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.
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.
Last edited by varmit; 05-06-2011 at 06:43 AM.