Protecting AC Motors From Destructive Shaft Currents
- Friday, 01 February 2008
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.