AC MACHINES-1 (66761) Theory

• If the X₁ terminal is directly across from the H₁ terminal, the transformer has subtractive polarity.

• If the X₁ terminal sits diagonally across from the H₁ terminal, the transformer has additive polarity.
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We can categorise the polarity of the transformer to two types,

1. Additive Polarity
2. Subtractive Polarity

Additive Polarity

In additive polarity, the voltage (V_c) between the primary side (V_a) and the secondary side (V_b) will be the sum of both high voltage and the low voltage, i.e. we will get V_c = V_a + V_b

Subtractive Polarity

In subtractive polarity, the voltage (V_c) between the primary side (V_a) and the secondary side (V_b) will be the difference of both high voltage and the low voltage, i.e. we will get V_c = V_a – V_b

In subtractive polarity, if V_c = V_a – V_b, it is a step-down transformer and if V_c = V_b – V_a, it is a step-up transformer.

We use additive polarity for small-scale distribution transformers and subtractive polarity for large-scale transformers.