POLARITY TEST OF A SINGLE-PHASE TRANSFORMER

Polarity testing of transformers is vital before connecting them in parallel. Otherwise, with incorrect polarity, it is not possible to connect them in parallel. The rated voltage is applied to the primary and its two terminals are marked as A₁ and A₂, respectively, as shown in Figures 1.44(a) and 1.45(b), respectively. The secondary winding terminals are also marked as a₁ and a₂, shown in Figures 1.44(a) and 1.45(b), respectively. Now a voltmeter is connected across A₂ and a₂. if it measures the difference of E₁ and E₂, A₂ and a₂ are of the same polarity. If it measures the addition of E₁ and E₂, A₂ and a₂ are of opposite polarity.

Figure 1.44 Polarity Test of a single-phase Two Winding Transformer

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Practical Commutation

The e.m.f. induced in each coil of armature is alternating in nature. If load is connected, the current flowing will also be alternating. But the flow of current in a d.c. generator must be undirectional. This can be achieved by the use of commutator. When the armature conductors are under the influence of one pole they carry current in one direction whereas the current is reversed when the conductors are under the influence of other pole. This reversal of current takes place along the magnetic neutral axis. Fig. 1 Note : The reversal of current is likely to take place in short interval when a coil is short circuited by a brush so that transfer of current from one direction to other is carried out without any sparking. This process is called commutation. Thus a process by which current in the short circuited coil is reversed while it crosses the MNA is called commutation. The time during which the coil remains short circuited is known as...

Demagnetising and Cross Magnetizing Conductors

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Winding Pitches

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