Unlike DC system, there are no fixed positive and negative poles in AC system, and hence, transformers cannot have fixed positive and negative terminals.
The relative direction in which primary and secondary windings of a transformer are wound around the core determines the relative direction of the voltage across the windings.
Polarity of Transformer Windings
If the windings of the high and low voltage coils are in the opposite directions, the applied voltage and the induced voltage will have opposite directions and the transformer is said to have subtractive polarity.
If the windings of the two coils are in the same direction, the applied voltage and the induced voltage will have the same direction and the transformer is said to have additive polarity. It should be noted that the secondary voltage waveform is in phase with the primary waveform in additive polarity. This condition is known as “no phase shift.”
Similar polarity ends are indicated by a dot convention as shown in Figure. Usually the ends of LV winding are denoted by small letter of the alphabet and are suffixed 1 and 2 while HV windings ends are denoted by the correspondung capatial letter and are suffixed 1 and 2 . The ends suffixed 1 (H1, x1) have same polarity and so have the ends denoted by 2 (H2, x2).
Knowledge of polarity of transformer windings is essential when single-phase transformers are connected in parallel or three-phase configurations. An understanding of polarity is also required to connect potential and current transformers to power metering and protective relays.
Polarity Test on Single Phase Transformer
Assume, we want to determine the polarity of 220/12 V transformer. It may be determined by a simple voltage measurement, as follows:
- Place a connection between the high-voltage and low-voltage terminals as shown in Figure.
- Apply a low voltage, 220 volts, to the two high-voltage terminals.
- Measure the voltage between H2 and X1 terminals.
- If the voltage is lower than the voltage across the high-voltage terminals, the transformer has subtractive polarity. If it is higher, the transformer has additive polarity.
Thanks for reading about polarity of transformer windings .
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