Electromagnetic induction relays operate on the principle of induction motor and are widely used for protecting relaying purpose involving a.c. quantities. They art not used with d.c. quantities owing to the principle of operation. The induction relay essentially consists of a pivoted aluminium disc placed in two alternating magnetic fields of the same frequency but displaced in time and space. The torque is produced in the disc by the interaction of one of the magnetic fields with the currents induced in the disc by the other. In order to understand the production of torque in an induction relay, refer to the elementary arrangement shown in fig 6 a. The two ac. fluxes Ф₁ and Ф₂ differing in phase by an angle α induce e.m.f.’s in the disc and cause the circulation of eddy currents i₁ and i₂ respectively These current lags the respective fluxes by 90°.

Now

from fig 6 b,

Let Ф1 = Ф_1max sin ωt

Ф₂ = Ф_2max sin (ωt + α) where Ф₁ and Ф₂ are the instantaneous values of fluxes and Ф₂ leads Ф₁ by an angle α.

Assuming that the path in which the rotor currents flow have negligible self inductance, the rotor currents will be in phase with their voltages.

i_{1 α} d Ф₁/dt _α d(Ф_1max sin ωt)/ dt

_α Ф_1max cos ωt

and i_{2 α} d Ф₂ /dt _α Ф_2max cos (ωt + α)

Now, F_{1 α} Ф₁ i₂ and F_{2 α} Ф₂ i₁

Fig 6 b shows that the two forces are in phase opposition.

So net force F at the instant can be considered is

F _α F₂ – F₁

_α Ф₂ i₁ – Ф₁ i₂

_α Ф_2max sin (ωt + α) Ф_1max cos ωt – Ф_1max sin ωt Ф_2max cos (ωt + α)

_α Ф_1max Ф_2max [sin (ωt + α) cos ωt – sin ωt cos (ωt + α)]

_α Ф_1max Ф_2max sin α

_α Ф₁ Ф₂ sin α (i)

Where Ф₁ and Ф₂ are the r.m.s. values of the fluxes.

The following points may be-noted from exp. (i)

a) The greater the phase angle α between the fluxes, the greater is the net force applied

to the disc. Maximum force will be produced when the two fluxes are 90º out of phase.

b) The net force is same at every instant.

c) The direction of net force and hence the direction of motion of the disc depends upon

which flux is leading.

The following three types of structures are commonly used for obtaining the phase difference in the fluxes and hence the operating torque in induction relays:

(a) shaded-pole structure

(b) watt-hour-meter or double winding structure

(c) induction cup structure

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