How to determine whether the noise of a motor is normal or not

Whether the motor is making abnormal noise can be attributed to the following reasons: (1) A phase is broken in the stator circuit. ⑵ The system voltage has dropped. ⑶ Inter-winding short circuit. ⑷ The end rings of the squirrel-cage rotor windings are cracked or have poor contact with the copper (aluminum) bars. ⑸ The motor rotor core is damaged or loose, and the shaft is bent or cracked. ⑹ Some components of the motor (such as bearing end caps) are loose, or the connection between the motor base and the foundation is not secure. ⑺ The air gaps between the stator and rotor of the motor are uneven and exceed the specified values. Abnormal noise should be taken seriously.

1) Detect using the sensors in the portable device. It is possible to measure the decibels of sound and the time-domain waveform; the frequency spectrum waveform can also be viewed, allowing for further demodulation analysis and signal processing, as well as precise fault diagnosis to determine the source of the sounds with peaks at various frequencies. These time-domain waveforms and spectral waveforms can also be compared with the waveforms when the device is operating normally. 2) Simply use a listening rod, with one end in contact with parts such as the bearings of the device and the other end against the ear, to listen to the sounds inside the device while it is in operation. If there is a problem with the equipment, experienced engineers can still detect some abnormal sounds.

During on-site inspections, we usually use a long screwdriver to press against the end cover of the motor bearing, placing our ear against the handle of the screwdriver in order to hear the sound produced by the rotating bearing. The normal sound of a motor is a high-frequency, continuous noise, while a bearing that is lacking oil or has a fault produces intermittent clicking sounds. If there are slight intermittent clicking sounds mixed within the high-frequency noise, it indicates that the bearing is slightly short of oil, but this does not affect its operation; usually, it can continue to function fine for another 1–3 months. The louder the intermittent low-frequency clicking sounds, the more serious the problem with the bearing. The principle behind this is that sound is generated by friction between the bearing balls. When there is sufficient lubrication, the friction is minimal and uniform, resulting in a soft sound at a high frequency. When the bearing lacks oil, friction increases, and the clicking sounds come from the friction between the balls and the uneven surfaces inside the bearing, leading to a lower frequency and a louder sound. Additionally, the condition of the bearing can be determined based on the temperature and vibration levels at the site. Generally, these skills can be mastered after about one month of on-site inspection experience

That makes perfect sense; the pump operators and maintenance workers where I am work in exactly that way. We also learned to do it in the same manner. Whether it’s a sound-detecting rod or a long screwdriver, as long as it’s a long metal tool with good sound transmission properties, it will do. :)

At the same time, during on-site inspections, it is necessary to frequently check the temperature of the motor by touching it; just like with humans, if there is something wrong with the pump, its temperature will be abnormally high.

By listening to the sound, it is possible to roughly determine the operating condition of the motor. 1. The sound is even, with little variation in pitch, and there are no noises. This indicates that the motor is working properly. 2. It emits a buzzing sound, and at the same time the rotation speed decreases significantly. By measuring the current, if three-phase imbalance or instability is detected, it is likely that there is a missing phase in the power supply, severe unevenness in the three-phase voltages, or serious breaks in the motor rotor. 3. The sound is loud and a bit harsh, but relatively even. The main reason is an overloaded motor or too high a power supply voltage. 4. The sound level fluctuates because of load variations or power supply voltage fluctuations. 5. There is noise. The reasons include bearing damage, friction between the fan and the shroud, friction between the stator and rotor, and foreign objects inside the motor.

Back to floor 2: Sudden large changes in the motor load can also cause excessive noise from the motor. It’s not sufficient to simply use a stethoscope to check the motor; it’s necessary to regularly monitor the three-phase currents as well

To determine whether a motor is functioning properly, it can be assessed by observing and evaluating aspects such as operating current, motor temperature, vibration, and noise. Among these, temperature is the main parameter under observation; faults such as current abnormalities or bearing issues can all be manifested as abnormal increases in temperature.

Use a listening rod to press against the motor casing to listen; generally, there is a uniform sound, with no noise or clashing sounds

Abnormal vibration and unusual noises in asynchronous motors are mainly caused by mechanical and electromagnetic factors. 　Mechanical reasons: ① The motor blades are damaged or the screws that hold them in place are loose, causing the blades to collide with the blade cover; the sound produced as a result varies in intensity depending on the severity of these collisions ； ②If severe eccentricity of the motor rotor occurs due to bearing wear or improper shafts, the stator and rotor will come into contact with each other, causing intense vibrations in the motor as well as uneven rubbing noises ； ③The motor experiences abnormal vibrations due to loose foundation screws or an unstable base as a result of long-term use, which in turn causes electromagnetic torque to act on the motor ； ④Motors that have been in use for a long time may produce abnormal hissing or groaning sounds inside their bearing housings, as a result of a lack of lubricant in the bearings or damage to the steel balls within them. Electromagnetic-related reasons: ① If a motor operating normally suddenly emits abnormal noises and its speed drops significantly while under load, with a low-frequency roaring sound, it may be due to uneven three-phase current, excessive load, or operation in single phase ； ②In a motor that is operating normally, if there is a short-circuit fault in the stator or rotor windings, or if the bars in the squirrel-cage rotor are broken, the motor will emit a humming sound of varying intensity.