Industrial Motors / Predictive Maintenance / Preventive Maintenance

Back to basics: Fundamentals of motor testing

A balance of static and dynamic testing will help prevent catastrophic (and costly) motor failure.

By Timothy Thomas, Hibbs ElectroMechanical

A motor is one part of a machine system, and there are three areas of concern we must address: the power quality supplying the motor, the motor itself, and the driven load. Static electric motor testing looks at the motor’s insulation system, while online or dynamic electric motor monitoring can identify issues regarding power quality and load issues. Both applications are needed to obtain a complete report on the motor’s health.

When evaluating test equipment, it is important to make sure the equipment is capable of meeting all industry test standards. There are established engineering standards that dictate which tests must be performed, how often, and to what voltage level. Adhering to these standards will ensure better diagnostic capabilities and provide proof of potential weaknesses within a machine system in time to remedy the issue before a catastrophic failure. Once a motor has failed, it will have core damage, which may render the motor not repairable. At best, it will be less efficient and less reliable even when rewound.

Static motor testing


Static or offline testing is performed while the motor is not running and can be executed on site, in inventory, or in a motor repair facility. On-site testing is routinely implemented once or twice a year or during scheduled outages. Static testing measures and evaluates the integrity of the insulation system. A motor insulation system has two basic areas of concern: copper-to-ground (or ground wall) and copper-to-copper (or turn) insulation. A successful off-line test set will consist of 1) winding resistance, 2) meg-ohm, 3) polarization index, 4) step-voltage or high potential test, and 5) a surge test. This combination of tests will measure the integrity of the entire insulation system, identifying problem areas before they arise.

Consider a low-voltage motor (600 volts and under). The magnet wire that makes up the motor’s insulation system must be capable of withstanding a 6000-volt AC spike. This voltage level is required and every supplier of low voltage magnet wire must maintain this standard. Modern offline test equipment performs its test using on controlled DC voltage so we can multiply that AC voltage by the square root of two, resulting in a spike resistant level of 8,400 volts DC. The slot-cell liner is rated at more than 20,000 volts DC, providing a combined total of up to 28,400 volts DC insulation to ground.

The weakest part of the insulation system is the copper-to-copper or turn insulation. The magnet wire is coated with a thin film of insulation that is only about 1.5 mm in thickness. Statistically speaking, more than 80% of winding failures begin as a turn-to-turn weakness, and if allowed to run to failure, the fault will cause turns to fuse together, resulting in a failure to ground within the slot. The fused shorted turns will cause a very high current that can be as high as twice locked-rotor current to circulate within the shorted turns.

The vast majority of turn faults begin at the “early” turns – that is, the coils where the “T” leads are connected. That is a result of the spiking that occurs with each startup and shutdown, created by contactor bounce, breaker closure, or another switching device. Motor starts can result in spikes as high as five times the incoming voltage and with as many as six to 10 repetitions, and spiking at shutdown can be even worse.

It is well-documented that a motor’s insulation system begins deteriorating on its first startup, and the rate of deterioration depends upon on many operating and environmental hazards. Starts and stops, load situations, power quality, and mechanical issues such as misalignment – as well as localized environmental issues – all affect the longevity of insulation systems. The real question is not whether a motor will fail, but when.

So what can you do? The answer is periodic offline testing and trending of the motor’s insulation systems. To have an overall picture of motor insulation health, a complete set of tests must be performed on a routine basis. How often you should test calls, again, for considering many issues. Criticality, past history, size, repair and emergency costs, availability of spares, daily starts and stops, ease of testing, available technicians and probably dozens of other issues play a part in determining how often and when you should test.

A complete diagnosis of the insulation system requires a winding resistance test, a meg-ohm test, a polarization test, a step-voltage or high potential test, and a surge test. Each test provides a piece of the health puzzle, and the tests should be performed in that order.

All collected values should be compared with results from previous tests. To accurately compare current and past tests, it is necessary to temperature-correct all collected data. Temperature affects both resistance and meg-ohm values. The rule of thumb is that for every 10 degrees Celsius increase in temperature, the resistance value halves.

The winding resistance test can find high-resistance connections that lead to current unbalances and eventual failures. The meg-ohm test can determine if the motor is grounded or heavily contaminated. The polarization test finds old brittle insulation and deteriorated ground wall insulation. The step-voltage or high-potential test further stresses the ground wall insulation, and when performed at the motor control cabinet, it tests the cables as well. Cables should be able to withstand the spiking and voltage levels the motor sees; damaged cables can be a nightmare.

These tests are known as the DC tests, and once they have been performed, it’s time for the surge test. The surge test is the only test that can locate weakness within the copper-to-copper insulation, where more than 80% of all winding failures begin. The surge test sends pulses at ever increasing voltage levels down one phase at a time looking for weak turn insulation. The pulses are generated in a fashion to simulate the spiking seen at startup and shutdown. We know that the spikes can be as high as five times the line voltage, so the preferred surge-test voltage of twice the line voltage plus 1000 volts is generally accepted as adequate. Weak turn insulation can easily be determined by the generated wave patterns and the digitized pulse-to-pulse comparison.

Dynamic motor testing


Dynamic or online motor testing is performed with the motor running in its normal environment and load condition. Equipment connects directly to the load side of low-voltage motors or in the low-voltage cabinet of medium- and high-voltage motors collecting voltage and current information. This data is analyzed and processed through numerous algorithms, displayed in categorized layouts and diagnosed with preset pass/fail criteria. Red, yellow, and green “flags” quickly and clearly indicate conditions of concern.

Today, we can test motors safely and quickly with the new state-of-the-art equipment available. Besides the usual portable cable, a newer, safer and quicker method makes data collection easier. A device that mounts inside the motor control cabinet with a collection port mounted through the door ensures the technician’s safety and prevents costly shutdowns for equipment connection and removal.

Dynamic testing provides information regarding power quality, the motor (including many mechanical issues), and the load. State-of-the-art equipment will calculate torque not only in a numerical value but also in a spectrum and provide an instantaneous view of “torque ripple.”

Power quality issues, including voltage levels, voltage unbalances, harmonic distortion, current levels and current unbalances are all gathered, displayed, and “graded” against preset values.

Many mechanical conditions, including rotor bar issues, cavitations, bearing faults, and others identified by current and torque spectra, are definable.

Conclusion


A predictive maintenance program is the most cost-effective approach for maintaining a safe and efficient operation. The cost savings from predicting imminent failures in advance more than offsets the cost of administering and implementing a PdM program.

A successful program requires state-of-the-art equipment, qualified technicians, routine monitoring, vigilance, and continuous adherence to details. A successful program also requires motor ownership where one person or a small group of people see all collected data and are allowed or obligated to make decisions regarding the asset’s future.

The best option for implementing an in-house PdM program may be to outsource some or all of the activities to an experienced partner. Selecting a complimentary partner may take some research, but the time spent will be well worth the effort. Experienced vendors may be able to take total control of management of your assets, allowing your staff more freedom to address production and safety issues. Qualified contractors will have all necessary equipment along with skilled, trained personnel to provide you with quality results and the data you need to make intelligent decisions about which equipment needs attention.