- Make sure that any motor to be used with a VFD is truly inverter-ready — equipped at the factory or retrofitted not just with extra winding insulation but also with a shaft grounding ring and, in certain cases, an insulated bearing.
- The repair or replacement of failed motor bearings can wipe out any savings that a VFD yields and diminish the reliability of an entire system.
Manufacturers of three-phase AC induction motors offer inverter-duty or inverter-ready models, but, while these motors have inverter-rated insulation to protect the windings, the bearings, their most vulnerable parts, are too often ignored. Unfortunately, many customers who purchase these motors do not understand this. The National Electrical Manufacturers Association (NEMA) does not recommend that new motors have bearing protection against damaging electrical discharges.
Inverters, also known as variable frequency drives (VFDs) and adjustable speed drives, can induce unwanted motor shaft voltages that, without effective mitigation, can destroy bearings, causing premature motor failure.
Although current NEMA standards highlight the possible need for extra bearing protection for VFD-driven motors, the language is often not specific enough to guide motor manufacturers and has not been updated to include new research results and developments in shaft grounding technology. Stronger, reworked standards, calling for effective mitigation in the form of reliable long-term shaft grounding technology would go a long way toward cautioning motor users of the need for such mitigation and would help to clear up common misconceptions.
VFDs are often used to make HVAC systems or automated assembly lines more energy-efficient. Whether they’re used to control a motor’s speed or torque, VFDs can induce voltages and currents that can damage bearings. In fact, the repair or replacement of failed motor bearings can wipe out any savings that a VFD yields and diminish the reliability of an entire system.
One way to minimize electrical bearing damage and make these systems sustainable is a motor shaft grounding ring, combined with insulation for motors greater than 100 hp.
New grounding rings encircle a motor’s shaft with contact points for far greater effectiveness.
Figure 1. Viewed under a scanning electron microscope, a new bearing race wall is a relatively smooth surface, marked by nothing but mechanical wear where bearings contact the wall.
Damage to windings and bearings is caused by repetitive and extremely rapid pulses applied to the motor from a modern VFD’s non-sinusoidal power-switching circuitry. The names used to describe this phenomenon include harmonic content, parasitic capacitance, capacitive coupling, electrostatic buildup, and common mode voltage. Regardless of the name used, high peak voltages and fast voltage rise times can cause cumulative degradation of insulation, bearings, and coil varnish. If the load impedance is higher than the 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.
Figure 2. frequent discharges can leave the entire bearing race riddled with pits known as frosting.
Without some form of mitigation, shaft currents discharge through bearings, causing unwanted electrical discharge machining (EDMing) that erodes the bearings and race walls and leads to premature bearing/motor failure (Figure 1). Before long, these frequent discharges can leave the entire bearing race riddled with pits known as frosting (Figure 2). In fact, electrical damage has become the most common cause of bearing failure in VFD-controlled AC motors.
In a phenomenon called fluting, the operational frequency of the VFD causes concentrated pitting at regular intervals along the race wall, forming washboard-like ridges (Figure 3). Fluting can cause excessive noise and vibration, which, in an HVAC system, can be magnified and transmitted by ductwork throughout the entire building. By the time this is noticeable, bearing failure is often imminent.
Figure 3. Taken from a failed motor, the fluted bearing race wall (left) resulted from VFD-induced bearing currents.
In its key role as information provider, 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. Since no other entity is in such a position of authority, NEMA could at the same time address the overall problem of electrical bearing damage more clearly and firmly.