Testing to overcome motor failures

Modern industrial motors are expected to run more quickly than ever before – and for longer – while using less power. Strange as it seems, this is not an impossible task. The answer is to ensure a strict maintenance regime that identifies problems immediately and corrects them.

There are many reasons for a motor to fail, and it’s the job of maintenance personnel to prevent this from happening – or at least delay it for as long as possible.

One common problem is bearing failure. Small imperfections on the raceway can magnify over time, leading to catastrophic failure. Constant monitoring through techniques such as vibration monitoring can help to avoid this. Also, motors are working at higher temperatures – which can cause early degradation of lubricant. This can be addressed using ‘lubrication-free’ bearings, or by re-lubricating using an automatic system.

However, a large number of failures are also directly related to the motor – either through internal electrical faults, or failings in the wider motor system. Problems with motor windings, for example, can be a real headache: they cannot be replaced as easily as a damaged bearing, or corrected by adding extra lubricant. But there are methods to detect motor-related faults as early as possible – whether they are in the motor itself, or in the wider motor system.

Motor failures
Around 40% of motor failures are caused by insulation problems, such as coil windings in the motor or loose connections. These are assessed using static testing – in which the motor is disconnected from the power supply. A separate technique – dynamic testing – tests the motor is tested in situ. Both can be performed automatically, using dedicated test equipment.

For automatic static testing, a device such as an SKF Static Motor Analyzer Baker AWA-IV removes operator error and inconsistency – caused by issues such as different operators using different test parameters, or applying a different test sequence to the motor. It comprises several specific tests, which need to be carried out in a specific sequence. These include:

• Meg-ohm test: this shows if the motor has a dead short to earth, or is wet or contaminated.

• PI test: a 10-minute test that incorporates the 1-minute Meg-ohm test. It is generally used on high voltage motors, to measure the ability of an insulator to polarise.

• DC step voltage test: typically performed at double the line voltage, plus 1000V. Voltage is increased in five or more steps, and leakage current is plotted. Good insulation will show a linear plot, while insulation weakness is revealed by non-linearity.

• Hipot test: usually the last step in a DC step test. This is the groundwall insulation resistance in meg ohms at 2 x line voltage + 1000V.

Static test results are clear and unambiguous, and produce a specific result that requires little interpretation. Results can also be trended over time: if a step voltage shows increasing non-linearity, this suggests a weakening of the groundwall, for example.

A dedicated testing programme can highlight these types of problems. One pulp and paper company, for example, worked its way through the 800 motors in its testing database. It identified – and solved – a range of problems, including: blown holes in insulating boots; bad lug connections in junction boxes; and stator coil turn-to-turn shorts. In all, the new testing regime, using an SKF test rig, cut annual motor costs by around one-third.

Dynamic approach
Problems in the wider machine system can be identified using dynamic testing. Here, a device such as an SKF Dynamic Motor Analyzer – EXP4000 can reap huge benefits – as a US-based utility company discovered. Dynamic testing revealed that a motor on a screen refuse pump was overheating and drawing excess current, though there were no signs of bearing problems, current imbalance or rotor bar problems. The bill to replace the motor was around £16,000 ($23,000). However, searching through the maintenance history showed the pump was using an oversized impeller, which was overloading the motor.