| Induction motors are the most common motors in use | | | | geometry, many parameters must be reviewed. A |
| today. Motor noise can be a challenge for most motor | | | | closed rotor slot will cause less permenace variation, |
| manufacturers. So what causes motor noise and what | | | | however, thickness of the bridge must be reviewed to |
| can be done to reduce the noise? In this article we will | | | | ensure the lamination does not saturate. The width of |
| discuss where motor noise comes from and how it | | | | the stator slot opening should be kept to a minimum |
| can be minimized. | | | | however; you still need it large enough to insert the |
| The three basic types of motor noise are: | | | | coils. The length of the air gap also affects permeance |
| 1. Magnetic Noise | | | | variation such that the larger the gap the less the |
| 2. Bearing Noise | | | | variation. But keep in mind, the air gap also is |
| 3. Unbalance Noise | | | | dependent on the other design criteria such as power |
| MAGNETIC NOISE: | | | | factor and efficiency. The motor design engineer |
| The main source for magnetic noise in induction | | | | needs to take into account all requirements when |
| motors comes from force waves created by the | | | | designing laminations. |
| rotating magnetic field. Since this field is the vehicle by | | | | Utilizing the proper stator coil pitch is additionally |
| which power is transmitted from the stator to the | | | | important. The stator coil pitch ratio should be selected |
| rotor, elimination of this noise is not possible. However, | | | | to give small pitch factors for lower order of air gap |
| magnetic noise can be minimized by utilizing several | | | | harmonics. The motor designer will need to keep this in |
| techniques: | | | | mind when designing the winding. Skewing of either |
| 1. Using the correct stator and rotor slot combination | | | | rotor or stator slots is another way to reduce motor |
| 2. Designing the stator and rotor lamination geometry | | | | noise and provide smooth acceleration. The degree of |
| 3. Applying the proper coil pitch | | | | skew also affects motor performance, therefore the |
| 4. Formulating the appropriate rotor skew | | | | skew is limited to performance parameters such as |
| The proper selection of stator and rotor slots is one of | | | | breakdown torque, starting torque and starting |
| the most important aspects of a quiet motor design. | | | | currents. |
| This selection also affects manufacturing costs. To | | | | BEARING NOISE |
| keep manufacturing costs low, the number of stator | | | | Bearing noise can be influenced by several factors |
| slots should be the number of poles times the number | | | | such as preload, shaft to bearing fits/tolerances, and |
| of phases within a given frame size. This number of | | | | bearing to bore fits/tolerances. The decision to which |
| stator slots should be such that a balanced winding | | | | bearing to use should always depend on the |
| can be accommodated. There are hardly any slot | | | | application of the motor. It is highly recommended to |
| combinations that are perfect; however, over the | | | | involve bearing manufactures when choosing the |
| years there have been some combinations that have | | | | bearing. |
| proven to be successful. The following general rules | | | | Proper preloading is required to ensure the bearings |
| should be used as a guide when designing the | | | | run smooth. Too much preloading will damage the |
| lamaintions:a. The number of rotor slots should not | | | | bearings. Same philosophy goes with the bearing fits to |
| equal the number of stator slots. Acceptable | | | | the motor shaft and housing and end bell bores. |
| performance has been attained when the number of | | | | Bearing manufactures have recommended fits |
| rotor slots differs by the number of stator slots by 15 | | | | tolerances to implement during the mechanical design |
| to 30%.b. To avoid dead points or cogging, the | | | | of the motor. |
| difference between stator and rotor slots should not | | | | UNBALANCE NOISE |
| equal +/- number of phases times number of polesc. | | | | Unbalance noise is created by an unequal weight |
| To minimize noise and vibration the difference | | | | distribution of the rotor assembly around the motor |
| between the number of stator slots and rotor slots | | | | axis of rotation. It has been found that vacuum |
| should not equal +/-1, +/-2, +/- (number of poles+/-1) | | | | impregnation and/or encapsulation of motor stator |
| and/or +/- (number of poles +/-2). For example: if you | | | | windings has also helped in coil vibration and noise. |
| have a 6 pole motor and the stator lamination has 18 | | | | There are many ways to reduce motor noise. All |
| slots then the rotor laminations should not have the | | | | motor parameters must be taken into account when |
| following number of slots: 17, 19, 16, 20, 13, 23, 14, or 26. | | | | designing the unit. |
| When designing the stator and rotor lamination | | | | |