Electrical testing – the missing link in motor condition monitoring?
2017 January 31, 09:00 GMT
After bearing problems, electrical faults are the most common cause of motor failure. Yet many organisations perform only the very basic of electrical tests, which are incapable of detecting all of the potential failure modes. Michael Herring, Channel and Platform Development Manager, Electric Motor Condition Monitoring at SKF describes the vital role that modern electrical testing technologies play in predicting and preventing motor failures.
Monitoring the health of an electric motor should entail measurement of both mechanical and electrical parameters in order to gain a comprehensive assessment of its operating condition. Vibration signature analysis, thermography and even oil analysis will provide early indications of mechanical problems, but these are unlikely to reveal the onset of insulation failure, phase imbalances, torque anomalies and the like, which will have a significant impact on motor performance and life expectancy.
Vibration measurement is commonly applied in motor condition monitoring programs and this, alone, can identify approximately 70 per cent of the issues that might indicate pending failure, including problems with bearings, unbalanced loads and shaft misalignment. Identifying the remaining 30 per cent of potential problems requires the use of electrical testing techniques, which fall into two broad categories: static tests, conducted on a motor when it’s not running, and dynamic tests, conducted on the motor in service.
Some 80 per cent of the electrical failures in motors begin with a failure of the thin insulation covering the motor winding, otherwise known as the turn-to-turn insulation. As this insulation degrades, the inrush of voltage during motor starting and stopping can cause arcing, further degrading the insulation and creating a conductive carbon path that will ultimately lead to a short-circuit and motor failure.
Static electrical testing of motors uses industry-accepted standards to identify these weaknesses within the motor windings. Testing can be automated using specialist portable instrumentation such as the SKF Static Motor Analyzer Baker AWA-IV, which removes operator error and inconsistencies arising from variations in test sequencing.
The test sequence is defined by the AWA-IV software and is configured to measure winding resistance and verify the dielectric strength of the insulation system via a series of specific test routines. One of these is the DC step voltage test, carried out over five or more incremental voltage steps, with the leakage current being plotted for each. Clean, dry insulation will show a linear plot, while insulation weakness or contamination is revealed by a non-linear increase in leakage. A surge test is also carried out to detect weak turn-to-turn and phase-to-phase insulation. Such weakness can be present without the motor displaying any signs of a problem during operation, yet the winding resistance and megohm tests cannot identify such problems. Static test results are clear and unambiguous, and produce a specific result that requires little interpretation.
Static testing should also be performed annually on spare motors that are being held in stock. These motors often go unused for many years, yet are held solely for use when a production motor fails, or requires replacing. If the condition of a stock motor is unknown prior to installation, there is a risk of failure during production.
Dynamic motor testing involves the measurement of the three phase voltages and currents which are then processed via a set of algorithms to assess the quality of the power supplied to the motor, certain issues within the motor, such as cracked and broken rotor bars, and the condition of the driven load. It can be undertaken using instrumentation such as the SKF Dynamic Motor Analyzer - EXP4000, which is capable of multiple tests to determine power condition, motor health, motor load and energy profile. As many as 40 parameters are monitored and recorded, including voltage, current, frequency, torque, speed and percentage load. The EXP4000 is also able to monitor the performance of inverter driven motor applications.
Also worth noting is motor current signature analysis, which, unlike dynamic analysis, measures only the phase currents to detect cracked and broken rotor bars and other mechanical issues. Also performed with the EXP4000, this test is normally carried out when the motor cannot be stopped to allow voltage connections to be made, or in the case of high voltage motors, when low voltage secondary connections are not available in the panel to allow safe connection of the equipment.
While these tests are relatively easily conducted by trained in-house staff, the task can be outsourced to specialist providers such as SKF, fielding engineering teams to undertake surveys of plant-wide industrial motor installations. Comprehensive reports are made available to customers advising them of any actions or remedial work that may be required.
And for those sites that are either difficult to reach or pose a hazard to personnel, SKF can also offer remote dynamic motor monitoring based on the SKF Online Motor Analysis System – NetEP, which is capable of monitoring up to 32 motors, 24 hours a day, seven days a week, with data being sent directly to SKF for analysis. Multiple NetEP devices can be networked, to increase the number of motors being monitored.
Modern instrumentation and specialist services mean there really is no excuse for companies to leave the health of their critical motors to chance.
The above technical article was originally published in the January 2017 edition of Automation magazine.