Peter53
Excellent!
2900 rpm is indeed a
2-pole motor ....
i.e.,3000 rpm sync. speed (-) 100 rpm (nominal)slip at full load.
2-pole motors are notorious for their
vibration problems.
One only needs to do a search— i.e., Google — on ...
Vibration in 2-pole motors .....
You'll find several articles / papers on the subject, some in PDF format. There are also some IEEE papers which, unfortunately are not free, even for those of us that are members thereof.
As a Prologue to my comments below, I offer excerpts from Neil Mulji's excellent article ... entitled—
Vibration in Electric Motors
Vibration in Electric Motors
Neil Mulji | Saturday, August 06, 2011
.... A basic electric motor has a magnetized stator that surrounds a rotor; the magnetic interaction between the two causes the rotor to spin.
Faults in motors can develop due to resonances, imbalance, misalignment and foundation problems or improper mounting. These faults can lead to bearing failures and cause vibration.
Some of the electrical and mechanical faults unique to motors include:
Eccentric or loose rotors or stators
Broken rotor bars
Bowed rotors
Uneven air gap between the rotor and stator
Loose laminations
Electrical discharges between the various electrical components
When analyzing vibration data for motors, it is typical to observe a vibration component at twice the line frequency (at 100/120Hz). The line frequency (50/60Hz) is the frequency at which AC power is supplied to the motor; this causes changes in magnetic attraction between the rotor and stator at twice the line frequency. The varying magnetic forces cause small dimensional changes in the iron material, leading to vibration.
In 2-pole motors, it may be difficult to discern differences between the 2X frequency and twice the line frequency(100/120Hz) and it is important to use a high enough resolution when collecting data. One method to verify the existence of a peak in the vibration at the 2X frequency is to take measurements while the motor is running, and then cutting off power; any peak at twice the line frequency should disappear once the power is turned off, leaving the 2X frequency if it exists.
Frequencies that may show up on the spectral data include:
- Slip frequency - the difference between the motor’s running speed and the synchronous speed; it is typically small in value and will be present as a sub-synchronous frequency.
- Pole-pass frequency - it is equal to the number of poles times the slip frequency.
- Sidebands spaced at intervals equivalent to the to the pole-pass frequency.
- Running speed:
3600 or 3000 RPM for a 2-pole motor
1800 or 1500 RPM for a 4-pole motor
1200 or 1000 RPM for a 6-pole motor
Eccentrically positioned rotors cause a variable air gab between the rotor and stator which leads to a pulsing vibration. Once again, a vibration component at twice the line frequency will be present, however it will have sidebands spaced at intervals equivalent to the pole-pass frequency; the sidebands will also surround the 1X frequency.
Comments:
In re-reading your post of 8 Jun 12 ... (quoted below) —
...another company do the vibration analysis what i find using ultrasound is a buzzing sound which is backed up by a spectralyzer showing dB and hz, basically i find peaks every 100hz with a small peak every 50hz which indicates fluting. As we are 50hz supply thats where i'm looking for electrical faults compared to mechanical.
— I would take umbrage with the conclusions drawn as well as the methodology by which the testing was performed.
The 50-Hz and 100-Hz components from which you draw conclusions are most likely related to— 1X & 2X— Mains Supply Frequency, consequent to testing while mains excitation was applied to the MUT (Motor Under Test). The better way to conduct these tests is while the motors are coasting-down
immediately upon removing excitation therefrom. In this mode, you eliminate any and all influences from the magnetics— (electromagnetic & electromechanical)—consequent to motor excitation.
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Bearing fluting resulting from dV/dt of the source voltage is not as likely on medium-voltage 50-Hz supplies as 60-Hz supplies because 50-Hz produces a lower value of dV/dt. Somewhere in my literature collection there is a reference to dV/dt causing bearing fluting on medium voltage 60-Hz supply back in the 1920 era. Also it is more likely on ball type bearings than on roller bearings. (Check with UR MTR MFGR. to see what type bearings are installed in affected machines of your concern.)
With induced electromechanical vibration, one is more likely to see bearing problems from brinelling as opposed to fluting. Most positive way to be sure is to remove the bearing exhibiting vibration during coast-down ... cutting it open and inspecting the races. Expensive... but informative; particularly where more than one motor is affected.
Other causes of low frequency vibration can be attributed to installation practices. Here are but a few for consideration:
1) Misalignment
2) Soft Footing
3) Weak Foundation
4) Shaft Coupling Eccentricities
5) Loose bolts of pump, gear box etc.
6) Bearing loose in housing (The bearing housing will be heated up)
Take a look at this article, which offers diagnostic advice for pursuing the above installation problems leading to vibration issues.
Basic Motor Vibration Troubleshooting Tips
Hope this helps..
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