What’s Up Doug? Mitigate damage to VFD-controlled electric motors
At a recent training session we discussed the topic of insulated bearings used on electric motors controlled with variable frequency drives (VFDs). The generation of a voltage across the shaft and housing of the motor is inherent to VFD-controlled motors. But this causes current leakage through the bearing and damages the bearing.
We discussed the strategies to mitigate this damage, which includes a ceramic coating on the inner ring or outer ring, carbon fibre brushes and bearings with ceramic rolling elements.
The question that I was asked was: “We have an existing motor that we are going to convert to being controlled by a VFD. Do we need to rebuild the motor with insulated bearings?”
The choice that the customer had was to rebuild the otherwise perfectly good motor with insulated or hybrid bearings, or install it and run it knowing that the bearing would likely be damaged.
Fortunately, I was onsite at a very reputable electric motor service shop and I was able to consult on this question with some of the experienced personnel. Together we came up with an answer:
- Run the un-insulated bearings and you will likely suffer damage to the bearings
- Use a device that counts the number of discharges to determine the rate at which the bearing surface is being damaged
- Use vibration analysis to monitor the progression of the surface damage and then schedule an appropriate time to replace the bearings
Because the damage from current leakage is extremely easy to diagnose and it is a gradual progression, it is rare that such damage leads to unplanned, catastrophic failure.
So the question now is: Should you refit the motor today or run it knowing that during a planned outage it can come out and be replaced with a newly rebuilt motor that addresses the current leakage?
Nothing in life is guaranteed, but in my discussion with the motor shop colleagues the minimum life of a motor exposed to leakage is six months. An unprotected bearing will often last longer than six months.
The user of this motor knows that the motor will have to be rebuilt now or in six months or longer. So this cost will be the same. Is the cost of replacing the motor in six months (or longer) feasible? If the motor is needed right away, can they wait for the rebuild to be done immediately? These then are the questions that need to be answered to make the decision to fix now or later.
Douglas Martin is a heavy industry engineer based in Vancouver. He specializes in the design of rotating equipment, failure analysis and lubrication. Reach him by email at firstname.lastname@example.org.