Polarity Test for Operation of Two Single Phase Transformers in Parallel

Experiment No.: 6(A)

Experiment Name:

Polarity Test for Operation of Two Single Phase Transformers in Parallel.

Objective:

To test polarity of two single phase transformers.

Theory:

We do polarity test on parallel transformers to ensure that we connect the same polarity windings and not the opposite ones. If we accidentally connect the opposite polarities of the windings, it will result in a short-circuit and eventually damage the machine. We can categorise the polarity of the transformer to two types,

1. Additive Polarity
2. Subtractive Polarity

Additive Polarity

In additive polarity, the voltage (V_c) between the primary side (V_a) and the secondary side (V_b) will be the sum of both high voltage and the low voltage, i.e. we will get V_c = V_a + V_b

Subtractive Polarity

In subtractive polarity, the voltage (V_c) between the primary side (V_a) and the secondary side (V_b) will be the difference of both high voltage and the low voltage, i.e. we will get V_c= V_a – V_b

In subtractive polarity, if V_c = V_a – V_b, it is a step-down transformer and if V_c = V_b – V_a, it is a step-up transformer. We use additive polarity for small-scale distribution transformers and subtractive polarity for large-scale transformers.

Circuit Diagram:

Procedure:

1. Connect the circuit as shown above with a voltmeter (V_a) across primary winding and another voltmeter (V_b) across the secondary winding.
2. If available, take down the ratings of the transformer and the turn ratio.
3. We connect a voltmeter (V_c) between primary and secondary windings.
4. We apply some voltage to the primary side.
5. By checking the value in the voltmeter (V_c), we can find whether it is additive or subtractive polarity.
6. Mark the terminals with ‘dot’ sign to indicate the polarity.

If additive polarity – V_c should be showing the sum of V_a and V_b.
If subtractive polarity – V_c should be showing the difference between V_a and V_b.

Observation Table:

Sl. No.	Voltage across Primary Side (Va)	Voltage across Secondary Side (Vb)	Voltage between Primary & Secondary Side (Vc)	Result
Transformer 1	229.86	126.3	355.23	Vc=Va+Vb Additive Polarity
Transformer 2	230.2	124.0	354.4	Vc=Va+Vb Additive Polarity

Apparatus Used:

Sl. No.	Name of the Apparatus	Specification	Quantity	Maker’s Name
1	Transformer	Single Phase, 230/115 V, Shell type	2
2	Variac	240/0-270 V AC	1
3	Digital Multimeter as Voltmeter	0-750 V AC, Digital	3	Akademika

Caution:

Be careful that the max. measuring the voltage of voltmeter V_c should be greater than the sum of V_a (Primary winding) and V_b (Secondary winding) otherwise during the additive polarity, the sum of V_a and V_b comes across it.

Remarks:

If we require additive polarity, but we have subtractive polarity, we can simply change it by keeping any of the primary or secondary windings in the same fashion and reversing the winding connection of the other one. Similarly, if we require subtractive polarity but have additive polarity, we could do the same procedure as above.