How does the noise in the motor come from?

- Oct 16, 2019-

 

Motor noise mainly comes from three aspects, namely air noise, mechanical noise and electromagnetic noise, but sometimes the internal noise of the circuit is included in one of the noise sources. The internal noise of the circuit mainly comes from the self-excitation of the circuit, the noise of the power supply, and the fluctuation of the electron flow in the circuit components and the thermal motion of the free electrons.

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Air noise


Air noise is mainly caused by the rotation of the fan, causing air to flow, impact, and friction. The noise level is determined by the fan size, shape, motor speed and wind resistance.


Basic frequency of air noise


How does the noise in the motor come from?


Among them, N - the number of fan blades; n - motor speed (RPM).


The larger the diameter of the fan, the larger the noise, and the 10% reduction in fan diameter can reduce the noise by 2-3dB. But then the amount of cold will also decrease. When the gap between the edge of the blade and the plenum is too small, a flute sound (like a piper) is produced. If the shape of the blade and the structure of the fan are unreasonable, causing eddy currents, noise is also generated. Due to the insufficient stiffness of the fan, vibration occurs when it is hit by the airflow, which also increases noise. In addition, turning to a protruding portion can also cause noise.


For the above reasons for air noise, the following measures help to reduce air noise:


(1) Reasonably design the fan structure and the shape of the vane to avoid eddy currents; ensure that there is sufficient clearance between the edge of the vane and the ventilating chamber, and minimize the diameter of the fan when permitted;


(2) In the case of permission, the airflow is turned and then blown (sucked), which can significantly reduce the noise, which has been used in vacuum cleaners;


(3) Ensure that the wind path is smooth and reduce the impact and friction of the air.


If the ventilation noise cannot be controlled from the sound source, it is necessary to use sound insulation or noise reduction, and it is also possible to prevent the sound absorbing material from being placed near the radial air passage of the stator. The simplest and most effective method of sound insulation is to use a sound insulation cover made of steel plate, wood board or plastic plate to surround the whole motor, which can reduce the noise by about 20 decibels. Of course, this is unfavorable for the overall heat dissipation, and it is occupied. The space is also relatively large.


The wind angle of the wind blade has a great influence on the noise, and the noise can be reduced by 1 to 2 db by increasing the angle by 5 to 10 degrees.



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2. Mechanical noise


Air noise is mainly caused by the rotation of the fan, causing air to flow, impact, and friction. The noise level is determined by the fan size, shape, motor speed and wind resistance.


Basic frequency of air noise


How does the noise in the motor come from?


Then the rotation frequency of the ball


How does the noise in the motor come from?


The corrugations, pits and ultra-roughness in the inner and outer ring raceways of the bearing are the main causes of noise.


Tests have shown that the noise sound pressure level is proportional to the product of the ripple height and the number of ripples on the rolling surface. In addition, the size of the radial clearance also affects the noise, reduces the radial clearance, and reduces the noise, but the bearing with small radial clearance is required to be equipped with a casing and an end cover with high concentricity in the two bearing chambers, and The requirements for rotor coaxiality are increased.


At the same time, the quality of the grease is also the main reason for the noise. The noise is related to the viscosity of the grease. The test shows that the noise decreases with the increase of the viscosity, but after the viscosity increases to a certain value, the noise increases. This is because the oil film has a cushioning effect on the vibration, and the viscosity is large and the noise is high. Low, but when the viscosity is too large, stirring sounds when turning.


The effect of mounting error on bearing noise. If the bearing installation error exceeds a certain critical value, the bearing noise will increase sharply, and the critical angle will decrease as the radial clearance of the bearing decreases. Figure 1 shows the relationship between the installation error angle and noise for a single row of inner core bearings with different radial clearances.


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In summary, in order to reduce the bearing noise enough, it is necessary to ensure the finishing quality of the inner and outer ring raceways of the bearing, and if possible, use a small radial clearance, and select a good grease and a suitable viscosity to control The bearing tooling error angle is below the critical angle, which can also reduce the noise of the rolling bearing.


In the structure of the motor, a wave spring is applied to the outer ring of the bearing to apply an axial preload force, which can reduce the click sound with a frequency of about 400 Hz. Because the bearing does not have the preload force when the motor rotates, the steel ball in the bearing is free to vibrate in a free state. This vibration is random, and it may also collide with the cage, the ferrule, etc. Additional noise. If a preload is added to the outer ring of the bearing by a wave spring, it properly adjusts the bearing. When there is an axial component, the spring acts on it to reduce the vibration, thereby weakening the free vibration of the ball, thereby weakening Additional noise.


