A troubleshooter’s guide to motor maintenance and repair
14 July 2014
A failed or under-performing motor can bring a busy production line to a halt, compromise a critical airflow, devastate a water supply or lead to any number of other issues. Here, Andrew McIntyre suggests some simple solutions to potential problems.
Induction motors are the workhorse of the manufacturing and process industries, power buildings’ heating, ventilation and air-conditioning systems, drive cranes and conveyors, pump drinking water, power medical equipment, run lifts and elevators. If a piece of machinery is moving, chances are that there is an induction motor providing the muscle.
Induction motors are not the only type of motor, but because they are robust, versatile and relatively cheap, it is not surprising that they are probably the most commonly used in the world today.
Once installed many motors receive little or no maintenance, yet despite this, most give sterling service, year after year. However, it is inevitable that sometimes a motor will break down or its performance will drop off. This may be due to a parts’ failure, lack of maintenance, overload, shock load or other abuse.
If there is a major problem, a replacement motor and/or rewind may be required. However, minor problems are usually easy to sort out and prompt attention will minimise disruption to normal operations. In this article, we will run through some of the common problems associated with industrial motors and suggest simple solutions. But before following any of these procedures, we recommend that you perform some preliminary safety steps, namely:
- Disconnect power to the motor
- Discharge all capacitors
- Ensure moving parts cannot harm the operative or trap clothing
- Replace safety guards after servicing
Motor fails to start on installation
This is most likely caused by an installation error. Firstly, check that all the electrical connections are correctly wired. (A related issue is that incorrect connections may cause the motor to run in reverse.)
It is also possible that the cover over the cooling fan has been squashed into the fan and is preventing the motor from turning, in which case the guard will need to be repaired or replaced.
Less likely but possible is that the rotor is contacting the stator, so unable to turn or that bearings have seized; checking alignment and lubrication may help, but serious issues will probably require a replacement motor.
Motor has been running, then fails to start
The first thing to check is the fuse or circuit breaker, then check for physical jamming of the load or excessive loads, then check wiring and switches for obvious failures. If the motor is fitted with capacitors, discharge these and check with an ohmmeter.
If all of these are OK, then there is probably a problem with the stator or rotor; you will have to dissemble the motor and check for burn marks, failed wiring, loose coils, etc.
Motor runs but dies down and cuts out
A likely cause is an increase in the load. This is fairly obvious, if say, the motor is driving a conveyor that is overfull, but it could also be due to a tightening bearing in the motor or drive train.
Another possibility is a voltage drop in the power supply; this could be due to a failing or undersized supply cable, which is easily fixed. If it is the actual supply, other motors will also run slow.
Motor acceleration is poor
The motor may be undersized, considering the load it is driving; this can be checked with a few fairly easy calculations and addressed.
The problem may be mechanical, e.g. a tightening bearing, in which case there is likely to be noise, vibration and possibly, local heating. Lubrication and alignment checks may set everything straight.
Alternatively, there may be an electrical issue, so check wiring, switches, etc. The capacitor may have failed; this will need to be checked with a meter and replaced if necessary.
Finally, it may be a supply problem, as above
Motor overload protector continually trips
Firstly, the overload protector may not be at fault, but doing its job well. So the first thing to check is the actual load; as previously, check for jamming, excessive load and mechanical problems in the drive line.
The protector may also trip if the motor is getting too hot, so make sure the area is well-ventilated, the fan is effective, etc. The stator or rotor may be failing, so check for loose or broken windings or other signs of problems.
If the protector is defective, it is relatively easy to replace.
Motor vibrates or is noisy
A humming noise is likely to indicate an electrical fault such as a failing coil or a loose connection. Vibrations are generally mechanical problems.
The simplest mechanical problem is a misaligned load, which is easily rectified. Then there is the possibility of an out-of-balance load, which may suggest a bent motor shaft, necessitating a new shaft. Vibration may also be introduced if the shaft has longitudinal play, in which case its mountings need to be tightened.
Bearings are another likely cause, so check their lubrication or if necessary, replace them. If the bearings are fine, it may be that the rotor needs rebalancing.
Motor sounds rough at start-up
Many motors seem to take a minute or two to settle into a rhythm, and this is indicative of a problem that will get worse if left unaddressed. It is often the case that some sensible maintenance will sort out the issues, perhaps realigning moving parts, lubricating dry bearings, tightening screws, etc.
Capacitors fail frequently
Poor capacitor life may be indicative of an undersized motor or frequent overload of an optimally sized motor. Other simple problems include a low voltage supply, a failing start switch or overheating due to poor ventilation.
Another possibility is stopping and starting too frequently. You may be able to adjust the duty cycle, add a mechanical clutch to the drive chain or fit a soft start unit to the supply side of the motor.
Industrial motors can be their own worst enemy. They are so robust and reliable that they are often mistreated or not maintained. A programme of regular checks will avoid many problems. The common problems listed above are generally minor and easily fixed. More major problems are often best addressed by swapping out the motor and sending it for a professional rewind and refurbishment.
Andrew McIntyre is with TSJ Industrial, Cleckheaton, West Yorkshire
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