Energy-saving VFDs can induce electrical bearing damage in pump motors
Grounding ring part of solution for mitigating damage that can lead to premature failure
Through trial-and-error and much hard work, a Kansas motor repair shop says it has developed a virtually foolproof process for protecting vertical hollow-shaft motors from the electrical bearing damage caused by stray shaft currents.
For six years, Scott Wilkins, manager of motor shop operations at Independent Electric Machinery Co. (IEMCO) in Kansas City, Mo., has overseen reconditioning of hundreds of these vertical motors — most of which run pumps — through a what it calls the “vertical motor solution.” None has had a repeat bearing failure.
After replacing the ruined, pitted bearings, his team installs a shaft grounding ring next to the motor’s guide (lower) bearing and, using proprietary techniques, applies ceramic insulation to the carrier that holds the thrust (upper) bearing in place at the motor’s drive end.
Although destructive currents can occur in any motor, Wilkins says most bearing damage he sees is in motors controlled by variable frequency drives (VFDs), also known as inverters or simply as drives. VFDs can save 30% or more in energy costs, but whether used to control a motor’s speed or torque, they often induce shaft voltages that discharge through the bearings, leaving fusion craters — pits in the bearing balls and race walls. Concentrated pitting at regular intervals along a race wall can form washboard-like ridges called fluting, which causes excessive noise and vibration. By this point, bearing failure is often imminent.
The cumulative degradation of bearings in VFD-controlled motors is well documented and believed to be caused by repetitive and extremely rapid pulses applied to the motor from a VFD’s non-sinusoidal power-switching circuitry. Names used to describe this phenomenon include parasitic capacitance, capacitive coupling and common mode voltage. The costly repair or replacement of failed motor bearings can wipe out any savings a VFD yields and severely diminish system reliability.
“The energy-saving potential of drives has led to a dramatic increase in their use, especially in new construction,” Wilkins explains. “We often see the problem in motors at new water or wastewater treatment plants, for example. As a result, general contractors and consulting-specifying engineers (CSEs) frequently end up with unhappy customers, who discover only after bearings fail that most warranties don’t cover electrical bearing damage. This leads to a lot of finger pointing, and typically the CSE and the end-user get stuck with the repair costs.”
Case in point
Dan Biby learned the hard way. An electrical engineer with Professional Engineering Consultants (PEC) of Wichita, Kan., Biby helped design a new water treatment plant for a city in Kansas. Shortly after the plant opened, Biby found himself in charge of a motor remediation project that lasted more than two years. One after the other, the motors, all of which are controlled by VFDs to adjust flow rate and pressure, developed serious electrical bearing damage.
IEMCO’s process saved the day, but only after months of frustration. The motor manufacturer replaced only the first motor that went bad. A local repair shop botched repairs on other motors. Eventually, Biby found IEMCO. By the end of 2011, all 17 of the plant’s pump motors had been refurbished with properly installed shaft-grounding devices. To date, they are all still running without any problems.
“We’ve learned a lot,” says Biby. “We’ve adjusted our motor specifications so that all new motors connected to VFDs are equipped with shaft grounding rings like AEGIS. We insist that the shaft grounding devices be factory installed or installed by a reputable motor shop with expertise in their proper installation. We also specify that if these devices are not factory installed, a third party shall be engaged to test the installation to ensure no shaft currents are present. And lastly, we require a warranty against VFD-induced bearing damage or failure for the life of the motor.”
IEMCO frequently retrofits brand new motors before they are put into service. Wilkins has even performed the Vertical Motor Solution for some motor manufacturers that sent him complete new motors or motor components prior to shipment to end-users.
“We have seen an increase in specs that include shaft current mitigation for VFD-driven motors,” says Wilkins. “Sometimes, the OEM or the general contractor might not catch it in the bid, and it has to be fixed later, to equip the motors with what was specified — better late than never.”
The “elephant in the room” is the growing awareness throughout the industry that these motors — all motors, in fact — could be built to withstand shaft currents in the first place. A few forward-looking motor manufacturers have recently added the AEGIS SGR Bearing Protection Ring — the same brand IEMCO uses — as a standard feature on certain models, but retrofitting is still the most common way to prevent electrical bearing damage.
Some failed motors IEMCO has reconditioned were originally marketed as “inverter-rated,” “inverter-duty” or “inverter-ready” models. Most of these motors have extra insulation to protect the windings but nothing to protect the bearings.
Spreading the word
In December 2011, while continuing to service motors from its own customers, IEMCO for the first time offered its Vertical Motor Solution to a distributor, Philadelphia-based Bartlett Bearing Co.
