Bimetal type protectors with normally closed contacts are widely used mainly in the case of single-phase induction motors, in order to protect against overheating caused by overloads, causing the rotor to be blocked, as well as possible voltage drops. Thus, thermal elements are normally applied to electric motors of the three-phase type, although their application only becomes feasible in motors with Y-connection. The advantages to be highlighted are the combination of current-sensitive protector, as well as temperature .
However, it should be noted that a disadvantage is that the current is limited by the protector being directly connected to the coil of the single-phase motor hammond power. The thermal protector is basically composed of a bimetallic disc, which has two moving contacts, plus a resistance and a pair of fixed contacts. Thus, the protectors are driven in series and due to the thermal dissipation, which is caused by the passage of the current, through the internal resistance, a deformation of the disc can occur, resulting in the opening of the contacts and thus causing the motor power is interrupted. After a lower than specified temperature has been reached, this protector must be reconnected.