Noise is a combination of chaotic sounds of various frequencies and intensities, and its harmful effects on humans are well-known. Noise is often accompanied by vibration, and excessive vibration can damage other equipment. Vibration noise levels reflect the quality of product design and manufacturing, and are an important indicator of product quality. The importance of noise reduction is now widely recognized. Motor designers, manufacturers, and operators need more knowledge about motor noise. For example, they need to understand how motor noise is generated, what factors it is related to, and how to predict the noise generated during actual operation of the motor during the design phase, while also knowing methods to reduce noise.
Electromagnetic noise is generated by the vibration of certain mechanical parts or spatial volumes caused by alternating changes in electromagnetic fields. For electric motors, unstable power supplies can also induce stator vibration and generate noise. The main characteristics of electromagnetic noise are related to factors such as the characteristics of the alternating electromagnetic field, the size and shape of the forced vibrating parts, and the space itself. Higher-frequency electromagnetic noise is also known as electromagnetic howling.
Analysis of the causes of electromagnetic noise
When the motor is running, there is a fundamental magnetic field and a series of harmonic magnetic fields in the air gap. These magnetic fields interact to generate tangential force, which in turn generates tangential electromagnetic torque and radial force that varies with time and space.
Under normal circumstances, rotating radial electromagnetic force waves of various frequencies and frequencies exist in the air gap of a motor. Each radial force wave acts on the stator and rotor cores respectively, causing the stator core, frame, and rotor to undergo radial deformation that changes periodically over time, i.e., vibration. The vibration frequency is the frequency of the force wave. Because the rotor core has high stiffness, the resulting vibration is very small; therefore, generally only the vibration of the stator core and frame is considered. Electromagnetic noise is mainly airborne noise caused by the pulsation of the surrounding air due to the stator vibration.
The lower the order of the radial force wave, the greater the distance between two adjacent support points of the core bending deformation, the relatively poor the core stiffness, and the greater the radial deformation. The amount of stator core deformation is approximately inversely proportional to the fourth power of the force wave order and directly proportional to the force wave amplitude. Therefore, low-order radial force waves with large amplitudes are the main source of electromagnetic noise. In addition, it should be noted that the core and the frame have certain natural vibration frequencies. When the radial force wave frequency is close to or even the same as this natural frequency, resonance will occur, and the core vibration and radiated noise will increase significantly.
Because the fundamental magnetic field has a large amplitude, and the motor cannot operate without it, the harmonic noise it generates is unavoidable. However, due to its high force wave number (except for 2-pole motors) and low frequency, the noise radiation efficiency is relatively low. Therefore, except for high-power 2-pole motors, harmonic noise is generally low.
Analysis of the main noise sources of motors
The main noise sources of motors are electromagnetic noise, mechanical noise, and ventilation noise.
●Electromagnetic noise
The interaction of magnetic fields in the air gap of the motor generates radial forces that vary with time and space, causing the stator core and frame to deform periodically over time, i.e., the stator vibrates; electromagnetic noise is mainly caused by the vibration of the stator, which causes the surrounding air to pulsate, resulting in airborne noise.
●Mechanical noise
Mechanical vibration and noise caused by centrifugal force due to rotor mechanical imbalance, bearing vibration noise, sliding contact noise between brushes and slip rings or commutator, and axial vibration noise of end caps excited by bearing vibration, etc.
●Ventilation noise
Noise from air vortexes generated by the rotation of fans or other ventilation components and rotors; single-frequency noise caused by the periodic pulsation of cooling air due to fan rotation or by gas impacting obstacles; and "whistling" noise caused by resonance of thin-walled parts in the airflow path or by unreasonable airflow design.
Electromagnetic noise identification of electric motors
The electromagnetic noise of an electric motor varies with the strength of the magnetic field, the magnitude of the load current, and the speed. This characteristic can be used to identify the noise.
● Sudden Power Cut-off Method. Since mechanical inertia is much slower than electromagnetic transients, a sudden power cut-off, with no electromagnetic influence, will cause the motor speed to remain almost unchanged. If the motor noise suddenly disappears or significantly decreases at this time, it can be concluded that the noise was caused by electromagnetic factors.
● Voltage changing method. Since the speed of an asynchronous motor does not change much with voltage, the mechanical noise and ventilation noise remain basically unchanged when the voltage is changed, but the electromagnetic noise changes greatly with voltage.
● Drag test. Use a low-noise motor to drive the noisy test motor. If the noise decreases or disappears, it indicates that the noise of the driven motor is electromagnetic noise.
Electromagnetic noise calculation
Electromagnetic noise is one of the main noise sources in motors. It is particularly prominent in multi-pole motors or motors with low ventilation noise. Generally, it increases with motor power and is the root cause of increased noise under load. This noise is closely related to the motor's electromagnetic design parameters. Improper design can lead to significant electromagnetic noise, potentially becoming the primary noise source, surpassing other noise levels. Therefore, studying the causes of electromagnetic noise, the relationship between motor design parameters and electromagnetic noise, and methods for calculating electromagnetic noise are crucial for predicting and controlling motor noise during the design phase.
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