What is polarity, when associated with a transformer?

Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity — that is, when tested the terminals of the high voltage and low voltage windings on the left hand side are connected together, refer to diagram below:

Polarity Diagram

This leaves one high voltage and one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings.

This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.

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