Controlling noise is an important part of environmental protection, and motor noise is an important technical indicator for measuring the quality of motor products. Therefore, controlling motor noise has become an important issue for the survival and development of domestic and foreign motor manufacturing enterprises. This article analyzes and discusses the main causes of noise and the noise reduction measures taken.

When a motor is running, there are usually multiple noise sources that coexist at the same time. Different noises are generated by various parts of the motor. The different parts of the motor that generate noise are generally unrelated to each other, so they can be studied separately and special noise reduction measures can be taken.

1. Electromagnetic noise

Electromagnetic noise is mainly caused by changes in time and space and caused by the magnetic pull between various parts of the motor. The magnetic field in the air gap space is a rotating force wave. Its radial force wave causes radial deformation and periodic vibration of the stator and rotor, which is the source of noise. Most of the sound waves are radiated to the surrounding space by the vibration of the stator and other components, becoming “airborne noise”, while most of the electromagnetic noise is “airborne noise”.

There are many other design and fault reasons that will also cause an increase in electromagnetic noise, such as unbalanced magnetic pull, the impact of core saturation, the impact of open slots, noise generated by magnetic flux oscillation, dynamic eccentricity of the air gap, and pulsation in the thyristor power supply components, harmonic components in the power grid, broken bars of asynchronous motors, short circuits between DC motor armature and main pole turns, AC motor cores that are not pressed tightly, uneven assembly air gaps, etc.

Therefore, appropriately reducing the air gap flux density of the motor and increasing the air gap, such as using armature skew slots, uneven air gaps of DC motors, and magnetic slot wedges of AC motors, can reduce tooth harmonics and electromagnetic noise. effective measures. Increasing the stiffness of the machine base can reduce the vibration and noise caused by the fundamental wave rotation force in the air gap field of the stator and rotor. Improving the uniformity of the air gap assembly and the stacking quality of the core will help reduce electromagnetic noise.

2. Aerodynamic noise

There are two types of aerodynamic noise from motors: eddy current noise and whistling noise.

Eddy current noise is mainly caused by the turbulence of the cooling air caused by the rotor and fan and the alternating eddy currents on the rotating surface; whistling noise is caused by compressed air, or air rubbing against fixed obstacles. The whistling noise in the motor is mainly caused by the radial ventilation trench. The whistling noise increases as the gap between the rotating parts and the fixed parts decreases, so a sealed soundproof cover is used to “seal” the noise in the soundproof cover, increase the gap between the rotating parts and the fixed parts, and improve the guidance The shape of the windshield and the use of unevenly distributed and unequal length blades are effective ways to reduce whistling noise. In addition, reducing the rotor surface peripheral speed, reducing the surface area of the motor and the rotor surface roughness can also reduce aerodynamic noise.

3. Commutation noise

Commutation noise, also known as brush noise.

In motors with slip rings and commutators, commutation noise is inevitable and sometimes becomes a major noise source. Causes of commutation noise:

Friction noise: Friction noise is bound to occur at the sliding connection between the brush, slip ring and commutator. The size of the friction noise is related to the surface condition of the slip ring and commutator, the friction coefficient of the brush, the absolute humidity of the air and the brush pressure. Therefore, methods to reduce friction noise include: improving the smoothness and roundness of the working surfaces of slip rings and commutators, ensuring that the absolute humidity of the air is not less than 5g/m³, cleaning the brush surface, and the brush material hardness should not be too large, appropriate Reduce commutator diameter, etc.

Impact noise: This is because there is a mica groove between the commutator segments. Due to the deformation of the commutator and poor engraving and chamfering of the mica groove, the brushes tend to hit the commutator segments when the motor rotates, causing the commutator to slice and reduce this noise.

4. Mechanical noise

The noise of rotating motors is mainly mechanical noise, which is easily produced by large and high-speed motors.

Poor rotor dynamic balance is one of the most common causes of mechanical vibration and mechanical noise. Improving the accuracy of rotor dynamic balance can effectively reduce this noise. If the installation and adjustment are poor, and the natural frequency of the stator and rotor components is consistent with the rotational frequency, mechanical noise will also be generated. When the motor is equipped with an end-shield type windshield, the hood is often shaken by the vibration of the motor, causing vibration and noise. In this case, the vibration of the motor stator is often the source of excitation for the end cover or wind cover. To reduce this vibration noise, the method is to increase the dynamic stiffness of the end cover and cover. Adding vibration-absorbing materials, such as felt, at the junction of the end shield and the stator can reduce the vibration amplitude of the stator.

5. Load noise

The main reasons for this kind of noise are manufacturing errors, assembly gaps, damage to the working surface, and electrical corrosion caused during operation, transportation, and installation, which will cause the bearing to run unbalanced and cause irregular impacts. For example, the surface roughness of the shaft does not meet the requirements, the roundness of the bearing hole is out of tolerance (stator or end cover), the material of the bearing hole is poor, the extension end of the armature core shaft is slightly scratched, etc.

Noise reduction countermeasures include: During the bearing processing and factory inspection, it is necessary to strengthen the test and inspection of the smoothness, roundness, and surface roughness of the inner wall of the bearing to confirm whether the oil hole or material of the inner wall of the bearing meets the requirements and whether the roundness of the bearing hole exceeds If the shaft head is poor, carefully check the shaft head to confirm whether the shaft head on the end cover side is qualified; when stacking and riveting the shaft, check whether the shaft extension end is scratched.

In short, in order to effectively control and reduce motor noise pollution, in addition to controlling quality during the production process, it is also necessary to master the monitoring, diagnosis, and identification technology of motor noise in order to take effective measures to reduce noise.