When the primary winding of a transformer is excited with suitable rated voltage then e.m.f. gets induced in both the windings. The polarities of these e.m.f.s depend on how the windings are wound on the core. It is usual practice to determine which ends of the two windings acquire simultaneously positive or negative polarity. This polarity determination is carried out by conducting the polarity test on a transformer.
Consider a transformer shown in the Fig. 1. Usually the ends of the L.V. winding are lebelled with small letters as a1, a2 while the ends of the H.V. windings are lebelled with capital letters as A1, A2.
In determining the relative polarity of the two windings of a transformer using polarity test, the two windings are connected in series across a voltmeter. The voltmeter is connected across a1 - A1.
One of the windings is excited by suitable voltage source. So a1 - a2 is excited by voltage V1. Let E1 and E2 are the induced e.m.f.s.
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| Fig. 1 Polarity test |
Key Point : if the voltmeter reads E1 - E2, thus voltmeter reading is less than V1, then the polarities are called subtractive in nature.
The net voltage acting around the local circuit consisting the voltmeter is E1 - E2. In such case the ends a2, A2 are simultaneously positive or negative. This is indicated by dots, as shown in the Fig. 1.
Key Point : But if the windings are wound in such a way that the voltmeter reads E1 + E2. The polarities are said to be additive.
In such case the voltmeter is more than V1. This confirms that if a2 is positive, terminal A2 is negative and vice-versa. In such case, the polarity markings of one of the windings must be interchanged.
Key Point : In practice the transformer windings are wound in such a way that the relative polarities are subtractive which is indicated by dots, as per the dot convention.
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