On the primary side of a two-winding transformer, one terminal is positive with respect to the other one. At the same time, one terminal of the secondary winding is positive with respect to the other one. A polarity test is performed to determine whether the terminals have the same instantaneous polarity. The relative polarities of the primary and secondary terminals at any instant must be known for connecting windings of the same transformers in parallel or series or for interconnecting two or more transformers in parallel or for connecting single-phase transformers for poly-phase transformation of voltages.
Explanation of Polarity Test with Circuit Diagram
For the determination of the relative polarity of the two windings of a transformer the two windings are connected in series across a voltmeter, and one of the windings is excited from a suitable ac voltage source.
We can categorize the polarity of transformers in two types:
- Additive polarity
- Subtractive Polarity
If the polarities of the windings are as marked on the diagram i.e when the transformer has a subtractive polarity, the voltmeter will read the difference between E1 and E2. If the voltmeter reads E1+E2 , the polarity markings of one of the windings must be interchanged.
In the subtractive polarity, the voltage between A1 and a1(or A2 and a2) 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 A1, A2, a1 and a2 are subjected to high voltage stresses. Thus subtractive polarity is preferred over additive polarity.
The above test may not be convenient for the determination of the relative polarities of the terminals of the two windings of a transformer in the field. Polarity tests in the field can be conveniently carried out by using a dc battery, a switch, and a dc voltmeter.
As the switch on the primary side is closed, the primary current increases and so do the flux linkages of both the windings, inducing emfs in them. The positive polarity of this induced emf in the primary is at the end to which the battery is connected (according to Lenz’s law). The end of the secondary which simultaneously acquires positive polarity as determined by the dc voltmeter is the similar polarity end. The reverse happens on the opening of the switch i.e the similar polarity end is that end which acquires negative potential.
Steps for performing polarity test
- Connect the circuit as shown in the above figure and must set the autotransformer to Zero position.
- Single phase supply is switched on.
- Voltmeter V1, V2 & V3 voltages are recorded.
- If V3 shows addition of the value V1 & V2 i.e V3=V1+V2 i.e additive polarity.
- Otherwise, if V3 shows the subtraction of the value V1 & V2 i.e. V3=V1-V2 i.e. Subtractive polarity or negative polarity.