The Emf induces in the transformer due to the rate of change of flux linkages. For the dc supply, the change of flux linkages rate is zero.
∴ The induced EMFs in the primary (E1) and secondary winding (E2) is zero. The primary winding current is given by
The current drawn by the primary winding is very high when rated dc supply voltage (V) is given.
∴ The primary winding will be burnt if we were continuously given the supply for a short time, but secondary winding is safe because the secondary winding is not connected to any active source. Hence the statement, DC power is never applied to the transformer is true.
The moment we connect the dc supply, the initial impulse voltage will be induced because of the sudden change in the step input. If we connect any galvanometer across the windings, it will deflect at a single instant and later It shows null deflection continuously.
At the time of disconnection of dc source also galvanometer shows deflection because of step change in the supply.
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