Transformer windings are marked to identify terminals with the same polarity. Polarity marks may either be dots or + marks. Alternatively, polarity marks may be shown by assigning the same subscripts to corresponding primary and secondary labels, H and X, respectively. Thus, when the primary terminals are named H1 and H2, the corresponding secondary terminals are identified as X1 and X2, respectively.
Primary and secondary terminals have the same polarity when, at a given instant of time, current enters the primary terminal and leaves the secondary terminal as shown in Fig.1.
Fig.1: Transformer Polarity Markings
The polarity marks also indicate that at the instant the primary dotted terminal H1 is positive with respect to the un-dotted end H2, so is the secondary terminal X1 positive with respect to its un-dotted end X2.
The terms additive polarity and subtractive polarity merely have reference to the relative position of the locations of the H and X terminals. Figure 2 illustrates both conditions.
Fig.2 Transformer Polarity (a) Subtractive Polarity; (b) Additive Polarity
If the top terminals of the transformer of Fig.2 are connected together and one winding is excited by a sinusoidal voltage source, the voltage measured across the bottom terminal will be either
1.The difference between the induced voltage across the H and X windings as in Fig.2 (a) or
2. The sum of the induced voltages as in Fig.2 (b)
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