FEATURE MOTORS Is your motor telling you something?
The extensive experience of some seasoned plant and maintenance engineers enables them to predict pending problems with machinery and take pre-emptive action to prevent them. Here, Bill Bertramof motor maker Marathon Electric, part of the Regal group, explains how different motor sounds can be interpreted
be bent if it is subjected to excessive load. It is worth noting that drive shafts often experience their load as being asymmetric i.e. they are subject to a constant bending moment. A slightly misaligned or bent motor
shaft will produce a humming sound. A similar noise can be created if there is a minor fault in the transmission equipment attached to the motor shaft. The latter can be confirmed by disconnecting the motor shaft from the load and turning it on. If the noise disappears, the fault is not in the motor. If the noise is still present, there is a
P
ut simply, a piece of plant that is running will be making a noise. And,
on closer inspection with a trained ear, individual elements within the original sound can be picked out, such as the whirring of a fan, the thumping of a pump or the rumbling of a conveyor. It is not surprising then that an experienced plant engineer will be able to pick out individual electric motors and come to know their ‘sound signatures’. If a motor’s sound starts to change, this may be an indication of a problem, so the astute plant engineer will take the time to investigate and may thus nip a potentially major breakdown in the bud. There are two main classes of extraneous noise in motors - mechanical and electrical. The most likely mechanical causes of noise are worn bearings, moving parts rubbing together or colliding, a bent shaft, a loose or missing screw, or another minor part. The type of noise may well indicate the problem, and the relevant part of the motor can be inspected and repaired if necessary. The most likely electrical causes of noise
are the loss of one of the three phases leading to a phase imbalance (three-phase motors only), or harmonics caused by the use of an inverter. Again, the route of the problem may be found in the character of the noise - the solution may be simple, but could also be a bit more complicated. Far from just having an ear for the
28 APRIL 2014 | ELECTRICAL ENGINEERING
Above: knowing the noises of a motor can be a key tool in predictive maintenance and problem identification
correct noise, sound signature analysis is in fact a highly developed field of study. It is generally only viable to apply in very special situations, such as the main drive motor on a nuclear submarine and the enormous pumping motors used in deep shaft mines. In major industrial applications, such as power stations, the similar discipline of vibration analysis is sometimes used as a way of monitoring the ‘health’ of large motors. However, in most cases motor noise is assessed intuitively by engineers familiar with the plant on a day to day basis.
CAUSE AND EFFECT Physical impact, electrical or mechanical overload and poor maintenance are all common causes of motor damage - the most common being damage to the relatively fragile fan cover which impacts the fan itself. While damage to the cover will be immediately apparent, a fan blade may also be broken or bent or the fan’s mounting or shaft may suffer. A simple visual inspection will show all of these problems apart from a slight bend in the shaft, which will probably lead to a whirring or humming noise in operation. A larger impact could bend the main shaft, damage the bearings, dislodge a minor part or even damage the casing. Most of these could involve a major repair or even require the scrapping of the motor. The motor’s central drive shaft can also
second test to do. Turn the motor on, then off - if the motor stops turning instantly, the problem is almost definitely electrical, not mechanical. A burning smell or carbon deposits indicate a failed connection, which may be easily repaired. It is possible that one of the coils of the rotor has failed (become unwound or disconnected), causing the electromagnetic field to become asymmetric and creating rotor wobble. If one of the coils feels loosely packed, a rewind is probably required. Rewinds will nearly always need to be undertaken by a professional, as will replacement of damaged shafts and worn bearings. Many other repairs could be completed in-house, although it may make more sense economically to simply replace the motor. It is increasingly common to use a motor
in conjunction with an inverter or variable speed drive. A drive can be used to reduce energy consumption by running the motor at a slower speed or to provide an extra level of operational control. However, it should be noted that an
inverter may increase both the electrical and mechanical stresses in a motor, so increased maintenance and monitoring could be required.
CONCLUSION Industrial electric motors are robust and reliable pieces of equipment, which require remarkably little maintenance during their working lives. However, one of the best ways to check on a motor is to get to know its sound signature and to listen to it regularly. It is not only a simple thing to do, but it becomes almost intuitive to a dedicated plant engineer, and it is probably the best early warning system available.
Regal UK
www.rotor.co.uk T: 01933 230 900
Enter 215 / ELECTRICALENGINEERING
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