Suppressing the Mechanical Noise of Motors from Design and Manufacturing

Motor mechanical noise has many reasons, now from the motor design and manufacturing point of view to discuss how to inhibit or improve the mechanical noise of the motor.

Motor bearing noise reduction

The use of high-quality bearings, the selection of bearing models and sizes, the appropriate design of bearing components, the improvement of the manufacturing accuracy of the parts matching the bearings, the reasonable selection of the matching grade, the clean installation environment, and the civilized installation process have significant effects on reducing motor vibration and noise. At present, in large motors or micromotors with fractional horsepower and below, sliding bearings are generally used, especially in micromotors. Most of them require low noise, and the cost of using high-precision rolling bearings is too high. The powder metallurgy oil bearing developed in recent years can ensure that the bearing noise is not more than 40dB, even up to 30dB.

For rolling bearings the following measures can be taken to reduce their noise.

·Spring coil (such as corrugated spring) is used to apply axial preload to the bearing, which can reduce the vibration caused by unconstrained movement of rolling elements. However, the corrugated spring must have enough thrust to track the excitation of axial vibration, and it should not be too hard. The force it acts on the outer ring of the bearing should be relatively uniform, or it may have the opposite effect or fail to work. The optimum value of axial preload can be determined by experiment. The experimental data show that when the axial preload is 196-490N, the vibration speed is low.

·Add an elastic shock absorber to the supporting device. This usually helps to reduce vibration and noise with frequencies higher than a few hundred Hz.

·Choose bearings with synthetic plastic retainers or strong mechanical retainers that are quieter at high speeds.

·Use bearings with shrouds or gaskets to prevent dirt or foreign matter from intruding into the bearings.

·The inherent frequency of the outer ring of the bearing shall be such that it is detuned from the inherent frequency of the supporting member.

- Choose the appropriate working clearance. From the noise point of view, the working clearance than the bearing corrugation, roughness, motor machining accuracy, with tolerances and installation process are secondary, but it should have a relative optimal value range.

Brush - commutator noise reduction

In a certain operating environment, reasonable design can reduce brush noise. In order to minimize the noise, the optimal length of the brush extending out of the brush box (calculated by the center line of the carbon brush) is 2~4mm. Brush noise varies with brush pressure, and the optimal pressure is 0.0392~0.0490MPa; When the gap between the brush and the brush holder is 0.2mm, the noise is minimum.

In order to make large motors or small high-speed motor brush - commutator noise minimization, the manufacturer must also carry out a series of experiments, from the perspective of noise to select the appropriate brush material and brush device parameters in line with the noise technical requirements.

For medium and large motors, one method of controlling brush-commutator noise is to install a sealing cover on the commutator end.

Rotor unbalance noise reduction

The key to reduce the noise caused by rotor imbalance is to improve the accuracy of dynamic balance calibration or limit the rotor imbalance to a reasonable limit. The International Organization for Standardization (1S0) has issued the most economical and reasonable balance quality grade of different motor rotors and the allowable residual unbalance amount per unit rotor mass (kg). Generally speaking, improving the rotor dynamic balance accuracy may reduce the noise by 3-5 dB.

The technical documentation specifies the permissible residual unbalance value per unit of rotor mass or the permissible total eccentricity value. The higher the speed, the more stringent the requirements. However, this only applies to "rigid" rotors, i.e. rotors running at speeds well below their first critical speed. For the actual "flexible" rotor, that is, the operating speed between the first and second critical speed rotor, the total permissible eccentricity value should also be smaller.

It can also be pointed out that, for a completely balanced motor if with other equipment sets, then due to the shaft system connection, bearing support stiffness and vibration form of change and other reasons, for example, is connected to the equipment itself is not balanced, did not find a good center, etc., still may not allow the occurrence of vibration and noise. Therefore, the entire rotating system should be calibrated and balanced under actual operating conditions.

In addition, attention should also be paid to the stability of the balance, especially after the operation of the heat. At the same time, can not be ignored on the coaxial installation of other components such as fans, keys, etc. Dynamic balance. 2 pole turbine generator rotor, due to the slot embedded in the windings, the two main axes of the bending stiffness is not the same, in this case, even if the rotor school balanced often can not be completely eliminated with two times the rotor rotating frequency of the mechanical noise. In particular, if the bearing support seat or some components of the inherent frequency is equal to or close to twice the rotor rotational frequency, the frequency of the noise and vibration may be high to an impermissible degree.

Control of aerodynamic noise

·Fan noise control. Fan in the motor due to space constraints, can not be installed to improve the efficiency of some of the additional components, the efficiency is much lower than the industrial fan, the noise is higher. In principle, to reduce the motor ventilation noise can be reduced from reducing the air volume, reduce wind resistance, improve efficiency (from the fan type, etc.), as well as possible to reduce the fan diameter and speed and other aspects of efforts. Such as: (1) fan blades with uneven spacing fan blades with uneven spacing, can reduce the intensity of regular pressure disturbances caused when the uniform; (2) fan blades using porous materials; (3) the use of special fan - rotor coupling. The fan of the variable speed motor supplies enough cooling air at low speeds, while at high speeds the cooling air is excessive, thus increasing the aerodynamic noise. By using a rotor coupling, the fan speed can be kept constant.

·Control of aerodynamic noise. In medium-sized and large motors, cooling air through the axially aligned stator and rotor ducts can produce whistle noise; an effective way to reduce this noise is to place sound-absorbing materials near the stator radial duct opening.

·The whistle noise can also be caused by other reasons, such as the gap between the rotor bar and the end of the stator winding of the high-speed induction motor. The main noise can be reduced by 20dB or more by using a closed hood or a closed ventilation system. However, due to the reflection of the cover wall, the sound pressure level in the cover will be higher than the value at the same point without the cover.

·Control of air inlet and outlet noise. The use of sound-absorbing materials made of mufflers to reduce the air inlet and outlet power noise, in many cases are very effective. China's new series of medium-sized motors, the power per unit mass has increased substantially, so it is necessary to increase the flow of cooling air. Enhanced ventilation will inevitably lead to an increase in motor ventilation noise. For this reason in the box-type structure of the anti-dripping medium-sized high-voltage motor on the upper part of the muffler.