When a motor is running, there are usually multiple noise sources coexisting at the same time. Different noises are generated by various parts of the motor, while different parts of the motor are generally unrelated to each other. Therefore, special noise reduction measures can be taken separately.
1. Electromagnetic noise
Electromagnetic noise is mainly caused by the variation in time and space and by the magnetic tension between the parts of the motor.The magnetic field in air gap space is a rotating force wave, whose radial force wave makes the stator and rotor produce radial deformation and periodic vibration, and is the sound source of noise.Most of its sound waves radiate into the surrounding space by vibration of stator and other components, and become "airborne noise", while most of electromagnetic noise belongs to "airborne noise".There are many design and fault reasons, also can cause the increase of electromagnetic noise, such as: magnetic tension imbalance;The effect of core saturation;Effect of opening slots;Magnetic flux oscillation produces noise;Dynamic air gap eccentricity;Pulsating component in thyristor power supply;Harmonic components in power network;Asynchronous motor broken bar;Short circuit between armature and main pole of dc motor;Ac motor core pressure is not tight;Uneven assembly air gap and so on.Therefore, it is an effective measure to reduce the tooth harmonic and electromagnetic noise to properly reduce the air gap flux density and increase the air gap of the motor, such as using the armature chute in the design, the uneven air gap of the dc motor and the magnetic slot wedge of the ac motor.Increasing the stiffness of the frame can reduce the vibration and noise caused by the fundamental wave rotation force in the air gap field between stator and rotor.Improving the uniformity of air gap assembly and the quality of core stacking can reduce the electromagnetic noise.
2. Aerodynamic noise
The aerodynamic noise of motor includes eddy current noise and whistle noise.
Eddy current noise is mainly caused by alternating eddy current of cooling air turbulence caused by rotor and fan on rotating surface.Whistling noise is produced by compressed air or by air passing over a fixed obstacle.Whistling noise in the motor is mainly caused by radial ventilation trench.The whistle noise increases with the decrease of the clearance between rotating parts and fixed parts.Therefore, it is an effective way to reduce the piping noise to adopt the sealed sound-proof hood, "seal" the noise in the sound-proof hood, increase the clearance between rotating parts and fixed parts, improve the shape of the wind-guide hood, and adopt the wind blade with uneven distribution and unequal length.In addition, the aerodynamic noise can be reduced by reducing the circumference velocity of the rotor surface, the surface area of the motor and the roughness of the rotor surface.
3. Commutating noise
Commutating noise, also known as brush noise.
In motor with slip ring and commutator, commutation noise is inevitable and sometimes becomes a major source of noise.Commutating noise is caused by three reasons:
Friction noise
Friction noise is inevitable at the sliding joint between the brush and the slip ring and the commutator.The size of friction noise is related to the surface state of slip ring and commutator, friction coefficient of brush, absolute humidity of air and brush pressure.Therefore, the methods to reduce friction noise include: improving the smoothness and roundness of the working surface of slip ring and commutator;Ensure that the absolute humidity of the air is no less than 5g/m3;Clean brush surface;Brush material hardness should not be too large;Reduce commutator diameter appropriately.
The impact noise
This is because there is a mica groove between the commutator plates. Due to the deformation of the commutator, the process of mica groove undercutting and chamfering is not good, so that the brush will often impact the commutator plates when the motor rotates, making the commutator plates and reducing the noise.
4. Mechanical noise
Rotary motor noise is mainly mechanical noise, large, high - speed motor easy to produce this noise.
Poor rotor dynamic balance is one of the most common reasons for mechanical vibration and noise.The noise can be effectively reduced by improving the accuracy of rotor dynamic balance.Poor installation and adjustment, the natural frequency of stator and rotor parts having the same frequency with speed will also produce mechanical noise.When the motor is equipped with end cover type wind cover, the cover is often shaken by the vibration of the motor, will also produce noise and vibration.In this case, the vibration of the motor stator is often the excitation source of the end cover or wind cover. To reduce the vibration noise, the measure is to increase the dynamic stiffness of the end cover and cover.The vibration amplitude of the stator can be reduced by adding a vibration absorbing material, such as felt, at the junction of the end cover and the stator.
5. Load noise
The main causes of this noise are manufacturing tolerance, assembly gap, and damage of working surface and electric corrosion caused in the process of operation, transportation and installation, which will make the bearing run unbalance and have irregular impact.For example, the shaft surface roughness is not up to the requirements, bearing hole roundness is out of tolerance (stator or end cover), bearing hole material is poor, armature core shaft extension is slightly scratched and so on.Noise reduction countermeasures include: during bearing processing and incoming inspection, the test and inspection of bearing inner wall smoothness, roundness and surface roughness must be strengthened;Confirm whether the oil hole or material on the inner wall of the bearing meets the requirements, and whether the roundness of the bearing hole is out of tolerance;Carefully check the shaft head to confirm whether the end cover side shaft head is qualified;Whether there is any scratch on the shaft extension when the serial shaft is riveted.
In short, in order to effectively control and reduce motor noise pollution, in addition to good quality in the production process, but also must master the motor noise monitoring, diagnosis and recognition technology, to take effective measures to reduce noise.
electric vehicle driving electromotor producer , LEGO, China
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