Variable frequency drive causing motor bearing problems (vfd)


If youre using a VFD, and getting short motor bearings life, youre probably having problems with drive induced currents discharging through, and damaging the bearings.

  • Due to the high speed switching frequencies used in PWM (pulse width modulated) inverters, variable frequency drives induce shaft currents in AC motors. These currents produce voltages on the motor shaft during normal operation through electromagnetic induction.
  • During the VFD cycle, induced shaft voltage can reach a level high enough to discharges from the motor shaft to the frame via the bearings
  • These discharges are so frequent that eventually the entire bearing race will become marked with numerous pits, causing premature bearing failure
  • The failure rate in these bearings often occurs in as little as 3 to 12 months after start up.

So, what can you do about it?

You’re going to have to divert the current from the bearings by insulating them or coming up with another path to a ground.

You have a couple of options if you’re looking for an alternative path to ground:

  • Conductive grease“ unfortunately the conductive particles can also cause wear of he bearing ball and aces.
  • A grounding brush – a metal brush contacting the metal shaft and connected to ground.
  • Shaft grounding ring (SGR)“ Check with the manufacturer for the details of this arrangement

Insulating the bearing often shifts the problem to some other place because the current continues to look for a path to ground.

Ceramic, non-conducing bearings are another choice.

Another possibility of premature bearing failure in motors or pumps, after you are satisfied that proper installation and preventative maintenance is being done, is that a welder is using your motor or pump for his ground connection.