The thermal protector is applicable to three-phase electric motors, but only to machines with a Y-connection. The advantages are current and temperature sensitive protector adjustment and the possibility of automatic reclosing. The drawbacks are based on current limitation, because the protector is connected directly to the coil of the single-phase motor and has its application facing three-phase motors only in the center of the Y-connection.
These protectors are bimetallic type elements that have closed contacts, used mainly in single-phase induction machines, with the task of preserving the same against overheating caused by overloads, which can generate the rotor locking and voltage drops.
The thermal protector on a bimetallic disc has two moving contacts, a resistance and a pair of fixed contacts. The protector is connected in series, with a power supply, through thermal dissipation, which is caused by the passage of current, through the internal resistance a deformation of the disk causes the contacts to open and the motor power is automatically interrupted.
After a lower than specified temperature is reached, this protector must be reconnected. Due to this reclosing, you can have two types of protectors, the protector with automatic reclosing and the protector with manual reclosing.