Article supplied by Avo New Zealand’s General Manager Sales & Marketing, Daniel Hurley.
Energy costs are a major part of the operating expenditure for any plant or facility, and in almost every case, electric motors are among the largest consumers of energy. Properly implemented testing and monitoring of motor performance, which will help to improve reliability and extend the life of motors, is therefore an excellent investment, as it will reduce overall operating costs.
To be truly effective, condition monitoring must include not only tests like vibration analysis, oil analysis and thermography, which mainly detect mechanical problems, but also structured testing regimes for electrical faults. All too often, other than the basic tests, electrical testing is deemed unnecessary. This is unfortunate as studies have shown time and again that after bearing failure, electrical winding faults are the most common mode of motor failure. A structured electrical testing regime is, therefore, not simply an optional extra – it’s a vital requirement for achieving plant reliability.
The insulation resistance test – which is often called a Megger test: after the trademarked name of one of the most popular instruments used to perform it – has long been the tool of choice for engineers, and this simple test is often the only electrical test performed on a motor. There’s no doubt that this test has a valid role, but it is simply not capable of detecting all of the faults that can occur within the windings of a motor.
Let us now look in more detail at electrical tests that should form part of an effective motor condition monitoring program.
Static motor tests
Static, or offline, motor tests should ideally be carried out in the sequence shown below:
- Winding resistance test – This test is used to detect loose connections and dead shorts.
- Insulation resistance (Megger) test – this test will show if the motor is wet or contaminated, or if it has a dead short to earth. An insulation resistance test cannot, however, confirm that a motor is in good condition, as it does not test the entire insulation system.
- Polarization Index (PI) test – the PI test is a 10 minute test that is used to measure/quantify (interfacial) polarisation effects occurring in the ground wall insulation of a motor or generator.
- DC Step voltage test – This test is typically performed at twice line voltage plus 1,000 volts. The voltage is increased in steps – ideally five steps or more – and the leakage current plotted. Good insulation to earth will show a linear plot whereas a non-linear plot suggests contamination or an insulation weakness at that voltage. A DC step voltage test provides considerably more information than a DC hipot test (see next point) and is therefore recommended for in-service motors.
- The hipot test simply applies a voltage, measures leakage current and calculates insulation resistance. If the insulation resistance is higher than the accepted minimum, the motor passes.
- Surge test – This final test is used to verify the turn-to-turn, coil-to-coil and phase-to-phase insulation condition and is typically performed at twice line voltage plus 1,000 volts. It can detect weak insulation both between phases and within one phase, dead shorts, loose connections, and unbalances caused by incorrect winding. When motors start and stop, high voltage spikes are generated and, over time, these spikes can damage winding insulation, as can the effects of heat, contamination, and the movement of the windings due to magnetic forces they experience during motor starting. Of all the tests described, the surge test is the only method of detecting the resulting weakness in turn-to-turn insulation. As studies have shown, 80% of electrical failures start as turn-to-turn weakness, making it clear that surge testing is essential if incipient winding faults are to be detected before they lead to complete failure of the motor.
Why test at high voltages?
Questions are frequently asked about the voltages applied to motors during testing. Why, for example, should a 415V motor be subjected to a 2,000V test? The answer relates to the large voltage spikes that motors see during starting and stopping. A typical 415V motor, particularly if it is started direct online, can see voltage spikes of up to 2,000V. To ensure motor reliability, therefore, testing needs to be carried out at similar voltage levels. International standards, including those from the IES, IEEE, NEMA and EASA, apply to all of these tests.
Electric motors are the almost indefatigable workhorses of industry and modern motors are exceptionally reliable. Nevertheless, when a motor does fail without warning, consequences can be costly and disruptive. Condition monitoring provides insurance against such failures, by giving advance warning of developing fault conditions. This gives time to plan the required maintenance or motor replacement. To be truly effective, however, condition monitoring must include ‘the missing link’ – electrical tests.