even in a dc transformer, the cause varying current though not in polarity, can cause a change in the magnetic flux linked with a coil.
Transformers do not work with direct current (DC), where the current constantly flows in the same direction.
Because the voltage induced in the secondary winding depends on the *rate of change* of current in the primary winding: E M * di/dt. induced voltage mutual inductance * rate of change of current in other winding.
Using a transformer with a varying DC signal was done all of the time with vacuum tube amps. Checkout the output transformer in the attached schematic and you will see its primary is being fed from high voltage(B+) DC. The output from the secondary is AC. Using a transformer is another way of blocking DC and passing the AC component of a signal. Note this doesn't mean that the DC is being pulsed on and off, rather it is varying with the applied signal. You can liken this to the algebraic sum of an AC signal with a DC bias voltage. 50VAC signal goes from +50 to - 50 volts Place that on a 100VDC and the result is a DC voltage that varies from 150V to 50V.
Transformers are used for obtaining a voltage which is different from available supply voltage. The supply voltage on the primary induces a magnetic field in the primary coil. This magnetic field is shared by the secondary coil, in which depending on the number of turns ( coils), a voltage is induced. To induce magnetic field, we need a relative movement of supply source and the medium where it is induced. In AC, since the voltage is varying in magnitude and direction, a stationary primary coil can get induced to produce a magnetic field. In DC, since there is no change in direction, voltage will not get induced in a stationary primary coil ( except during the switching transient conditions. i.e On and off times). Note that the fundamental requirement for induction is rate of change of flux.
