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Postby ksushantk » Sun Sep 22, 2013 7:47 pm

a 50hz transformer having equal hystersis and eddy current losses at rated excitation is operated at 45hz at 90% of its rated voltage compared to rated operating point, the core losses under this condition will be
a> reduce by 10%
b> reduce by 19%
c> reduce by 14.5%
d> remain unchanged

can anyone explain answer with steps,
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Re: transformers

Postby madhuadroit » Fri Oct 04, 2013 10:35 am

Iron Loss is given by = Hysterisis and Eddy currnt losses

Wi = We +Wh

Wh = A*(V/f)^1.6 *f --------(1)

We =B*(V/f)^2 *f^2 ------------(2)

Under Normal conditions

both Wh and We are same

so Wh=We= let us say = P

Wi = 2P

Now when the txmr is operated at 90 percent of both voltage and frequecy (45 hertz is 90 percent of 50 hz)

the above equations become

Wh = A*(.9V/.9f)^1.6 *0.9*f = .9*P

We =B*(.9V/.9f)^2 *(.9f)^2 =.9*.9*P

Wnewi =.9*P+.9*.9*P = 1.71P

So the percent reduction = (2P - 1.71P )/2p*100 =.145 *100 =14.5 percent

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