How an MOV Protects a Drive – A Galco Blog Tech Tip

What is an MOV and how does it protect drives and other equipment? Let’s find out.

An MOV, or metal-oxide varistor, is a ceramic mass of zinc oxide grains in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese). These grains are sandwiched between two metal plates. The boundary between each grain and its neighbor forms a diode junction, which allows current to flow in only one direction.

The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs.

When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows.

In an electronic application, varistors remain non-conductive as long as the voltage across it remains below its rating, or “clamping voltage.” When the voltage across the MOV exceeds its rating, the varistor fails.

This type of behavior, specifically with VFD applications, allows the MOV to protect other power devices downstream by suppressing line voltage surges.

Consider a power surge with 400 V going through a 200 V drive. If you open up the drive to see what happened to it, it is highly likely that one of the input MOVs has burnt out to protect the drive from overvoltage. This protects not only the rest of the drive, but all valuable equipment down the line.

In this case, you would replace the MOV, inspect the rest of the drive, and finally check the input rectifier by measuring from the L1, L2, and L3 input terminals to the DC+ and DC- terminals. If everything tests correctly, you can reassemble the drive and connect the power terminals back to the power source.

Running the drive at the correct voltage will confirm it works correctly.

 

For more information about MOVs, variable frequency drives and protecting industrial equipment, contact our experts today!

 

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