VFD Motors Failing? Think Twice Before Replacing The Motor….
May 11, 2012 Leave a comment
A funny thing happened on the way to work one day. I received a call from a contractor that they had several motors failing on a project they were working on. They advised the motors had just been installed and commissioned one month previous and their electrical contractor determined that motor failure was due to winding failure. The funny thing about it was that the contractor demanded we order new motors and have them shipped overnight.
In short, several questions were asked and in the end it was determined that it was not the motors fault nor was it the fan manufacturer’s fault. Motor winding failure can and will occur if certain steps are not followed when installing VFD’s.
First and foremost, the electrical engineer should verify the distance between the VFD and the motor it is controlling. Voltage spikes caused by excessive distance can kill a motor and it can happen in a short period of time. To prevent premature winding failure, review the VFD manufacturer’s recommendations for proper distances. A rule of thumb is less than 150 feet. The shorter the distance the better chance there will be less voltage spiking during operation. The longer the distance, the greater the voltage spikes from the VFD.
Second, and just as important, make sure the motor that is selected for the fan is inverter duty rated and has the correct insulation class rating for the windings. There are three common insulation classes that most manufacturers provide. Class B would be the lowest insulation factor and is often recommended for non-inverter duty applications. The insulation on the windings is not capable of handling voltage spikes from a VFD control. Class F would be the second highest insulation rating and is capable of handling voltage spikes. Class H would be the highest rating and is recommended for severe high heat conditions and in applications where the VFD control will be adjusting speeds on a continual basis during operation.
Lastly, if the motor speed will be adjusted continuously by the VFD, such as with variable air volume systems, it is recommended to use a motor shaft grounding device. Shaft grounding will eliminate induced motor shaft currents which discharge through the bearings. Motor shaft currents cause pitting, fluting, and catastrophic motor failure. Providing motors with shaft grounding rings will prevent these harmful currents from destroying the motor bearings and extend the life of the motor.
For more information on VFD motor failure and shaft grounding rings please clink on the links below.
VFD and Motor Failure – http://www.yaskawa.com/site/dmdrive.nsf/link2/MNEN-5JFQPL/$file/AR.AFD.05.pdf
Shaft Grounding – http://www.est-aegis.com/markets/hvac.htm
- By Jeff Dubecky
