What is the procedure to polarity test a 3 phase transformer?

Answer 1

A 'polarity test' is conducted on a single-phase transformer, not a three-phase transformer (or transformer bank). The polarity of a single-phase transformer being important if two Transformers are to be connected in parallel, or three transformers are going to be connected to form a three-phase transformer bank.

'Angular displacement' is, to a three-phase transformer, what 'polarity' is to a single-phase transformer. So you really should be asking about angular displacement, rather than polarity. Angular displacement, or 'phase displacement', is the angle by which the secondary line voltage lags the primary line voltage.

Angular displacement can be determined either by drawing a phasor diagram of the three-phase connection and measuring it, or by looking up the connection in a vector-group chart/table -you would nor normally 'calculate' angular displacement.

Answer 2

You don't. 'Polarity' applies to single-phase transformers, and knowledge of polarity is an essential requirement before you can connect three single-phase transformers to form a three-phase transformer bank. The 'equivalent' of polarity, for a three-phase transformer, is called 'angular displacement'.

Answer 3

A three-phase transformer doesn't have 'polarity'. Single-phase transformers have polarity, and knowing their polarity is important when they are connected to form a three-phase transformer bank. The equivalent of 'polarity' for a three-phase transformer, or a three-phase transformer bank, is termed 'angular displacement' -defined as the angle by which the secondary line voltage is displaced from the primary line voltages. While there is a test for polarity, angular displacement is determined by constructing a three-phase phasor diagram for the transformer and measuring the angular displacement.

Answer 4

They don't have a polarity as such because that alternates with the current, but in some designs the windings are coupled, so the manufacturer has to be sure the alternating poles do correspond.

Answer

Polarity is one of the characteristics you need to know if you are to parallel two or more single-phase transformers. Polarity describes how the secondary voltage relates to the primary voltage, and may be 'additive' or 'subtractive'. These terms describe whether the secondary voltage is in phase or in antiphase with the primary voltage, and depends on how the two windings have been wound, relative to each other.

A polarity test is normally performed from the high-voltage side of the transformer. To perform a polarity test, the primary and secondary windings are connected in series with each other, and a small a.c. voltage is applied to the high-voltage winding. The resulting voltage across both windings is then measured. If this voltage is higher than the applied test voltage, then the transformer is said to have 'additive polarity'; if the voltage across both windings is lower than the applied voltage, then the transformer has 'subtractive polarity'.