What is a relay Coil?  The relay  coil has many turns of copper wire wound around a bobbin in which iron core is situated. The resistance of the coil depends on the length of the wire and cross-sectional area of wire. When the  voltage is applied across the coil, the coil set up a magnetic field which attracts the armature. The armature controls contact movement. The AC and DC relay are different in construction.

The coil resistance of the AC and DC relay for the same voltage is different. The resistance of the AC coil is  less as compared to the DC coil. The AC relay/contactor coil offers reactance when AC supply is applied across it. The inductive reactance of the AC coil offers more impedance and thus it limit the current. The DC coil has more number of turns of thin wire because the current through the DC coil is limited by resistance only. In case of DC, reactance is zero.

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