PRACTICAL NO: 4

AIM: To perform parallel operation and polarity test of two single phase transformer.

APPARATURS:

Sr.

No. Apparatus Range Quantity

1 Wattmeter 300V, 10A 01

2 Voltmeter 0-300 Volt 02

4 Ammeter 05 A 02

5 Ammeter 10 A 01

6 Lamp load 1phase 5KW,230V 01

THEORY:

The two transformers to be connected in parallel should be connected in a manner that their

primary and secondary have the same polarities. This can be accomplished by conducting a

polarity test.

PROCEDURE:

1. Polarity check:

The two transformers to be connected in parallel should be connected in a manner that their

primary and secondary have the same polarities. This can be accomplished by conducting a

polarity test -for which the transformers should be connected as shown in Fig.:1

The test is conducted as follows:

1. Connect the circuit as shown in Fig.:1

2. Switch on the supply.

3. Note the reading in the voltmeter connected on the secondary side. It may be either zero

or twice the secondary terminal voltage.

4. If the volt meter reads double the secondary voltage, then in the secondary opposite

polarities have been connected. Then the terminal S1 should be connected to S'2 and S'1

should be connected to S2.

CIRCUIT DIAGRAM:

Fig.:-1

Fig.:-2

2. PARALLEL OPERATION OF TWO SINGLE PHASE TRANSFORMERS FOR

EQUALVOLTAGE RATIO:

1. Connect the circuit as per the circuit diagram shown in fig.:-2. Ensure that the two

secondary have been connected properly as per polarity determined in part (1).

2. With S2open, close the switch S1. If the voltage ratio of the two transformers is same no

circulating current would be observed. But, if some circulating current is observed,

means that the voltage ratio is not equal. Record the reading of this circulating current.

3. Next, switch S2, on the secondary side should be closed.

4. Change the position of load. For various positions of the load, note the readings of all the

instruments connected.

5. Switch off the load in steps. Open the switch S2 and then open the switch Sl.

OBSERVATIONS:

The observations made above should be recorded in a tabular form as shown below:

Sr.

No.

VL

Volt

IL

Ampere

Measured Values Calculated Values

I1 (Amp.) I2 (Amp.) I1 (Amp.) I2 (Amp.)

1.

2.

3.

4.

5.

CALCULATION:

21

1 2

*lZ

I IZ Z

12

1 2

*lZ

I IZ Z

Here,

Z1&Z2 are the secondary impedances of the two transformers.

Load shared by 1st transformer is

21

1 2

*lZ

S SZ Z

Load shared by 2nd

transformer is

12

1 2

*lZ

S SZ Z

Where lS is load rating i.e 2l lS V I

GRAPH:

Plot the graph of IL versus I1 and I2.

Conclusion: