4. Observe the direction of rotation of the motor. In case, it is reverse, change the phase sequence of the applied voltage.

5. Take-down the readings of all the meters and the speed under no load running.

6. Increase the load on the motor gradually by turning of the hand wheels, thus tighting the belt. Record the readings of all the meters and the speed at every setting of the load. Observations may be continued upto the full load current rating of the motor.

7. Reduce the load on the motor and finally unload it completely.

8. Switch-off the supply to stop the motor.

9. Note-down the efficiency diameter of the brake drum.

CIRCUIT DIAGRAM

Fig. (8.6) shows the circuit diagram of load test on 3 phase squirrel cage induction motor. Instruments connected in the circuit serve the function indicated against each.



3 phase variac – to limit the starting current of the motorAmmeter – to measure the current drawn by the statorVoltmeter – to measure the voltage across the stator phasesWattmeters – to measure input power and power factor.


S. No. Line Voltage Input current W1 W2 S1 S2 speed

CALCULATIONSLIP : The speed of the rotor, Nr droops slightly as the load on the motor is increased. The synchronous speed, Ns of the rotating magnetic field is calculated field is calculated, based on the number of poles, P and the supply frequency, i.e.

120 Synchronous speed, Ns = r.p.m.

P

NS - Nr

Then, slip, S = X 100 percentNs

Normally, the range of slip at full load is from 2 to 5 per cent.

TORQUE : Mechanical loading is the most common type of method employed in laboratories. A brake drum is coupled to the shaft of the motor and the load is applied by tightening the belt, provided on the brake drum. The net force exerted at the brake drum in kg is obtained from the readings S1 and S2 of the spring balances i.e.

Net force exerted, W = (S1 – S2) kgThen, load torque, T = W X d/2 kg –m

= W X d/2 X 9.8 Nw-m

where, d- effective diameter of the brake drum in meters.

OUTPUT POWER, P0 : The output power in watts developed by the motor is given by,

2 N TOutput power, P0 = watts

60



where, N is the speed of the motor in r.p.m.

INPUT POWER : Input power is measured by the two wattmeters, properly connected in the circuit i.e. Input power = (W1 + W2) wattsWhere, W1 and W2 are the readings of the two watttmeters.

INPUT POWER FACTOR : Input power factor can also be calculated from the readings of two wattmeters for balanced load. If φ is the power factor angle, then

W1 – W2

tan φ = 3 W2 + W2

Knowing the power factor angle, φ, from the above, power factor, cos φ can be calculated. It may be noted clearly at this stage, that the power factor of the induction motor is very low at no load, hardly 0.1 to 0.25 lagging. As such, one of the wattmeter will record a negative reading, till the power factor is less than 0.5, which may be measured by reversing the connections of either the current coil or pressure coil of this wattmeter.

EFFICIENCY :

Output powerPercentage efficiency of the motor, = X 100

Input powr

Full load efficiency of 3 phase induction motor lies in the range of 82 % (for small motors) to 92 % (for very large motors).

RESULTS


ZERO SEQUENCE REACTANCE, Xd

AIM : To measurement zero sequence reactance of synchronous machine.



INSTRUMENTS

S. No. Name Type Range Quantity

1.

2.

3.

4.

Ammeter

Voltmeter

Wattmeter

Single phase variac

MI

MI

Dynamometer

-

0-15 A

0-60 V

10/20 A, 75/150 V

230/0-270 V, 15 A

1

1

1

1

PROCEDURE

1. Connected the circuit as per fig (10.16), with the three stator phases in series.

2. Ensure that the variac is at zero position.

3. Switch on the single phase ac supply to the circuit.

4. Apply a reduced voltage to the circuit, so that the current in the series circuit is rated full load current.

5. Record the readings of all the meters connected in the circuit.

6. Switch off the supply.

Procedure for the parallel circuit is on similar lines.


S. No. E I P Z0 X0

Circuit diagram



CALCULATIONS(i) Series Connection

Series connection of all the three phase is possible, only when both the terminals of each phase are accessible. For measuring zero sequence reactance, reduced single phase voltage is applied across the stator winding with three phases connected in series with the field winding short circuited. The synchronous machine if desired may be run as an alternator at rated speed. However, the magnitude of zero sequence reactance is not much affected by the rotation of the machine. As such, test may be performed with the synchronous machine stationary. Zero sequence reactance can then be found out by recording the current, applied voltage and input power and proceeding as per the following.

