Thermal Fault Protection – Thermistors

What is a Thermal Fault?

One of the most common faults in a motor control system, and surprisingly not well understood, is the thermal fault. When troubleshooting drives with customers over the phone, we often will get the complaint that they need help because their drive is “overheating”. When pressed for details we often will find out that the fault was not the drive “overheating” but rather the temperature sensor to the motor doing its job – alerting the operator to a temperature problem in the motor. Why the confusion? Mainly it is because in a lot of computing systems, such as motherboards with CPU’s in a home PC, such a fault would be caused by a sensor detecting an overheat in the circuitry itself. misdiagnosis of this kind come from the supposition that most electrical alarms or faults are somewhat ubiquitous in nature. This is why we stress to always keep all literature on hand and read over the descriptions of any faults you may see in the troubleshooting section before making assumptions.

What is a Thermistor?

In the motor control world, a fault indicating a thermal failure is usually being detected in a device installed within industrial use motors called a thermistor. The word “thermistor” is a portmanteau of the words thermal and resistor. A thermistors function is to increase or decrease resistance depending on temperature. As temperature changes within the environment the thermistor is engaged. For example a motor windings beginning to overheat due to a phase imbalance, mechanical failure etc- the amount of resistance can be detected by an external device.

How do Variable Frequency Drives use Thermistors?

In most cases modern variable frequency drives have a special set of circuit terminals where the thermal circuit can be installed. In addition, many drives now have programmable settings within the drives control parameters which can be used to customize exactly how high a temperature gets before a certain degree will trip the circuit causing the fault. This is helpful to application engineers as they can set up the drive system to adapt to a wide range of potentially harsh climates be it hot or cold.

Remember, if you have any thermal problems with your devices, be it Motor or Drive, give us a call! Also be sure to visit us online at or call us at 1-877-249-1701 to learn more about our services. We’re proud to offer Surplus, Complete Repair and Maintenance on all types of Industrial Electronics, Servo Motors, AC and DC Motors, Hydraulics and Pneumatics. Please subscribe to our YouTube page and Like Us on Facebook! Thank you!

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