How to identify the source of motor noise —— Part one?

Motors play a pivotal role in modern industrial production. During daily equipment inspections, we will find that the running motor emits various abnormal noises, and this long-term "abnormal operation" state seriously threatens the safe operation of the motor. In order to detect and eliminate abnormal phenomena in time, we must understand the motor manufacturing and assembly process in detail, and accurately identify the main noise source.

1. Noise source

General motor noise sources can be divided into mechanical noise, electromagnetic noise, aerodynamic noise and so on.

1.1 Mechanical noise

Motor stator and rotor friction, dynamic balance damage, bearing and bearing sleeve wear, and motor body resonance form mechanical noise. The detailed reasons are as follows:

(1) The bearing is damaged or poorly assembled. When the motor rotates, place one end of the listening rod on the bearing end cover, and place the other end on the earlobe with your finger to listen to whether the sound of the bearing rotation is uniform, and whether there is periodic "gulong," "Crumbling" sound, if there is any abnormal sound, it means that there is a problem with the bearing, which is generally caused by a serious lack of oil in the bearing, impurities in the oil, unqualified product quality or bearing wear. For large high-voltage motors, such as improper assembly of motor bearings, wear of bearing sleeves, and loosening of bearing lock nuts will cause abnormal noises from the bearings.

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

(3) The stator and rotor cores are loose.

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

(5) For a newly wound motor, the interphase insulating paper or slot protrudes out of the slot and rubs against the rotor.

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

(7) The iron core is loose or short circuit between sheets, and the groove teeth are damaged.

(8) The fan is rubbing against the hood or the fan is unbalanced and the wind is loose.

(9) There are debris in the machine, and foreign objects enter the motor.

(10) The coupling connection is loose.

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

(12) The fasteners are loose.

(13) The carbon brush commutator is rubbed.

(14) The foundation is uneven or the installation is not good, and the footing is unstable. During installation, the motor is not aligned (or the alignment is not good), and the motor shaft and the mechanical load shaft are not concentric.

1.2 Electromagnetic Noise

Electromagnetic noise is a low-frequency noise generated by the structure and vibration of the stator, rotor and the entire motor caused by the alternating magnetic field in the air gap of the motor. Electromagnetic noise is mainly due to the periodically changing radial force or unbalanced magnetic field force generated by the magnetic field in the air gap (including the fundamental wave magnetic field and various high-order harmonic magnetic fields), which causes the stator and rotor cores to produce hysteresis contraction and vibration. The 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 core is not tightly stacked.

(2) The lengths of the stator and rotor are not well matched (too much difference).

(3) The radial vibration of the rotor core.

(4) The winding pitch is wrong.

(5) The inclination of the rotor slot is not enough.

(6) The coil in a certain pole phase group is reversed.

(7) There is a branch circuit in the parallel winding, the stator winding is asymmetrical or short circuit between turns.

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

(9) The voltage and frequency change greatly. The voltage is seriously unbalanced and the frequency is too high, causing the electromagnetic sound to increase.

1.3 Aerodynamic noise

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