EZero sequence impedance, Z0 =

3 I

1/2P 2

And, zero sequence reactance, X0 = Z0 1 - E I

Where, E – Voltage applied to the series circuit of stator phases I – Current flowing in the series circuit of stator phasesP – Input power to the series circuit.

Zero sequence reactance is the smallest out of all the reactance defined for synchronous machine.

(ii) Parallel Circuit

Zero Sequence reactance can also be determined by connecting all the three phases of stator winding in parallel. This is adopted, only when four terminals of the stator winding are available with neutral terminals of the three phases connected together internally and only one terminal for neutral brought out. Three line terminals of the stator winding are connected in parallel externally. A reduced single phase voltage is applied between the line terminals and the neutral terminal. Zero sequence reactance is hardly affected by the rotation of the machine, as such the test may be performed with the rotor at stand till. In case, heating is excessive, the machine may be driven at normal speed with the field short circuited. Readings of voltmeter and ammeter connected in the circuit may be recorded for various value of applied voltage. Then,

3 EZero sequence impedance, Z0 =

IWhere E – Voltage applied to the parallel circuit of stator phases.



I – Total test current

Zero sequence reactance can then be found out as already stated in series test.

Zero sequence reactance, X0 of synchronous machine is the least among the various reactances assigned to it. Its value is hardly 8 to 10 percent of the direct –axis synchronous reactance, Xd.

Results:




SUMPNER’S TEST AIM : (a) To perform Sumpner’s (Back to Back) test on two identical transformers. (b) Determine the efficiency at 1/4 , 1/2, 3/4, full load and 1.25 times the full load and at 0.85 p.f. lagging. (c) Plot efficiency Vs output characteristic.

INS TRUMENTS

S. No. Name Type Range Quantity1.2.3.4.5.6.7.8.

AmmeterAmmeterVoltmeterVoltmeterVoltmeterWattmeterWattmeter

Single phase variac

MIMIMIMIMI

DynamometerDynamometerFully variable

2.5/5 A15/30 A300 V600 V30 V

2.5 A, 200 V15 A, 75 V

230/0-270 V, 15 A

11111111

PROCEDURE

1. Connect the circuit as per fig (6.4).

2. Ensure that switches S2 and S3 are open.

3. Energize the primaries by closing the switch S1.

4. Observe the reading of voltmeter V1, which should be zero for correct connection of the secondaries. In case, the voltmeter reads twice the rated voltage of each transformer, open the switch S 1 and interchange the connections at the secondary terminals of one of the transformer. Close the switch S 1 again and verify that the voltmeter V1 now reads zero. Important caution : Even if the voltmeter V1 reads zero at the first instance, it is advisable to check the reading of Voltmeter V1 by interchanging the connections at the secondary terminals of one of the transformer

5. Adjust the setting of the variac, to give nearly zero output voltage.

6. Replace the voltmeter V1, by a low range voltmeter.

7. Close the switch S3 and then S2.

8. Adjust the output voltage of the variac, so that the current flowing in the secondaries is

full load secondary current of each transformer.

9. Record the readings of all the instruments connected in the primary and secondary

circuit. Only one set of reading is sufficient to calculate the efficiency at different loads.

10. Switch off the supply to primary and secondary circuits.

CIRCUIT DIAGRAM:



OBSERVATION : May be tabulated as follows,

S. No. Primary side Secondary sideV0 I0 W0 Vsc Isc Wsc

RESULTS:




PARALLEL OPERATION OF TRANSFORMER

AIM : (a) To separate the two transformers in parallel. (b) To study the load sharing by each transformer.


1.2.3.4.5.6.

AmmeterAmmeterVoltmeterWattmeterWattmeter

Load

MIMIMI

DynamometerDynamometer

Inductive

0-15 A0-30 A

0-300/600 V200 V, 15 A200 V, 30 A

250 V, 7.5 Kw

211211

PARALLEL OPERATION OF TRANSFORMER CONDITION MUST BE SATISFIED

(a) For single phase transformers

(i) The same polarity

(ii) The same voltage ratio.

