Thermal Relay

Thermal Relay Points : Definition of Thermal Relay Relay used in control circuits; operated by a change in one electrical circuit to control a device in the same circuit or another circuit. Overload means operation of equipment in excess of normal, full load, rating or of a conductor in excess of rated current which, when excess current passing through it, it would cause damage or dangerous over-heating. Overload protection is the result of a device that operates on excessive current, but not necessarily on a short circuit, to, cause the interruption of current flow to the device governed. A relay is a small control switch that operates automatically whenever a certain specified condition arises. Thermal overload relay consists of a thermal device; this thermal device is usually a bimetallic strip, which is heated by current passing directly through the strip or by a heather coil surrounding the strip. This current generated heat. The bimetallic strip consists of two thin layers of different metals welded together. The metals are chosen to have quite different temperature co-efficient of expansion and since the strip is firmly fixed at one end, the other end deflects as the temperature rises. The heater coil is in series with the circuit that to be protected. Such a device will not operate on momentary overloads, such as occur during the starting of motor, but will operate if the overload on a motor continues for a period of approximately one to two minutes.

The excess heat developed by the heater units causes the bimetallic strips to expand. As each bimetallic strip expands, it causes the normally closed contacts in the control circuit to open. The main coil is de-energized and disconnects the motor by opening the main and auxiliary contacts.

Before the motor can be restarted at the push button, the overload contact in the control circuit must be allowed to cool before being reset. When we press the reset button in the magnetic starter, the overload contacts in the control are reset to their normally closed position. The overload protection relay consists of a three thermal overload units inserted in series with the 3-phase leads feeding to the motor. Overload protection should be set preferably at the working load current and not according to the motor current shown on the motor name plate. One of the difficulties encountered in designing an overload relay, which can be set at, say 15% to 25% overload after a short time intervals is to prevent it from, tripping on high starting currents before the starting operation has completed. The overload relay unit may be either an individual Unit or a common block containing the three heaters and only one or tips witch contact. The overload heaters units are made of a special alloy.