Sound creates vibrations, and without vibrations there would be no sound. Therefore, when it comes to motor noise, we must first understand the causes and types of vibrations produced by the motor.
The main reasons for abnormal vibration and abnormal sound produced by asynchronous motors are mechanical and electromagnetic.
Mechanical reasons:
1. The motor fan blades are damaged or the screws fastening the fan blades are loose, causing the fan blades to collide with the fan blade cover. The sound produced will vary from loud to small depending on the severity of the impact;
2. Due to bearing wear or improper shaft, serious eccentricity of the motor rotor will cause the stator and rotor to rub against each other, causing the motor to produce violent vibrations and uneven scratching sounds;
3. The anchor screws of the motor are loose or the foundation is weak due to long-term use, so the motor produces abnormal vibration under the action of electromagnetic torque;
4. The motor that has been used for a long time will cause abnormal hissing or grunting sounds in the bearing chamber of the motor due to the lack of lubricating oil in the bearing due to grinding operation or damage to the steel balls in the bearing.
Electromagnetic reasons:
1. A motor that is running normally suddenly makes abnormal noises. When running with a load, the speed drops significantly and makes a low roar. It may be that the three-phase current is unbalanced, the load is too heavy, or it is running in a single phase;
2. For a motor that is running normally, if a short-circuit fault occurs in the stator or rotor windings or the squirrel cage rotor is broken, the motor will make a high and low buzzing sound. The fuselage also vibrates.
Basic types and characteristics of motor noise
1. Electromagnetic noise
Frequency multiplier noise: multiples of the power frequency
Tooth harmonic noise: Noise caused by eccentricity of the stator and rotor, improper slot matching, and unbalanced rotor.
Slip noise: caused by loose fit between the blades and the shaft.
2. Air duct noise
Whistle noise: The rotation of the fan causes the gas to pulsate periodically and the airflow to collide with solid objects to produce a modulating sound, which is what we usually call the "whistle effect"
Eddy noise: The vortex sound generated by the surrounding gas when the fan blades rotate, and its spectrum range is wide.
3. Mechanical noise
It mainly includes bearing noise, friction sound generated by mechanical components such as chamber scanning.
Summary
1. The peak frequency of frequency multiplication noise is generally 2 or 4 times the power frequency and does not change with the change of motor speed.
2. The peak frequency of tooth harmonic and slip noise increases or decreases correspondingly with the change of motor speed. The noise will disappear after the motor is powered off.
3. The peak frequency of air duct noise increases or decreases with the change of motor speed, but the noise of the whole machine will continue for a period of time after the motor is powered off.
4. Mechanical noise is relatively easy to distinguish. It is usually caused by motor sweep, bearing damage or poor assembly of the whole machine.
Methods to reduce electromagnetic noise of asynchronous motors:
1. Reasonably select the air gap magnetic density.
2. Select the appropriate winding form and number of parallel branches
3. Increase the number of stator slots to reduce the harmonic distribution coefficient
4. Suitable groove matching
5. Use magnetic slot wedges
6. Rotor chute
For products that have been produced, the electromagnetic noise is relatively large:
1. Appropriately increase the moment of inertia of the machine base section to avoid the resonance zone;
2. The synchronous salient pole machine can appropriately increase or decrease the pole shoe width through calculation to improve the magnetic field distribution, making the fundamental wave closer to the sine wave, thus reducing the high-order harmonic components and achieving the effect of reducing electromagnetic noise;
3. Choosing a more appropriate number of stator winding wiring rotations can effectively reduce the reversal wave generated by the motor winding, thereby reducing noise;
4. For those with high tooth harmonic content, magnetic slot shoes can be used.
As for the newly designed motor:
1. Choose the appropriate number of slots to match;
2. Choose the appropriate pole distance;
3. Increase the number of parallel branches;
4. For salient pole machines, the appropriate pole shoe width must be selected;
5. When the motor performance is guaranteed, appropriately reduce the air gap magnetic density;
6. The concentricity of the stator and rotor is ensured through technology, so that the unilateral magnetic pulling force tends to zero.
The main contradictions in the electromagnetic noise of the motor are the stator and rotor slot fit, the rotor chute and the concentricity of the stator and rotor. The main noise reduction measures are to select high-quality stator and rotor slots and a reasonable rotor chute width, while making the motor air gap as uniform as possible.
So,It is impossible to completely eliminate electromagnetic noise, and this is true for any motor;
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