| Preventing Drive Induced Electrical Damage to AC Motor Bearings |
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| Written by Adam Willwerth of Electro Static Technology | |
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Currents are induced on the motor shafts when AC motors are employed with VFDs to drive air pumps, chillwater pumps and compressors in HVAC/R systems. Without a grounding device, these currents typically discharge through the motor bearings, causing frosting, pitting, fusion craters, and fluting. Due in large part to an increased focus on energy savings, the use of pulse width modulated (PWM) variable frequency drives (VFDs) to control AC motors has grown dramatically over the last few years. While they offer low operating costs and high performance, VFDs are not without their problems. Shaft currents induced by VFDs can lead to motor failures. Without some form of mitigation, shaft currents travel to ground through bearings, causing pitting, fusion craters, fluting, excessive bearing noise, eventual bearing failure, and subsequent motor failure. This is not a small problem. Consider:
Electrical Damage to Bearings Due to the high-speed switching frequencies used in PWM inverters, all variable frequency drives induce shaft current in AC motors. The switching frequencies of insulated-gate bipolar transistors (IGBT) used in these drives produce voltages on the motor shaft during normal operation through electromagnetic induction. These voltages, which can register 70-V or more (peak-to-peak), are easily measured by touching an oscilloscope probe to the shaft while the motor is running (see Figure 1). Once these voltages reach a level sufficient to overcome the dielectric properties of the grease in the bearings, they discharge along the path of least resistance - typically the motor bearings - to the motor housing. (Bearings are designed to operate with a very thin layer of oil between the rotating ball and the bearing race.) During virtually every VFD cycle, induced shaft voltage discharges from the motor shaft to the frame via the bearings, leaving a small fusion crater in the bearing race. These discharges are so frequent that, before long, the entire bearing race becomes marked with countless pits known as frosting. As damage continues, the frosting increases, eventually leading to noisy bearings and bearing failure. A phenomenon known as fluting may occur as well, producing washboard-like ridges across the frosted bearing race. Fluting can cause excessive noise and vibration that is magnified and transmitted by the ducting in heating, ventilation, and air-conditioning systems. Regardless of the type of bearing or race damage that occurs, the resulting motor failure often costs many thousands or even tens of thousands of dollars in downtime and lost production. Failure rates vary widely depending on many factors, but evidence suggests that a significant portion of failures occur only 3 to 12 months after system startup. Because many of today's AC motors have sealed bearings to keep out dirt and other contaminants, electrical damage has become the most common cause of bearing failure in AC motors with VFDs. If half of all AC motor failures are due to bearing failure, almost 80 percent of these are caused by electrical damage to bearings. Strategies for Mitigating Shaft Current Damage As demonstrated above, electrical damage to VFD/AC motor bearings begins at startup and grows progressively worse. As a result of this damage, the bearings eventually fail. To prevent such damage in the first place, the induced shaft current must be diverted from the bearings by insulation and/or an alternate path to ground. Insulation Insulating motor bearings is a solution that tends to shift the problem elsewhere as shaft current looks for another path to ground. Sometimes, because of the capacitive effect of the ceramic insulation, high-frequency VFD-induced currents actually pass through the insulating layer and cause bearing failure. If attached equipment, such as a pump, provides this path, the other equipment often winds up with bearing damage of its own. Insulation and other bearing-isolation strategies can be costly to implement. Alternate Discharge Paths When properly implemented, these strategies 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 a very low resistance path from shaft to frame, would be low-cost, and could be broadly applied across all VFD/AC motor applications, affording the greatest degree of bearing protection and maximum return on investment. Shaft-Current Mitigation Technologies There are a number of technologies now available to protect AC motor bearings from damage due to shaft currents: Faraday Shield The shield prevents the VFD current from being induced onto the shaft by effectively blocking it with a capacitive barrier between the stator and rotor. However, this solution can be extremely difficult to implement, very expensive, and has been generally abandoned as a practical solution. Insulated Bearings Insulating material, usually a nonconductive resin or ceramic layer, isolates the bearings and prevents shaft current from discharging through them to the frame. This forces current to seek another path to ground, such as through an attached pump or tachometer or even the load. Due to the high cost of insulating the bearing journals, this solution is generally limited to larger-sized NEMA motors. Sometimes, high-frequency VFD-induced currents actually pass through the insulating layer and cause bearing damage anyway. Another drawback is the potential for contaminated insulation, which can establish a current path through the bearings over time. Ceramic Bearing The use of nonconductive ceramic balls prevents the discharge of shaft current through this type of bearing. As with other isolation measures, shaft current will seek an alternate path to ground. This technology can be very costly and, in most cases, motors with ceramic bearings must be special ordered and have long lead times. In addition, because ceramic bearings and steel bearings differ in compressive strength, ceramic bearings must be resized in most cases to handle mechanical static and dynamic loadings. Conductive Grease In theory, because this grease contains conductive particles, it would provide a lower-impedance path through the bearing and would bleed off shaft current through the bearing without the damaging discharge. Unfortunately, the conductive particles in these lubricants can increase mechanical wear to the bearing, rendering the lubricants ineffective and often causing premature failures. This method has been widely abandoned as a viable solution to bearing currents. Grounding Brush A metal brush contacting the motor shaft is a more practical and economical way to provide a low-impedance path to ground, especially for larger NEMA-frame motors. However, these brushes can pose several problems of their own:
Shaft Grounding Ring (SGR) This approach involves using a ring of specially engineered conductive micro fibers to redirect shaft current and provide a reliable, very low impedance path from shaft to frame, bypassing the motor bearings entirely. The ring 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. With hundreds of thousands of discharge points, an SGR can channel shaft currents around the AC motor bearings and protect them from electrical damage. It is a low-cost solution that can be applied to virtually any size AC motor, in virtually any VFD application. Benefits include:
Motor manufacturers and process engineers in industries using VFDs are keenly aware of the problems and expense caused by electrical damage to bearings. They have expended significant time, effort, and money to find a solution to this problem. Because of this investment, end-users can now choose from a myriad of options to protect against electrical damage to bearings. Adam Willwerth is the sales and marketing manager at Electro Static Technology, 31 Winterbrook Road, Mechanic Falls, ME 04256-5724, 207-998-5140, Fax: 207-998-5143, www.est-aegis.com.  |
