The Induction type relay operates based on the electromagnetic principle therefore, the relay can be used only on AC circuit and not on DC circuit. Depending upon the types of the rotor relay being used this relay is categorised. 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:

1. Induction Disc type

   1. Shaded pole structure

   2. Watt hour metre or double winding structure

Shaded Pole Structure:

• The general arrangement of shaded pole structure is shown in figure 2.12. It consists of a pivoted aluminium disc free to rotate in the air gap of a C shaped electromagnet. One half of each pole of the magnet is surrounded by a copper band known as a shading ring. The alternating flux Φs (Φ1 – Auxiliary flux) in the shaded portion of the poles will, owing to the reaction of the current induced in the ring, lag behind flux Φu (Φ2 – Main flux) in the unshaded portion by an angle α. Thus two alternating fluxes displaced in space and time, cut the disc and produce eddy currents in it. These two ac fluxes interact with eddy current and will produce the necessary torque to rotate the disc. The resultant torque causes the disc to rotate.

a = ΦU; b = ΦS

2.12 Shaded pole structure

Watt-hour Metre Structure:

• This structure is used in watt-hour metres, which is as shown in figure 2.13. It consists of a pivoted aluminium disc arranged to rotate freely between the poles of two electromagnets. The upper electromagnet carries two windings; the primary and the secondary. The primary winding carries the relay current I1 (Main current and produces main flux Φ1) while the secondary winding is connected to the windings 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 (Auxiliary flux) induced in the lower magnet by the current in the secondary winding of the upper magnet will lag behind Φ1 by an angle α. These two fluxes produce a driving torque on the disc proportional to Φ1Φ2sin α.

a = Φ1; b = Φ2

Fig. 2.13 Watt-hour-meter structure

• 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 primary current may be and consequently no torque will be produced. Therefore, the relay can be made in-operative by opening its secondary winding circuit.

2. Induction Cup Structure:

• Figure shows 2.14 the general arrangement of an induction cup structure. It closely resembles an induction motor, except that 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 voltage in the rotor cup and hence the eddy currents. The eddy current due to one flux interacts with the flux due to another flux in the cup to provide the necessary driving torque. If Φ1 & Φ2 represent the fluxes produced by the respective pairs of poles, then torque produced is proportional to Φ1Φ2sin α. Where, α is the phase difference between the two fluxes. A control spring and the back stop or closing of the contacts carried on an arm are attached to the spindle of the cup to prevent the continuous rotation.

Fig. 2.14 Induction cup structure

• Induction cup structures are more efficient torque producers than the above two types of structures. Therefore, this type of relay has very high speed with operating time less than 0.1 second.