(b) For three phase transformers

(i) The same polarity

(ii) Zero-relative phase displacement

(iii) Same phase sequence

(iv) Same voltage ratio.

In addition to the above essential requirements, the transformers to be operated in parallel should have the following for better load sharing and operating power factor.

(i) Equal per unit impedances

(ii) Equal ratios of resistance to reactance.

Polarity : Polarity of the two transformers connected in parallel can be either right or wrong. The wrong polarity of the transformers connected in parallel, would result in a dead short circuit on the transformers.

Phase Sequence : The question of phase sequence occurs only in case of 3 phase transformers. The phase sequence of the two transformers to be operated in parallel must be the same; otherwise during the cycle each pair of phases will be short circuited.

Zero relative phase displacement : This is also applicable only with 3 phase transformers. The connections of the primary and secondary windings of the two transformers to be connected in parallel should be such that there is zero relative phase displacement between the two. Thus the following three phase transformers can be operated in parallel.

Transformer 1 : star/star, star/delta

Transformer 2 : delta/delta, delta/star

However transformers with + 300 phase displacement may be operated in parallel with those having – 30 0 phase displacement after reversing the phase-sequence of both the primary and secondary winding.

Voltage ratio

An equal voltage ratio of the two transformer operating in parallel is necessary to avoid no load circulating current. In case of unequal voltage ratio, circulating current will flow in the closed circuit formed by the secondaries and also the primaries, even at no load, which results in additional I2 R loss. The no load circulating current should not be permitted to exceed 10 percent of its



normal rated value for satisfactory parallel operation. In case of equal voltage ratio, the load shared by the two transformers operating in parallel is given by,

Z2

Load shared by transformer 1, Q1 = QZ1 + Z2

Z1

Load shared by transformer 2, Q2 = QZ1 + Z2

The above equations indicate clearly that the load sharing on the two transformers can be equalized, if their per unit impedances are equal. However in case of unequal voltage ratio, the transformer of smaller rating will be over load, if the combined full load output is drawn from the parallel combination, resulting in excessive heating of one of the transformer.

Equal per unit impedance

For better load sharing, two transformers of the same rating should have equal per unit impedance for operating in parallel. If two transformers of rating in the ratio 2:1 are to be operated in parallel, then to carry double the current, former transformer must have half the impedance of the latter for the same regulation. In case, this condition is not full filled, the transformer of the lower rating will be over loaded, while that of higher rating will be operating at comparatively lesser load than its rating.

A difference in the quality of percentage impedance i.e. resistance/reactance ratio, results in a divergence of the phase angle of the two current shared by the transformers. Hence, one transformer will be working with a higher and the other with a lower power factor than that of the normal operating power fact

CIRCUIT DIAGRAM

Fig. (6.9) shows the circuit diagram for operating two single phase transformers in parallel, which is self explanatory.

PROCEDURE

(a) Polarity check

1. Connect the circuit as shown in fig (6.9 a).

2. Switch-on the supply to the primary circuit, where the primaries of both the transformers are

connected in parallel.

3. The voltmeter connected in the secondary circuit of the transformers wil read either zero or twice

the secondary terminal voltage of each transformer.

(i) If the voltmeter reads zero-connect a1 to a`1 and a2 to a`2 for the two secondaries to be in parallel

(ii) In case, the voltmeter reads twice the secondary terminal voltage, then connect a1 to a`2 and a2 to a`1 for parallel operation of the two transformers.

(b) parallel operation



1. Connect the circuit as per the circuit diagram shown in fig (6.9 b). Ensure that the two secondaries have been connected properly as per the polarity determined in part (a).

2. Close the switch S1 to energize both the primaries. Ensure that the switch S2 is kept open. In case the voltage ratio of the two transformers are unequal, there will be a circulating current, which may be recorded.

3. Close the switch S2. Adjust a particular load on the secondaries and record the readings of all the instruments connected in the circuit.

4. Repeat step 3 for various values of load current upto the rated capacity of the two transformers operating in parallel.

5. Switch off the load slowly. Open the switch S2 and then switch off the supply to the primaries of the transformers.

OBSERVATIONS : May be tabulated as follows.S. No. VL IL WL I1 W1 I2 W2

RESULTS:


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