Simple solutions can prevent premature damage
Most motor failures stem from damaged bearings or stator windings. Lack of lubrication, over lubrication, misalignment and bearing (shaft) currents often dramatically shorten bearing life. The culprits responsible for premature failure of stator windings include mechanical or thermal overload, poor ventilation and transient voltages from variable-frequency drives. Fortunately, most premature motor failures can be prevented using straightforward solutions to protect bearings and stator windings.
Bearings are small compared to other major motor components, making them particularly vulnerable to damage and wear. More than half of all motor failures are due to bearing failures, most of which result from too little or too much lubrication. The key to avoiding these conditions is to establish a lubrication program using bearing and motor manufacturer guidelines to determine the frequency and amount of lubrication for the motor application, duty, environmental conditions and bearing size.
Another significant cause of bearing failure is misalignment, the effect of which increases by the cube of the change. An alignment value that is twice the new installation tolerance will reduce bearing life by a factor of eight. The solution is simple: align the motor and driven equipment to new installation tolerances, or better.
Bearing currents are typically caused by dissymmetry in the motor frame or powering the motor from a VFD. Decades ago, bearing currents were only an issue on very large motors due to their inherent lack of magnetic symmetry. VFDs subject these motors to a chopped output waveform. The resulting magnetic dissymmetry produces a current path from stator frame to shaft, and through the bearings at each end.
Although no solution to bearing currents exists, some remedial measures are available. Among the most common of these are insulated bearing housings, ceramic rolling element bearings and shaft-grounding brushes. Other methods include insulating the shaft bearing journal, installing completely ceramic bearings and using conductive grease. Applying filters or reactors to the VFD also helps by reducing the magnitude of the bearing current.
Stator winding failures run a distant second to bearings as a cause of motor failures. Yet the extent of damage, repair cost and downtime from a winding failure is often much greater than for bearing failures.
Mechanical overload is the leading cause of stator winding failure. Operating a motor at 15% above rated load (equal to the 1.15 service factor of many motors) can reduce winding thermal life to 25% of normal. A common misunderstanding is that motors can be loaded to their service factor continuously. Actually, service factor capability is intended for short-term, intermittent use only. The solution to mechanical overload is to reduce the load to no more than the motor’s power rating.
Thermal overload is caused by overvoltage, under voltage and unbalanced voltages. A variation in voltage of more than 10% from rated or a voltage unbalance greater than 1% from the average results in excessive winding heating. The solution is to bring the voltages at the motor to within tolerance. This may require special transformers or adjusting the load on each phase.
Motors require both internal and external airflow to extract heat from windings. Accumulation of contaminants on the stator windings or externally on the frame and the fan cover inhibits airflow. Damaged or missing fans reduce cooling air flow. The solution is to repair or replace damaged or missing fans and to clean the motor. If it’s an open enclosure motor in a dirty environment, replace it with a TEFC model. It’s easier and faster to remove dirt from the exterior of a TEFC motor than from the inside of an open enclosure motor.
Transient voltages reach magnitudes of many times line voltage within microseconds. The ideal solution for transients is to prevent them from occurring. The practical solution is to install transient voltage protection in the motor terminal box. The only true solution for repetitive transients from VFDs is a VFD output without transient voltages. Until that becomes available, common preventive measures include installing filters or line reactors and inverter-duty motor windings.
Fluting of the bearing can be caused by shaft current due to use with a VFD.
|Thomas H. Bishop, P.E. is a technical support specialist at the Electrical Apparatus Service Association (EASA) in St. Louis, MO. He can be reached at (314) 993-2220. EASA (|