Abstract: This paper mainly describes the noise measurement and noise limit of cage three-phase asynchronous motor, and analyzes the noise failure caused by no-load and load operation of the motor. The hidden trouble of mechanical and electromagnetic noise and the repair preventive measures of cage three-phase asynchronous motor in no-load operation are discussed.

Key words: three-phase asynchronous motor; Bearing noise; Mechanical fit noise; Electromagnetic noise; Dynamic mechanical properties

Introduction

As a main power source of mechanical equipment, the motor, especially the cage three-phase asynchronous motor, is widely used in all walks of life because of its simple structure, easy maintenance and reliable operation. In order to ensure the quality of the motor and ensure its long-term safe operation, the motor test and operation should master the various hidden faults of the motor, among which the motor noise is the most intuitive manifestation of hidden faults in the motor operation process, and is the initial signal of the motor failure. If the fault is not handled in time, the fault will be further expanded. Abnormal noise occurs, which damages the motor and affects production. The following on the motor noise test methods, limits and test and use of common noise failures to do an analysis and explanation for reference.

1. Measurement methods and limits of motor noise

Motor operation will produce different sounds, motor size, type, protection installation structure type, running speed is different, the main sound source of its noise is also different. The main noise source of high-speed motor is ventilation noise. The electromagnetic noise and bearing noise of the motor running at medium speed and low speed are more obvious. Under normal circumstances, the noise generated during the operation of the motor is required to be within the range specified in the national standard GB10069-2008 "Rotating motor noise measurement methods and limits Part III: Noise limits", and the abnormal sound is noise beyond the standard range and should be dealt with. Cage three-phase asynchronous motor noise measurement method, according to the regulations, the shaft center height of the motor is not greater than 180mm, the hemisphere method is used to measure, the shaft center of the motor is higher than 355mm, the parallelepiped method is used to test, the center height between this method can be selected, the measurement point from the motor surface vertical distance of 1m. In no-load state, A counts the sound power level, and the cooling mode is IC411, IC511, IC611 single-speed three-phase cage asynchronous motor. The noise limit specified by the center height is shown in Table 1, where the sound power level limit of 60HZ motor is increased by 5dB(A) for 2-pole motors, 3dB(A) for 4, 6, and 8-pole motors, and the cooling mode is IC01. IC11, IC21 motor acoustic power level limit 2, 4 pole motor increased by 7dB(A), 6, 8 pole motor increased by 4dB(A). Motor noise is divided into mechanical noise and electromagnetic noise according to its different mechanism.

Table 1 The noise limit table of the motor with different pole number of center and high power

2 Analysis of mechanical noise fault of cage motor during factory inspection

The mechanical noise faults of squirrel-cage three-phase asynchronous motor in factory inspection mainly include bearing noise, mechanical matching noise, friction noise, ventilation noise and vibration noise during motor operation. In general, when testing the dynamic mechanical properties of the motor, the noise of the motor is checked by hearing and the fault is judged, and the motor runs smoothly, the sound is light and uniform, and there is no harmful noise.

2.1 Bearing noise

In the assembly process of the motor, if the bearing is damaged or the bearing is subjected to undue external force after assembly, the bearing noise will be significantly increased, and accompanied by abnormal noise, then it is necessary to find out the cause of the bearing damage or the harmful external force, to eliminate, and replace the bearing if necessary to ensure the good operation of the motor.

Since the bearing is the main mechanical force component of the motor operation, there is a gap between the rolling body, the support frame and the inner and outer rings of the bearing during the bearing operation, if the bearing is mixed with debris, it will cause the bearing components to collide with each other, resulting in vibration and noise. The noise value is related to the dimensional accuracy, surface roughness and form and position tolerance of the grooves of the rolling element and inner and outer ring. The use of precision bearings can reduce bearing noise, but sometimes after the use of precision bearings, the noise increases, the reason may be that the shaft and the bearing inner ring is too tight, so that the inner ring deformation of precision bearings is greater than the deformation of ordinary bearings, resulting in pulsation, vibration, resulting in increased motor noise, temperature rise. Therefore, the dimensional tolerance of bearings, bearing chambers and shafts is an important factor in controlling bearing noise.

The following methods are usually adopted to reduce bearing noise: using sealed bearings to prevent debris from entering; In the bearing assembly, ensure strict demagnetization cleaning process, wash away oil and iron filings, the cleaned bearing noise is lower than the bearing before cleaning; The grease shall be clean and shall not contain any iron filings, dust and impurities; The match between the bearing outer ring and the bearing chamber, the match between the inner ring and the shaft, generally should not be too tight, the match between the bearing outer ring and the bearing chamber, the radial clearance should be in the range of 3 ~ 9μm; In order to eliminate the axial clearance of the rotor, it is necessary to apply appropriate pressure to the bearing. Usually, waveform spring washers or three-point elastic washers are selected and placed on the extension end of the shaft. For motors with particularly strict noise requirements, low noise bearings should be selected; When the load is not too large, oil bearing can be used, which is lower than the noise of rolling bearings of the same size.

