Bearing damage in electric vehicles

As motor vehicle manufacturers embrace hybrid-electric and all-electric drive systems to meet fuel-economy mandates and reduce fuel use, they introduce a potential new problem into drive systems – bearing damage.

The heart of every electrified vehicle is an alternating-current (AC) traction motor. Batteries provide direct current (DC), requiring inverters, also known as variable frequency drives (VFDs), to convert the DC to AC. But inverters can induce unwanted voltages on motor shafts. Without effective, long-term grounding, these voltages will erode and eventually destroy motor bearings.
 

Damage from poor grounding

Inverter-induced shaft voltages discharge to ground along the path of least resistance, often running through the bearing system. With electric motors operating as high as 16,000rpm, the very thin grease layer between the rolling elements and race in a bearing can break down due to voltage discharges of 5V to 40V. Every time the grease dielectric is overcome, an electrical arc burns the grease and blasts a tiny fusion crater in the steel surface of the bearing.

At inverter carrier frequencies of more than 12kHz, 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 burnt, black color.

Before long, frequent discharges 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.

Partial mitigation measures, such as insulated motor bearings, can shift the damage to gearbox bearings, transmission gears, wheel bearings, or other components.
 

Grounding inverter/motor systems

To nip the problem of electrical bearing damage in the bud, engineers need only look to other industries. For years, design and maintenance engineers in manufacturing, processing, HVAC, and materials handling have turned to inverters as a way of controlling the speed of AC motors to save energy. They found that without an effective method of channeling inverter-induced shaft voltages safely to ground, motor-bearing replacement costs could quickly wipe out savings from reduced energy consumption. Also learned was that a grounding ring installed on a motor shaft can divert harmful currents before they damage bearings. Such rings should prevent bearing damage in electrified vehicles and improve overall reliability.

Engineered with conductive microfibers, the Aegis SGR Bearing Protection Ring channels currents to ground, bypassing the bearings entirely. Scalable to any motor regardless of shaft size or horsepower, the ring has been used on motors powering pumps, fans, turbines, and conveyors in installations worldwide.

More recently, makers of electric trucks, trains, trolleys, and construction equipment, such as EVAG-Essener and HAVAG-Hallesche, have used the Aegis ring in inverter-controlled electric motor systems.
 

Inverter-induced shaft voltages

The main culprit in electrical damage to bearings is common-mode voltage arising from the non-sinusoidal current produced by an inverter’s power-switching circuitry. The fast voltage rise times (dV/dt) associated with the insulated gate bipolar transistors (IGBTs) found in today’s inverters can cause charges to build up on the motor shaft. These voltages can discharge through bearings, causing unwanted electrical discharge machining (EDM) that erodes ball bearings and race walls.
 

Cost-effective shaft grounding

The Aegis SGR is more robust than 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 rpms, and the latter are susceptible to hotspotting – an arc briefly fusing a brush to the motor shaft. The Aegis ring typically lasts for the service life of the motor without maintenance.

Non-grounding methods of mitigating electrical bearing damage tend to be expensive. Multilevel inverters and harmonic filters, for example, can cost thousands of dollars, while ceramic bearings are costly and can still pass harmful voltage discharges on to other equipment.
 

Bearing protection ring design

Patented conductive microfibers, arranged along the ring’s inner circumference, are key to its effectiveness. These fibers surround the motor shaft and can flex without breaking. A deep channel also protects the fibers from dust, liquids, and other debris. Tests on multiple motors show surface wear of less than 0.001" per 10,000 hours of continuous operation and no fiber breakage after 2 million direction reversals.

As electric vehicle become more prevalent, the potential for bearing failure will grow as well. The Aegis SGR Bearing Protection Ring offers vehicle designers a way to improve reliability as they engineer the next generation of products by protecting electric motors from bearing damage, extending product life, and preventing inverter-induced voltage-damage to other components.

Electro Static Technology
www.est-static.com

Adam Willwerth is development manager for Electro Static Technology and can be reached at 207.998.5140.