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Voltage on the Shaft

One of the issues to contend with when using variable frequency drives is the potentially damaging voltages that may be on the drive shafts of both DC and AC motors. As the variable drive is kicking on and off, the change of speeds creates the voltage problem. When this happens, the voltage can "ride" on the motor shaft through the coupling between the rotor and the stator. The voltage tries to find a way to get to ground, usually through the motor bearings. To prevent this from happening, shaft grounding rings may be necessary to prevent bearing damage.

The voltage that is on the shaft is going to seek its way to ground and it does this through the bearings if at all possible. Of course, that is reduced when you have shaft grounding rings in place, and it can help to reduce the wear on the bearings and from them breaking down and failing prematurely.

Another problem that occurs in both DC and AC motors operating above 100 HP is bearing failures, but the cause is different than shaft currents. Rather, this is caused by what is known as high-frequency circulating currents that generally occur as a result of variable frequency drives. When this happens, it is usually seen more readily in higher horsepower motors. So the higher the horsepower, the more likely it is that you're going to need to employ shaft grounding rings to avoid the possibility of this damage occurring.

Contact us to find out additional information about these problems and how the bearing damage occurs in more detail. There is, however, one solution we can offer: Shaft Ground Rings.

Shaft Grounding Rings

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Prevent Shaft Current


VFD Induced Shaft Currents
Damaging voltages are induced on the shafts of AC and DC motors controlled by variable frequency drives (VFD). The extremely high on/off switching speeds of the pulse width modulation (PWM), generated by the insulated gate bipolar transistors (IGBT), induce damaging voltages onto the motor shaft through parasitic capacitive coupling between the stator and rotor. This common mode shaft voltage seeks a path to ground, usually through the motor's bearings.


EDM Currents Damage Bearings

Damage to the Bearing
Damaging currents arc through the dielectric oil film between the rolling elements and the bearing race. This is known as electrical discharge machining (EDM) effect. EDM causes fusion craters, severe pitting, and eventually bearing fluting (a washboard-like pattern in the bearing race) which results in premature bearing failure.



Bearing fluting, "washboard" pattern on bearing race

High Frequency Circulating Currents in Large AC and DC Motors
In addition to potential bearing failures in motors from VFD induced EDM currents, AC and DC motors above 100hp (75 kW) may also experience bearing failures caused by high frequency circulating currents. VFD induced high frequency circulating currents are in the kilohertz or even megahertz range and circulate through the motor's bearings because of magnetic flux imbalances in the stator. This type of VFD induced current becomes the more dominant destructive current in higher hp/kW motors.




High Frequency Currents Damage Bearings

For more information on shaft currents, contact L&S Electric. Information courtesy of AEGIS
Motor Sales from L&S Electric!