Induction type relays are most widely used for protective-relaying purposes involving ac quantities.
They are not usable with DC quantities,owing to the principle of operation
An induction type relay is split-phase induction motor with contacts.
Actuating force is developed in a movable element,that may be a disc or other form of rotor of non-magnetic current-conducting material,by the interaction of electromagnetic fluxes with eddy currents that are induced in the rotor by these fluxes.
Induction Disc Relay
In this type of relay,a metal disc is allowed to rotate between two electromagnets.The electromagnets are energized by the interaction of the flux of one of the magnets and the eddy current induced in the disc by other.
The different types of structure that have been used are commonly called
Shaded Pole Structure
The shaded pole structure is generally actuated by current flowing in a single coil on a magnetic structure containing an air gap.
The air gap flux produced by this current is split into two out-of-phase components by a so called "shading ring" generally of copper,that encircles part of the pole face of each pole at the air gap
Watt-hour meter structure
It contain two separate coils on two different magnetic circuits,each of which produces one of the two necessary fluxes for driving the rotor,which is also a disc.
Induction Cup Relay
There are two structure in which the most close resemble induction motor,except the rotor iron is stationary,only the rotor-conductor portion being free to rotate.
The cup structure employs a hollow cylindrical rotor.
The double-loop structure employs two loops at right angles to one another.
It is practically suited as directional or phase comparison units.this is because,besides their sensitivity,induction cup relays have study non-vibrating torque and their parasitic torque due to current or voltage alone are small.
Induction cup structures are more efficient torque producers than either shaded-pole or watt hour meter structure
Advantages of Induction Relay
They have fast operation and fast reset
High operating speed with high operating time in few milliseconds can also be achieved.
Modern relays are compact,simple,reliable and robust.
The directional feature is absent
Due to the fast operation,the working can be affected by the transients