Protecting e-motor bearings against shaft voltage

  • 14-Jun-2011 11:56 EDT
FIGURE 1 -- EDM pit on bearing race wall.jpg

EDM pitting of a bearing race wall (magnified) is shown, the result of inverter-induced electrical discharges from the motor shaft.

The greater the reliance on electric motors for automotive propulsion, the greater the potential for electrical bearing damage. At the heart of every battery electric vehicle (BEV) and hybrid is an alternating-current (ac), three-phase traction motor/generator. Since batteries provide direct current (dc), inverters (also known as variable frequency drives or VFDs) are needed to convert the dc to ac.

These inverters have an unfortunate side effect: they induce unwanted voltages on motor shafts. Without effective, long-term grounding, this shaft voltage will erode and eventually destroy motor bearings.

Because electrical bearing damage is a lurking problem in electrified vehicles, automotive design engineers face a new set of challenges. Inverter-induced shaft voltages jump to the path of least resistance wherever it leads, so partial mitigation measures such as insulated motor bearings can just shift the damage to other components such as gearbox bearings, transmission gears, or wheel bearings. Even the bearings of a hybrid’s gasoline engine are vulnerable to such damage when the vehicle is operating in electric mode.

To solve the problem of electrical bearing damage, automotive engineers need only look to other industries experienced in the use of ac motors. For years, design and maintenance engineers and contractors in manufacturing, processing, HVAC, and materials handling have turned to inverters as a way of controlling the speed of ac motors and thereby reducing their energy costs. They found that, without an effective method of channeling inverter-induced shaft voltages safely to ground, any savings due to reduced energy consumption could quickly be wiped out by the high maintenance costs of replacing damaged motor bearings.

In short, an effective, long-term method of grounding motor shafts is needed to make inverter-driven systems reliable. A shaft-grounding device can divert harmful currents before they can cause bearing damage. Applied to the traction motor in a BEV or hybrid, such a device should prevent bearing damage and thus contribute to overall vehicle reliability.

One of the most reliable and cost-effective grounding devices is a ring that fits over the motor’s shaft. Engineered with specially designed conductive microfibers, the AEGIS SGR Bearing Protection Ring developed by Electro Static Technology safely channels damaging currents to ground, bypassing the bearings entirely.

Recognizing that the best solution is to design motors from a clean sheet to survive the damaging effects of inverters, a handful of forward-looking motor manufacturers including Baldor Electric and GE now install the AEGIS ring internally on certain models. A growing list of other motor manufacturers, such as Marathon Electric, TECO-Westinghouse, and WEG Electric, offer it as a factory-installed option.

For motors already in service, the ring can be retrofitted. Scalable to any National Electrical Manufacturers Association (NEMA) or International Electrotechnical Commission (IEC) motor regardless of shaft size or power, the ring has been installed successfully on motors powering pumps, fans, turbines, conveyors, etc., in hundreds of thousands of installations worldwide including the Time-Life Building in New York, Eli Lilly and Co., and Purdue University’s Birck Nanotechnology Center.

The AEGIS ring also has proven itself effective in the inverter-controlled traction motors of electric trucks, trains, trolleys, and construction equipment. It is now being tested by several automotive OEMs.

Cause and effect

For electrical damage to motor bearings, the main culprit is common-mode voltage arising from the nonsinusoidal waveforms produced by an inverter’s power-switching circuitry. The extremely fast voltage rise times (dV/dt) associated with the insulated gate bipolar transistors typically found in today’s pulse-width-modulated inverters can cause charges to build up on the motor shaft.

Without mitigation, these voltages discharge through bearings, causing unwanted electrical discharge machining (EDM) that erodes ball bearings and race walls and leads to premature bearing/motor failure.

Electric motors in vehicles operate in a range from 1000 to over 16,000 rpm, and at such speeds the very thin grease layer between the rolling elements and race in a bearing can break down due to voltage discharges of 5 to 40 V. Every time the grease dielectric is overcome, an electrical arc through the bearing burns the grease and blasts a tiny pit (fusion crater) in the steel surface. At inverter carrier frequencies of over 12 kHz, many millions of pits can be created in a very short time.

This process also generates steel and carbon particles that contaminate the grease, further reducing its lubrication properties and giving it a black “burnt” color.

Frequent discharges eventually can leave the entire bearing race riddled with pits known as frosting. In a phenomenon called fluting, the operational frequency of the inverter causes concentrated pitting at regular intervals along the bearing race wall, forming washboard-like ridges that result in noise and vibration.

EST’s technology key

Electro Static Technology contends that the AEGIS ring is superior to conventional spring-pressure grounding brushes, which corrode, become clogged with debris, and require regular maintenance. Neither metal brushes nor carbon-block (graphite) brushes work as well at high rpm, and the latter are susceptible to “hotspotting,” in which an arc briefly fuses a brush to the motor shaft. In contrast, in-house testing has shown that the AEGIS ring requires no maintenance and lasts for the life of the motor, regardless of rpm.

Nongrounding methods of mitigating electrical bearing damage tend to be expensive. Multilevel inverters and harmonic filters, for example, can cost thousands of dollars, while the EST offering solves the problem at a comparatively low cost. Ceramic bearings are also costly and, like bearing insulation, can pass on harmful voltage discharges to other equipment.

Key to the ring’s success are the patented conductive microfibers arranged along the entire inner circumference of the ring that completely surrounds the motor shaft. Secured in the patented FiberLock channel, these fibers can flex without breaking. The deep channel also protects the fibers from dust, liquids, and other debris. Tests of the ring on multiple motors show surface wear of less than 0.001 in (0.0254 mm) per 10,000 hours of continuous operation and no fiber breakage after 2 million direction reversals.

The effectiveness of the AEGIS ring can be seen on an oscilloscope. Without shaft grounding, damaging inverter-induced shaft voltages show up as peaks and valleys. After the installation of an SGR ring, the nearly straight line demonstrates how the ring diverts these voltages, channeling them safely to ground.

As the transition from gasoline-powered to all-electric cars gathers momentum, the EST solution offers automobile designers a way to improve the reliability of hybrids and BEVs now and in the future.

Matthew Roman, Engineering Manager for Electro Static Technology, wrote this article for SAE Magazines.

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