“We often have our customer send the carriers from motors that have experienced bearing damage to Scott Wilkins,” says Bill Potts, Bartlett’s VP of operations. “In addition to applying the ceramic coating, he checks each carrier for mechanical integrity (i.e., axial and radial runouts) and maintains the correct finished bearing tolerance. We benefit from his knowledge, experience, expertise and workmanship.”
As for the motor’s guide bearing, Bartlett offers the AEGIS shaft grounding ring, in addition to options such as insulated bearings — most commonly coated with ceramic, but sometimes with another nonconductive compound — and hybrid bearings with ceramic rolling elements. When using insulated bearings, shaft grounding rings are recommended to divert the bearing currents to ground, thus protecting attached equipment.
Whether installed by IEMCO, a contractor or the end-user, the AEGIS ring should be mounted internally, to the lower bearing retainer or cap, Potts explains. Depending on the size and shape of the retainer/cap, Bartlett Bearing recommends one of three mounting methods: press fit, bolt through or “top hat.” In the “top hat” method, a special fixture is created for the bearing retainer/cap, and the ring goes in the fixture.
“Whenever we have used an AEGIS ring in this formula, we’ve had 100% satisfaction,” says Potts. “We deal primarily with over a thousand electric motor repair shops that are trying to solve end-user problems created by VFDs. So, we get IEMCO’s services and the AEGIS ring in front of the faces of a lot of potential customers.”
Key to the grounding ring’s effectiveness is its “Nanogap” technology, which ensures superior contact/noncontact grounding protection for the normal service life of the motor’s bearings.
The ring includes conductive microfibers arranged in a continuous circle around the motor shaft, providing contact and noncontact voltage discharge points. When the ring is installed, its conductive microfibers overlap the motor shaft, and over time, slowly wear to fit the shaft surface, maintaining electrical contact throughout the bearing life. Electron-transfer technology includes three distinct current-transfer processes that work simultaneously:
- The ability of electrons to “tunnel” across an insulating barrier, which works for gaps smaller than 2 nm.
- Field emission of electrons is a form of quantum tunneling whereby electrons move through a barrier in the presence of a high electric field. It provides grounding across gaps of 2 nm to 5 μm.
- Townsend avalanche of gaseous ions results from the cascading effect of secondary electrons released by collisions and the impact ionization of gas ions accelerating across gaps greater than 5 μm.
These noncontact nanogap processes provide highly effective electron transfer — even in the presence of grease, oil, dust and other contaminants — and are unaffected by motor speed. No other grounding product works with both contact and noncontact electron transfer.
Systematic approach, long-term solution
Virtually all VFD-driven motors are vulnerable to bearing damage. To make the savings generated by VFDs sustainable, an effective long-term method of shaft grounding is essential. Although a grounding ring safely bleeds damaging currents to ground, vertical pump motors need something more.
Some carriers conduct electricity, but Wilkins is convinced that a carrier should be electrically isolated, disconnecting the motor from the pump shaft electrically though not mechanically. In addition to protecting the motor’s thrust bearing from electrical damage, this keeps shaft currents from jumping to the bearings of the pump itself, or to the bearings of a gearbox, tachometer, encoder or other component.
“It’s the combination that does it,” says Wilkins. “The grounding ring does a great job, but the ring in a vertical motor is competing for the current that exists in the possible path of the thrust bearing. We’ve found that in vertical applications the thrust bearing can be a lower-impedance path to ground, because of the Hertzian point contact of the thrust bearing and the load that it is placed under. So we have to eliminate that current path via insulation on the carrier.”
Carriers fabricated from inferior metals, inappropriate coatings or application protocols that fail to provide long-lasting protection won’t work. To apply the ceramic coating, IEMCO uses a tightly controlled flame-spray welding procedure. To minimize subsequent wear on the coated surface, proper bearing fit is of the utmost importance, so Wilkins’ team grinds each newly coated carrier to very tight tolerances. The finished carrier has a hardness of Rockwell 50C and provides a resistance of more than 1 gigohm at 1,000 volts. The National Electrical Manufacturers Association (NEMA) standard for carrier isolation is only 1 megohm at 500 volts.
“Frankly, it’s a technique that we feel we have perfected,” says Wilkins. “With a vertical hollow-shaft motor, after we’ve added the grounding ring and upgraded the carrier, the motor is truly inverter-ready.”
For more information, contact Adam Willwerth, Sales & Marketing Manager, Electro Static Technology, 31 Winterbrook Road, Mechanic Falls, ME 04256-5724, TEL: (207) 998-5140, FAX: (207) 998-5143. www.est-aegis.com