Fulfilling the promise of variable frequency drives
By Adam Willwerth
 Without some form of mitigation, VFD-induced shaft currents can cause considerable motor/bearing damage. |
Mitigating Bearing Damage
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As explained above, electrical damage to AC motor bearings often begins at startup and grows progressively worse. As a result of this damage, the bearings eventually fail. To guard against such damage and thus extend motor life, the induced current must be diverted from the bearings by means of mitigation technologies such as insulation, shielding, and/or an alternate path to ground. These technologies vary in terms of their cost and effectiveness.
Insulating motor bearings is a partial solution that more often than not shifts the problem elsewhere. Blocked by insulation, shaft current seeks another path to ground. Attached equipment, such as a pump, often provides this path, and it frequently winds up with bearing damage of its own. In addition to being expensive, insulation is subject to contamination. Worse yet, some types of insulation can be totally self-defeating: In certain circumstances, the insulating layer has a capacitive effect on high-frequency VFD-induced currents, allowing them to pass right through to the bearings it was supposed to protect.
A Faraday shield can be created by installing grounded conductive material such as copper foil or paint between the stator and rotor. If built to the proper specifications for the motor, this can block most of the harmful currents that jump across the motor’s air gap. However, this mitigating measure is often expensive and difficult to implement, and attached equipment could still be vulnerable to deflected currents.
 The AEGIS SGR™ effectively reduces these currents (and the damage they cause) by channeling them safely to ground. |
Likewise, nonconductive ceramic ball bearings divert currents from the main motor’s bearings but leave attached equipment open to damage of its own. Ceramic bearings can be costly and usually must be resized to handle mechanical static and dynamic loadings.
Yet another mitigation attempt comes in the form of conductive grease, which, in theory, bleeds off harmful currents by providing a lower-impedance path through the bearings. In practice, however, the conductive particles in the grease increase mechanical wear.
Metal grounding brushes certainly help. They contact the motor shaft to provide alternate paths to ground. Unfortunately, they also wear out and corrode, thus requiring regular maintenance.
As explained above, alternate discharge paths to ground, when properly implemented, 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 an effective, low-cost, very-low-resistance path from shaft to frame and could be broadly applied across all VFD/AC motor applications, affording the greatest degree of bearing protection and maximum return on investment. The AEGIS SGR meets all these criteria. Its patent-pending Electron Transport Technology™ 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.
 The AEGIS SGR with NEMA adaptor plate |
The versatile AEGIS SGR is scalable to any NEMA or IEC motor regardless of shaft size, horsepower, or application. SGRs have been successfully applied to power generators, gas turbines, AC traction and break motors, cleanrooms, HVAC systems, and a long list of other industrial and commercial applications.
For VFD-equipped motors of less than 100 HP (75 kW) with shaft diameters of less than 2” (50mm), a single SGR on the drive end of the motor shaft is typically sufficient to divert harmful shaft currents.
 When installing a single SGR, the motor’s drive end is the preferred location. |
Large AC motors (100HP/75kW or more) and even large DC motors, especially those with shaft diameters of more than 2” (50mm), are more likely to have high-frequency circulating currents (as well as EDM-type discharges) that can damage bearings. Motors with roller bearings are also more vulnerable to damaging circulating currents because roller bearings have a greater surface area and their lubricant layer is usually thinner. Such motors benefit from the combination of an SGR on the drive end and insulation on the non-drive end to break the circulating current path. This may also be the solution in situations where installing an SGR on the non-drive end would be impractical because of encoders, fans, or other special circumstances.