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:
Most motor bearings are designed to last for 100,000 hours, yet motors controlled by VFDs can fail within one month (720 hours).
An HVAC contractor recently reported that of the VFD-controlled 30-hp to 60-hp vane axial fan motors he installed in a large building project, all failed within a year (two within six months). Repair costs totaled more than $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 U.S. motor manufacturer cited eliminating drive-related motor failures as its number one engineering challenge.
Almost a dozen blogs now on the Internet focus on discussing the problems presented by VFD-induced shaft currents, sharing information and experiences, and suggesting solutions.
Motor failures caused by VFD-induced shaft currents result in hundreds of thousands of hours of unplanned downtime in the U.S. each year. These failures affect the performance and mean time between failure (MTBF) of the OEM systems in which they are used.
With recent motor-price increases (approximately 16 percent over last year) due to rising copper prices, this problem will become even more costly.
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.
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:
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.
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.
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.
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.
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:
They are subject to wear because of the mechanical contact with the shaft.
They collect contaminants on their metal bristles, which destroys their effectiveness.
They are subject to oxidation buildup, which decreases their grounding effectiveness.
They require maintenance on a regular basis, increasing their cost.
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:
1. Scalability. The technology is scalable to all sizes of NEMA-frame and larger motors regardless of shaft size or application. Introduced in 2005, these rings were designed for motors with shafts from 0.311-in to 6.020-in, including NEMA and IEC frames as well as high-horsepower AC and DC motors.
They have been applied to power generators, gas turbines, wind turbine generators, AC traction and break motors, cleanrooms and HVAC systems, and a long list of other industrial and commercial applications.
2. Installation and maintenance. An SGR can be easily installed by sliding the ring over either end of the motor shaft and locking it in place with simple screw-on mounting brackets. Because no machining is required, the ring can be installed in minutes - even in the field.
Once installed, the ring requires no maintenance. With no parts to wear out, it can last as long as the bearings. A split-ring design allows installation around the shaft without disassembling attached equipment.
3. Low cost and high return on investment. Typically, an AC motor coupled with a VFD costs from $2,400 to $100,000 or more and may be part of a manufacturing process that generates revenues from $10,000 to $1,000,000 or more per hour. The cost of installing an SGR in a VFD/AC motor system can be very low when compared to the cost of the overall system, usually less than 1 percent of the equipment cost.
By preventing electrical damage to bearings, an SGR can protect the VFD system from the costly downtime of unplanned maintenance. In some production applications, even a momentary stoppage due to motor failure can cost more than $250,000, excluding the cost of repairing the motor.
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.
Pumps & Systems, September 2007