Why electric motors fail

Sunday - 12/11/2017 11:12

Why electric motors fail

Despite the dependence on these motors and their vital roles in industry, electric motors can fail for many reasons leading to losses in productivity and associated profitability.
Electric motors top the industrial landscape as the most widely deployed asset driving processes and productivity. In fact, industrial motor use accounts for about 25 percent of all electricity usage nationwide, according to the U.S. Department of Energy. Despite the dependence on these motors and their vital roles in industry, electric motors can fail for many reasons leading to losses in productivity and associated profitability.
 
Nevertheless, the health of electric motors might not be perceived as a top priority in day-to-day operations, even where predictive maintenance programs have been implemented to make timely maintenance fixes on critical machinery before catastrophic failures can occur.
 
The universal reliance on electric motors underscores the inherent value in detecting, identifying and evaluating operating abnormalities. Without proper attention, the likelihood of failure increases, and it will likely come without warning and at an inopportune time.
 
How and why do electric motors fail, and what can be done to optimize their performance? Possible causes include turn-to-turn insulation faults, premature bearing failure modes, and even improper installation, especially in applications that involve small motors. As safeguards, testing protocols, technologies and best practices can enhance motor reliability.
 
 
Testing & monitoring
 
Identifying hidden problems in electric motors that can be repaired or replaced can optimize a motor’s efficiency and performance and save time, money and headaches.
 
Static testing, which is performed when a motor is off-line, can determine the condition of a motor’s insulation and circuit, while dynamic motor analysis, which is performed while a motor is operating, can pinpoint issues related to power quality, motor performance and load. Together, these tests will paint a picture of motor health and provide information to accurately diagnose and predict imminent failures.
 
Industry studies have shown that 80 percent of motor failures electrically develop from turn-to-turn, end-turn insulation system faults. In general, insulation in a motor begins to wear as turns rub together from movement generated during motor startup. Insulation can further degrade due to the introduction of chemical deposits that are usually found when a motor is over-greased. In the end, a compromised insulation system in an electric motor will increase the chances for failure from an electrical perspective.
 
While the dielectric strength of a new motor is high, normal aging can be expected from thermal, chemical and/or mechanical causes. Dielectric strength will then diminish until in-rush voltage causes electrical arcing, and every time the motor starts and stops it will become more severe until it fails. In short, insulation deterioration gets worse, dielectric strength drops below operating voltage, arcing action causes high levels of induced current and high heat, and the surefire outcome is rapid failure, sometimes within minutes.
 
Static testing supported by enabling technologies can help determine whether a motor is running toward electrical failure. Testing equipment can focus on off-line testing domains including:
 
Winding resistance test confirms windings are balanced with no connection issues.
Meg-ohm test verifies ground wall integrity and the presence of moisture and/or contamination.
 
Polarization test determines winding cleanliness, potential thermal degradation, and contamination issues indicating embrittlement and insulation deterioration.
 
Ramp voltage/step voltage tests highlight ground wall integrity and contamination issues and are useful in determining severity of insulation breakdown.
Surge test identifies turn-to-turn insulation integrity, coil shorts and inductance.
 
Resulting test values can further be trended over time with the application of fully automated route-based testers that can track the various domains throughout a motor’s service life.
 
Looking at dynamic motor analysis, a toolbox of technologies for on-line testing includes portable analyzers and network-connected monitor systems. They have been engineered to measure the following: power quality (including voltage levels, voltage unbalance and any distortion of incoming power), motor performance (how hard a motor can work through speed torque and operating temperature), overcurrent or current imbalances, torque characteristics (whether a motor is over- or underworked and related levels of energy consumption), connections (verifying all phases are operating symmetrically without unbalance) and variable frequency drives (usually installed to improve overall plant energy consumption and efficiency but liable to create bad power feeds leading to premature wear and possible failure).
 
With new technologies, a network-connected and permanently installed electrical processor analysis system can automatically monitor motors from anywhere in the world and continuously collect data on their health and performance. Such a system can monitor up to 30 motors per unit, measure more than 40 electrical parameters, compare results with standard limits and display alerts if limits have been exceeded.
 
