Solving motor vibration problems

When motor vibration problems occur, the magnitude and direction of the vibration can give a good indication of where to look for the cause. When vibration is higher in the vertical plane, one of the first things to examine is the base/foundation of the motor. If the high vertical readings are compounded by indications of an eccentric air gap, such as high axial vibration and a predominant twice-line-frequency vibration, a ‘soft foot’ or twisted frame is often to blame.

Construction basics

To correct soft foot conditions, alignment technicians commonly use prefabricated shims (sized to accept the hold-down bolts) under motor feet. What they may not realize is that a cast iron or steel motor frame isn’t as solid as it appears.

The feet might be more than an inch (25 mm) thick, so some technicians mistakenly assume distortion isn’t possible. As a result, they do not place shims to the greatest benefit. A review of motor frame construction basics can be helpful in determining where to put shims to obtain the best support, and therefore the lowest vibration readings.

Fig. 1 shows the typical fabricated-frame construction; the holes through the motor feet are sized for the hold-down bolts. Depending on motor size, there will be at least four bolts and sometimes more. Each foot may be a standard pad at one corner of the motor, or it may run the full length of the frame, in which case there are usually four or more bolts along each side of the motor.

Frame bulkheads

An exploded view of any fabricated-frame motor would clearly show that the bulkheads do much more than hold the stator in the frame or channel the internal air flow, as in the case of a weatherproof (WP) enclosure machine. They also stiffen the entire frame, helping dampen vibration. Vibration levels will therefore increase if one or more of the bulkheads is not properly supported by the motor base.

Although most motors have solid-looking feet that are securely attached to the bulkheads, the mounting bolts must be far enough from the bulkheads to provide access for tightening. Therefore the bulkheads (as well as the feet) may require shimming to achieve the rock-solid support that is critical for minimizing frame distortion and vibration. If loosening or tightening base bolts changes the vibration levels, the motor is not shimmed properly.

Fig. 2 illustrates the critical areas for shimming. When a corner requires X amount of shim, that shim should also extend to the area directly beneath the bulkheads near the bolts. If the base and supporting foundation are not parallel, the shim thickness required beneath the bulkhead may differ from that required at the bolt.

Minimizing vibration

The shims beneath the bulkheads should not be loose after the mounting bolts have been tightened. If the soft-foot check procedure (see sidebar) of loosening the bolts indicates a soft foot, the shim pack beneath the bulkhead may be too thick. Vibration will be minimized once all feet are correctly supported, including the area directly beneath each bulkhead (see Fig. 3). 

Note that these same basic principles also apply to other rotating equipment, for example, generators, blowers, screw compressors and so forth.

Avoiding frame distortion

When installing, removing or repairing a fabricated-frame motor, always use every lifting point provided by the manufacturer. If a motor with a welded steel frame is designed to be lifted at all four corners, picking it up at only two diagonally opposite corners is likely to twist the frame. The resulting irregular air gap may affect assembly and testing; a twisted frame will also cause future soft-foot problems, both in the service centre and on the jobsite.

Prudent engineering often uses a design safety factor of five (depending on many considerations, including the consequence of failure). It is a mistake to infer that it is ‘safe’ to lift a motor with only half of its lifting points; frame distortion is still a very real possibility. (Caution: An assumed design safety factor never makes it safe to lift a motor using only half of its lifting points.)

Summing it up

The bottom line is that minimizing vibration often depends as much on shim placement as it does on using the correct amount of shimming. Using proper lifting techniques can also help prevent frame distortion that could worsen soft-foot and vibration problems. 

Alignment technicians who understand and apply these basic principles can be more effective in reducing the vibration levels of both motors and driven equipment. MRO 

Chuck Yung is a senior technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, MO. EASA is an international trade association of more than 1,900 firms in 56 countries that sell and service electrical, electronic and mechanical apparatus. For details, visit www.easa.com.

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