What are Electromagnetic relays?
Electromagnetic relays are widely used for protective relaying purposed. Induction relays are used only for A.C quantities. The induction relays are used on the principle of split-phase induction motor. The operating forces will be developed due to interactions of fluxes in the rotor which are displaced in time and space. The induction type relays are mostly used for the protection of transmission lines and substations equipment. The induction relays are consists of pivoted aluminium discs which is placed in two alternating magnetic fields of the same frequency but displaced in time and space.
The operating forces are produced in the aluminium disc by the interaction of electromagnetic fluxes with eddy current which is induced in the Disc by these electromagnetic fluxes.
The induction type relays are classified as
1. Induction Disc Relay
The shaded pole type induction relay consists aluminium disc, electromagnetic current coil, shading ring, control spring, damping magnets, fixed contacts and moving contacts.
Advantages of Induction Relay:
The induction type relays are classified as
1. Induction Disc Relay
2. Induction Cup Relay
The induction Relays are classified as
1. Shaded Pole structure
2. Watt Hour Meter Structure
1. Induction Disc Relay
Principle of Induction Disc Relay:
The induction disc relays work on the principle of electromagnetic induction same as the induction motor. In this relay, aluminium discs are rotating between two electromagnets.
The coil of electromagnets is energised by alternating currents. when the alternating currents flow through the electromagnetic coils, magnetic flux is produced in the disc. This magnetic flux produces an eddy current in the aluminium disc which creates its own magnetic field.
The torque in induction disc type relay is produced when the magnetic field produced by the electromagnetic coil interacts with the magnetic flux produced by the eddy currents. These two magnetic fields have the same frequency but are displaced in time and place. Induction disc relays are only used for A.C quantity only and these relays are not used for D.C quantity.
Construction of Induction Disc Relay:
The induction disc type relay consists of an aluminium disc mounted on the shaft and the disc is placed between the two electromagnets. The electromagnet is made by wounding a wire on an iron core. This electromagnet coil is energised when alternating currents are flowing through the coil. The fixed contacts of the relay are mounted on the stationary relay frame and moving contacts are mounted in the aluminium rotating disc.
The induction type relay consists of one control spring which is used to provide retraining torque to the rotating aluminium disc. When the current not flowing the electromagnet coil control spring keeps the aluminium disc at its reset position.
The induction disc relay consists of one damping contact which is provides damping torque to the aluminium disc, i.e control the speed of the aluminium disc.
There are three types of torques are produced in induction type relays
1. The operating torque is produced by the electromagnetic coil.
2. The restraining torque is produced by the control spring.
3. The damping torque is produced by the damping contacts.
Working of the Induction disc relay:
When a fault occurs in the power system, the alternating current flowing through the electromagnetic coil and coil get excited and magnetic flux is produced in the aluminium disc. This magnetic flux produces eddy currents in the aluminium disc which creates its own magnetic flux. When magnetic flux produced by electromagnetic coil interacts with magnetic flux produced by eddy currents results in the production of torque on the disc.
This torque causes to rotates the disc and moving contacts comes towards the fixed contacts and the trip circuit is completed. The trip circuit gives the commands to the circuit breaker and circuit breaker trips and faults current are interrupted.
When the fault in the circuit is cleared, the current through the electromagnetic coils falls below the pickup value. The control spring provides restraining torque to the disc and the disc comes back to its reset position. The moving contacts go away from the fixed contacts and the trip circuit will get open. The time of contacts close depends upon the contact travel distance which is set by time delay.
Shaded pole Type Induction relay:
The aluminium disc is mounted on the shaft which is placed between the shading rings and electromagnetic coil. One half of each pole of a magnet is surrounded by a copper ring is called a shading ring. The shading ring split the main flux into the two fluxes displaced in time and space. The shading ring is made from copper and encircles the part of the pole face of each pole at the air gap. Hence the two magnetic fluxes are produced which is displaced at time and space.
The shading flux lag behind the air gap flux. The torque is produced in the aluminium disc by the interaction of one flux with another flux is produced due to eddy currents. Due to this torque aluminium discs rotates and moving contacts are come near to the fixed contacts and the trip circuit close. The damping magnets are used to control the speed of the disc. The control spring is used to provide restraining torque to the disc.
Watt-Hour Meter Induction Type Relay:
The construction of a watt-hour meter induction relay is the same as a watt-hour meter that is used to measure power flowing through the circuit. The watt-hour meter relay consists of two electromagnets and an aluminium disc.
The aluminium disc rotates freely between two electromagnets. In watt-hour meter type relay upper electromagnet is E shaped and the lower electromagnets are U shaped. The upper electromagnets consist of two windings one is primary winding and the second is secondary winding on the central limb. The lower electromagnet consists only of secondary winding on both limbs.
When the fault occurs on the circuit, current I1 flows through the primary winding of the relay and due to this current I1 flux φ1 is produced. The e.m.f get induced in the secondary winding due to flux φ1 and current I2 flows through the secondary winding.
Due to this current I2, Flux φ2 is Produced in the lower magnet of the relay. The flux produced in lower magnets lags behind the flux produced in the upper magnet by phase angle θ. Due to the interaction of two fluxes, torque is exerted on the aluminium disc and the aluminium disc rotates. The torque produced in the aluminium disc is proportional to Φ1 Φ2 sinθ.
In watt-hour meter, relay tappings are provided in the primary winding and suitable primary turns can be selected hence the current setting of the relay can be adjusted.
Advantages of Induction Relay:
1. The induction relay have robust construction, fast operation and fast reset.
2. The operation of the induction relay can easily be controlled during abnormal conditions by opening the secondary winding of the relay.
3. In the induction relay time setting and current setting are easily done.
4. Induction disc relay is reliable, accurate, compact and instantaneous.
5. The induction relay is mostly used for overcurrent protection.
7. The induction relays are lower in cost.
8. The induction relay can withstand higher voltages.
Disadvantages of induction relay:
1. In the induction relay, periodic maintenance and testing are required.
2. The relay operation is affected by the ageing of components and pollution dust.
3. The operation speed of the induction relay is limited due to the mechanical inertia of the component.
4. The directional feature is absent in induction relays.
5. In induction disc relay, a high burden current transformer and potential transformer are required.
Applications of Induction relay:
1. These relays are used for differential relay.
2. Induction relay are used where reliability and robustness are required.
3. These relays are used for over current, over voltage and under voltage, under-voltage protection of various equipment.
4. The induction relays are used where slow-speed operations are required.
5. The induction disc relays are used where adjustable operating time and time delay features are required.
6. The induction disc relays are used where a high reset to pick up ratio is required.
7. The induction relays are used for motor control and automotive applications.
8. The induction relays are used to control large power loads in the industry.