This category of protector consists of a bimetallic component that presents normally closed contacts, normally used in single-phase induction machines in order to avoid overheating resulting from overloads, which can cause the rotor to lock and voltage drops. The protector is activated in series with a power supply, and due to the thermal dissipation caused by the passage of the current through the internal resistance, a deformation of the disc occurs, leeson 120554.00 causing the contacts to open and the motor power interrupted. Therefore, the thermal protector is based on a bimetallic disc, with two moving contacts, a resistance and a pair of fixed contacts. After reaching a considerably lower temperature than specified, the protector should be activated automatically.
Due to this reclosing, two types of protectors can occur, with automatic reclosing and the protector with manual reclosing. The thermal protector is also suitable for three-phase motors, but only for Y-connected motors. The advantages are a combination of current and temperature-sensitive protector and the possibility of automatic reclosing. The disadvantages are the current limitation, because the protector is connected directly to the coil of the single-phase motor and the application is aimed at three-phase motors only in the center of the Y-connection.