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The VFD Answer Man, Volume 7: VFD's and Motor Bearing Failure

December 20, 2015 |By: Terry M. Smith

IGBT’s can cause a host of problems with motors and electrical systems with improper installation. The IGBT switching scheme can cause high dV/dT and dI/dT in the output waveform. This can lead to high frequency iron losses in the stator and the rotor of the electric motor. These losses must go to ground. In a properly grounded motor they are discharged from the stator, but not the rotor. The only path to ground is through the shaft, and in most cases the bearings.

The bearings are subject to several factors that lead to their early demise:  

1. Carrier frequency actually pulses the motor on/off and the bearings are mechanically vibrated by the switching. This helps generate the fluting pattern shown in the picture below.

2. Stray current and voltage in the rotor must go to ground. The path is through the bearing. The current heats the lubricant and can cause arcing between the balls and the raceway. Typical bearing failures show evidence of heat.

3. Improper installation of ground. Ungrounded systems have no path for the stator losses to go to ground. They will try to go to ground through the motor frame and can reinforce the voltage and current present from the rotor. They can also induce larger currents and voltage in the rotor from inductance and resonance. Typically they will show up in the motor as early winding failures as well.

4. Improper electrical installation. Systems that do not have individual metal conduits for AC power into the VFD and from the VFD to the motor may have one of the 3 deficiencies that commonly add to the bearing failure issue:

  • Wires in the same conduit
  • Wires in trough (worst of all possible installations)
  • Wires in PVC conduit

Any of these installation issues will add harmonics to the AC line side, the VFD, and the motor. From these installations the additional harmonic content will increase motor and bearing heating, increase the stray currents from iron losses, and create issues with the controls as well. When we examine bearing failures we need to look for additional problems in the VFD system.

 Solutions:

1. Make sure the entire electrical system is properly grounded. There should be a linear grounding connection from the Voltage Source disconnect to the VFD and from the VFD to the motor.

2. Ensure the system is wired with individual metal conduit for the AC power from the source breaker to the VFD, and a separate metal conduit from the VFD to the motor. Specify only metal conduit, and also specify that trough is not to be used!

3. Install a shaft grounding system. Grounding the motor shaft with a system of brushes creates a low-impedance path to ground for the discharging currents. Aegis Shaft grounding rings are by far the best solution available.

4.  Reduce the drive's switching frequency. Try to avoid frequencies higher than 10 kHz. You can do this by adjusting the switching frequency on your PWM drives. Industry experience shows this is the threshold of the problem.

5. Motor Selection. New motors for VFD service should always be Premium Efficient, NEMA Design B (A is OK). Premium Efficient motors are inherently optimized for VFD service. They provide the best heat dissipation, and have the largest bearings (other than IEEE-841 service motors) available for a given motor. Where possible, TEFC-Severe Duty motors should be employed. They are the most rugged and durable, with the same size bearings on both ends. They can be expected to outlast any other motor in VFD service by a factor of 2. Motors designated NEMA Design E should not be used. They are often incapable of starting loads across the line in applications where they replace NEMA Design B motors.

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