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For temperature monitoring

Basic information

The thermistor motor protection relay monitors the temperature of windings in electrical machines (as motors, generators and transformers) in conjunction with PTC/ thermistors. It provides safety protection against thermal overload.



The thermistor motor protection relay is used for:

  • Heavy duty starting (Overloading)
  • Restricted cooling
  • Frequent switching
  • Blocked rotor
  • Phase failure, with a consequent increase in the windings temperature
  • Heating and ventilating
  • Operating with a frequency converter


General function

The thermistor motor protection relay monitors up to six PTC-thermistors connected in series. Three phase AC motors are usually equipped with three sensors, polechangeable three phase AC motors, with separate windings, require six sensors.
In normal operation the relay is activated, e.g. there is no thermal overload of the device. When the temperature of the PTC thermistor exceeds the tripping point, the motor protection relay is deactivated, (e.g. relay switches off) and triggers the power cut-off to the equipment, which it is monitoring. A fall in temperature causes re-activation of the relay (e.g. the relay switches on).
According to the internationally standardized tripping points the device provides switching-on and switchingoff automatically. Breaks in the sensor circuit, are indicated by the motor protection relay is deactivated.



  • Internationally standardised on and off switch points.
  • Universal interchange of the PTC thermistors.
  • PTC thermistors, of all standardized operating temperatures, and conforming to DIN 44081 und DIN 44082 standards, are suitable for connection.
  • Wire breakage in the sensor circuit indicated.
  • The device operates according to the closed circuit current principle. This guarantees the secure stopping of the electrical machine, even in the event of a power blackout.
  • Extremely cost effective.



Data Sheet
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