In an industrial climate where overall equipment efficiency (OEE) is becoming increasingly important, preventing unexpected failures of equipment is paramount. Electric motors as the prime movers in all industry sectors are a major example. Generally regarded as highly reliable, due to tried and tested construction techniques, they are, nevertheless, subject to number of harmful conditions that can affect their performance and reduce operational life. One of the most common of these conditions is vibration, resulting from either electrical or mechanical imbalance in the motor.
Often vibration is confused with noise, which is understandable as the two are in a causal sequence. Under normal running conditions, around 1 kHz is the generally accepted boundary line to separate vibration from noise: i.e. the frequency range of 1 kHz or less will be treated as vibration, while that above this range will be treated as noise.
Vibration can cause damage to electric motors in several ways. First, it can accelerate bearing failure by causing indentations on the bearing raceways at the ball or roller spacings. Secondly, it can loosen windings and cause mechanical damage to insulation by fracturing, flaking or eroding of the material. Third, the excessive movement it causes can result in lead wires becoming brittle. Fourth and final, it can cause brush sparking at commutators or current collector rings. As a result of these problems, whenever vibration is located in an electric motor, its source should be located quickly and corrected. However, this is not always the case.
In some applications the fact that the overall amplitude of vibration from electric motors is usually low compared to other equipment, means that vibrations caused by internal defects in the motors are either not detected, or not considered important enough for rectification measures to be undertaken. In the latter case, the reasoning is that defects in the motor grow slowly and, hence, have a significant effect on the motor’s performance only in the later stages of its operating life. Such a reduction is often considered not to be significant, and is therefore acceptable. This was certainly the case before the availability of sophisticated monitoring and detection tools that would have enabled engineers to isolate such problems.
Moreover, even today with the availability of such tools, it is often easier to assign problems to external causes such as incorrect motor mounting, misaligned shaft mounting and external disturbances from other connected machinery.
So how does the motor user who is experiencing vibration go about isolating its cause? The first step is to check that the motor shaft mounting is level, and that the motor shaft is properly aligned. These are particularly important because misalignment can cause additional unnecessary loading on motors and, hence, increased energy consumption, plus reduced plant life through additional mechanical loading.
If the mounting is uneven, use shims to bring it back to the level; also ensure that the mounting mountings are tight. If the shaft is misaligned, realign it using laser alignment tools. If neither of these problems applies, then disconnect the motor from its driven load. If it then runs smoothly, the likely source of the vibration is in the driven equipment. If this is the case the motor will need to be isolated from the driven source using vibration absorbing components such as elastomeric couplings. However, if the vibration persists, cut off the supply to the motor: if this has the effect of stopping the vibration, then the problem is most likely an electrical unbalance within the motor. If the vibration continues as the unpowered motor winds down to a stop, then look for a mechanical unbalance.
Electrical imbalances usually denote failures in motor components such as rotor windings, stators and rings. They also result from uneven air gaps, due to worn bearings, and from motor shaft deflection due to uneven magnetic attraction between the motor stator and rotor. This causes the shaft to deflect as it rotates, creating a mechanical unbalance.
Using today’s sophisticated vibration analysis tools and software both mechanical and electrical unbalances can be detected and rectified. However, no matter how good the rebalancing, there will also be a residual imbalance, however slight. This fact is well known by motor manufacturers who publish specification tables showing balancing limits. Most manufacturers balance their motors to the N, or normal grade, with grades R (reduced) and S (special) available on request.
Motors can be either statically or dynamically balanced. Moreover, balancing can be done in 1, 2 or more planes. Since the rotor is a dynamic component, static balancing is not the best method. On the other hand, dynamic balancing requires substantial investment. Some companies dynamically balance their motors, but only a few carry this out at motor rated speed.
Solving Vibration – and General Motor Problems
Where the user is experiencing motor vibration – or any other type of motor problem – Deritend can help. The company repairs or replaces thousands of motors each year, and specialises in all aspects of motor management from condition monitoring to conducting motor usage surveys, supplying motors off the shelf to repairing motors of any type or size to installing/maintaining the units and providing impartial advice in all these areas. What Deritend can do with regard to vibration problems, is to implement a monitoring programme that establishes trends, thus giving early warning of worsening conditions, and allowing corrective measures to be undertaken before a possibly catastrophic failure occurs.
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