Why do you do polarity test of transformers?

Answer 1

Yes, Transformers have polarity.

ADDED. They do in the sense of Live and Neutral or earth-tapping arrangement in a mains wiring system, but otherwise they are not considered to do so because they handle only alternating, not direct, current. So their voltage and magnetic polarities alternate, rather than staying constant.

Answer 2

Knowing a transformer's polarity is important if you wish to parallel that transformer with another transformer, in much the same way that you must know a battery's polarity before you can parallel it with another battery.

Transformers are either 'additive' or 'subtractive' polarity. In North America, for example, a simple, two-winding, transformer has its higher-voltage winding terminals labelled H1 - H2, and its lower-voltage winding terminals labelled X1 - X2. If terminal X1 goes positive at the same time as terminal H1 goes positive (in other words, if voltage X1 - X2 is in phase with voltage H1 - H2), then the transformer is additive polarity. If terminal X2 goes positive at the same time terminal H1 goes positive, then the transformer is subtractive polarity.

If the terminals are unmarked, then you perform a simple test to determine the transformer's polarity and then mark the terminals yourself.

(In the UK, higher-voltage windings are labelled with capital letters, e.g. A1 - A2, and lower-voltage winding are labelled with lower-case letters, e.g. a1 - a2. Otherwise, the same rules apply.)

You can parallel an additive transformer with either an additive or a subtractive polarity transformer once you have correctly identified its terminals. However, in addition to polarity, the transformers must satisfy various other requirements before they can be paralleled... but that's the subject of another answer.

Answer 3

In order to determine how the secondary windings of two or more transformers must be connected together when the transformers are to be operated in parallel.