What Will Happen If a DC Voltage is Given to the Transformer Primary?
Effects of giving a constant DC input to a transformer:
1. We know, for an inductor v=Ldi/dt, since v is constant here, the i(current) will follow a ramp
waveform and will keep on increasing till the core saturates.
2. This means, i will rise to a dangerously high value and then stop changing, if i stops
changing then the induced voltage across the primary will be zero(as di/dt=0) thus short
circuiting the DC source.
3. Heavy short circuit current will flow, overheating the coils, resulting in i squared R losses.
Temperature rise will exceed safe values and eventually the apparatus will catch fire.
4. Short circuit currents will result in radial magnetic forces that will rip apart the winding and
the windings become exposed to the atmosphere. Transformer oil may catch fire under short
5. So we must never apply a DC voltage to a transformer during operating conditions.
P.S. Pulsating DC can be applied to transformers, such the flux reset occurs in the negative
pulse period. The concept is to make the time integral of voltage zero in one complete period,
this helps in resetting the flux. The ubiquitous Switched Mode Power Supply(SMPS) forms the
quintessence of this concept.