2.2 Mechanical fit noise

If the motor emits a low-frequency "buzz" sound during operation, it is generally due to eccentric processing or installation of the end cover, there is stress, resulting in uneven air gap of the motor rotor, and the sound is continuous, and the end cover can be knocked to eliminate stress or the end cover can be refastened at the exchange Angle; If there is a intermittent "buzz" sound at start or stop, it may be running, because the rolling friction is relatively low at low speed. In some cases, when the rolling body of the bearing rotates in the inner and outer ring raceway, the force that drives the outer ring or the inner ring to rotate is relatively large, such as the bearing chamber size is too large, or the journal size is too small, the outer ring or the inner ring may rotate, which will produce intermittent friction sound. This is often accompanied by intermittent vibration. ; If there is a high-frequency sound, it may be the outer small cover of the bearing stop ring friction bearing. As the running time increases and the temperature increases, the friction sound will tend to be obvious. At this time, the outer small cover should be removed and the inner contact surface of the car small cover should be repaired.

2.3 Phase erase noise

The friction noise of the motor during the bare machine test is divided into real friction and virtual friction, real friction is the mutual friction of the stator core and the rotor of the motor, commonly known as "sweep boring", sweep boring will cause local heat of the motor, long-term operation is easy to cause local turns of the motor and burn out, which is harmful to the motor. The main cause of motor boring is uneven stator overlaying or local deformation in the process of stator pressing, which should be dealt with according to the area of the boring or on the lathe. Scratch is the insulation paper of the motor winding, slot or binding tape rub with the rotor, the motor end insulation cut is not uniform, the joint part of the binding tape is too long, the paint is tilted in the inner side of the motor stator end, and the individual groove of the motor is skewed, which may cause the motor scratch. For the motor with virtual friction, the motor is generally disassembled for trimming and leveling.

2.4 Ventilation noise

The ventilation noise of the motor is caused by the flow of air caused by the rotating rotor and the cooling fan rotating with the shaft. The faster the flow and the more drastic the change, the greater the noise. The ventilation noise is related to the speed, the shape of the fan and rotor, roughness, unbalance and the change of the air duct section and the shape of the air duct. The fan noise of high-speed motor often occupies the main position in the noise of motor.

The main measures to reduce the ventilation noise are as follows: eliminate the fan as far as possible for the motor with good heat dissipation or low temperature rise to eliminate the noise source; For external fans, try not to leave ventilation margin in the design, and preferentially use axial flow fans; The connection between the external fan and the rotating shaft is not connected by a key, but by knurled straight grain technology; The external fan should have uniform thickness, no distortion, uniform spacing, and should be dynamically balanced; The obstacles in the air duct should be reduced as far as possible, and the flow shaped air duct should be used if there is a special air duct, and the cross section change of the air duct should not be sudden; The surface of the rotor should be as smooth as possible.

2.5 Vibration and Noise

Motor vibration will produce noise, especially resonance, to deal with such problems need to be eliminated by solving the vibration problem. The cause of motor vibration can be simply judged from the vibration value of three directions, the horizontal vibration is large, the rotor balance is poor; The vertical vibration is large, the installation is not in place; High axial vibration and poor bearing assembly quality. More than 4 pole multi-pole motors generally do not cause vibration noise because of motor manufacturing quality problems, vibration noise is common in 2-pole motors.

3 Electromagnetic noise fault analysis of cage motor factory inspection

Electromagnetic noise is a kind of eddy sound caused by the uneven air gap between the stator rotor, which is mainly produced by the radial component of the air gap magnetic field acting on the stator core. The stator core vibrates and deforms by propagating outward through the yoke. The second is the tangential component of the air gap magnetic field, which is contrary to the electromagnetic torque and causes the core teeth to deform and vibrate. When the radial electromagnetic wave is close to the natural frequency of the stator, it will cause resonance, which will enhance the vibration and noise and endanger the safety of the motor. For some motors due to technical process limitations, a certain amount of electromagnetic noise is allowed, but it is not allowed to exceed the noise limit of the motor itself. At the same time, pay attention to whether there is a constant rotor friction phenomenon in the motor with electromagnetic noise.

