Electrical Machine - IES Academy
Electrical Machine - IES Academy
Electrical Machine - IES Academy
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India’s No. 1<br />
<strong>IES</strong> <strong>Academy</strong><br />
Transformer<br />
Chapter-1<br />
induction motor starters, interconnection of HV systems at voltage levels with ratio less than 2<br />
and in obtaining variable voltage power supplies.<br />
Advantages of auto- transformer over two-winding<br />
transformer<br />
They require less copper in winding, hence are more economical. Further, they have lower<br />
reactance, lower losses, smaller exciting current, better voltage regulation and higher<br />
operating efficiency because in an auto-transformer a part of energy transfer is through<br />
conduction process.<br />
The given auto-transformer has N1 turns primary with N2 turns tapped for a lower secondary<br />
voltage. The winding section BC is common to both primary and secondary circuits.<br />
For a two-winding transformer, turn-ratio a = (V1 – V2)/V2<br />
= (N1 – N2)/N2 where, (N1 > N2)<br />
= N1/N2 – 1<br />
Auto- transformer turn ratio, a' = V1/V2 = N1/N2 (>1)<br />
Therefore, a' = 1 + a<br />
VA Ratings<br />
For two-winding transformer: (VA)TW = (V1 – V2)I2 = (I2 – I1) V2<br />
For auto- transformer: (VA)auto = V1 I1 = V2 I2<br />
From above, (VA)auto = 1/[1– (1/a')] (VA)TW ; a' > 1<br />
Therefore, (VA)auto > (VA)TW; (since part of VA is transferred conductively in autotransformer).<br />
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