What is Electromagnetic attraction Relays?
The Electromagnetic attraction relay works on the principle of electromagnetic attraction. A coil is wound on the armature which is works as an electromagnet. During normal operating conditions current flowing through the coil is small and is not sufficient to produce a strong magnetic field and not attract the plunger. So tripping contacts are remain in open conditions and the breaker is not tripped.
When a fault occurs in the power system, more operating current flows through the coil and a strong magnetic field is produced by the magnet. Which is attracts the plunger or armature, making contact with the trip circuit breaker. The circuit breaker tripped and the fault current interrupted.
The electromagnetic attraction relays are further classified as
1. Attracted armature type relay
2. Solenoid type relay
3. Balanced bream type relay
1. Attracted armature type relay:
The attracted armature type relay consists electromagnet with Coil and a pivoted laminated moving armature. The armature or iron core of the attraction armature type relay is balanced by a counterweight and a pair of spring contacts fingers at its one free end and one free end consists of moving contacts for the trip circuit.
Under normal operating conditions, the current through the coil is not sufficient to attract the armature and the counterweight holds the armature. When a short circuit fault occurs in the power system, the current through the relay coil increase sufficiently and the relay armature attract upwards as shown.
The armature attraction type relay consists of fixed and moving contacts for the tripping circuit. The fixed contacts are attached to the relay frame and moving contacts are attached to the armature of the relay.
When the electromagnet attracts the armature, the moving contacts are touched to fixed contacts and the trip circuit is completed and which gives commands to the circuit breaker. The circuit breaker tripped and disconnect the faulty section from the circuit.
In the attracted armature type consists a number of tapping are provided in the relay coil so that the number of turns in use and the setting value at which the relay operates can be varied.
The minimum current in the relay coil at the magnetic field produced and armature attracted to close the tripping circuit is called pickup current.
The attracted armature type relays are utilised as magnitude relay of ratio relay. These relays are used in over voltage, under voltage, auxiliary relay, control relay, over current, under current and impedance measuring relay.
2. Solenoid Type relay:
The solenoid type relay consists of solenoids which are also called electromagnetic coils with a hollow centre core and movable plunger.
The plunger is attracted axially in the magnetic field of the solenoid. The moving contacts are attached with the iron movable plunger and fixed contacts are attracted with a relay frame.
Under the normal operating conditions, the current through the solenoid coil is not sufficient to attract the movable iron plunger. so iron plunger remains in the lower position due to gravity or spring. the spring is attached inside the iron plunger between the solenoid and plunger which is taking back the plunger to its first position after the solenoid is de-energised or fault clearing.
When a short circuit fault occurs in the power system, the current flowing through the solenoid is sufficient to attract the plunger. The movable plunger is attached and fixed contacts and moving contacts touch each other. The trip circuit gives commands to the circuit breaker and the circuit breaker tripped and disconnects the faulty section from the system.
3. Balanced Beam type relay:
The balanced bream type relay consists of an iron armature fastened to the balanced bream. the fixed contacts are connected to the stationary frame of the relay and moving contacts are connected to the iron armature as shown in fig.
Under the normal operating condition, the current through the relay coil is not sufficient to attract the armature and beam held in a horizontal position by the spring.
When the fault occurs in the circuit, the current through the relay coil becomes greater than the pickup current and beams are attracted to close the trip circuit.
The relay gives the commands to the circuit breaker and circuit breaker trips and faults current interrupted. These relays are fast in operations and robust. Only one cycle is enough for operation. Due to d.c transient, these relays are not more accurate and now a days these relays aren't used.
Advantages of electromagnetic relays
1. The electromagnetic relays are used for both A.C and D.C.
2. The electromagnetic relays have fast speed and fast reset.
3. The attraction armature type relays have a fast operating speed which has an operating time of a few milliseconds.
4. The electromagnetic relays are instantaneous relays.
5. The pickup value of electromagnetic relay is very high for D.C operation 90-95% and for A.C operation 60 to 90 %.
6. Modern electromagnetic relays are robust, reliable, simple and compact.
Disadvantages of electromagnetic relays
1. The directional feature is absent in the case of electromagnetic relays.
2. Due to the fast operations of electromagnetic relays working can be affected by transients.
Applications of electromagnetic relays
1. The electromagnetic relays are used in the protection of various a.c and d.c equipment.
2. The electromagnetic relays are used in over current, under current and over-voltage, under-voltage protection of various a.c and d.c equipment.
3. The differential relays are also used for differential protection.