Motion System Design
Shielding bearings: Grounding helps  inverter-duty motors live up to their name

Shielding bearings: Grounding helps inverter-duty motors live up to their name

Inverters used to control a motor’s speed or torque — also known as variable frequency drives (VFDs) or adjustable speed drives — can induce unwanted motor shaft voltages that cause premature motor failure. To fully leverage the potential savings of VFD-driven designs, effective long-term shaft grounding is essential.

All major manufacturers of three-phase ac induction motors offer inverter-duty models with insulation sufficient to protect the windings, but the bearings are often left vulnerable to damage — something designers specifying or purchasing these motors don’t always realize.

For motors without adequate bearing protection, the term inverter-duty is misleading, as the potential for electrical discharge machining (EDM) remains a major cause of premature bearing and motor failure. In fact, most motors labeled inverter-duty or inverter-ready are inadequately protected. It is therefore incumbent on savvy specifiers to ensure that any motor to be used with a VFD is equipped at the factory or retrofitted not only with extra winding insulation, but also with a shaft grounding ring and, in certain cases, an insulated bearing.

How VFDs cause motor failure

Repetitive, rapid electrical pulses applied to the motor from a modern VFD’s non-sinusoidal power-switching circuitry cause winding and bearing damage. This phenomenon — also called harmonic content, parasitic capacitance, capacitive coupling, electrostatic buildup, and common mode voltage — is associated with high peak voltages and fast voltage rise times that cause cumulative degradation of insulation, bearings, and coil varnish. If load impedance is higher than line impedance, current is reflected back toward the VFD, creating voltage spikes at the motor terminal that can be twice as high as the dc bus voltage. The cumulative bearing damage caused by VFD-induced currents is often overlooked until it is too late to save the motor.

Elusive sustainability

Because VFDs can save 30% or more in energy costs, they are a key technology for those aiming to boost the efficiency of automated assembly lines, commercial HVAC systems, and other equipment. However, current-damaged motor bearings necessitate costly repairs or replacement that can severely diminish system reliability and wipe out any savings that a VFD yields. What’s more, warranty claims against motor and VFD manufacturers are often rendered invalid, as systems that use VFDs are so varied and liability can be hard to determine.

Bearing grounding technology

Some traditional single-point contact brush devices designed to provide a path to ground rely on direct contact to transfer current and can fall short at high rpms. They exhibit wear, in addition to other problems:

• Metal spring-pressure grounding brushes can be contaminated by corrosion or clogged by debris, requiring regular maintenance or replacement.
• Carbon-block (graphite) brushes are susceptible to hotspotting, in which an arc briefly fuses the brush to the motor shaft.
• Other contact brush designs quickly wear out, allowing shaft currents to return to discharging through the bearings.

Unlike older single-point contact brushes, some of today’s grounding rings encircle a motor’s shaft for improved contact and effectiveness. Continuous circumferential rows of engineered microfibers boost electron transfer rates and lower shaft-to-frame impedance — safely bleeding currents to ground and bypassing motor bearings entirely. Microfiber brushes work with little or no contact, so do not wear like conventional brushes. These fibers flex without breaking, and a deep protective channel can protect against dust, liquid, and debris ingress. Typical surface wear is less than 0.001 in. per 10,000 hours of continuous operation and no fiber breakage after two million direction reversals.

In fact, some designs work with or without direct fiber contact with the motor shaft, using electron transport to discharge. Such rings will not wear out and require no maintenance, regardless of rpm. Ring and installation cost is very low when compared to overall system cost, usually less than 1% of equipment cost.

Manufacturers incorporating shaft grounding rings include Baldor Electric Co. (part of the ABB Group), Fort Smith, Ark., which offers inverter-ready NEMA Premium motors with protection rings installed. In 1 to 100 hp in open drip-proof (ODP) and totally enclosed fan-cooled (TEFC) configurations, the pre-protected motors are used as conveyor motors for airport baggage and other material handling applications; general-purpose and brake motors for machinery used in manufacturing; and HVAC motors that run fans or chill water pumps, condensers, and variable-flow refrigerant pumps.

