Induction Motor Noise

Induction motors are the foremost common motors in use today. Motor noise can be a challenge for most motor manufacturers. Thus what causes motor noise and what can be done to cut back the noise? In this article we can discuss where motor noise comes from and how it can be minimized.
The 3 basic types of motor noise are:
1. Magnetic Noise
2. Bearing Noise
3. Unbalance Noise
MAGNETIC NOISE:
The most source for magnetic noise in induction motors comes from force waves created by the rotating magnetic field. Since this field is the vehicle by that power is transmitted from the stator to the rotor, elimination of this noise is not possible. However, magnetic noise can be minimized by utilizing many techniques:
1. Using the proper stator and rotor slot combination
2. Planning the stator and rotor lamination geometry
3. Applying the right coil pitch
4. Formulating the suitable rotor skew
The right selection of stator and rotor slots is one in all the most necessary aspects of a quiet motor design. This selection also affects manufacturing costs. To stay producing prices low, the number of stator slots should be the quantity of poles times the quantity of phases within a given frame size. This variety of stator slots ought to be such that a balanced winding will be accommodated. There are hardly any slot mixtures that are good; but, over the years there have been some mixtures that have proven to be successful. The following general rules ought to be used as a guide when coming up with the laminations:
a. The number of rotor slots ought to not equal the quantity of stator slots. Acceptable performance has been attained when the quantity of rotor slots differs by the quantity of stator slots by fifteen to thirty%.
b. To avoid dead points or cogging, the distinction between stator and rotor slots ought to not equal +/- number of phases times number of poles
c. To minimize noise and vibration the difference between the amount of stator slots and rotor slots should not equal +/-one, +/-two, +/- (number of poles+/-one) and/or +/- (number of poles +/-2). For example: if you've got a 6 pole motor and therefore the stator lamination has eighteen slots then the rotor laminations should not have the following number of slots: 17, nineteen, 16, 20, 13, 23, fourteen, or 26.
When coming up with the stator and rotor lamination geometry, several parameters must be reviewed. A closed rotor slot can cause less permenace variation, however, thickness of the bridge must be reviewed to ensure the lamination does not saturate. The width of the stator slot opening should be kept to a minimum but; you continue to need it massive enough to insert the coils. The length of the air gap conjointly affects presence variation such that the larger the gap the less the variation. But bear in mind, the air gap conjointly relies on the other style criteria like power factor and efficiency. The motor design engineer wants to require under consideration all needs when designing laminations.
Utilizing the correct stator coil pitch is additionally important. The stator coil pitch ratio ought to be selected to present tiny pitch factors for lower order of air gap harmonics. The motor designer can want to stay this in mind when designing the winding. Skewing of either rotor or stator slots is another manner to reduce motor noise and offer smooth acceleration. The degree of skew additionally affects motor performance, therefore the skew is limited to performance parameters like breakdown torque, beginning torque and beginning currents.
BEARING NOISE
Bearing noise can be influenced by several factors like preload, shaft to bearing fits/tolerances, and bearing to bore fits/tolerances. The choice to that bearing to use should continually depend on the appliance of the motor. It is highly counseled to involve bearing manufactures when selecting the bearing.
Proper preloading is needed to confirm the bearings run smooth. Too much preloading will harm the bearings. Same philosophy goes with the bearing fits to the motor shaft and housing and finish bell bores. Bearing manufactures have counseled fits/tolerances to implement throughout the mechanical design of the motor.
UNBALANCE NOISE
Unbalance noise is made by an unequal weight distribution of the rotor assembly around the motor axis of rotation. It's been found that vacuum impregnation and/or encapsulation of motor stator windings has additionally helped in coil vibration and noise. There are a number of ways to reduce motor noise. All motor parameters must be taken into consideration when coming up with the unit.

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