Multi-stage centrifugal blower
Thread Content
It is a multi-stage centrifugal blower of model C60-1.3; the motor has no nameplate. The outlet vent valve is fully open, and the inlet valve is opened to 5–10% of its full opening. When the motor is started, its speed does not reach the rated value. There are abnormal noises and severe vibrations at both the motor drive end and the blower drive end. The starting current of the motor is 360 A (electrical technicians say it should be around 80 A under normal operation); the current drops to 300 A before further decreasing, after which the motor shuts down. Upon inspection, the barring gear is functioning properly – it rotates smoothly without any sticking, and can be turned easily with just one hand. Rechecking the alignment of the couplings with a dial gauge showed no issues; the couplings are of the elastic sleeve pin type. The elastic collar is in good condition, the coupling bolts are fine, the bearings of the fan contain clean grease, and there is no axial movement of the fan rotor. The motor was tested with the coupling bolts removed; the current drawn by the motor was 86A. There was no vibration in the motor’s front or rear bearings, nor any abnormal noises. When using a crowbar to move the motor’s coupling, the rotor could move axially by 1-2 mm. After reinstalling the coupling, the motor was started again. Whenever the bearings began to vibrate or produce abnormal noises, the motor was stopped. It was then restarted only after the vibrations and noises ceased (with the motor still running). This process was repeated 4 times, after which the motor, along with the fan, reached its rated speed, with a current of 86A. The fan showed no vibration or abnormal noises. I hope experts can help by analyzing the causes of vibration, abnormal noises, and overcurrent. I personally believe that the accident was caused by axial movement of the motor – is that correct? Also, what are the reasons for axial movement of the motor rotor in motors equipped with small deep-groove ball bearings, and what are the consequences of this? Could it lead to vibration and abnormal noises in the fan?
This phenomenon should be examined under a microscope. It’s very likely to be a bearing issue, from a mechanical perspective. Additionally, from a process perspective, before starting up the system, the opening degree should be as low as possible, provided that surge does not occur. Whenever the machine is shut down, the inlet is closed to the valve opening corresponding to the minimum current at which surge does not occur. This opening degree serves as the preparation opening degree before the next start-up. Your 5–10 is too broad. For large centrifugal fans (compressors), even a 1% opening degree has a significant impact. For reference.
Thank you. For the small blower, I asked again about an electric motor of around 45 Kw
Just as I thought. Could you please continue with the analysis? Thank you
Easy rotation of the rotor indicates no shaft jamming, while vibration at high speeds suggests poor dynamic balance – there are two possible causes: poor dynamic balance of the rotor or damaged fan bearings. The shaft is bent, the impeller is corroded, and there are debris blocking the air duct.
The analysis of CHANGBAISHAI earlier is quite reasonable. This phenomenon is clearly due to an excessive starting load on the motor, resulting in an extremely high starting current that causes it to trip. The inlet valve should be as small as possible before startup. Increase the inlet valve once the rated speed is reached.
Okay, I’ll try reducing the opening degree again tomorrow. But with another fan of the same type, using the same motor and with the same fan inlet opening degree, this problem didn’t occur when it was started; it started smoothly, without any abnormal noises or vibrations, with a starting current of at most 300, and an operating current of less than 80
The motor starts in a soft-start manner; it vibrates during startup, and it shakes when its speed is low. Additionally, the current level is very high, which causes the motor to shut down immediately. Once it reaches the desired speed, there is no more vibration or abnormal noises, and the current level returns to normal
The motor shaft has no axial positioning; an axial movement of 0.30 mm is already quite significant!
I’ve heard a story similar to yours: the rotation speed was in the tens of thousands; even when the new equipment was started up, it didn’t reach its rated speed – it was around 8,000 revolutions per minute, I can’t remember exactly. There was severe vibration, and the motor experienced overcurrent, preventing the speed from increasing. The manufacturer was called upon, and they suggested trying to force the motor to run at higher speeds (I forget whether it was done by us or at the manufacturer’s request). The rotor got damaged as a result, and the manufacturer replaced it. After installation, there was still severe vibration, and ultimately no solution could be found. The boss found an expert, and after learning about the situation, the expert said that the motor was too small; the manufacturer refused to agree to that. The boss asked, Are you sure it’s the motor that’s too small? The master said it was confirmed, and then a larger motor was installed. When the machine started up, it still had vibration at that speed, but it managed to reach that speed easily and operated smoothly once it reached its rated speed.