International Islamic University,

Islamabad

Electrical Machines LAB

EXPERIMENT # 03:

Testing of Single Phase Transformers

(Turn Ratio and Polarity Test)

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Experiment 3

Testing of Single Phase Transformer and Equivalent Circuit

Determination

Turn Ratio Test

Apparatus

Theory

The induced emf in primary and secondary winding in transformer is given by following equations:

max11 44.4 fNV

max22 44.4 fNV

Taking ratio of these equations, we would have

KN

N

E

E

V

V

1

2

1

2

1

2

If the secondary and primary voltages are measured on no-load, their ratio is very nearly equal to the true

value. Measurement of primary and secondary currents in short circuit test also gives fairly accurate

results, especially if the transformer has little leakage flux and low core reluctance.

Procedure

1. Make the connections as shown in figure.

2. Now connect the primary winding of the transformer to the variable AC supply, connect a

voltmeter on the secondary winding at no-load.

3. Make sure that for various voltages the turn ratio comes out to be

394.0127

50

1

2

1

2 N

N

V

V

Circuit Diagram

Readings and Observations

S. No Primary Voltage ( 1V ) Secondary Voltage ( 2V ) 1

2

V

VK

1 20V

2 40V

3 60V

4 80V

5 100V

6 115V

7 127V

S. No Primary Current ( 1I ) Secondary Current ( 2I ) 2

1

I

IK

1

2

3

4

5

6

7

Polarity Test

Apparatus

Theory

On the primary side of a two winding transformer, one terminal is positive with respect to the other one at

any instant. At the same instant, one terminal of the secondary winding is positive with respect to the

other one. Polarity test is performed to determine the terminals having the same instantaneous polarity

(assuming that the terminals are not marked). The relative polarities of the primary and secondary

terminals at any instant must be known for connecting windings of the same transformer in parallel, or

series, or for interconnecting two or more transformers in parallel, or for connecting single phase

transformers for polyphase transformation of voltages.

For determination of relative polarity of the two windings of a transformer the two windings are

connected in series across a voltmeter, while one of the windings (preferably HV winding) is excited from

a suitable AC voltage source. IF the polarities of the windings are as marked on the diagram i.e., when the

transformer has subtractive polarity the voltmeter will read the difference of 1E and 2E . If the voltmeter

reads 21 EE , the polarity markings of one of the windings must be interchanged.

In subtractive polarity, the voltage between 1A and 1a is reduced. The leads connected to these

terminals and the two windings are, therefore, not subjected to high voltage stress. On the other hand in

additive polarity the two windings and leads connected to 2121 &,, aaAA are subjected to high voltage

stress. This is the reason that subtractive polarity is preferred over additive polarity.

Procedure

1. Make the connections as shown in figure.

2. Now apply some voltage on the primary from variable AC voltage source.

3. If the polarities of the windings are as marked on the diagram, the voltmeter should read the

difference of 1E and 2E (subtractive polarity).

4. If it reads 21 EE , the polarity marking of one of the windings must be interchanged.

Circuit Diagram

Subtractive Polarity

Sr. 1V (Primary) 2V (Secondary) V (Voltmeter)

1 20

2 40

Additive Polarity

Sr. 1V (Primary) 2V (Secondary) V (Voltmeter)

1 10

2 20