In addition, General Electric, Erie, Pa., equips its constant-torque variable speed drive motors (1.5 to 300 hp) with internally mounted shaft grounding rings, and offers the rings as an externally mounted option on ODP and TEFC motors for metal processing, material handling, heavy-duty, HVAC, and general-purpose applications.

These motors are exceptions to the rule, though many manufacturers will (upon request) add grounding rings to motors before shipping; on motors already in service, the customer or contractor must retrofit the rings.

Need for updated standards

The National Electrical Manufacturers Association (NEMA) has yet to recommend that new motors sport protection against electrical discharges. Current NEMA standards highlight the possible need for extra bearing protection for VFD-driven motors, but the language isn’t specific enough to guide motor manufacturers and doesn’t yet reflect the effect of new innovation in shaft grounding. Stronger standards calling for effective mitigation would go a long way toward cautioning motor users of the need for such mitigation.

In its key role as an industry leader, NEMA is in the unique position to update its MG1 standard to more clearly state that common mode shaft voltages are present in virtually all motors fed by pulse-width-modulated (PWM) VFDs. No other entity is in such a position of authority, so NEMA may at the same time address the overall problem of electrical bearing damage more directly.

The association’s current standards acknowledge the potential damage from VFD-induced voltage spikes: They state that motors controlled by modern VFDs containing insulated gate bipolar transistors (IGBTs) should be designed to withstand repeated spikes (at the terminals) of up to 3.1 times the motor’s rated voltage, at rise times not less than 0.1 microsecond. When addressing the potential for bearing currents, the language is less prescriptive.

NEMA Standard MG1-2009 (Revision 1-2010), Section IV, Part 31, Definite-Purpose Inverter-Fed Polyphase Motors, correctly states: “Shaft voltages can result in the flow of destructive currents through motor bearings, manifesting themselves through pitting of the bearings, scoring of the shaft, and eventual bearing failure.”

Subsection of Part 31 recommends bearing insulation at one end of a larger motor (defined as “usually 500 frame or larger,” horsepower unspecified) if the peak shaft voltage is greater than 300 mV. Unfortunately, the paragraph dealing with these larger motors only mentions circulating end-to-end shaft currents caused by magnetic dissymmetries under sinusoidal operation. It fails to add that the bearings of large motors can also be plagued by VFD-induced, high-frequency capacitively coupled common mode voltages.

In a paragraph on “much smaller motors” (frame size and horsepower unspecified), the same subsection recommends insulating both bearings or installing shaft grounding brushes to divert damaging currents around the bearings. For these motors, the standard correctly explains, a VFD can generate high-frequency common mode voltage, which shifts the three-phase-winding neutral potentials significantly from ground. Because the damaging voltage oscillates at high frequency and is capacitively coupled to the rotor, the current path to ground can run through one bearing or both. But here the standard neglects to mention that high-frequency circulating currents may also be present in VFD-driven motors as small as 100 hp.

In short, NEMA omits mention of common mode voltages from its paragraph on larger motors and omits circulating currents from its paragraph on smaller motors. Another issue with the wording is that neither a grounding brush nor insulation is a reliable, long-term solution to the problem of electrical bearing damage at the system level, which includes motors and attached equipment.

The NEMA standard correctly indicates, “Insulating the motor bearings will not prevent the damage of other shaft connected equipment.” When the path to the bearings is simply blocked by insulation, damaging current seeks another path to ground — which can flow through a gearbox, tachometer, encoder, pump, or other subcomponent, which consequently can suffer bearing damage of its own. One economical and field-tested solution is a maintenance-free, long-life shaft grounding ring that protects attached equipment as well as the motor’s bearings.

In future standards, it could be noted that for motors above 100 hp, in which both circulating currents and common mode voltages could cause bearing damage, combining an insulated bearing on one end with a shaft grounding ring on the opposite end provides optimal protection from electrical bearing damage.

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