# VFD Buss overvoltage



## backstay (Feb 3, 2011)

Check connections, how big is this VFD? could be a cap or I think they have a snubber circuit(like a shock system). If a cap goes bad and explodes you get damage.


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## McClary’s Electrical (Feb 21, 2009)

Dave R said:


> During the past few weeks, our vfd that is used to control an exhaust fan is shutting down 5-10 times an 8 hour period on a "Buss Overvoltage" fault. This vfd/motor combo has been problem free for 13 years. What would case this fault to occur while up and running?:blink:


Make and model? What size motor? How far from drive?


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## don_resqcapt19 (Jul 18, 2010)

If this is an older installation and this is a new problem, what has changed? Has there been a change in how the speed is controlled? Have you added any power factor correction to the system that supplies the building power?


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## nez (Jan 16, 2010)

I had this problem once on a new installation. The supply fan was pulling the return fan faster than the vfd was trying to run the motor. Once the system was balanced the problem went away. Maybe you have a louver closed that should be open.


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## tommu56 (Nov 19, 2010)

We had a building with a lot of exhaust fans pulling air out of it and the make up air unit would windmill and do it under the wrong conditions adding a chopper / dynamic brake and interlocking a couple of the exhaust fans fixed it.


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## wired480 (Jan 14, 2011)

I had this same issue just two days ago with a PowerFlex 70 vfd. Turns out there was a wirenut melted in the motor peckerhead causing a resistive short to ground...not enough to show continuity on a ohmmeter but enough to megger bad from t2 to ground.


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## varmit (Apr 19, 2009)

A "Buss Overvoltage" is USUALLY caused by a problem external to the drive. Something is causing the motor to run faster than the drive is commanding. If this fan starts and stops instead of running constantly, you could set the "Stop Mode" to "Coast" to avoid overvoltage trips on stopping the motor.

Other possibilities:
If there are other VFDs or DC drives, on the same power distribution buss, one, or more, of the other drives could be failing and sending higher frequency harmonics into the power distribution system. Check the fan drive LINE SIDE voltage with the fan not running. If the voltage is over about 63 hertz, look for the problem at another drive. A line reactor or isolation transformer can, and should, be used in this situation.

Power factor correction capacitors can cause a similar problem by causing line voltage surges.

Are there other large motors that start and stop? If so, it could be a combination of voltage dips and noise caused by the other motor(s) starting.

There are other possibilities.

A line reactor is always cheap insurance on a VFD.


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