Electric motor maintenance best practices

A proper preventive maintenance plan must cover the electric motors and controls in service within the plant. A correctly designed and implemented program will help ensure many hours of operation without unexpected downtime of any equipment. 

Following these suggestions will set the plant on the path to a well-designed program, which should include the controls portion of the motors system.

Start with a plan

A good plan compares the costs of preventive maintenance (PM) and unexpected downtime. It also includes the cost of proper test equipment and tools. Training for in-house teams is needed for testing to be efficient and thorough. In some cases, it may be more cost-effective to use outside sources to perform the more complex tests.

The plan must be documented; plan implementation can be quantified to ensure that costs are minimized while the systems are kept in a healthy operational state.

Motor and control center maintenance

Begin maintenance with the motor and control center (MCC). This step aims to check component health, including the motors, starters, protective equipment, and circuit breakers (see Figure 1). The following tests should be performed annually or more frequently if the equipment is subjected to severe environmental issues such as heat or vibration. This will improve motor performance and help with UL508 conformance.

Recommended testing and inspection

1. Tighten all electrical connections as the device manufacturer recommends using a calibrated torque wrench.
2. Perform thermal imaging on each device—replace any device that shows hot spots. Be sure to record the results for future reference.
3. Conduct an insulation test on all incoming and outgoing cables. 
4. Conduct an insulation resistance test of all contactors. Test from phase to ground, phase to phase, and across any open contacts. Ensure that they meet the manufacturer’s specifications.
5. Clean all filters and remove any accumulated dust within the enclosure. Do not use compressed air to clean; use a vacuum or soft cloth. Avoid using spray contact cleaners; they can cause more issues than they resolve with the pole faces.
6. Check all disconnect handles and breaker mechanisms for wear and replace them as required.
7. Conduct a contact resistance test using the four-wire DC voltage drop method.
8. Measure the resistance across any fusing and replace the fuses outside the manufacturer’s recommendations.

If you do not have the manufacturer’s recommendations on any of the above tests or inspections, use InterNational Electrical Testing Association (NETA) recommendations. (For more information visit www.netaworld.org.)

Component maintenance

This step ensures the motor itself is healthy and in good operating condition. Proper practices are vital for many years of problem-free motor operation. Recording all maintenance steps and test results is essential for setting a baseline and seeing trends that determine when a motor is failing.

Bearing lubrication requirements vary greatly from motor to motor. Lubrication intervals are affected by the motor type, its operating rpm and, most importantly, the environment. Severe environments will require more frequent lubrication.

The proper procedure is to clean the grease fitting first, open any purge fittings, and wipe off the grease fitting when done.

It is best to use the manufacturer’s recommended grease, but if that isn’t possible, ensure that the one used is compatible with the original grease.

Never overlubricate. This is one of the top reasons for premature motor failure.

Inspections   

Simple inspections, such as the following steps, can provide favorable results in motor health (see Figure 2):

1. Clean excessive dust, dirt, or debris that may have accumulated on the motor’s frame. A clean motor dissipates heat properly. If the motor is located in a severe environment and nearly impossible to keep clean, consider using protective motor covers.
2. As with the MCC testing, check that all electrical connections are tightened to the manufacturer’s recommendations using a calibrated torque wrench.
3. Visually inspect all power cables and perform a resistance test. Replace any worn or damaged cables.
4. Periodically remove critical motors on variable frequency drives (VFDs) from service. If bearing inspection reveals pitting, this signifies shaft current leaks caused by the pulse with modulation (PWM) sine wave of the VFD. Install shaft grounding rings to mitigate this problem. Consider upgrading to VFD-rated cable and adding shaft grounding devices if bearing damage is an ongoing issue.                     

Electrical tests

The following electrical tests need to be performed. For accuracy, the tests must be conducted by qualified technicians, and all results must be recorded:

1. Winding phase-to-phase resistance test—Use a low ohm meter; the resistance of each winding should be the same.
2. IR-to-ground test—This measures the resistance between each winding and the frame ground. Use an insulation resistance tester because of the high resistances expected. It is important to remember that temperature is a factor in these tests, and the winding temperature needs to be part of the record, so the resistance can be adjusted to the 40 °C standard.
3. Surge tests—Check the incoming and outgoing surge values. This test will point out insulation weakness, shorts in the windings, and internal damage or improper connections within the motor. A multifunction surge tester is needed to perform this test.
4. High potential (hi-pot) test—This overvoltage test is used as a stress test for the insulation. Voltages as high as 75,000 V can be reached. A dc hi-pot tester is required.
5. Polarization index test—This insulation-to-ground test is performed over 10 minutes. When completed, the ratio of the 10-minute value over the 1-minute value is calculated and recorded. The results can predict insulation failure. This test needs to be conducted to IEEE 43-2000.

Thermal imaging

Thermal imaging is an important component in preventive maintenance. It can point out many issues that other tests cannot easily do.

Thermal surveys of the motor should be scheduled based on its operating characteristics. However, this should be done at least once annually.

All surveys need to be recorded and a baseline set. Reviewing the most recent tests to the baseline will present any impending problems with the motor (see Figure 3).

Well-trained inside technicians can be used for all the testing and inspections discussed. Each organization is different, with specific needs and cost controls to consider. But in many cases, using an outside certified testing company is more cost-effective. This should be explored if training and equipment costs are prohibitive.

What does the future hold?

The advent of the connected world and the industrial internet of things (IIoT) has caused incredible changes to predictive maintenance. Data collection and thorough analysis are essential for today’s smart factories to run at top efficiency. Accurately planning maintenance is based on information directly provided by machines.

A complete discussion of this process is beyond the scope of this article, but here are some highlights:

Condition monitoring devices are essential to a modern predictive maintenance data collection plan. Two kinds of devices provide feedback from motors to the data collection points and then the data analysis software:

1. Vibration sensors collect data based on the vibration of the motor. Over time, this will show a trend line that can predict bearing failure, shaft alignment issues, or mounting integrity.
2. Temperature sensors collect data based on the temperature of the motor. Winding sensors and bearing sensors each provide data showing trend lines in the motor’s health.

Machine learning software takes all the data collected and sets the baseline for the motor and machine. This facilitates the maintenance schedule based on actual machine usage as opposed to general manufacturer suggestions.

When the software notices that a temperature or vibration threshold has been exceeded, different warnings or alarms can be issued to:

1. Turn on a stack light to inform the operator of a problem.
2. Email the maintenance staff about the issue.
3. Sound an alarm and shut down the machine.

Remember that a well-planned, executed, and recorded preventive/predictive maintenance program will ensure a long, reliable life for motors and control systems. Start with a plan, document, routinely maintain the MCC and other components, and leverage predictive maintenance tools.