1. Mechanical noise

Mechanical noise is caused by friction between the motor's stator and rotor, destruction of dynamic balance, wear of bearings and bearing sleeves, and resonance of the motor body. The detailed reasons are as follows:

(1) The bearing is damaged or poorly assembled. When the motor is rotating, place one end of the listening stick on the bearing end cover, hold the other end against your earlobe with your finger, and listen to see whether the bearing rotation sound is uniform and whether there are periodic "rumblings, rumblings, or rumblings". "Gurgling" sound. If there is any abnormal sound, it means there is a problem with the bearing. It is usually caused by a serious lack of oil in the bearing, impurities in the oil, substandard product quality, or bearing wear. For large high-voltage motors, if the motor bearing is not assembled in place, the bearing sleeve is worn, and the bearing lock nut is loose, it will cause the bearing to make abnormal noise.

(2) The dynamic balance of the motor rotor is damaged. The rotor is unstable or the shaft is bent, causing the rotor to vibrate. At the same time, the machine base vibrates and produces noise.

(3) The stator and rotor cores are loose.

(4) The uneven air gap between the stator and the rotor causes mutual friction.

(5) In newly wound motors, the interphase insulating paper or slots protrude outside the slots and rub against the rotor.

(6) Vibration of components (end shield, wind shield, outlet box cover, etc.).

(7) The iron core is loose or short-circuited between pieces, or the grooves are damaged.

(8) The fan rubs against the windshield or the fan is unbalanced and loosened by the wind.

(9) There are debris inside the machine and foreign objects have entered the motor.

(10) The coupling connection is loose.

(11) The bearing is poorly installed or the bearing is damaged.

(12) Fasteners are loose.

(13) The carbon brushes and commutators rub against each other.

(14) The foundation is uneven or poorly installed, and the footing is unstable. During installation, the motor is not aligned (or is poorly aligned), and the motor shaft is not concentric with the mechanical load shaft.

2. Electromagnetic noise

The alternating magnetic field in the air gap of the motor causes the structure and vibration of the stator, rotor and the entire motor to produce a kind of low-frequency noise called electromagnetic noise. Electromagnetic noise is mainly caused by the magnetic field in the air gap (including fundamental magnetic field and various high-order harmonic magnetic fields) that produces periodically changing radial forces or unbalanced magnetic field forces that cause hysteresis contraction and vibration of the stator and rotor cores. Caused by electromagnetic noise, electromagnetic noise accounts for about 20% of the total motor noise. The specific reasons are as follows:

(1) The stator and rotor slots are not properly matched, and the iron cores are not tightly stacked.

(2) The stator and rotor lengths do not match well (too much difference).

(3) Radial vibration of the rotor core.

(4) The winding pitch is incorrect.

(5) The rotor slot slope is not enough.

(6) The coils in a certain pole phase group are connected reversely.

(7) There is a branch circuit break in the parallel winding, the stator winding is asymmetrical or the turns are short-circuited.

(8) The cage bars of the cage rotor are open or disconnected.

(9) Voltage and frequency change greatly. Seriously unbalanced voltage and excessive frequency cause electromagnetic sound to increase.

3. Aerodynamic noise

When the motor rotates, certain protruding parts on the fan and rotor cause air impact and friction to form aerodynamic noise. It increases with the increase of fan and rotor peripheral speed. The rotation of the fan forms a broad-band continuous noise, which accounts for a large proportion of this noise (especially high-speed motors with a speed above 1500r/min). The intensity of fan noise is determined by improper design of the fan (blade angle, width, motor speed), air inlet and outlet, and air duct. The main part of aerodynamic noise accounts for about 75%.

How to control motor noise?

1. Reasonably design the structure of the motor to reduce noise

(1) Correct selection of fan material and structure: For a single-rotation high-speed motor, a streamlined backward-inclined centrifugal fan can be used. For centrifugal fans, the noise of the centrifugal fan with a reverse ring is lower than that without a reverse ring; in addition, the basin type The fan has lower noise than the blade fan; the aluminum fan has lower noise than the nylon fan.

(2) Improve the air path: Increase the gap between the outer edge of the fan and the fan cover or the inner cavity of the end face, remove obstacles in the air duct, smooth the air flow direction, and improve noise.

(3) The stator winding adopts reasonable short torque.

