Difference between AC and DC Relay

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 AC coil has fewer number of turns as compared to the DC coil.

The AC coil draws high initial current due to low inductance when armature is open and once armature is closed it requires a shader ring to retain the armature at the cross over point to avoid chatter. Since alternating current decreases to zero every half-cycle , the relay armature tends to release magnetic energy in every half-cycle. This continual movement of the armature not only causes an audible “buzz,” but will cause the contacts to open and close as the armature moves.

In order to operate a relay from AC, a shader ring or shader coil is placed on the top of the core. The magnetism developed in the the part of the core lags somewhat with the remainder of the core and, as result the phase displacement established between the magnetism of the part of the core and remainder of the core.

Thus, when the magnetic energy decrease to zero every half-cycle in unshaded region, the magnetic energy still present in the shaded region of the core and, when the magnetic energy decrease to zero in shaded portion of the core, the unshaded magnetic energy starts to increase. In this way, the core remains magnetized throughout the cycle and chattering problem is avoided.

The DC relay has more number of turns of thin wires and, it offers high resistance. The resistance of the DC coil is kept more than the AC coil. It is because the current drawn by DC coil is determined by the resistance of the coil. In case of DC the inductive reactance is zero.

The current of a DC relay is directly affected by the resistance of the coil. The current in the DC coil is governed by Ohm’s law.According to Ohm’s Law, for a given amount of resistance, current is directly proportional to voltage.

I = E/R

where; I = current in amperes

E = voltage in volts

R = resistance in ohms

Thus, a 24 volt DC coil that has 240 ohms of resistance draws 0.1 ampere current.The DC relay has slightly lower pull in time , however once the armature is closed, the current to keep the relay in closed position requires lower current. In DC relay, shading ring is not required to avoid chattering.

The AC relay can be used with DC supply with a series current limiting device.