When electric motors fail, they bring critical assets to a halt. Yet predicting such problems before they occur is notoriously challenging. Michael Herring, Channel and Platform Development Manager at SKF Electric Motor Condition Monitoring, explains why a comprehensive electrical testing strategy should be part of any organisation’s maintenance processes.
Condition-based maintenance is now widely accepted as a route to improved reliability and lower total lifecycle costs for critical assets. By monitoring the performance of equipment for the early warning signs of potential problems, companies can reduce planned and unplanned downtime, increase plant availability and cut maintenance labour and component replacement costs.
Many companies have built their condition-based maintenance strategies around tools and techniques that identify mechanical problems. Vibration analysis can spot issues with failing bearings or poor shaft alignment in machinery, for example. Oil analysis is useful to identify contamination in lubrication systems, or to spot the telltale signs of excessive wear.
Some of these problems afflict motors too. Vibration analysis can identify 70 to 75 per cent of the issues that can eventually cause motor failure, including problems with bearings, unbalanced loads and shaft misalignment. To identify the remaining 25 to 30 per cent of potential problems, however, teams must make use of electrical testing techniques. These tests fall into two broad categories: static tests, conducted on a motor when it is not running, and dynamic tests, conducted on the motor in service.
Static testing uses industry accepted standards to identify insulation weaknesses within the motor windings. A sequence of tests is conducted to measure the winding resistance (motor circuit), insulation resistance (insulation to ground) and turn-to-turn insulation. Two of these tests are performed at higher voltage levels to ensure the dielectric strength of the insulation can withstand the switching surges that motors typically see on start-up and shut down. Approximately 80 percent of the electrical failures in motors begin with a failure in the thin insulation around individual wires in the motor coils. As this insulation degrades, the inrush of voltage during motor starting can cause arcing, further degrading the insulation and creating a conductive carbon path that will eventually lead to a short circuit and eventual failure. A special test known as a surge test is used to detect weak turn-to-turn insulation before a short occurs, giving the team time to plan when to replace the motor – thus avoiding loss of production.
The tests may seem simple, but they can have a huge effect on productivity and maintenance. One pulp and paper company has a plant with 800 motors – aiming to test each one twice a year. It began using SKF Baker AWA-IV testers to perform these tests, which helped it to cut its annual motor costs by nearly one-third. Among the many problems it found – and solved – were blown holes in the insulating boots that covered cable lugs, cables partially shorted to ground in motor control equipment and developing turn-to-turn shorts in stator coils.
By its nature, static testing requires motors to be taken out of service for approximately twenty minutes, and even a six month test frequency leaves plenty of time for problems to establish themselves. Dynamic motor test equipment, by contrast, collects voltage and current information from the motor during normal operation and uses a set of algorithms to assess the quality of the power supplied, certain issues within the motor such as rotor bar condition, and a host of mechanical issues with the load. This equipment can also provide torque information, both in real time and as a spectrum across a pre-determined frequency range. This provides a wealth of information, as well as helping to distinguish a mechanical problem in the wider machine from an electrical problem in the motor.
The payoff from the effective use of dynamic test equipment can be dramatic. Technicians at a US coal-fired power plant used an SKF Baker EXP4000 test unit to investigate why one of three submerged water pumps was requesting less input power and – as a result – running faster. 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 of interest had a torque level of around 75 per cent that 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 (around $3.5 million), which would have otherwise have resulted from falling output.
While dynamic testing is most often performed using portable test units, there is an increasing trend towards permanently installed online motor analysers such as SKF’s NetEP, which monitor multiple motors on a continuous basis.
These systems allow data to be accessed centrally, at any time, which helps technicians and managers make better decisions – and much more quickly. As an added bonus, these types of online dynamic analyser can improve safety, by removing the need for technicians to visit hazardous areas. Alerts can flag up the need to investigate or replace a critical motor, while trend data from months of monitoring can help to prioritise maintenance planning.
