Pump Over-Temperature Detection Methods
Either a resistive device, known as a Thermistor, or a thermal switch is used to interface with a controller
to indicate or alarm on a motor over-temperature condition. Usually the control system is designed to
shut down the motor until the temperature is back into an acceptable operating range and the controls
are re-set.
Thermistors are usually wound into the windings of the motor during manufacture, and leads are brought out for the user to connect and monitor the resistance. Sometimes several of these are wound into the motor winding and internally connected in series, giving a higher cold starting resistance but allowing temperature hot spots to be detected quickly, presumably before the motor can be damaged.
The initial "cold" resistance value of a single Thermistor as used in motor windings is around 120 to 160 ohms. When several of these are wired in series, the resistances are added together. Hence R * n = Rt, where R is the resistance of a single Thermistor, n is the number of thermistors in the motor, and Rt is the total resistance of the circuit.
As the temperature of the motor rises, the thermistors increase in resistance until the control to which they are connected determines that an intolerable temperature for the motor has been reached and turns off the control circuit shutting down the motor. The hot resistance of the thermistors appears nearly as an open circuit to the control, as the hot resistance can be millions of ohms.
If there is a failure of one of the thermistors in the motor windings where there are several thermistors wired in series, then a complete repair of the motor must be undertaken. Thermistors are not intended to carry large currents, and only controls intended for resistance temperature detection should be used. Control devices should never be switched directly with the leads from the Thermistors.
Quality Manufacturing Company, 518 18th St. SW, Roanoke, VA 24016. Tel: 800-428-6699; Fax: 540-985-3720.