3.1 Electromagnetic noise generated by tooth harmonics

Due to the influence of the tooth slot on the stator or rotor, the permeability of the force wave generated by the first order tooth harmonics will produce periodic changes, which will cause the size of the air gap magnetic density to change periodically, resulting in tooth harmonics. The vibration and noise caused by tooth harmonics can be weakened by the method of inclined groove. In general, when the rotor is tilted to a stator slot, the radial force generated by the tooth harmonics is much smaller than that generated by the straight slot.

3.2 Electromagnetic noise generated by unilateral magnetic pull

The force wave generated by the unilateral magnetic pull due to the eccentricity of the fixed and rotor, or the asymmetry of the magnetic circuit, will cause the asymmetry of the magnetic flux distribution, and there is a phenomenon of large force and small force on the one side, which changes periodically with the speed. Therefore, when we design or process, the roundness of the fixed and rotor must meet the requirements, and the magnetic circuit must be symmetrical and evenly distributed. In the motor assembly, the coaxial degree of the rotor should be checked to make it within the range of accuracy requirements. In this case, electromagnetic noise is usually reduced by replacing the rotor or end cap and adjusting the coaxial degree of the rotor.

3.3 Cage motor easy to produce electromagnetic noise

The electromagnetic noise of cage single-speed motor is mainly generated in 6,8,10,12,16,24 low speed motor. The multi-speed motor is mostly generated in the low speed of the motor, such as the 6 poles of the two-speed 4/6 pole motor, the 8 poles of the 4/8 and 6/8 pole motors, the 24 poles of the three-speed 4/6/24 pole motor, and the 32 poles of the 4/8/32 pole motor. Variable frequency motor is easy to produce electromagnetic noise in low or high frequency band.

3.4 Methods for reducing electromagnetic noise

In summary, according to the cause of electromagnetic noise, different methods can be used to reduce electromagnetic noise. Sinusoidal winding is adopted to reduce harmonic components; When selecting the appropriate air gap magnetic density, the magnetic flux density should not be too high when considering the material utilization rate; Select appropriate groove fit to avoid low order force wave; The rotor groove is used to incline a stator groove distance; The stator and rotor magnetic circuit are symmetrical and uniform, and the superposition is tight. Fixed, rotor processing and assembly, should pay attention to their roundness and coaxial degree; Be careful to avoid their resonant frequencies.

4. Analysis of noise fault during load operation of cage motor

When the cage three-phase asynchronous motor is put into operation, if the motor is transported long distance or the ambient temperature is low, there is usually a significant bearing abnormal noise at the beginning of the trial operation, and the noise of the motor can generally disappear for a period of time (3 to 5 minutes). If there is still noise, it can be run by adding grease or reversing the motor, which is because the motor is in the process of long-distance transportation due to turbulence. Caused by uneven distribution of grease in bearing parts or low ambient temperature (below zero degrees Celsius), insufficient grease lubrication during operation. The bearing noise that appears after the motor runs for a period of time must be carefully listened to with a metal rod or screwdriver on the bearing outer cover. If the sound of the bearing runs evenly, the refueling can be solved. If there is an obvious "click" sound in the bearing operation, the bearing must be replaced and the cylindricity of the bearing room must be checked. Because the end cover of cast iron or steel plate will produce stress during the manufacturing process, after the motor runs for a period of time, with the release of stress, the bearing chamber will be deformed, and the cylindricity may be out of alignment. The best way to check whether the bearing chamber is deformed is to use the old bearing. Several angles back loading.

After the electric machine is used for a long time, the quality of the motor is defective or overloaded, and the noise of the motor will be larger than normal; Motor switch or contactor contact one phase is not connected, lack of phase operation, the noise will be particularly large, at this time can be powered off and then closed, to see whether the motor can start normally, if not start, it may be caused by the motor lack of phase; When the cage rotor guide bar breaks, sometimes high and low "buzzing" sound, the speed becomes slow, and the current increases. In this case, the cage rotor guide bar should be checked and treated; The stator winding first and end wiring is wrong, the motor will have muffled noise, and the speed will drop. After the stator winding first and end wiring is corrected, the motor can return to normal; The stator groove wedge is loose or broken, and the "sizzling" sound can be heard. The loose and broken stator groove wedge should be replaced.

5 Summary

Motor noise, as an important indicator of the dynamic mechanical performance of the motor, is a must-test item of the motor type test. Factory test judges the motor produced in batches by human perception, and conducts regular sampling and testing with noise instruments. For the motors with high noise requirements, the noise instrument is used for full inspection during the factory test. In the process of use, the motor with sudden abnormal noise is usually distinguished first whether it is caused by mechanical reasons or electromagnetic reasons, and then processed to eliminate the fault.