The thermal protector consists of a bimetallic disc, which contains two moving contacts, plus a resistance and a pair of fixed contacts. The protector is driven in series and due to thermal dissipation, which is generated 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 to stop .
After a lower than specified temperature has been reached, this protector must be reconnected. Depending on this process, two types of protectors can be used, one with automatic reclosing and the other with manual reclosing. These protectors are basically bimetallic type devices which have normally closed contacts, used mainly in single-phase induction motors, in order to protect against overheating caused by overloads, causing the rotor locking and voltage drops.
The thermal protector is applicable to electric motors GLP3348 of the three-phase type, but only to Y-connected motors. The advantages are a combination of current and temperature-sensitive protector. The drawback is the current limitation by the protector being directly connected to the single-phase motor coil.