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Current affairs: dynamic motor testing

Current affairs: dynamic motor testing

Your electric motors are already producing a rich stream of information on their own health, and that of the wider machine; you just need to know how to read the signs, says Michael Herring of SKF.

Motor health matters. The unexpected failure of an electric motor in a complex manufacturing line can bring it to an abrupt halt. That means delays, lost production and possibly scrapped products. It’s no wonder that the prevention of such failures is an increasing priority for operations and maintenance teams.

Today, many companies are adopting condition-based maintenance strategies in an effort to reduce the occurrence of both planned and unplanned down time. These approaches are based on an understanding of the likely causes of failures, and the monitoring of equipment in service to spot the earliest symptoms of potential issues. For motors, a condition-based approach will often involve a combination of temperature and vibration analysis to identify problems in bearings, and periodic static electrical tests to find electrical issues, like insulation failures in motor windings.

Alongside these techniques, there’s another approach that can provide powerful insights into motor condition. Dynamic, or in-service, testing is the measurement of electrical flows in a motor during its normal operation. Dynamic testing provides information on power quality and unbalances or distortions in voltage and current levels. Monitoring these characteristics is important. Even a small imbalance, together with minor harmonic voltage distortion can severely affect motor power and performance. And such issues can’t be detected with simple multimeters and amp probes. To be really effective, dynamic testing should be performed more often than off-line static testing. Testing is fast and non-intrusive, and typically conducted every few months – a similar frequency to that used for vibration analysis. 

Dynamic electrical testing can also provide insights into problems beyond the electrical characteristics of the motor itself. Many mechanical issues with a motor and its system are discernible from the data that dynamic analysers can collect. Torque and current spectra, for example, can help to determine a number of mechanical issues, including bearing faults, looseness (vibration or misalignment) and eccentricity. By considering a motor as part of a motor/machine system with three aspects (power source, load source, and the motor itself), a good dynamic analyser provides relevant condition information about all three. 

Many motor problems are created by adverse or mismatched loads, or by poor supply power. By measuring a full range of motor performance characteristics dynamic analysis can allow maintenance staff to separate mechanical issues from electrical ones, and help them decide whether repair or replacement would be the more appropriate mitigation approach. 

One challenge with electric motors, for example, is tracking the condition of their rotors. Today’s dynamic motor analysers help predict rotor bar failures or potential failures if the load is relatively steady. A pump, fan or blower operating at a steady frequency will show very clear rotor bar signatures that make rotor fault diagnoses easier than before. During normal operation, a motor’s rotor is stressed by its load. Torque waveform analysis provides a picture of those stresses, which when they exceed specified levels, can be an indicator of a number of mechanical problems. Pump cavitation and belt flapping, for example, are easily seen in a torque waveform signature. Motor analyser manufacturers are continually improving the ability of test equipment to discern other mechanical motor system issues earlier and with greater accuracy.

The payoff from faster, more accurate diagnosis of mechanical problems can be dramatic. Technicians at a US coal-fired power plant, for example, used an SKF EXP4000 dynamic motor analyser to investigate why one of three submerged water pumps was requesting less input power and running faster than the others. They used the device to capture the torque signature of all three pump motors, giving a snapshot of the load demands of each one. The pump in question had a torque level of around 75% of a healthy pump, but the torque was also fluctuating severely. It was examined by a diver – who discovered that the end bell had fallen off, causing the problem. The company estimates that identifying the problem – and repairing the pump – prevented losses of almost £2.5 million, which would have otherwise have resulted from falling output.

Dynamic testers are usually portable devices, allowing them to be located close to the motor under evaluation or its control cabinet. Emerging new technology has spawned an alternative dynamic motor analysis tool known as an online motor analyser that is permanently installed and connected to multiple motors via the buses in their control cabinets. These devices perform all of the same tests as portable dynamic motor tester, but on a continuous basis. As an additional benefit, their output is available remotely, allowing a company to view the status of a given motor from anywhere in the world.

This technology enables maintenance professionals to make better decisions faster than the “spot-testing” method of testing that is characterised by route-running once or twice a year. It captures information that can’t be obtained in a single testing session performed with a portable tester. Alerts can be set to flag maintenance professionals of the need to investigate and/or replace critical motors the online analyser is monitoring. Moreover, the trend data from months of monitoring provides valuable insight that aids maintenance planning and helps prioritise resources and actions. Finally, because the monitoring is effectively performed remotely, online dynamic analysers all but eliminate safety hazards associated with testing in-service motors in the field.

Dynamic monitoring also provides efficiency information, allowing maintenance professionals to make informed decisions about whether to repair an ailing motor, or replace it with a higher efficiency version. Since motors are often the largest energy consumer at a manufacturing site, even single figure percentage increases in efficiency can result in annual savings of many thousands of pounds.

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