Answer / dinesh lokhande

If question is where is DC voltage used in transformer then
it is correct, used for measuring resistance.

To elaborate,

DC voltage is used to counter effect inductive reactance
of winding. As winding put on core coil assembly consist of
resistance as well inductance, for measuring pure reistance
we pass DC current through winding.

Impedance = resistance + Inductive reactance.

Inductive reactance = 2*pi*f*L where,
L = Inductance
f = frequency which is zero for DC and hence Inductive
reactance becomes zero for DC supply.

measured impedance at winding terminal is pure resistance.

Answer / arun

winding resistance testing used for dc voltage in transfomer

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