PTC (Positive Temperature Coefficient) thermistor and relays and how they work

PTC thermistors are a very commonly used option in our Siemens and Fabrika motors to ensure that the temperature of the windings does not exceed the permissible limit in order to prevent the windings from burning and the motor from becoming damaged.

PTC thermistors are electronic circuit elements whose electrical resistance increases as the temperature of the environment or the contact surface increases. They are thermally sensitive semiconductor resistors.

Electrical resistances increase at a certain temperature. The temperature at which the resistances rise very suddenly is called the “nominal trip temperature” (NAT). The nominal trip temperature is selected according to the insulation class of the motor to be protected and the permissible limit temperature value.

For our standard Siemens and Fabrika motors, the windings are insulation class F (155 °C), utilised according to class B (130 °C). This means that the windings can reach up to 130 °C with a reserve allowing them to reach 155 °C, for example when driven by frequency converter or at higher ambient temperatures.

The set of PTC thermistor elements are placed between the stator windings of the motor. The PTC relay is located on the motor control panel. The relay connection may differ depending on the manufacturer. The relay controls the power contactor of the motor. The thermistor elements issue the relay with a warning signal close to the permissible limit temperature of the windings and an open signal at the permissible limit temperature.

The relay that receives the open signal starts to operate and opens the energy contactor. In doing so, it protects the windings from burning, except for sudden overcurrent rises and sudden overvoltage changes. Three PTC thermistors, one for each of the three phases, are the most commonly used option.