PQ Tip: PWM Drives May Have Bearing on Bearing Failures

Sep 1, 1998 12:00 PM, By John DeDad, Editor-in-Chief

Your or your client's motors seem to have an inordinate amount of bearing failures. But, the motors aren't that old, and in some cases, are relatively new. You've checked for proper alignment, yet the problem begins anew after each bearing repair job. What's causing it?

Induced motor shaft voltage may be the culprit of premature bearing failures. According to a Power Quality Testing Network paper (Application No. 8), the insulated-gate bipolar transistors, or IGBTs, used in some PWM drives act as high frequency switches. Their switching may cause a bearing discharge current. Basically, this high frequency switching, coupled with the fast rise times of the IGBT inverter output, causes induced voltage in a motor's rotor.

This voltage capacitively couples to the motor's shaft, and may exceed the dielectric strength of the lubricant in the shaft bearings. When this happens, current flows from the shaft, through the bearing lubricant, to the grounded motor frame. This current, in turn, causes pitting and fluting of bearing races. A shaft voltage of just 6V can cause arcing through a bearing lubricant.

Of course, this depends on the type of bearing lubricant used and the clearing between the ball bearings and their race. The resulting high rolling resistance leads to the premature failure of shaft bearings.

This may also affect other equipment. For example, the bearings of any devices connected to the motor shaft, such as direct-connected tachometers and gearboxes, can be damaged. In fact, the discharge current will attack a tachometer's bearings instead of a motor's bearings.

Why? Because the path of least resistance is the smaller tachometer bearings. When this happens, the tachometer will begin to vibrate and generate an erratic signal.

Look for these symptoms. Usually, audible motor noise and vibration are your first indications of premature bearing failure. But, these are symptomatic of other problems as well, such as motor misalignment or poor motor mounts.

Another, and more incriminating, symptom is damage to the smaller idle bearings opposite to the shaft end connected to the motor load. The current arcing across shaft bearings damages these bearing first. Idle bearing failure can happen within six months of a motor's installation.

You have two ways to find out if you have excessive shaft voltage:

Measure the shaft-to-ground voltage with a specialized shaft-monitoring device, or
Inspect failed bearing races for fluting.

The latter method is the easiest. Basically, you verify race fluting through visual inspection. If you're using a PWM-type drive with a switching rate above 10 kHz, you most likely have excessive shaft voltage.

You have three options to choose from to reduce or eliminate the problem.

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. You can use several commercially available brush systems. But, you should follow these "do's" and "don'ts:"

Don't use soft carbon brushes. They create a nonconductive film that eventually will reduce electrical contact between the brushes and the motor shaft.
Do use brass or stainless steel brushes, because they don't create the nonconductive film.
Do make sure, at every routine maintenance, the brushes are in electrical contact with the motor shaft.

Also, if the motors are in a clean room environment, make sure you install a sealed shaft grounding system. After all, you don't want to contaminate the environment with airborne particles from a standard grounding-brush system.

Install insulated motor bearings. Yes, these bearings will stop the flow of discharge current, which will save the motor's bearings. But, they won't prevent damage to the bearings of other shaft-connected equipment, such as tachometers or fans.

Also, there's a personnel safety issue here. If the insulated bearing motor has no shaft-connected equipment, anyone touching its rotating shaft will be exposed to a mild shock.

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.

If you have no choice but to use a higher frequency, try buying motors having warranties against bearing failures caused by discharge current.

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