July 21, 2014
Most motor failures are due to bearings or windings, so ensure proper maintenance of these critical components.
by John Malinowski
Motors are critical to the successful operation of most processes today, which means that proper motor maintenance is important to reduce the downtime and increase the productivity and reliability of an operation. Proper motor maintenance consists of several key tests and procedures. The majority of motor failures can be divided into two categories: bearings and windings. Therefore, we will focus on the maintenance of these two critical components.
More motors fail due to bearing problems than for any other reason. The leading cause of bearing failures relates to a variety of issues surrounding lubrication. Anti-friction bearings should be relubricated on a regular basis. The lubrication schedule depends greatly on the motor’s operating environment and service conditions. Operating conditions can be divided into three main categories, including the following:
• Standard Conditions: 8 hours per day, normal or light loading, clean ambient air at 104°F (40°C) maximum.
• Severe Conditions: 24 hours per day operation or shock loading, vibration, ambient air containing dirt or dust at 104 to 122°F (40 to 50°C).
• Extreme Conditions: Heavy shock or vibration, and dust.
Once the operating condition is determined, the recommended relubrication schedule can be determined by using relubrication intervals shown in Table 1.
It’s also important to select the right grease when relubricating the bearings. The motor nameplate or manual will specify which type of grease should be used. Different greases contain different soap bases and additives, and are designed for specific applications. They also have different lubricating characteristics. When different greases are mixed, a reaction can take place between incompatible ingredients, altering the characteristics of the grease and interfering with its ability to lubricate properly. This can often result in a catastrophic bearing failure in only a few hours. In an attempt to provide a guide for mixing greases, see Table 2. If greases must be mixed, the grease manufacturers should be contacted to determine whether the greases are compatible.
While failures may occur due to lack of lubrication, bearings may also fail due to grease that is contaminated by water or other materials. In severe or extreme applications, bearing seals should be considered. The amount of water or dirt present often will determine if a shaft slinger, contact seal, or non-contact seal is required. Installation of a seal on the motor bearings may dramatically extend the life of the motor.
Routine vibration measurements of the entire equipment train should be taken at regular intervals so that problems can be found well in advance of a bearing failure. The frequency at which these measurements are taken will depend on the importance of the equipment being monitored. The foundation and equipment base should be checked regularly for movement or looseness.
Another common cause of motor failures is stator winding failures. To ensure long motor life, it is important the motor operate within the temperature class of its insulation system. The motor should be kept clean and free of particle build up on the frame surface, air inlet, and fans. Totally enclosed fan-cooled (TEFC) motors depend on heat transfer from the frame to the ambient air in order to conduct heat out of the stator winding. A build up of dirt and particles on the frame surface will increase the motor’s temperature rise. Excessive motor temperatures may lead to premature winding failures.
There are several simple tests that can be performed to detect and prevent premature failure of your motor winding. First, motor current can be measured to determine if a motor is overloaded. Measuring current is performed with a portable clamp-on current transformer with an appropriate voltage insulation level. If the equipment has a panel-mounted amp meter, that instrument may be used. Current levels should be less than, or equal to, the nameplate values. Current levels in excess of the nameplate rating should be reviewed. Measurement of voltage imbalance is the second test. Voltage imbalance between phases may increase motor temperature and cause the motor to exceed rated temperature. For most industrial installations, voltage measurements at the motor starter are close enough to the motor so that measurements are satisfactory. The phase-to-phase voltage of all three phases should be measured. Each measured value should be within 10 percent of the motor’s nameplate voltage, and all measured values should be within 1 percent of each other. If voltages are not within 1 percent of each other, the voltage motor horsepower should be derated per Graph 1.
The third test uses non-contact infrared pyrometers. Non-contact infrared pyrometers help identify potential motor temperature problems by identifying abnormal hot spots, bearing problems, air flow problems, and cooling problems.
Standard motors are designed to operate with a maximum total winding temperature rise of 176°F (80°C) by resistance for NEMA class B or 221°F (105°C) by resistance for NEMA class F above 104°F (40°C) ambient. If the surface of a motor exceeds 248°F (120°C) for a class B motor or 293°F (145°C) for a class F motor, it may indicate a winding or cooling problem; the motor winding temperatures should be measured and monitored, or the motor should be taken out of service so that the problem can be fixed. The motor’s insulation class can be found on the motor’s nameplate.
Insulation resistance testing is another common test that can be performed. Motor insulation systems may deteriorate because of contamination, mechanical movement, cracking, attack by solvents, mechanical impact, or many other factors. A megger test places a voltage across the ground insulation. The megger test voltage is usually direct current (DC) and larger than the normal operating voltage, but not large enough to damage the insulation. A megger test is not a pass/fail test, but a measurement of condition. The recommended test voltages for this test and minimum winding resistance are found in Table 4.
While proper motor maintenance is critical, selection of the right motor for the application is actually more important and can prevent many of the failures outlined in this article. Applications in severe and extreme environments require a more rugged motor. Selecting the proper motor for a specific application will provide better system performance and reduce unscheduled down time.