An electromagnetic attraction type relay is the simplest form of protection relay. The relay uses an electromagnet to perform its function. Its basic construction consists of a magnetic core made of laminated, cold-rolled, grain-oriented silicon steel or laminated steel. Lamination reduces eddy current losses, while the cold-rolled structure of the iron minimizes hysteresis loss. We wrap a very thin, insulated copper wire around the limb of the core to form the operating coil. An iron piece is hinged to the core in such a way that an air gap is maintained between the core and the iron piece.
Basic Working Principle of an Electromagnetic Attraction Type Relay
Under normal conditions, current flows through the coil, but its amplitude is too small to produce a strong magnetic field capable of attracting the plunger or iron piece against the spring pressure or gravity. During fault conditions, however, the fault current becomes high. Consequently, the current through the secondary of the CT also increases. Since we connect the operating coil across the CT secondary, the current through the coil also increases.
As a result, the magnetic attraction produced by the coil on the plunger or iron piece also increases. Once this magnetic attraction force exceeds the restoring force of the spring or gravity, the movable plunger moves toward the core of the coil. The plunger holds a set of normally open and normally closed contacts. When the plunger moves, all the normally open contacts close, and all the normally closed contacts open. This means the relay has operated. This is the basic working principle of an attraction-type relay.
Applications of an Electromagnetic Attraction Type Relay
Although this type of relay has become almost obsolete nowadays, it is still in use in some low-voltage and medium-voltage applications within the power system.
Over Current Protection
Previously, power system engineers widely used this type of relay for overcurrent protection in medium-voltage and low-voltage networks. Whenever an overcurrent or short-circuit current flows through the relay coil, the coil attracts the iron plunger more strongly. As a result, the plunger rotates about its hinge and pushes the moving contact upward. This action closes the normally open (NO) contacts and opens the normally closed (NC) contacts. Consequently, the relay trips the circuit breaker and actuates the associated annunciation and interlocking functions.
Backup Protection
Sometimes, engineers also use this type of relay as a backup relay. If the primary (advanced) relay fails to operate for any reason, this relay will at least trip the circuit breaker to protect the system.
Contact Multiplier
Another important use of the electromagnetic attraction armature type relay is contact multiplication. This means engineers can use this relay as a simple contact multiplier. Suppose the main relay has only a few contacts, which are not sufficient to provide all the required annunciation signals and interlocking. In that case, the coil of this attraction armature-type auxiliary relay is connected across a DC power circuit. A normally open (NO) contact of the main relay is placed in that DC circuit in such a way that, when it closes, the DC supply energizes the auxiliary relay coil.
As soon as the auxiliary (contact multiplier) relay coil receives DC power, it attracts its armature and operates its own NO and NC contacts. Thus, from one contact of the main relay, we can control multiple contacts of another relay. This is the function of a contact multiplier relay.