Issues regarding vibration of screw compressor motors
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This post was last edited by 156026692 on 2019-8-16 at 13:59. I have a screw ammonia compressor here; the motor vibrates heavily. It has been sent out for repair, and dynamic balancing has also been done. Upon returning it, it was first tested individually in the electrical workshop; the vibration level was around 2 mm/s (a level of no more than 2.8 mm/s is considered normal). However, after it was reinstalled on site and tested individually, the vibration level was very high, at 5.2 mm/s for vertical vibration (the normal limit being no more than 4.5 mm/s). Since the motor of this screw compressor is installed on a horizontal tank (see the left side of the figure below), electrical technicians believe that factors such as insufficient rigidity of the foundation of this horizontal tank are responsible for the high vibration levels observed during the motor’s trial run on site. But we also measured the spectrum on site; the vibration spectrum of the motor consists mainly of the first harmonic. From the perspective of vibration theory, the main possible cause is related to balance issues. Although it relates to motor vibration, I thought if I posted it in the electrical section, people there might not be as familiar with vibration issues as those in our mechanical equipment section; that’s why I’m posting here for advice. Please take a look; the motor exhibits high vibration during individual tests. What could be the cause? Thank you
The support stiffness of the motor decreases, similar to structural loosening
The doubling of frequency could also be due to looseness; it’s possible to measure whether the vibration level of the base beneath the foot bolts is roughly the same
This post was last edited by chentonghao123 on 2019-8-16 12:01
The issue has been resolved; it was indeed a problem with loose fasteners in the basic machinery. Thank you all! :lol:victory::hug:
What standard do you use to measure the vibration of your electric motors? Why are the vibration standards for motors much stricter than those for compressors?
I’m not entirely sure which specific standard applies; in the electrical field, 2.8 mm/s is used as a criterion to determine whether the testing is proceeding normally. For compressors, our unit commonly uses the vibration standard of SHS01003, whereby a value not exceeding 4.5 mm/s is used as the criterion to determine whether it is within normal limits.
Install the motor under normal operating conditions, that is, install all the foot bolts and tighten them properly (they were not tightened before and were simply placed on the base of the horizontal tank, in a loose state). After aligning the motor and the compressor correctly, disconnect the coupling and conduct a separate test to ensure that any vibrations are not caused by insufficient rigidity of the base on which the motor is mounted. Well, after giving it a try like this, it worked fine: lol. This also gave me new insights and experience in dealing with such vibration problems in the future, especially those arising from conflicts between electrical and mechanical aspects.
I also thank you for sharing, as it has deepened my understanding of this aspect of the knowledge. :handshake
A simple analysis shows that there are generally several issues: 1. The alignment of the axes of the main motor and the compressor needs to be adjusted； 2. Caused by the clearance of the motor bearings or improper installation ； 3. The mounting bolts of the main motor are not tightened sufficiently.