The use of variable-frequency drives (VFDs) to control AC motors has increased dramatically in recent years. In addition to low operating cost and high performance, they save energy. Today, the challenge facing system designers and engineers is to minimize damage to AC motors from shaft current.
Existing strategies to mitigate this damage have been costly, technically unfeasible, or narrow in application. However, a new technology employs a circumferential ring of conductive microfibers to discharge harmful currents and provides a low-cost solution.
Path of Least Resistance
From the onset of operation, a VFD induces destructive voltages that build up on the motor shaft until they find discharge paths to the frame (ground). In most cases, the motor bearings present the path of least resistance. Once voltage is sufficient to overcome the resistance of the oil film layer in the bearing, shaft current discharges, causing electrical discharge machining (EDM) pits and fusion craters in the race wall and ball bearings. This phenomenon continues until the bearings become so severely pitted that fluting, excessive noise, and failure occur.
This is not a small problem. Consider:
- Most motor bearings are designed to last for 100,000 hours, yet motors controlled by VFDs can fail within one month.
- An HVAC contractor recently reported that, of the VFD-controlled 30 to 60 HP vane axial fan motors he installed in a large building project, all failed within a year (two within 6 months). Repair costs exceeded $110,000.
- Several large pulp and paper companies surveyed noted that the VFD-controlled AC motors used in their plants typically fail due to bearing damage within six months.
- The largest United States motor manufacturer has cited eliminating drive-related motor failures as its number-one engineering challenge.
- Motor failures caused by VFD-induced shaft currents result in hundreds of thousands of hours of unplanned downtime, in the United States alone, each year. In addition, these failures affect the performance and mean time between failure (MTBF) of the original equipment manufacturing (OEM) systems in which they are used.
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 volts or more (peak-to-peak), are easily measured by touching an oscilloscope probe to the shaft while the motor is running.
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. 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.
The frosting eventually produces 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, in heating, ventilation, and air-conditioning systems, is magnified and transmitted by the ducting. 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 stem from bearing failure, almost 80% of these are caused by electrical damage to bearings.
New bearing race: Viewed under a scanning electron microscope, a new bearing race wall is a smooth surface. As the motor runs, a track eventually forms where the bearing ball contacts the wall. With no electrical discharge damage, this type of mechanical wear would be the only cause of degradation.
Pitting: After 5400 hours of continuous use in a VFD/AC motor system, early damage typically takes the form of pitting. These fusion craters increase in number and size as each cycle of induced voltage discharges from the shaft through the bearings to the frame and ground. Soon the entire race is covered with millions of pits. As new fusion craters form over old ones, eventually a “frosted” surface — easily visible to the naked eye — appears.
Fluting: In a phenomenon known as fluting, the operational frequency of the VFD causes concentrated pitting at regular intervals along the bearing race wall, forming a “washboard” pattern. This pattern results in vibration and noise. In an HVAC system, this noise can be transmitted throughout a facility via air ducts.
Electrical damage to VFD/AC motor bearings begins at startup and eventually produces bearing failure. To prevent such damage, 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.
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, be low-cost, and could be applied across all VFD/AC motor applications.
Few current technologies to protect AC motor bearings from shaft current damage meet all the criteria of effectiveness, low cost, and application versatility.
- 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 is difficult to implement, 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 arrangement 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 damage bearings anyway.
- Ceramic bearings: 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 is very costly, and in most cases motors with ceramic bearings must be special ordered and have long lead times. Moreover, because ceramic bearings and steel bearings differ in compressive strength, ceramic bearings often must be resized to handle mechanical static and dynamic loading.
- Conductive grease: In theory, because this grease contains conductive particles, it would provide a lower-impedance path through the bearing and bleed off shaft current through the bearing without the damaging discharge. Unfortunately, the conductive particles in these lubricants 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 NEMAframe motors. However, these brushes are subject to wear because of the mechanical contact with the shaft. Their metal bristles collect contaminants that destroy their effectiveness, and are subject to oxidation buildup.
- Shaft grounding ring (SGR): This approach uses a ring of specially engineered conductive microfibers to redirect shaft current and provide a reliable, very low impedance path from shaft to frame, bypassing the motor bearings entirely. The ring uses ionization principles 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, the shaft grounding ring channels route currents around the AC motor bearings bearings and protect them from electrical damage. The technology can be applied to any AC motor.
The shaft grounding ring technology is scalable to all sizes of NEMA-frame and larger motors, regardless of shaft size or application. It is designed for motors with shafts from 0.311 to 6.020 in., including NEMA and IEC frames as well as high-horsepower AC and DC motors. The technology has been applied to power generators, gas turbines, wind turbine generators, AC traction and break motors, cleanrooms and HVAC systems.
The shaft grounding ring is simply slid over either end of the motor shaft and locked in place with simple screw-on mounting brackets. It can be installed in minutes, and requires no maintenance.