Overload is the commonest fault in motors. It is revealed by an increase in the current absorbed by the motor and by thermal effects.

The importance of proper protection against overload is easy to understand:

• it preserves the lifetime of motors by preventing them from working in overheating conditions
• it ensures operating continuity by preventing motors from stopping abruptly
• after tripping, enabling restart in the best conditions of safety for people and equipment

Depending on the level of protection required, overload protection can be provided by relays:

• overload relays, thermal (bimetal) or electronic relays, which provide minimum protection against:
  • overload, by controlling the current absorbed on each phase
  • unbalanced or missing phase, by a differential device
• positive temperature coefficient (PTC) thermistor probe relays
• overtorque relays
• multifunction relays