Motor Noise Analysis and Noise Reduction Measures
Controlling noise is a vital aspect of environmental protection, and motor noise serves as a significant technical indicator of motor product quality. As a result, controlling motor noise has become a crucial issue for the survival and development of motor manufacturing enterprises both domestically and internationally. The article analyzes and discusses the main causes of noise and the measures taken to reduce it.
When an electric motor operates, various noise sources often coexist simultaneously. Different noises are generated by various components of the motor, and the different parts of the motor that produce noise are generally unrelated to each other. Therefore, they can be studied separately and specific noise reduction measures can be taken individually.
1Electromagnetic Noise
Electromagnetic noise is primarily caused by the varying magnetic forces between different parts of the motor over time and space. The magnetic field in the air gap is a rotating pressure wave, whose radial force waves induce radial deformation and periodic vibrations in the stator and rotor, serving as the sound source of the noise. The majority of the sound waves are radiated into the surrounding space by the vibration of the stator and other components.“Aerodynamic Noise”The electromagnetic noise is mainly of“Aerodynamic Noise”Many design and fault causes can also lead to an increase in electromagnetic noise, such as: magnetic pull force imbalance; the influence of core saturation; the effect of open slots; noise generated by flux oscillation; dynamic eccentricity of air gap; pulsating components in thyristor power supplies; harmonic components in the power grid; broken bars in asynchronous motors; armature and main pole winding short circuits in DC motors; loose core assembly in AC motors; uneven assembly air gaps, and more. Therefore, appropriately reducing the magnetic flux density in the air gap of the motor, increasing the air gap, and designing with skewed slots for the armature, uneven air gaps in DC motors, and magnetic slot wedges in AC motors are all effective measures to reduce tooth harmonics and electromagnetic noise. Increasing the rigidity of the base can reduce vibrations and noise caused by the fundamental frequency rotating field in the stator and rotor air gaps. Improving the uniformity of air gap assembly and the stacking quality of the core also contribute to a decrease in electromagnetic noise.
2Aerodynamic Noise
The aerodynamic noise of motors includes two types: vortical noise and whistling noise. Vortical noise is primarily caused by the turbulent flow of cooling air around the rotor and fan, which alternates with vortices on the rotating surface. Whistling noise, on the other hand, is produced by compressing air or air passing over a fixed obstacle. The whistling noise within the motor is mainly caused by radial ventilation grooves. Whistling noise intensifies as the gap between rotating and fixed components decreases. Therefore, using a sealed noise-insulating cover can reduce the noise.“Sealed”Inside the sound-insulating hood, effectively reducing whistle noise can be achieved by increasing the gap between the rotating and fixed parts, improving the shape of the air guide hood, and using wind blades with uneven distribution and non-equal spacing in length. Additionally, reducing the circumferential speed of the rotor surface, the surface area of the motor, and the roughness of the rotor surface can also lower aerodynamic noise.
3Reversing Noise
Directional noise, also known as brush noise. In motors with slip rings and commutators, directional noise is unavoidable and can sometimes become a major noise source. Directional noise is caused by three factors:①Friction Noise. Friction noise inevitably occurs at the sliding connections between the commutator and the sliding rings and rectifiers. The magnitude of the friction noise is related to the surface condition of the sliding rings and rectifiers, the friction coefficient of the brushes, and the air's absolute...*Humidity is related to brush pressure. Therefore, methods to reduce friction and noise include improving the smoothness and roundness of the working surface of the slip ring and commutator; ensuring the air's insulation.*Humidity must not be below5g/m3Clean the brush surface; ensure the brush material is not too hard; appropriately reduce the commutator diameter.②Collision noise. This is due to the presence of mica grooves between the commutator segments, which, due to commutator deformation, have poor undercut and chamfering processes. This causes the brush to often collide with the commutator segments as the motor rotates, leading to periodic impacts between the commutator segments and the brush. This results in radial跳动 and oscillation of the brush, causing periodic vibration between the brush and its holder, generating commutation noise. Noise reduction countermeasures include confirming whether the commutator is eccentric or deformed; if the insertion dimensions of the commutator are within specification; if the brush height is out of tolerance; if the brush is severely deformed, has stacked electrodes, or three-electrode distortion, etc.③Spark Noise. It is caused by sparks generated during the contact conduction process between the commutator or slip ring and the brushes. Spark noise increases with the size of the commutator sparks. Factors such as commutator deformation, poor surface working condition, shock loads, frequent overloads and stalls, high rate of current change, low temperature around the motor, high dust content in the air, harmful gases, and high room temperature can all lead to an increase in commutator sparks. Therefore, confirming the surface working condition, checking for long-term overloads and stalls, testing the dust content in the air, ensuring the room temperature meets requirements, and checking for harmful gases are all methods that can reduce this noise.
4Mechanical Noise
The rotational noise of motors is primarily mechanical in nature, and large, high-speed motors are prone to such noise. Poor dynamic balance of the rotor can lead to mechanical vibrations and noise.*One common cause is improving the rotational balance accuracy of the rotor, which can effectively reduce this noise. Poor installation or adjustment, as well as the natural frequency of the stator and rotor components coinciding with the rotational speed frequency, can also generate mechanical noise. When the motor is equipped with an end cover-type fan shroud, the cover is often shaken by the motor's vibration and causes vibration, thus producing noise. In this case, the vibration of the motor's stator is often the excitation source for the end cover or fan shroud. To reduce this vibration noise, the measure is to increase the dynamic stiffness of the end cover and the shroud. Adding vibration-dampening materials, such as felt, at the junction of the end cover and stator can reduce the amplitude of the stator's vibration.
5Load Noise
The primary cause of this noise is due to manufacturing tolerances, assembly gaps, and damage to working surfaces during operation, transportation, and installation, which include wear and electrical corrosion. These factors lead to uneven bearing operation and irregular impacts. For instance, issues such as the shaft surface roughness not meeting specifications, the bearing bore roundness exceeding limits (stator or end cover), poor material quality in the bearing bore, and slight scratches on the armature shaft extension. Noise reduction strategies include: during the bearing processing and incoming inspection, it is crucial to strengthen the testing and inspection of the inner wall smoothness, roundness, and surface roughness of the bearings; confirm whether the oil holes or material of the bearing inner wall meet the requirements, and whether the bearing bore roundness is within the allowable limits; carefully inspect the shaft head to ensure the end cover side shaft head is qualified; and check for scratches on the shaft extension during the shaft stringing and riveting process.
In summary, to effectively control and reduce electrical motor noise pollution, it is essential not only to maintain strict quality control during production but also to master the techniques of monitoring, diagnosing, and identifying motor noise to implement effective noise reduction measures.
