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:
Shaded pole structure
Watt hour metre or double winding structure
Shaded Pole Structure:
a = ΦU; b = ΦS
2.12 Shaded pole structure
Watt-hour Metre Structure:
a = Φ1; b = Φ2
Fig. 2.13 Watt-hour-meter structure
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.