Electromechanical relays

In electromechanical relays, whenever the actuating quantity surpasses a predetermined value, a torque is developed. This torque causes the moving element of the relay to travel and close a set of contacts in order to energize the trip coil of the circuit-breaker. A few examples of electromechanical relays are attracted armature type, polarized moving iron type, induction disc type, induction cup type, moving coil type and printed disc type. Attracted armature type relay operates through a specially designed armature. This armature is attracted to the poles of an electromagnet through a plunger which is drawn into a solenoid. Here both ac and dc operations are possible. In polarized moving iron relay, polarization is achieved with the help of a permanent magnet and this increases the sensitivity of the relay. Electromagnetic induction principle is employed in induction disc relays. A disc is the moving element here and only ac operation is possible here. Two types of structures exist here. They are shaded pole structure and wattmetric structure. In shaded pole structure, disc is placed between the shaded and unshaded poles of the relay. Wattmetric structure is used in watt-hour meters. In induction cup relay, rotor iron is stationary and rotor conductor rotating. Moving coil relay is dependent on dc actuating quantity and hence is very sensitive. In printed disc relays, a permanent magnet is used to produce a magnetic field.