Pumps & Systems, September 2007
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:
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Most motor bearings are designed to last for 100,000 hours, yet motors controlled by VFDs can fail within one month (720 hours).
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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.
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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.
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The largest U.S. motor manufacturer cited eliminating drive-related motor failures as its number one engineering challenge.
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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.
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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.
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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.
Figure 2. 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.
Figure 3. A frosted bearing race wall 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.
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.
Figure 4. In a fluted bearing, 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.
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
