Polarity Test

Polarity Test

• 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. 6.17.1. 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

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. 6.17.1.

Key Point: But if the windings are wound in such a way that the voltmeter reads E₁ + E₂ polarities are said to be additive.

• In such case the voltmeter reading 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 in indicated by dots, as per the dot convention.

Review Question

1. What is polarity test? Why it is required to be performed? How it is performed ? Draw the vigg suitable circuit diagram.