Electrical testers predict motor maintenance

Electric motor problems occur for a variety of reasons, ranging from basic design faults and poor manufacturing quality to problems caused by application and site conditions.

Electric motor problems occur for a variety of reasons, ranging from basic design faults and poor manufacturing quality to problems caused by application and site conditions.

Specifically though, they are most likely to arise from bearing failures - probably the most common cause - with insulation deterioration following a close second.

While not trivialising the effect on plant operational efficiency that motor bearing failure will confer, established vibration analysis methods can readily pinpoint the potential for mechanical parts failure and trigger preventative measures.

However, up until now, the plant engineer has had a more complex task on hand, both in anticipating insulation failures and in determining the root cause of motor windings problems after they occur.

Early identification of a weak motor and its replacement or repair during an outage is demonstrably less expensive in comparison to plant downtime and rushed repairs.

Fortunately there are now specific condition monitoring techniques available that provide the ability to monitor motor performance, identify early indications of potential trouble and, better still, help to predict and proactively minimise problems by noninstrusive online methods.

One novel nonintrusive approach, developed by Glasgow Caledonian University, proposes the use of RF antennas to pick up radio interference generated during the motor's commutation process, then to store, process and analyse resultant data to reveal complex internal problems.

But before examining more conventional insulation resistance testing methods, a look at the causes of insulation deterioration, that can be detected by predictive maintenance testing, may go some way to help minimise their effects.

Poor cooling is a common cause of insulation overtemperature caused by plugged cooling ducts or accumulated dust on windings.

Elevated temperatures can cause insulation to become brittle and/or shrink.

Shrinkage allows the winding to vibrate, eventually allowing the copper winding to contact the steel core of the motor with catastrophic results.

This shrinkage can also lead to winding wedges being loosened or lost, or the coil side packing allowing too much movement between coils.

Wear between coil and slot leads to breakdown of the remaining insulation and partial discharges increase, causing oxidation and erosion.

Winding temperature can be elevated to a damaging level by cyclic, heavy-load starting.

Heat is retained in the windings and in the core material, leading to cracked rotor bars.

Moisture and electrical circuits are a combination to be avoided at all costs, but moisture is not the only contaminant that threatens performance.

Although reaction to chemical contamination is an obvious cause of insulation deterioration, oil contamination is the most common culprit.

It can cause current leakage to increase and it can encourage accumulation of abrasive dust, brought in with the cooling air, that erodes insulation.

Most general purpose induction motors are more tolerant of small overvoltage than they are of undervoltage.

Undervoltage creates insulation temperature stress.

A motor operating at lower than specified voltage runs hotter.

Conversely, significant repeated overvoltage exposure, and particularly transient voltage spikes, can lead to turn-to-turn and phase-to-phase short circuits.

When turn insulation fails, a short circuit loop ensues causing the motor winding to act as an autotransformer - the shorted turn serves as the secondary and the remainder of the phase as a primary.

The motor is not prevented from running but the undetected, underlying condition causes progressive heat migration and damaging consequences.

There are a number of simple tests to determine if motor windings are approaching failure - DC coil resistance tests, insulation resistance tests, polarisation index tests, DC high potential tests and surge comparison tests.

These procedures can reveal whether winding insulation is deteriorating or if there are localised flaws that will eventually escalate into total winding failure.

The DC coil resistance test compares the coil phases' resistance to each other in order to locate poor connections, open windings and shorted windings or turns.

However it is not capable of predicting turn-to-turn failures.

The insulation resistance test is the most common tool for diagnosing insulation breakdown.

In this procedure the motor frame is grounded and a megohmeter imposes a DC voltage, typically 500, 1000 or 2000V on the motor winding.

A sound winding will produce readout in hundreds or thousands of megohms.

A minimal acceptable reading will be 1Mohm plus 1Mohm/kV of the motor's rated voltage.

For example, the minimal acceptable resistance for a 460V motor is 1.46Mohm.

It is important to recognise that IR test readings are highly temperature and moisture sensitive.

Test motors should ideally be out of service long enough to assume ambient temperature and preclude condensation.

IR readings should be corrected in line with tables provided with test instrumentation.

The polarisation index test takes IR testing one step further.

As the DC test voltage is maintained, insulation molecules polarise, resulting in a gradual increase in the resistance reading.

This ability decreases with ageing.

By measuring leakage currents from windings to ground at specific intervals, the plant engineer can establish a polarisation index.

By observing trendable test results over a given period the ageing and subsequent loss of insulation integrity can be determined.

Weaknesses that might escape the scrutiny of IR and PI tests may well be identified by a DC high potential test as it provides information on the dielectric strength of the insulation.

The recommended test voltage - double the motor rating plus 1000V - is gradually applied in step increments up to the maximum.

The resultant plot should be displayed as a straight line; any abrupt upswing in the plot slope would indicate an insulation flaw and the test procedure should be halted in this event to prevent winding failure.

Probably the newest of the classic tests to determine insulation condition, the surge comparison test detects turn-to-turn, coil-to-coil, and phase-to-phase defects that cannot be determined by any other method.

It is premised on the principle that in a motor with no winding defects, all three-phase windings are identical.

Each is tested against the other with brief DC voltage pulses.

Reflected pulse images of two identical windings appear as a single trace.

The waveform deviations from this pattern clearly indicate any turn-to-turn shorts or winding-to-ground flaws.

Electric motor test instrumentation specialist Whitelegg Machines offers a comprehensive range of single- and multifunction testers.

Featuring digital output capability for interfacing with computer systems, these instruments allow the plant engineer to manipulate motor test data and integrate findings with other pertinent conditioning monitoring and maintenance data.

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