Induction Type Relay

Induction Type Relay

An induction type relay or induction relay operates on the principle of electromagnetic induction. The operating principle of an induction relay is the same as the split-phase induction motor with contacts.

This type of relay is widely used for the protection of lines or apparatus.

Operating force is developed by interacting two AC flux displaced in time and space in a movable element.

The induction type relays divide into two parts according to the rotor type.

  • Induction Disc Relay
  • Induction Cup Relay

Induction Disc Relay

The induction disc relay is also known as a shaded pole type induction relay, consisting of an aluminium disc. The construction of the induction disc relay is shown in the figure below.

Induction Disc Relay
Induction Disc Relay

This relay is operated by the current flowing in a single coil placed on a magnetic core having an air gap.

The aluminium disc is free to rotate in the airgap of an electromagnet.

The part of pole face of each pole is shaded with the help of a copper band or ring. This ring is known as a shaded ring.

Due to the shading ring, the total flux produced by the alternating current splits into two fluxes displaced in time and space.

Due to this flux, EMF gets induced in the shading ring. And the current produced in the shaded ring. This current causes flux in a shaded ring that lags behind the flux in an unshaded portion by angle α.

Now consider,

фS, фU = Flux induced in shaded portion and unshaded portion
ES, EU = EMF induced in the disc due to flux фS and фU, respectively
IS, IU = Induced current due to ES and EU

The phasor diagram of this arrangement is shown in the figure below.

phasor diagram of Induction Disc Relay
phasor diagram of Induction Disc Relay

EU lags behind фU by 90˚ and ES lags behind фS by 90˚.

The current IS and IU lags ES and EU by small-angle β. This angle is minimal, and we can neglect it in practice.

    \[ T \propto \phi_S \phi_U sin \alpha \]

фS and фU is proportional to the current I in the relay coil,

    \[ T \propto I^2 sin \alpha \]

    \[ T = k I^2 \]

Here, for this design, we can take sin α as constant. This torque is proportional to the square of the current through the coil.

Induction Cup Relay

The arrangement of the induction cup relay is similar to an induction motor.

Compared to the induction motor, the only difference is the rotor core is stationary, and only the rotor conductor portion is free to rotate about its axis. The rotor is a hollow cylindrical cup type in structure.

The arrangement of this relay is shown in the figure below.

Induction Cup Relay
Induction Cup Relay

The above figure shows the four pole construction of the induction cup relay. It can be two, four, or more—these poles energies by the relay coils.

The coils 1 and 1’ are connected, and coils 2 and 2’ are combined to form two pairs of coils.

By these pair of coils, the current and respective fluxes produce. These fluxes are displaced from each other by angle α.

Therefore, the resultant flux in the air gap is rotating, and two pairs of coils produce RMF.

The eddy currents are induced in this process, and it produces flux.

The interaction of the two fluxes produce torque, and the rotor rotates in the same direction as that of the rotating magnetic field.

To prevent continuous rotation, a control spring and backstop are carried on an arm attached to the spindle of the cup.

The operation of the induction cup relay is very fast (operating time is in order of 10 msec).

In this type of relay, the rotor is light, and it requires a very low moment of inertia to rotate.

The induction cup relay can be used for two quantities or a single quantity. In a single quantity relay, both coils are fed by the same actuating quantity with a fixed phase angle shift in between them.

A double induction loop structure reduces the rotor inertia and makes the system faster.

The induction relays are used as a protective relay, including AC quantities. The speed of these relays can be adjusted, and hence, it is possible to have a high and low-speed operation.

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