The use of sliding bearings instead of ball bearings is the most effective way to reduce bearing noise. Powder metallurgy oil-impregnated bearings are simple to introduce, low in cost, low in noise, uniform in spectrum and relatively stable, and gradually introduced into micromotors with noise requirements below 40 dB. . However, the porous structure of such bearings is prone to oil leakage, which affects its lubricity, and its load capacity is smaller than that of the same outer diameter rolling bearing, and if the shaft and the bearing bore surface have low precision and roughness Large degree, high porosity, insufficient oil supply and excessive free carburization ratio in iron-copper-based graphite oil-bearing bearings, uneven distribution, resulting in individual hard spots, will also produce some high-frequency noise.


Turning to noise caused by imbalance: The rotor of a high-speed motor must be strictly checked for dynamic balance to reduce the residual imbalance of the rotor. The frequency of the rotor unbalanced noise is equal to the rotor rotation frequency fa. Although the frequency is not high, generally below 400 Hz, the vibration of the motor is increased, so that the noise of each part is increased. When the dynamic balance accuracy of the rotor reaches G 6.3, the noise and vibration caused by the rotor imbalance can be significantly improved.


Noise caused by friction between the carbon brush and the commutator: Frictional noise is generated due to the brush being pressed against the rotating commutator. Generally, most of the motors use semi-plastic commutators, and the roundness of the commutator surface is not good. The mica between the sheets is slightly concave due to the engraving, thereby increasing the friction noise and increasing the frequency of the noise. The roundness of the commutator is not good and the surface of the commutator is not flat to increase the noise.


Frequency of carbon brush and commutator friction noise


How does the noise in the motor come from?


Where: k - number of commutators


In addition, the carbon brush holder structure is not strong enough, causing periodic vibrations to increase noise.


In summary, strict control of the roundness of the commutator, to ensure a good surface finish and beating, and the use of a strong and rigid carbon brush holder structure can reduce the noise caused by the friction between the brush and the commutator.


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3. Electromagnetic noise


The alternating electromagnetic force acting on the motor stator and rotor air gap will cause vibration and noise of the motor stator and rotor. Since the air gap magnetic field not only has a fundamental wave but also a series of higher harmonics, these magnetic field interactions will generate periodic forces, and both the fundamental wave and the higher harmonic electromagnetic force will cause vibration and noise.

       The electromagnetic sound frequency distribution is mostly between 100-4000 Hz. The magnitude of vibration and noise intensity is related to the magnitude of the electromagnetic force and the stator and rotor stiffness. When the electromagnetic force that excites the vibration matches the natural vibration frequency of the vibrating component, resonance will occur, and the vibration and noise will also increase significantly. The electromagnetic force has a radial component and a tangential component. The radial component of the electromagnetic force plays a major role in causing motor vibration and noise. It causes the stator core to generate radial vibration, and the noise generated by the radial vibration is the main component of the motor electromagnetic noise. When a single-slot rotor is used, the groove-induced noise becomes the most important part of electromagnetic noise. During the operation of the motor, the rotor core of the single-slot is periodically subjected to the change of the single-sided magnetic pull force, and the reason can be explained by FIG.


In Fig. 2(a), the upper magnetic pole arc covers three rotor slots, and the lower magnetic pole arc covers only two rotor slots. At this time, the upper magnetic pulling force is large, and the lower magnetic pulling force is small, so that the stator core has an upward movement. the trend of. When the rotor rotates half a slot distance, as shown in Figure 2 (b), the lower pole pole arc covers three rotor slots, and the upper pole pole chord covers only two rotor slots. The change is made, the lower magnetic pulling force is large, and the upper magnetic pulling force is small, so the stator core has a tendency to move downward. Therefore, during the rotation of the rotor, the stator core generates periodic up and down vibrations. In the same way, the rotor is subjected to a cyclical change of the single-sided magnetic pulling force, thereby causing the rotor to vibrate.


When the double-numbered slot rotor is used, the above situation does not occur, but the slot position changes when the rotor rotates, causing a pulsed magnetic field in the air gap, which may also cause vibration.


According to the above analysis, the generated electromagnetic noise frequency


How does the noise in the motor come from?


Where: Z - number of rotor slots


In the electromagnetic noise, in addition to the noise generated by the above reasons, due to the higher harmonic components in the current, the harmonic magnetic field is generated in the stator rotor gas, and uneven torque is generated, causing vibration to generate noise.


Since electromagnetic noise accounts for a small portion of the total noise of the motor, measures are often taken to reduce electromagnetic noise in a targeted manner in the design and manufacture of the motor. However, when the limit noise is too high (such as air-conditioning pump motor and some other places for indoor electrical and high noise requirements), and the air noise and mechanical noise have been effectively suppressed, the rotor can be used. Measures such as slots, increasing the stator air gap, and reducing the magnetic flux density reduce electromagnetic noise.