Question

Describe the normal criteria used for setting thermal protection relays and their advantages compared to magnetic type.

Setting of thermal protection relay

Protecting an electric motor basically involves preventing the motor from getting too hot. The best way to protect a motor against overheating is to directly monitor the temperature of the motor winding and its surrounding. Thermal relay are set to work for a maximum recommended temperature of the insulation.

[Bi-metallic thermal overload relays

This is the most common form of induction motor overload protection in present use. It consists of three bi-metallic strips each of which is heated by a small heater element connected in series with the motor supply or fed from a set of current transformers.

An increase in the temperature of the bi-metallic strips causes them to deflect due to the difference in coefficients of expansion of the two metals from which the strips are made. This deflection operates a mechanism linked to the trip contacts. The trip contacts operate when anyone or all of the phases heat up to a pre-set temperature. Additionally most mechanisms incorporate a different arrangement which initiates tripping owing to the unbalanced currents occurring under single-phasing conditions. ]

The advantage of thermal relays compared with magnetic types is that they can be designed to follow approximately the motor heating curve and that they are able to store heat.

The latter property protects the motor from overheating when it is started too frequently or becomes subject to a succession of relatively small overloads of short duration while running. Should the relay operate, the motor cannot be restarted until the thermal elements have cooled down.

This, however, is not advantageous when circumstances demand an immediate restart regardless of temperature considerations.