Through trial-and-error and much hard work, a Kansas motor repair shop has developed a virtually foolproof process for protecting vertical hollow-shaft motors from electrical bearing damage caused by stray shaft currents.
For six years, Scott Wilkins, manager of motor shop operations at Independent Electric Machinery Company (IEMCO) in Kansas City, has overseen the reconditioning of hundreds of these vertical motors (most of which run pumps) through a process known as the Vertical Motor Solution. None of them has had 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 reports that most of the bearing damage he sees is in motors controlled by energy-saving 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 time, 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 modern VFD’s non-sinusoidal power-switching circuitry. The many 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 that a VFD yields and severely diminish the reliability of an entire system.
“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 the 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 discovered only after bearings failed that most warranties do not 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 it all the hard way. An ordeal concerning 13 pump motors made him a believer in IEMCO’s Vertical Motor Solution.
An electrical engineer with Professional Engineering Consultants (PEC) of Wichita, KS, Biby helped design a new water treatment plant for a city in Kansas. The plant opened in 2009, but shortly thereafter Biby found himself in charge of a motor remediation project that would last more than two years. One after the other, the motors, all of which are controlled by VFDs to provide adjustability in flow rate and pressure, developed serious electrical bearing damage.
IEMCO’s process saved the day, but only after months of frustration. The motor manufacturer provided minimal help, replacing 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 have since adjusted our motor specifications so that all new motors that will be connected to VFDs will be 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 the proper installation of the devices. 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, who have sent him complete new motors or motor components prior to shipping them 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 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.
Sadly, some of the failed motors IEMCO has reconditioned were originally marketed as ‘inverter-rated,’ ‘inverter-duty,’ or ‘inverter-ready’ models. The perturbed users who purchased them did not understand that 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, the Philadelphia-based Bartlett Bearing Co. Inc. The relationship appears to be mutually beneficial.
“We often have our customer send the carriers from motors that have experienced bearing damage to Scott Wilkins,” says Bill Potts, Bartlett’s vice-president of operations. “In addition to applying the ceramic coating, he checks each carrier for mechanical integrity (axial and radial runouts) and maintains the correct finished bearing tolerance. We benefit from his knowledge, his experience, his expertise, and his workmanship. He does a fantastic job, like nothing else I’ve seen in the industry, including the motor manufacturers.”
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 sell to HVAC contractors and to end users, but we deal primarily with over 1,000 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.”
More success stories
Not all of the shops that get bearings from Bartlett
use IEMCO, however. Willier Electric, in Gibbsboro, NJ, coats the carriers themselves.
Jim Willier tells of a municipal water supply plant with six 150-hp vertical solid-shaft pump motors that were plagued with recurring fluting of the bearings.
“Insulating bearings on the top and bottom, and other methods, never worked,” he says.
Willier then tried something new, installing the AEGIS ring at the lower end of a motor, with a brass insert to make sure the ring was centred and protected from bearing grease. Willier’s crew also applied an extra-thick ceramic coating to the carrier at the upper end of the motor. Two of the six motors were upgraded this way and reinstalled early in 2012. “So far everything’s been working — no complaints,” says Willier. The end user plans to upgrade the rest of the motors as they come out of service.
“Now, when we sell a new vertical pump motor and a drive together, we sell them with the insulated carrier and the AEGIS grounding ring,” Willier adds.
Based outside of South Bend, IN, Precision Electric Inc., sells and services electric motors and industrial electronics. The company also designs and carries out automation projects. Most of their customers are in northern Indiana and southern Michigan.
For two ailing vertical hollow-shaft pump motors (60 hp and 150 hp) at a municipal water plant, Precision Electric sent the carriers for the top (thrust) bearings to Bartlett Bearing, which shipped them to Kansas City for IEMCO to coat and machine.
“It doesn’t mean we won’t do it ourselves in the future,” says Kerry Dodd, Precision’s vice-president and co-owner. “We’re just not geared up to do it right now.”
Dodd’s shop did replace the motors’ upper and lower bearings. In addition, the lower bearing caps were machined with a press fit and AEGIS rings installed.
“It solved the problem,” says Dodd. “This customer was going though bearings like crazy, and we had actually measured significant shaft currents. Once we made those modifications, the currents were gone.”
Both motors have been running fine since. According to Dodd, the water plant intends to rotate out four more motors for the same fix.
Key to the AEGIS Bearing Protection Ring’s effectiveness is its patented Nanogap Technology, which ensures superior contact/noncontact grounding protection for the normal service life of the motor’s bearings.
The ring’s unique design includes proprietary conductive micro-fibres arranged in a continuous circle around the motor shaft, providing hundreds of thousands or even millions of contact and noncontact voltage discharge points.
When the ring is installed, its conductive micro-fibres overlap the motor shaft and, over time, slowly wear to fit the shaft surface perfectly, continuing to maintain excellent electrical contact throughout the life of the bearing. Electron transfer technology includes three distinct current-transfer processes that work simultaneously:
1) Tunnelling of electrons: This mechanism is based on the ability of electrons to “tunnel” across an insulating barrier, and works for gaps smaller than 2 nm.
2) Field emissions of electrons: Field emission is a form of quantum tunnelling 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.
3) Townsend avalanche of gaseous ions: This process 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. Because no other grounding product works with both contact and noncontact electron transfer, no other product offers the long-term, maintenance-free performance of the AEGIS ring.
Systematic approach yields a 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 an AEGIS 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, etc.
“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.”
Many motor manufacturers and repair shops use carriers fabricated from inferior metals, inappropriate coatings, or application protocols that fail to provide long-lasting protection. To apply the coating of ceramic, 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.
IEMCO is by no means the first motor repair shop to realize that coating the bearing carrier with an insulating material is a good idea. But it may be the first shop to do it right.
“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 and marketing manager, Electro Static Technology, at 207-998-5140, or visit www.est-aegis.com.