The electromagnetic induction disc relay is frequently used where the time of relay operation should depend upon the amount of an overcurrent. The relay is essentially a small induction motor. This is probably the most widely used protective relay in the industry. It starts to turn when the current exceeds a (previously selected) threshold current, and rotates faster as the current increases. A typical set of characteristic curves for a relay of this type is shown on page 178 in the Tap Settings for Type CO Relay section.
This relay has one set of stationary contacts and one set which moves as the disc turns. The distance which the disc must travel to close the contacts is adjusted by setting the position of the time dial control. The magnitude of current which initiates disc movement is set by the choice of the tap on the current coil. The results is that relay contact operation is dependent upon the tap and the time dial settings. The relay timing can be varied from a few cycles to as long as 30 seconds. The actual speed-torque characteristic of the relay can be controlled by the designer to give a wide variety of operating characteristics.
