The most reliable protection is a sensor that measures the winding temperature directly. The inverter's motor protection function calculates a thermal model from the current and the motor parameters entered. This model does not know the motor's actual cooling condition: when the shaft fan's air drops at low speed, when the ambient temperature is high, or when the motor runs in a dusty cabinet, the winding may be hotter than the current suggests.

On HEM motors, PTC and PT100 winding sensors are order options:

  • PTC: Its resistance rises sharply at a set temperature; connected to the inverter's thermistor input or a separate PTC relay, it stops the motor.
  • PT100: Measures temperature continuously; warning and trip levels can be set separately.

Sensor cables should be run separately from the inverter output cable and shielded; otherwise interference may cause nuisance trips. Since the sensor is placed in the winding during manufacture, the need should be specified in the order together with the motor.

For how to specify protective devices in the panel, see the article on electric motor protection devices.