Variable Frequency Drive Protection – Metal Oxide Varistors

We have talked about in the past how fuses and circuit breakers protect a variable frequency drive from over-current conditions but what about over voltage? On the main supply lines coming into the drive often you will see a little colored disk that favors a capacitor. These are metal oxide varistors, or MOV’s and they serve to protect the device from an over voltage condition.

What is a varistor?

The word varistor is a combination of the words variable and resistor. This term is a little confusing as while resistance to current does vary with voltage, it does not act to regulate resistance variably such as a rheostat or potentiometer will. In any case, what a varistor does do is that at a rated voltage, the varistor will provide a set resistance. As voltage is raised the varistor will provide less and less resistance until either all voltage is absorbed or the varistor will fail by design, thus preventing greater damage to the circuit.

What does a varistor protect against?

The main condition a varistor is meant to protect against is the presence of transient voltages. Transient voltages are short, fast spikes in voltage that can come from many different sources. Things such as a high amount of switching can cause inductive spikes on the line. Weather phenomena such as lightning strikes or wind causing power flickers can cause transient voltages as well. Failing switch gear or transformer faults can also create this condition.

Often maintenance technicians will see these little colored disks burned or even look like they have exploded after a drive failure. While this may look bad, it does mean that the little guys did their job, and for that we thank them. The presence of metal oxide varistors often means a drive will be repairable.

If you have any trouble with any of your motor controls, especially from unexpected voltage spikes, 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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