Motors and Generators:Balancing


Electrical failures are often ascribed to deteriorated insulation, open circuit, short circuit, and so on, but in many cases, failure of insulation results from mechanical disturbances. Unusual noises in electrical apparatus may be the result of grounds, short-circuited coils, changes in voltage or frequency, rubbing or looseness of parts, vibration, defective bearings, and many other causes.

Any unusual amount of vibration or an increase in machine vibration should be investigated immediately. Common causes of undue vibration, other than imbalance, or bearing wear, dirt accumulation, misalignment, an incorrect or a settled foundation, uneven air gap, parts rubbing the rotating element, sprung shafting, a short-circuited field coil, or imbalanced stator currents in the case of AC machines. These should be investigated before balance weights are added or shifted. If at any time it should be necessary to remove the balance weights, they should be replaced in the same position.

Before disassembling a pole on high-speed machines, the axial position of that pole should be accurately marked so that it can be replaced in the same position. Should it become necessary to replace a field coil, or a complete pole, the balance must be checked.

Need for Balancing

Vibrations produced by unbalanced rotating parts may result in the following:

• Excessive bearing wear

• Noisy operation of the equipment

• Failure of structural parts

• Reduced overall mechanical efficiency

• Vibration of machine parts or the supporting structure

Imbalance Measurement

Imbalance is generally measured in ounce-inches (oz-in.). An imbalance of 1 oz-in. in a rotating body will produce a centrifugal force equivalent to that produced by 1 oz of weight 1 in. from the rotational axis. A rotor weighing 62.5 lb (1000 oz) whose mass center is displaced 0.001 from the rotational axis is 1 oz-in. out of balance.

Only force imbalance is measured by static balancing, which is a single- plane correction. The part being balanced is not rotated. Dynamic balancing of a part by rotation is required when there is appreciable axle length because, by this method, force imbalance, moment imbalance, or a combination of both may be measured. This is a two-plane correction.

The balancing process is not complete until corrections have been applied relative to the size and that the exact location indicated by the balancing machine. Corrections for balance may be made by the addition or removal of metal.

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