Rotor overtemperature protection for electric motors

Abstract From the Canadian patent

A system is provided for preventing overtemperatrue of the rotor bars of large squirrel-cage induction motors. A transmitter is mounted on the rotor and supplied with power from the rotor wind ing itself by a special current transformer. The transmitter is controlled by a temperature sensing element which causes it to transmit a signal in response to excessive temperature, and a slip frequency signal is also transmitted as a reliability check. A stationary receiver picks up the signal from the transmitter and includes circuitry for deenergizing the motor in response to the excessive temperature signal and for preventing energization of the motor if the slip frequency signal is not received.