Protecting motor bearings and windings is the first step in preventing motor failures.
By Thomas H. Bishop, P.E., Electrical Apparatus Services Association (EASA)
If you want to make a plant manager’s day, just say: “Production’s down because a motor failed.” Fortunately, such situations can usually be overcome with some relatively simple techniques and straightforward solutions to protect the bearings and stator windings. These two vulnerable components are root causes of the vast majority of motor failures.
Studies show more than half of all motor failures are due to bearing failures—most of which stem from too much or too little lubrication. Other contributing causes include contaminated lubricants, using the wrong lubricant or viscosity for the application and mixing incompatible greases or oils.
Fig. 1. Fluting of the bearing caused by shaft currents from a VFD.
The best way to avoid bearing-lubrication problems is to develop a lubrication program that uses motor and bearing manufacturer guidelines to determine the re-lubrication frequency, and type and amount of lubricant for the motor application, duty (continuous or intermittent), environmental conditions and bearing size. With sleeve-bearing applications, it is also important to mount the motor level to ensure proper lubrication and accurate level checks.
Misalignment of the motor and driven load also contributes to premature bearing failures, as does vibration. The effect of misalignment, for example, increases by the cube of the change, which means an alignment value that is twice the new installation tolerance will decrease bearing life by a factor of 8 (23). On direct-coupled machines, this is a recipe for increased vibration and early bearing failure. On belt-driven applications, poor alignment increases axial bearing load. It may also result in over-tensioning of belts, which places excessive radial loading on the drive-end bearing. The solutions are simple:
- Mount the motor properly (e.g., correct any soft-foot condition);
- Align the unit to new installation tolerance; and
- If practical, isolate the motor from external vibration.
Bearing life can also be shortened dramatically by bearing (or shaft) currents that flow from the stator frame to the shaft and through the bearings. Bearing currents are caused by the magnetic dissymmetry inherent in the frames of very large motors, or results from powering a motor with the characteristic “chopped” output waveform of a variable-frequency drive (VFD). The prevalence of VFDs today in new installations and retrofits has markedly increased the potential for bearing failures due to bearing/shaft currents (see Fig. 1).
While there’s no simple solution to the problem of bearing currents, common remedial actions include: insulating the bearing housings or shaft bearing journals; installing ceramic rolling-element bearings; and using conductive grease and shaft-grounding brushes. Applying filters or reactors to the VFD also helps by reducing the magnitude of the bearing current.
Stator winding failures may run a distant second to bearings as a cause of motor failures, but the extent of the resulting damage, repair cost and downtime is often orders of magnitude greater than that associated with bearing failures.
Mechanical overload is the leading cause of failure for stator windings. Operating a motor at “only” 15% above rated load (i.e., equal to the 1.15 service factor of many motors) can reduce winding thermal life to one-fourth of normal.
A common misunderstanding is that motors can be continuously loaded to their service factor. Actually, service factor capability is intended only for short-term, intermittent use. The solution to mechanical overload is straightforward, but not always easily executed: Reduce the load to no more than the power rating of the motor. For critical applications, it also pays to install overload protection that is sized correctly for the motor rating.
Fig. 2. Symmetrical overheating of the entire winding caused by overcurrent
Thermal overload results from steady-state electrical causes such as over-voltage, under-voltage, and unbalanced voltages (see Fig. 2). A voltage variation of more than 10% from rated, or a voltage unbalance greater than 1% from the average, can cause excessive heating of the windings. Here again, the solution is straightforward: Bring the voltages at the motor to within tolerance. Implementation can be daunting, however, as it may require special transformers or adjusting the load on each phase.
Excess heat is always the enemy when it comes to stator windings, because it can prematurely age the insulation system. For this reason, motors require the ventilation effects of internal and external airflow to extract heat from winding and other component losses. Accumulation of contaminants on the stator windings or externally on the frame and the fan cover (if applicable) may inhibit airflow. Damaged or missing fans also significantly reduce the flow of cooling air.
The solutions for these types of problems include keeping the motor clean and repairing or replacing damaged or missing fans. To ensure that heated air is not recirculated to the motor’s air inlet, it may be necessary to supply a sufficient volume of filtered air from an external source. If the motor is an open enclosure in a dirty environment, consider replacing it with a totally enclosed fan cooled (TEFC) model. It’s much easier and faster to remove dirt from the exterior of a TEFC motor than from the inside of an open-enclosure motor.
Winding failures can also result from transient voltages. These voltage “spikes” can reach levels many times higher than line voltage within microseconds. Transient voltages may occur as a single event (e.g., from lightning strikes, rapid switching of the motor or utility bus transfers) or continuously (e.g., high-frequency transients from the “chopped” waveform output that VFDs use to simulate variable-voltage and variable-frequency AC supplies). While the effect of single-event transients is often dramatic, the partial discharge (corona) from continuous VFD transients can literally eat away the insulation of a stator winding.
Although “prevention” would be the ideal solution for single-event transients, the practical remedy is to install transient-voltage protection in the motor terminal box. Similarly, the only true solution for repetitive transients from VFDs would be a VFD output without transient voltages. Until that becomes available, common preventive measures include installing filters or line reactors and inverter-duty (VFD-rated) motor windings. It is also important to use inverter-duty supply cables and to keep the cable length within the motor manufacturer’s guidelines.
Capturing the reward
Since most motor failures can be traced to damaged bearings or stator windings, it makes sense to leverage techniques and solutions to protect these components. Your reward: longer, more reliable motor life and increased productivity. MT
Thomas H. Bishop, P.E., is a Senior Technical Support Specialist at the Electrical Apparatus Service Association (EASA), in St. Louis, MO, 314-993-2220. EASA (easa.com) is an international trade association of more than 1900 firms in 62 countries that sell and service electrical, electronic and mechanical apparatus.