Protective Relays

Experiment No : 1                                                                                     Date 14-09-2017

Objective : 

(i) Study the principle of induction relays
(ii) Study construction & Working of different Induction relays

Electromagnetic induction relays operate on the principle of induction motor and are widely used for protective relaying purposes involving a.c. quantities. An induction relay essentially consists of a pivoted aluminum 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. To understand the production of torque in an induction relay, refer to the elementary arrangement shown in Fig. below (i). The two a.c. fluxes φ2 and φ1 differing in phase by an angle α induce E.M.F.s’ in the disc and cause the circulation of eddy currents I2 and I 1

respectively. These currents lag behind their respective fluxes by 90 Referring to Fig. below (ii)

where the two a.c. fluxes and induced currents are shown separately for clarity, let where

Ø1 = Ø1msinwt

Ø2 = Ø2msin (wt + Ɵ)

Fig.1 Principle of Working of Induction Relay

Image Courtsey : http://electricalbaba.com/principle-of-induction-type-relays/

Where Ɵ is the phase angle by which Ø2 leads Ø1. It may be assumed with negligible error that the paths in which the rotor currents flow have negligible self-inductance, and hence rotor currents are in phase with their voltages:

iØ1 ∝ d Ø1 / dt ∝ Ø1coswt

iØ2 ∝ d Ø2 / dt ∝ Ø2cos (wt+Ɵ)

As clear from the figure shown above, the two forces F1 and F2 are in opposition, and consequently we may write the equation for the net force (F) as follows:

F = (F2 – F1)

    ∝ (Ø2iØ1– Ø1iØ2) 

Putting the values of the quantities into equation (1), we get

F ∝ Ø1mØ2m[sin (wt + Ɵ)coswt – cos (wt + Ɵ)sinwt ]   

Therefore,

F ∝ Ø1mØ2mSinƟ

(i) Shaded-pole structure.

The general arrangement of shaded-pole structure is shown in Fig. Below It consists of a pivoted aluminum disc free to rotate in the air-gap of an electromagnet. One half of each pole of the magnet is surrounded by a copper band known as shading ring. The alternating flux φs in the shaded portion of the poles will, owing to the reaction of the current induced in the ring, lag behind the flux φu in the unshaded portion by an angle α. These two a.c. fluxes differing in phase will produce the necessary torque to rotate the disc. As proved earlier, the driving torque

(ii) Watthour-meter structure. 

This structure gets its name from the fact that it is used in watthour meters. The general arrangement of this type of relay is shown in Fig. 21.8. It consists of a pivoted aluminium disc arranged to rotate freely between the poles of two electro magnets. The upper electromagnet carries two windings; the pirmary and the secondary. The primary winding carries the relay current I1 while the secondary winding is connected to the winding of the lower magnet. The primary current Induces e.m.f. in the secondary and so circulates a current I2 in it. The flux φ2 Induced in the lower magnet by the current in the secondary winding of the upper magnet will lag behind φ1 by an angle α.The two fluxes φ1 and φ2 differing in phase by α will produce a driving torque on the disc proportional to φ1φ2sin α. An important feature of this type of relay is that its operation can be controlled by opening or closing the secondary winding circuit. If this circuit is opened, no flux can be set by the lower magnet however great the value of current in the primary winding may be and consequently no torque will be produced. Therefore, the relay can be made inoperative by opening its secondary winding circuit.

(iii) Induction cup structure.

Fig. aside shows the general arrangement of an induction cup structure. It most closely resembles an induction motor, except that the rotor iron is stationary, only the rotor conductor portion being free to rotate. The moving element is a hollow cylindrical rotor which turns on its axis. The rotating field is produced by two pairs of coils wound on four poles as shown. The rotating field induces currents in the cup to provide the necessary driving torque. If φ 1 and φ2 represent the fluxes produced by the respective pairs of poles, then torque produced is proportional to φ1 φ2 sin α where α is the phase difference between the two fluxes. A control spring and the back stop for closing of the contacts carried on an arm are attached to the spindle of the cup to prevent the continuous rotation. Induction cup structures are more efficient torque producers than either the shaded-pole or the Watt-hour meter structures. Therefore, this type of relay has very high speed and may have an operating time less
then 0·1 second.