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 />
Transformed VA<br />
Input VA<br />
( V − V ) I V<br />
1 (1 k)<br />
VI V<br />
1 2 1 2<br />
= = − = −<br />
1 1 1<br />
Conducted VA<br />
Input VA<br />
V − V<br />
VI<br />
2 1<br />
= =<br />
1 1<br />
k<br />
As a' = N1/N2 approaches unity, (VA)' >> (VA)<br />
Hence, auto- transformer is used for required turn-ratio is 2 or less.<br />
Copper requirement<br />
For a two-winding transformer and auto-transformer having same voltage-ratio and VA<br />
rating, the weight of copper required by both of them is as below:<br />
G<br />
G<br />
auto<br />
TW<br />
=<br />
I ( N − N ) + ( I −I ) N<br />
IN 1 1+<br />
IN 2 2<br />
1 1 2 2 1 2<br />
= 1 –<br />
N<br />
V<br />
2<br />
N = 1 – 2<br />
1 V 1<br />
Copper saving = GTW – Gauto = 1/a' GTW<br />
1<br />
R2' = ⎛ N ⎞ 2<br />
⎜ −1<br />
R2<br />
⎟<br />
and X2' = ⎛ N1<br />
⎞<br />
⎜ −1⎟<br />
⎝ N 2 ⎠<br />
⎝ N 2 ⎠<br />
2<br />
X2<br />
Three Phase Transformers<br />
There exist two arrangements:<br />
(i) A bank of three single – phase transformers<br />
(ii) A three – phase transformer (single unit)<br />
Fig. (a) Three-phase transformer bank, both windings in star (b) three-phase core-type<br />
transformer<br />
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