One cure
So how can you prevent these stray currents from chewing up your motor bearings? The simplest, though not necessarily the cheapest, way is to stop them flowing through the bearings in the first place. In other words, insulate the bearings from the shaft currents. This is the option offered by SKF, as Snyder explains: “We have two solutions: one is to put a ceramic coating on the OD [outside diameter] of the bearing, which provides a resistance to any current trying to go through it, while the other way is to actually replace the rolling elements within the bearing with ceramic balls or rollers — the same effect, really, but all the insulating material is contained within the bearing instead of being on the outside.”

The first solution comprises the Insocoat range (Figure 3). These bearings, in all common sizes above 70-mm bore diameter, are coated either on the outer or inner ring with a nominal 100-μm-thick aluminum oxide layer that is applied using a plasma spraying technique. The standard layer thickness is said to prevent most current passage problems.

SKF’s other option is its Hybrid design. These bearings combine silicon nitride ceramic rolling elements with steel rings. The result is a lighter, harder and more durable alternative to conventional all-steel bearings, capable of running at higher speeds and lower operating temperatures. And the natural insulating properties of the ceramic elements is said to make them ideal for large adjustable-speed motors. Standard types include single row, deep groove ball bearings with bore diameters from 5 mm to 110 mm, although SKF can customize hybrids to meet specific application requirements.
“We can work with the end users to try and solve their problems,” says Snyder, “and then we will take that knowledge and experience back to the original equipment manufacturer (OEM) to provide a value-added product for them to put in their motors — but that’s a tougher sell unless you can get the ‘pull-through’ from the end user.”

Right now, users are not doing much pulling because motor manufacturers generally have not exactly been shouting about the possible bearing problems associated with ASDs, notes Snyder. “We’ve had the products [Insocoat and Hybrid] on the market for six years now, but they’ve only recently been gaining momentum as more people are beginning to recognize the problem. We probably saw it before [the OEMs] because we were seeing bearings come back to us. Sometimes we would recognize the problem before the customer, who just looked upon it as bearing failure without appreciating the root cause. And the OEMs weren’t really seeing the problems either, because the motors were going through rebuild shops and the feedback loop to the OEMs wasn’t quite what it is now.”

It’s often said that for every problem there is a solution, but for this solution — insulated bearings — there can also be a problem. The common mode currents may well be prevented from flowing through the bearings, but they have to go somewhere. This is why ABB’s Brown says that insulated bearings should only be fitted to one end (the non-driven one) of the motor drive shaft. (ABB offers insulated bearings as a standard option on motors above 280 frame size, but doesn’t fit them if the motor is to be used at constant speed.) “Fitting such bearings at both ends of the shaft can cause problems as the stray currents can go into the driven device. We have seen examples of pump impellers being eroded in this way.”

Another option
John Malinowski of Baldor Electric in Fort Smith, Ark., makes a similar point: “We know a lot more about this phenomenon [stray shaft currents] today than in the past and can reduce its occurrence as long as correct installation, setup and wiring practices are followed. Obviously, there may be rare occasions where the fluting still occurs, or where the user wants to ensure he has no problems by building in a safety margin. This is when shaft grounding and isolated bearings may be used. Either may be retrofitted, but with the bearings the motor shaft must be insulated from the load so it doesn’t transfer the current into the driven load bearings.”

Baldor recommends the use of shaft grounding brushes as the simplest and most cost-effective way of dealing with the problem. It offers a grounding brush as standard on its 250-hp and larger Inverter Drive or Vector Drive motors, which are specifically designed to be powered from ASDs.

Operating on a similar principle is the recently introduced Motor Grounding Seal (MGS) from Inpro/Seal of Rock Island, Ill. An alternative to insulated bearings is to provide a path to ground for the shaft currents before they can get to the bearings. This is the principle behind the Motor Grounding Seal (MGS), recently introduced by Inpro/Seal of Rock Island, Ill. Developed in conjunction with Electro Static Technology (EST) of Mechanical Falls, Maine, the MGS essentially is a bearing isolator, or labyrinth seal, that includes a built-in grounding ring and brush from EST. The brush contains microfibers that completely surround the shaft to discharge any current flowing through it.

EST’s version of the same device has been dubbed the Aegis shaft grounding coupling (SGC) and, according to EST general manager Tony King, “is the first product of its kind to provide a solution to two significant industry challenges — shaft current elimination and bearing isolation — while delivering a maintenance-free and cost-effective solution that is truly revolutionary and makes all other technologies obsolete.” These are bold claims indeed to make to an industry that looks upon brush-based electrical connections as anything but maintenance free.

At the core of Aegis is EST’s patent-pending Electron Transport Technology, which the company says effectively creates an ultra-low resistance path between the motor shaft and frame to dissipate all of the damaging shaft current to ground. Aegis solves the problem, says engineering manager Willam Oh, by providing a “virtual short” between shaft and frame so that shaft voltages and currents cannot build up in the first place.

Good advice
Clearly, there is a potential problem surrounding the use of ASDs, but most observers agree that it is still relative rare and unpredictable, albeit becoming more visible simply because of the growing number of ASDs in operation. The advice given some time ago by David Kowal, application development engineer for Emerson Process Management’s CSI Machinery Health Management division, Knoxville, Tenn., almost certainly still holds good today: “Bearing damage resulting from EDM doesn’t have to be chronic or remain unexplained. Understanding what voltage sources result in and which machines are more susceptible to EDM damage, knowing what questions to ask, knowing how to identify EDM damage through visual inspection and vibration data, and acquiring shaft-to-ground voltage and current readings can assist in combating this phenomena.”
Process and electrical engineers alike should take note.