(4) The asynchronous motor rotor adopts a relatively inclined double chute structure to reduce axial force; the DC motor adopts an uneven air gap. AC motors use magnetic slot wedges, which can not only reduce harmonic losses and improve efficiency, but also reduce electromagnetic noise caused by harmonic magnetic fields.

(5) When the motor in use produces "sweeping", the air gap can be appropriately increased to reduce the air gap magnetic density. When the motor power has margin, part of the rotor circumference can be machined to increase the air gap and eliminate the noise caused by high-order harmonics. However, while reducing the no-load current, the no-load current will be increased and the power factor will be improved. reduce.

(6) Properly control the ripples, pits, roughness and radial clearance of the bearing rolling surface.

(7) Improve the surface processing accuracy and smoothness of the commutator to reduce brush noise.

(8) Increase the stiffness and balance of the machine base. If necessary, use a spirit level to check the level of the foundation; visually check whether the installation angle of the motor and the dragged machinery are suitable.

2. Ensure excellent assembly technology

(1) Choose high-quality bearings. The coordination between the bearing and the rotating shaft or the bearing and the bearing seat should be appropriate, and the temperature and time of the bearing hot sleeve should be controlled well. (Heat the bearing to about 100℃. The non-sealed bearing can be boiled in engine oil for about 5 minutes. When the inner ring swells, quickly put the bearing on the journal. After the bearing cools and shrinks, the inner ring of the bearing will be tightly Fixed on the journal. For sealed bearings, because the inside is already coated with grease, do not use oil to heat them. You can use electric heating to heat the bearings evenly and then put them on the bearings. Our company now uses electric bearing heaters for all heating)

(2) Poor dynamic balance of the rotor is the main cause of mechanical noise. Therefore, it is necessary to improve the accuracy of dynamic balance inspection of the rotor, minimize the influence of eccentricity, and ensure the concentricity of the coupling when the motor is installed.

(3) The lubricating oil should be selected appropriately and free of impurities. Grease has high viscosity and low noise. But when the viscosity is too high, there will be stirring sounds. Grease filling amount: For non-sealed bearings, the bearing must be filled with grease. The grease can be squeezed in from one end of the bearing and squeezed out from the other end of the bearing to fill the bearing with grease. Apply it with your fingers on both ends. Just flat. The amount of grease applied to the inner cavity of the bearing cover should be 1/3-2/3 of the internal space of the bearing chamber.

(4) Different types of bearings must be installed according to the requirements of their installation processes. In principle, the method of hitting the bearing with a copper rod is not allowed, otherwise the bearing will be damaged due to uneven stress on the inner ring of the bearing. When using the hot-sleeving method to assemble a bearing, you must carefully check the matching dimensions of the bearing and the journal in advance. Because hot-sleeving and cold-sleeving are different, it is not easy to detect the matching tolerance and interference between the journal and the bearing when the bearing is hot-sleeved. Whether the degree is appropriate. After the bearing is hot-sleeved, the motor should not be moved or other accessories should be assembled to prevent the bearing from shifting.

(5) Before assembling the motor coupling, the matching size with the shaft diameter must be measured (generally interference fit = 0). At the same time, check the tightness of the keyway and key at the end of the rotating shaft to prevent noise caused by loosening of the key after the motor rotates.

3. Other noise reduction methods

(1) For large-capacity high-speed motors with a capacity exceeding 10MW and a rotation speed exceeding 1000r/min, the most effective way to reduce noise is to use a rigid isolation cover (sound-absorbing material pasted on the inner surface) to cover the motor.

(2) Install a muffler in the part that generates the strongest airflow noise, which has low resistance to the airflow and does not affect the heat dissipation of the motor and facilitates installation and removal.

(3) Avoid mechanical impact on the machine base during transportation.

(4) The motor absorbs electrical energy from the power supply, converts it into mechanical energy and then outputs it from the shaft. Therefore, dynamic reactive power compensation and filtering devices are adopted in the power grid to make the harmonic components in the power supply meet the specification requirements, improve the quality of power supply , and ensure voltage and frequency. Qualified, three-phase voltage balance to control motor noise.

(5) The bearing cover should not be opened when the motor is running; keep the motor clean; replace the lubricating oil regularly; clean the commutator surface frequently to maintain good lubrication contact.