Quoted from PhilGreg:
Hey I'm not saying I want to modify my EM, I want to drive EM coils with DC power for a different application, so I do want to use diodes.
I understand. Regardless of your application, the coil is a current operated device. It doesn't care what voltage (AC or DC) is impressed on it as long as the voltage level does not exceed the voltage rating of its magnet wire insulation. As such, you can apply a DC voltage to an AC coil. To get the AC coil to work on a DC system requires a sufficient DC voltage be impressed on the coil so that the same amount of current is drawn as when the coil is operated on AC.
When you apply a voltage to a coil is creates a magnetic field. When you remove the voltage the magnetic field collapses and creates a reverse polarity voltage and can be many times the value of the original applied voltage. This creates a transient voltage pulse that can damage other components in the circuit that are not rated for this polarity or the higher voltage created, things like semiconductor bridge rectifiers have a maximum voltage limit and breakdown if exceeded. Having a reversed biased diode across the coil allows the diode to conduct for reverse polarity voltages and creates a 'short circuit' across the coil that allows the pulse to be dissipated in the resistance of the coil wiring.