Vertical motors run on variable frequency drives are susceptible to electrical bearing damage and premature failure. When any motor, vertical or horizontal, is run on an inverter, the drive’s output causes a capacitive voltage on the motor shaft that can discharge through the bearings, causing electrical damage and premature failure. This has brought grief to more than one water treatment plant; you can read one of their stories here.
GE Power Conversion is aware of this problem, so they offer their Ultra™ Vertical Pump Motors with an AEGIS® Shaft Grounding Ring as an option. AEGIS® shaft grounding gives VFD-produced shaft voltages a low-impedance path to ground, drawing damaging current away from the bearings, and so protecting them against electrical damage.
For more on GE Ultra™ Vertical Pump Motors, see this brochure. To learn how AEGIS® shaft grounding helped with a Kansas treatment facility’s chronic bearing failure problem, read this case study. And for fuller background on electrical bearing damage due to VFDs, see the AEGIS® Bearing Protection Handbook.
This video clearly demonstrates that common mode chokes (inductive absorbers) installed on a VFD do not eliminate damaging shaft voltage, and that these chokes are no substitute for AEGIS® Rings.
In this video, we installed the common mode chokes exactly as recommended by the manufacturer and then operated the motor, with no AEGIS® Ring installed, while monitoring shaft voltages to see if bearing discharges occurred. With common mode chokes and no AEGIS® Ring, the oscilloscope detected continual shaft voltage discharge through the bearings. These discharges cause cumulative bearing damage, and lead to eventual bearing failure.
Then we simply stopped the motor, installed the AEGIS® Ring, and turned the motor back on while again monitoring the shaft voltage. All the shaft voltage was gone – as expected – since now the AEGIS Ring was discharging the voltage away from the motor’s bearings to ground. By providing a low-resistance discharge path to shaft voltages, the AEGIS® Ring protects the bearings from electrical damage.
Conclusion: Common mode chokes (cores) have zero benefit in reducing or eliminating shaft voltage, which can discharge through motor bearings and cause electrical pitting damage.
In contrast, AEGIS® Shaft Grounding Rings were found to eliminate the bearing discharges and effectively protect the motor’s bearings.
Electro Static Technology-AEGIS® is proud to announce that BUILDINGS has selected the AEGIS® Shaft Voltage Tester™ as a 2016 Money-Saving Product winner in the Energy Measures category. The Shaft Voltage Tester™ is a member of an elite group of 76 products showcased in the June 2016 issue of BUILDINGS and online at www.buildings.com.
Finalists were evaluated by the BUILDINGS editorial staff for the money-saving qualities they offer to building owners and facility managers in areas such as energy efficiency, water savings, and maintenance. The AEGIS® Shaft Voltage Tester™ saves money by detecting the hazard of VFD-sourced electrical bearing damage to motors, allowing users to head off expensive repairs and downtime by taking preventive measures before damage passes the point of no return. Continue reading
Variable frequency drives (VFDs) enable precise speed control and significant energy savings, but they can also cause motor bearing failure. VFDs take smooth electric power and break it up into pulses. These pulses create a voltage imbalance in the motor that can build up on the shaft. This shaft voltage can discharge through the bearings, causing cumulative damage and premature motor failure.
These destructive discharges can be seen with a digital oscilloscope equipped with a conductive microfiber probe tip, like the AEGIS® Shaft Voltage Tester™. A typical bearing discharge is shown on the left below, where voltage gradually builds up to several volts (~20V here) and then suddenly collapses back to baseline.
Specially designed for VFD-driven motors in harsh washdown and processing environments, Flowserve’s new Bearing Gard™ bearing isolator protects against both contaminant ingress and VFD-induced electrical bearing damage. The isolator’s unique design offers both static and dynamic protection against contamination ingress. A unique double drive ring design, an innovative static labyrinth shut-off, and an oversized contamination channel provide superior protection against harmful contaminants. And Bearing Gard’s built-in AEGIS® Bearing Protection Ring channels VFD-induced shaft voltages away from motor bearings and safely to ground.
Bearing Gard™ Bearing Isolators are available from stock in standard and slimline versions for motors with shaft diameters of 0.875 – 6.000 inches (23 – 152mm). Custom sizes are also available.
For more information on Bearing Gard™ Isolators with AEGIS® Bearing Protection, click here.