Thermal Overload Relay
Thermal overload relay is a protective device, and that is mainly designed to cut the power whenever the motor uses too much current for an extended time period.
To achieve this, these relays include an NC (normally closed) relay. Once extreme current supplies throughout the motor circuit, then the relay gets open because of the improved temperature of the motor, a temperature of the relay, otherwise detected overload current, based on the type of relay.
Magnetic Overload Relay
Magnetic overload relay can be operated by detecting the magnetic field strength which is generated by the flow of current toward the motor. This relay can be built with a variable magnetic core within a coil that holds the motor current. The flux arrangement within the coil drags the core up. As the core increases far enough, then it trips a set of connections on the summit of the relay.