What is the difference between AC and DC relays?

AC and DC relays work on the same principle, that of, electromagnetic induction. However, there are some differences in construction. DC relays have something known as the freewheel diode which acts to discharge the emf built in the inductance when the coil is de-energized. (Click here to read about the phenomenon of freewheeling.) AC relays have cores which are laminated to prevent losses due to eddy current heating.

Another more conspicuous difference between a DC relay and an AC relay is presence of the Shading Coil. In AC relays, the alternating current supply changes direction about 100 times a second. At each instance, when the sine wave passes through zero, the current flowing through the coil becomes zero. This results in a loss of magnetism for a few milliseconds. When this happens about 100 times a second, the repeated drop and pickup of the coil produces a noise known as chattering. This also leads to the making and breaking of the relay contacts leading to disturbances in the connected electric circuits.

A shading coil is a coil with high remanence. thus when the magnetism of the coil collapses when the current becomes zero. The shading coil still retains the magnetism. Thus, ensuring that the contacts do not drop off.