I would like to understand why if the secondary of a transformer is short circuited the primary winding will also reflect the short circuit despite the fact that they are isolated. Would the electromagnetic flux through the core be increased with a short circuit on the secondary? If the secondary is shorted I guess it would be as if no turns were present, there's no voltage difference. How would that draw more current from the supply at which the primary is connected?
Hi Victor,
as long as the secondary coil is open (no current) and the primary coil is connected to an AC voltage, the magnetic flux will change in such a way that the voltage induced in the primary coil according to curl E = -d B / d t nearly equals the amount of the external AC voltage, but points in the opposite direction. The remaining small voltage difference causes the current through the (resistive) primary coil which in turn causes the flux.
In a rough approximation you can assume that both coils are perfectly coupled, i. e. both are pervaded by the same flux, and as a consequence, the induced voltage per turn is the same in both.
If the secondary coil is shorted, the current in this coil is so directed that it opposes a change of the flux. Therefore the variation of the flux is much smaller, and so is the induced voltage in the primary coil. As a consequence, the difference between external voltage and induced voltage is much larger, and so is the primary current.
To look at it from another perspective: The shorted coil will be heated by the large current, and the electrical power turned into heat has to be delivered from the external voltage source; so, the primary current must increase.
Another perspective:
One might consider the primary winding's inductance to be lowered by any current flowing through the secondary winding - which does easily explain the increase of primary current with any increase of the secondary current - up to short circuit conditions.
Imagine a 100% coupling of both windings, without stray inductance and without ohmic resistance. Then, just as much current is induced in the short-circuited secondary coil at any time to completely compensate for the magnetic field of the primary coil. Therefore, the current in the primary coil increases indefinitely. Then the iron core is unnecessary, the coils alone do the job. The magnetization of the iron core is then vanishingly small, it is no longer needed. Conversely, the magnetization of the iron core is maximal if no secondary coil flows through it.
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