Page 256 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Fig 1

A difference in the quality factor (i.e the ratio of reactance to resistance) of the per unit impedance results in a
divergence of the phase angle of the currents, so that one transformer will be working with a higher and the other
with a lower power factor than that of the combined output.

Verification of terminals or Polarity: When two or more transformers are to be connected in parallel on their
primary and secondary sides, the terminals of the same polarity only can be connected together, otherwise a
heavy circulating current will be produced between the windings.

Standard procedure to determine the polarity is explained below:-
• Connect one end of the high voltage winding to one end of the low voltage winding as shown in Fig 2a.
• Connect a voltmeter between the two open ends.
• Apply a voltage not greater than the rated voltage of the winding to either high or low voltage winding.

If V2 reads less than V1 ( Fig 2a) the primary and secondary emfs are in opposition. The marking on primary will
be A1 for +ve side and A2 for –ve side and a1 for +ve side of secondary and a2 for –ve side. If the connections are
made (Fig 2b) the voltmeter V2 will read more than V1. Thereby it is ascertained opposite ends are connected.

If in transformer has similar ends in one side (Fig 3a) the polarity marking is said to be subtractive polarity marking
on the other hand if the opposite ends are in one side ( Fig 3b) the polarity marking is called as additive polarity
marking.

Fig 2 Fig 3

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CITS : Power - Electrician & Wireman - Lesson 41-49 CITS : Power - Electrician & Wireman - Lesson 41-49