Pumps & Systems - September 2007

such damage in the first place, the induced shaft current must be diverted from the bearings by insulation and/or an alternate path to ground.

Insulation

Insulating motor bearings is a solution that tends to shift the problem elsewhere as shaft current looks for another path to ground. Sometimes, because of the capacitive effect of the ceramic insulation, high-frequency VFD-induced currents actually pass through the insulating layer and cause bearing failure.

If attached equipment, such as a pump, provides this path, the other equipment often winds up with bearing damage of its own. Insulation and other bearing-isolation strategies can be costly to implement.

Alternate Discharge Paths

When properly implemented, these strategies are preferable to insulation because they neutralize shaft current. Techniques range in cost and sometimes can only be applied selectively, depending on motor size or application.

The ideal solution would provide a very low resistance path from shaft to frame, would be low-cost, and could be broadly applied across all VFD/AC motor applications, affording the greatest degree of bearing protection and maximum return on investment.

Shaft-Current Mitigation Technologies There are a number of technologies now available to protect AC motor bearings from damage due to shaft currents:

Faraday Shield

The shield prevents the VFD current from being induced onto the shaft by effectively blocking it with a capacitive barrier between the stator and rotor. However, this solution can be extremely difficult to implement, very expensive, and has been generally abandoned as a practical solution.

Insulated Bearings

Insulating material, usually a nonconductive resin or ceramic layer, isolates the bearings and prevents shaft current from discharging through them to the frame. This forces current to seek another path to ground, such as through an attached pump or tachometer or even the load.

Due to the high cost of insulating the bearing journals, this solution is generally limited to larger-sized NEMA motors. Sometimes, high-frequency VFD-induced currents actually pass through the insulating layer and cause bearing damage anyway. Another drawback is the potential for contaminated insulation, which can establish a current path through the bearings over time.

Ceramic Bearing

The use of nonconductive ceramic balls prevents the discharge of shaft current through this type of bearing. As with other

isolation measures, shaft current will seek an alternate path to ground.

This technology can be very costly and, in most cases, motors with ceramic bearings must be special ordered and have long lead times. In addition, because ceramic bearings and steel bearings differ in compressive strength, ceramic bearings must be resized in most cases to handle mechanical static and dynamic loadings.

Conductive Grease

In theory, because this grease contains conductive particles, it would provide a lower-impedance path through the bearing and would bleed off shaft current through the bearing without the damaging discharge.

Unfortunately, the conductive particles in these lubricants can increase mechanical wear to the bearing, rendering the lubricants ineffective and often causing premature failures. This method has been widely abandoned as a viable solution to bearing currents.

Grounding Brush

A metal brush contacting the motor shaft is a more practi-
cal and economical way to provide a low-impedance path to
ground, especially for larger NEMA-frame motors. However,
these brushes can pose several problems of their own:
a) They are subject to wear because of the mechanical contact
with the shaft.
b) They collect contaminants on their metal bristles, which
destroys their effectiveness.
c) They are subject to oxidation buildup, which decreases their
grounding effectiveness.
d) They require maintenance on a regular basis, increasing
their cost.

Shaft Grounding Ring (SGR)

This approach involves using a ring of specially engineered conductive micro fibers to redirect shaft current and provide a reliable, very low impedance path from shaft to frame, bypassing the motor bearings entirely. The ring uses the principles of ionization to boost the electron-transfer rate and promote extremely efficient discharge of the high-frequency shaft currents induced by VFDs.

With hundreds of thousands of discharge points, an SGR can channel shaft currents around the AC motor bearings and protect them from electrical damage. It is a low-cost solution that can be applied to virtually any size AC motor, in virtually any VFD application. Benefits include:

1. Scalability. The technology is scalable to all sizes of NEMA-frame and larger motors regardless of shaft size or application. Introduced in 2005, these rings were designed for motors with shafts from 0.311-in to 6.020-in, including NEMA and IEC frames as well as high-horsepower AC and DC motors.