A circuit useful to protect motors and other dynamoelectric machines from overtemperature conditions caused by a fault condition such as locked rotor, overcurrent, or the like. The circuit comprises first and second tuned branch circuits coupled to the power lines of the motor or other machine along with an oscillator circuit and detector circuit and a switch to change the effective impedance of one of the tuned circuits. The tuned circuits each includes a series connected inductance and capacitance while the switch, which can be an electro-mechanical device such as a heat responsive thermostatic switch, or a solid state switch such as an NTC or PTC thermistor is connected around one of the components of one of the tuned circuits. The oscillator may be a conventional one adapted to provide a selected frequency, preferably high relative to that of the motor power supply. The detector, which also may be of a conventional type and the oscillator are both coupled to one of the power lines of the motor by respective toroid cores through which the power line passes intermediate the tuned circuits. When the switch is in one state the tuned circuits will resonate enabling the detector to detect the signal and to give an output thereby effecting indication of a fault condition or deenergization of the motor. When the switch is in its opposite state the tuned circuits will not resonate, and there will be no output signal given by the detector.