The induction type relays are also called magnitude relays.These relays work on the principle of the induction motor or an energy meter.In these relays, a metallic disc is allowed to rotate between the two electromagnets.The coils of the electromagnets are energised with the help of alternating currents.
The torque is produced in Induction relays due to the interaction of one alternating flux with eddy currents induced in the rotor by another alternating flux.The two fluxes have the same frequency but are displaced in time and space. As the interaction of alternating fluxes is the base of operation of Induction relays, these are not used for the d.c. quantities.These are widely used for protective relaying involving only a.c. quantities.
Types of Induction Relays:
1)Shaded pole Relays
2)Watt-hour meter Relays
3)Induction cup Relays
Before studying these types in detail, let us derive the torque equation for the induction type relays, which is same for all the three types of induction relays.
Torque Equation of Induction Relay – its Derivation
The induction relay works on the principle of electromagnetic induction similar to the induction motor and energy meter. It consists of two electromagnets and a rotating disc placed between the two electromagnets. The torque produced in these relays is by the interaction of alternating flux with the eddy currents induced in the rotor (disc) by another alternating flux.
The two electromagnets when excited produce two alternating fluxes Φ1 and Φ2. The two fluxes produced by the electromagnets will be of the same frequency but there exists a phase displacement α (required for the production of torque) between the two fluxes. The mathematical expression for the two alternating fluxes is given by,
Φ1 = Φ1m sin ωt …(1)
Φ2 = Φ2m sin(ωt + α) …(2)
The two alternating fluxes when links with the disc induce two emfs in the disc which lags their respective fluxes by 90°. The induced emfs leads to the circulation of eddy currents i1 and i2 in the disc. The interaction of eddy currents produced due to two alternating fluxes results in the production of force or torque in the disc.