Polarity Test Of Transformer

Transformer receives input A.C voltage and gives voltage at secondary terminals due to induction. So when primary terminals are connected to an A.C source the positive and negative polarity switches between the two terminals.

Polarity test to be done to identify which terminals of the primary and secondary are having same polarity i.e. positive or negative.

Suppose we have a transformer which High voltage terminals are marked as A1, A2 and secondary terminals are marked as a1, a2. Let us apply A.C voltage to A1, A2. So for one half cycle (10 mS for 50 Hz A.C) A1 will be positive, A2 will be negative. And for the next half cycle i.e. for the next 10 mS A1 will be negative and A2 will be positive. Our objective is to identify if the secondary terminals i.e. a1 & a2 will be induced with voltage in the same positive negative sequence or reverse sequence.

Execution of Polarity test is very simple. To understand this test first let us see an analogue with D.C circuit. 

Suppose you have two battery of voltage say V1 & V2. 

Now if you short negative terminals of the two battery and connect voltmeter to the two positive terminals the voltmeter will show V1-V2. i.e. the positive and negative terminals are in same order.

if you short positive terminal of a battery with negative terminal and measures voltage between the other two terminals you will see the voltmeter will display V1+V2. i.e positive and negative terminals are in reverse order.

Now let us perform the polarity test on transformer.


TEST CIRCUIT

Fig:1 CIRCUIT DIAGRAM OF TRANSFORMER POLARITY TEST/
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Step-1: 

To execute this test first short two terminals of transformer from each winding. We will short A2 & a2.

Step-2:

Apply reduced voltage between A1 & A2. You don't need to apply full rated voltage, reduced voltage will satisfy our objective. Applying voltage to L.V winding will result higher voltage at H.V winding, so it is better to apply voltage at high voltage winding.

Step-3:

Measure voltage between A1, A2; a1, a2; and A1, a1. Say these are V1, V2, & V.

Observation and Conclusion:

Now if V=V1-V2 then terminal A1 & a1 have same polarity. i.e when A1 is positive a1 is also positive.

Fig 2:TRANSFORMER TERMINAL A1, a1 HAS SAME POLARITY
ANALOGUE WITH DC CIRCUIT
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and

if V=V1+V2 then terminal A1 & a1 has different polarity, but A1 & a2 has same polarity.

Fig 3:TRANSFORMER TERMINAL A1, a1 HAS OPPOSITE POLARITY
ANALOGUE WITH DC CIRCUIT
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Same polarity terminals are indicated with a "Dot" as shown.

In practice the terminals are marked according to its polarity for easy identification of terminals at site.

i.e terminals shall be so marked that A1, a1 will have same polarity. So that at site it could be identified without testing.

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