What are the two types of thermal overload relays in magnetic controls?

The two types of thermal overload relays used in magnetic controls are indeed bimetallic and melting-alloy. Bimetallic relays operate based on the principle that two different metals expand at different rates when heated. This differential expansion causes the bimetallic strip to bend and break the circuit when the temperature exceeds a certain threshold, thereby protecting the motor from overheating.

Melting-alloy relays use a different method by incorporating a fusible link made from a specific alloy which melts at a predetermined temperature. When the temperature rises to a level that would indicate potential damage to the system, the alloy melts, breaking the circuit and stopping the motor. This method provides a resettable option once the system cools down and can be serviced.

Together, these two types of relays provide essential thermal protection for motors and other electrical devices, demonstrating the variety of mechanisms available for safeguarding against overheating conditions.