From a big picture perspective, a recommended bundled electric motor management program that embraces all these testing protocols can help reduce unscheduled motor-driven downtime, detect root causes of motor failure, minimize troubleshooting time, assist in maintenance and quality assurance objectives, and save energy and related costs.
 
Bearings & failure modes
 
Bearings are key components of electric motors. They support and locate the rotor, keep the air gap small and consistent, and transfer loads from motor shaft to motor frame. Electric motors typically incorporate a locating and non-locating bearing arrangement to support the rotor radially and locate the rotor axially relative to the stator. Locating bearings position the shaft and support axial loads, while non-locating bearings permit shaft movement in the axial direction and compensate for overload conditions when thermal expansion of the shaft occurs.
 
Proper selection, installation and maintenance of bearings can help contribute to extended service life in electric motors. However, bearings can fail prematurely for reasons including electrical erosion, inadequate or unsuitable lubrication, and/or loads that are heavier than expected or too light.
 
Electrical erosion, also known as electric arcing, develops when a stray current uses a bearing as its path to ground. Its most frequent causes include asymmetry in the motor’s magnetic circuit, unshielded power cables and/or fast-switching variable frequency drives.
 
Once electric arc bearing damage has begun, excessive vibrations, increased heat, increased noise levels and reduced effectiveness of the motor bearing’s lubricant will contribute to shorten a bearing’s service life. The extent of the damage will depend on the amount of energy and its duration, but the result is usually the same: pitting damage to the bearing’s rollers and raceways, rapid degradation of lubricant and premature bearing failure.
 
One solution for resolving electric arcing problems is to insulate the bearings from the shaft currents. Specialized ceramic coatings can be applied on the outside or inside diameter of a bearing to provide the insulation properties and prevent currents from flowing through the bearings. As another solution, hybrid bearing designs substitute ceramic balls or rollers for the metal rolling elements within a bearing. This effectively insulates bearings from the inside, and adding value, hybrid bearings possess a higher speed capability and can sustain longer service life than all-steel bearings in most motor applications.
 
Inadequate lubrication will cause either surface distress or abrasive wear in a bearing, substantially reducing its service life. If the lubricant film between a bearing’s rolling elements and raceways is too thin due to inadequate viscosity or contamination, the surfaces will no longer be fully separated, and that metal-to-metal contact can cause potentially dire consequences.
 
To correct it, first check whether the proper lubricant is being used for the bearing and that re-greasing intervals are adequate for the application. If the lubricant contains contaminants, check the seals to determine whether they should be replaced or upgraded. In some cases depending on the application, a lubricant with a higher viscosity could be needed to increase the oil film.
 
Insufficient bearing load occurs when a motor runs without a load, increasing the risk of damaging bearings because they always require a minimum load to function well. The damage will appear as smearing on the rolling elements and raceways over a long period of time. Unless preloaded bearings are used, external loads should always be applied to the bearings to keep this from happening.
 
Motor installation & setup
 
Unless an electric motor is installed and set up properly, it will not realize its expected service life, even if the motor integrates high-quality components. Typical installation and setup errors, especially as they relate to a motor’s bearings, include the following:
 
Poor alignment. If the shaft of an electric motor is not aligned carefully with the shaft of the driven component, the bearings in both applications will be subjected to additional forces that could significantly reduce bearing service life in both pieces of equipment. Precision alignment tools can confirm alignment is appropriate.
 
Unbalance. Substantial unbalance in the driven unit can be transferred to a motor, and these vibrations will shorten bearing service life. Checking the vibration level of the driven unit can point to the root cause of the problem.
 
Excessive belt tension. This is commonly encountered in premature bearing failures. In most cases, the excessive loads from the belt cause unnecessarily high loads on the motor bearings to significantly reduce the service life of the bearings and the belt. Higher loads also mean higher operating temperatures, which can reduce the effectiveness of lubricant and consequently bearing service life. A best practice is to check to make sure that the belts have the correct tension and that simple tools are available for the job.
 
Without electric motors, most machines would be out of service. Conducting proper static and dynamic testing, discerning adverse influences on motor bearings, and properly installing and setting up motors at the start can go a long way toward optimizing the health and performance of motors — and the productivity processes they drive.
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