Transformer Polarity Test – Additive, Subtractive and Transformation Ratio Test

Transformer Polarity Test

Transformer Polarity is the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals. The winding terminals of a transformer possess polarities. The polarity test can determine the relative polarity of the primary and secondary of a transformer at any instant.

The transformer has two types of polarity, that is additive and subtractive polarity.

Additive polarity 

In Additive polarity, the voltage across the primary (V1) and secondary (V2) of a transformer will be the sum of the voltages at low voltage and High voltage sides. That is, if  V3 measures a value equal to the sum of voltages measured at V1 and V2, then the polarity will be additive.

For an Additive Polarity, V3 = V1+ V2

 Subtractive polarity

In subtractive polarity, the voltage across two windings will be the difference between the voltages at two sides. That is if the voltage V3 is the difference in the voltage between V1 and V2, such a polarity can be termed as subtractive polarity.

For subtractive Polarity,

V3 = V2 – V1, for a step-up transformer.

V3 = V1- V2, for a step-down transformer.

As shown in the circuit, a transformer polarity dot denotes which terminals have the same relative polarity.

Transformation ratio calculation

For the test, the transformer supply should be connected to an autotransformer. Before switching, the autotransformer should set a minimum position. Then adjust the voltage to rated values and note the corresponding voltmeter readings.

The transformation ratio of a single-phase transformer can be determined by the equation,

The transformation ratio K= V2/V1

V2 > V1 for a step-up transformer and V1> V2 for a step-down transformer.

Thus the value of transformation ratio will be less than one for a step-down transformer and greater than one for a step-up transformer.

The maximum measuring voltage of the voltmeter V3 must be greater than the sum of primary and secondary voltages. Because in additive polarity the sum of V1 and V2 appears across the voltmeter V3.