What is the effect on a transformer when you apply dc supply?
1. If primary of the transformer is connected to rated dc voltage, the flux produced in the transformer core will not vary with time and remain constant in magnitude. Consequently, no emf is induced in the secondary winding except during the switching on the dc supply on primary side. As a result, no voltage appears across the secondary except during a small transient period at the start due to inrush current.
2. When primary side is connected to ac supply, the flux links both sides and there is also a self induced emf which opposes the applied voltage. This, in turn, limits the primary current during normal operation. But when dc supply is given to primary, no self induced emf is established. As a result high current flows through the primary winding (low resistance) and eventually burns it due to excessive heat. The primary and secondary winding in a physical transformer are wrapped one on top of the other with the low-voltage winding innermost. Subsequently, this may also cause damage to the secondary winding.
