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Motor Thermal Protection

All electric motors should be protected against overloading. Electric motors will develop a fault when overloaded as the temperature rises above the thermal limit of the insulation materials during the overload, which results in the loss of the mechanical and electrical properties.


Service life

The standard service life of the motor insulation material is 20,000 to 25,000 hours depending on the maximum limit temperature of the individual material. In real life this theoretical service life is exceeded many times. The insulation class B determines the maximum permissible winding temperature of 120°C (limit temperature 130°C) and class F determines the maximum permissible winding temperature of 145°C (limit temperature 155°C). The service life of the winding insulation is reduced by half every time the winding temperature exceeds the maximum permissible temperature by 10 K. Rotor nl® electric motors have been wound in class F (155°C) as standard but the motors’ temperature rise is within the class B, well below the class F thermal limit. The service life is therefore expected to exceed the standard service life many times.


Motor protection switch

The temperature of the winding is determined by the energy losses in the electric motor as well as other factors. The “copper losses” are the main contributor to the heat generation. The copper losses are in proportion to the square of the current (Pcu = I2 x R). It is often the case that the temperature does not rise to the ultimate value immediately when a specific electric current starts to pass through the winding. The temperature will rise gradually. It is possible to measure the current and thus determine the temperature in the electric motor. This method is used by applying a protection switch. The electric motor current passes through bi-metal in the switch, which gets warmer. The switch is designed such that the bi-metals will cause the switch to turn off when a certain temperature is exceeded. Therefore, an electric motor cannot be protected against overheating by fuses, as they cannot be adjusted for the electric motor current and do not heat up and cool down simultaneously. It is not recommended to set the fuses for a slightly higher current value. Should one fuse melt, the motor will run on 2 fases wherein it is possible that the thermal protection switch would trip off too late or not at all. In switches the fuses should only serve as protection against short-circuits. Protection tool for your driven equipment The thermal electric motor Protection Switch can also be used as a protection for the driven equipment. The current limit can be set lower than the electric motor rated current. The Protection Switch will trip the motor off even when the motor rated current has not been exceeded. It is not unusual that electric motors run at 30% to 80% of the rated load. It is therefore advisable to adjust the thermal Protection Switch based on the driven equipment requirements.



PT100 is a commonly used sensor for temperature measurement as part of a resistance thermometer. Another name is the RTD (Resistance Temperature Detector). Although this expression also includes other types of temperature sensors. 70% of all temperature measurements in electric motors are carried out by PT100s. The advantages are wide measurement range, the (almost) linear characteristics, long service life, its accuracy and ease of use and connectivity. The linear relation between temperature and resistance values is a significant difference when compared with PTCs and the positive temperature coefficient differs essentially from the NTC-resistance. The acronym PT refers to platinum being the material from which the very fine resistance wires in PT100s are produced. The number 100 refers to the electrical resistance of 100 Ohm (± 0.1 Ohm) that the sensors have at 0°C.


PTC thermistors

PTCs are used to protect the motor winding and trip when the maximum permissible winding temperature is reached. The PTC (Positive Temperature Coefficient) is a resistance sensor that has a small resistance value when cold and high resistance value when hot. The PTC has a thermistor effect. This means that the temperature characteristics on the resistance is not linear, but behaves in accordance with a special resistance/temperature curve (see image). If these PTCs are used in combination with a PTC thermistor relay in the auxiliary current circuit of the electric motor, the electric motor will be switched off when the limit temperature is reached. This method is independent of the motor current and responds only to the temperature of the winding.

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