The most direct way to monitor winding temperature is to embed a temperature sensor in the winding while the motor is being built. At HEM Motor, PTC thermistors and PT100 sensors are among the ordering options, together with an anti-condensation heater. Fitting the sensor during manufacture is far more practical than adding it to a wound motor later, so the need should be stated at the quotation stage.

The two sensors give different information. A PTC sharply increases its resistance at a set temperature and signals “limit exceeded” to a protection relay. A PT100 measures temperature continuously; the value can be shown on a display or in an automation system and logged as a trend.

Thermal protection that measures current does not see the actual winding temperature. A sensor makes a difference especially in these cases:

  • Closed enclosure, high ambient temperature or restricted air flow
  • Frequent starts or heavy, long starts
  • Long operation at low speed on an inverter

How the sensor is wired to a relay or drive and how it is used alongside a thermal overload relay is covered on the motor connection and starting questions page.