Sounds like you have two fans and one stack. Possible solution - could one fan rotate opposite direction to cancel sound waves? Many motors are reversible; one of the fans could possibly have blades reversed as well.
It may be that the frequency is equal to one of the axial mode "organ pipe"...
Still under investigation - not proven but most likely due to electrical grid intermittent faults increasing component of 50 Hz grid frequency. Transient monitoring may still be done in the future.
Centrifugal compressor runs about 11000 rpm with motor near 1500 rpm.
These large train components can't be mounted on rubber pads.
I'm thinking they still may have missed some grounding issue to give the odd difference equation.
To Greg Locock's comment: What are some possible causes of doubling for rectification in a 4-pole motor that should be more symmetric than a 2-pole - I'm not a EE.
Answers are below questions from Elecricpete:
I have some questions (which is not to say I'll have answers):
What is the machine...
A machinery train has vibration at the first torsional critical, but excited only near a certain speed. It is a "direct torque control" variable speed, 4-pole induction motor with 50 Hz utility grid frequency. The speed for high vibration events appears to be where:
Difference between 2 times...
(fixed title)
With a permanent magnet AC motor, for reducing noise, should the stator slots be completely filled? If so, is it best to use magnetic, non-magnetic, or semi-magnetic material? Also, are there service shops that can treat stators that have unfilled slots?
Perhaps it could be an interaction - the frequency was shown to be constant with motor operating speed. It likely is a natural frequency of the electrical circuit. Has anyone ever seen one that low - i.e. indicative of a long time constant?
See:
quinn.mech.uakron.edu/me340/notes/me340sum.ps
I believe it is isolated WYE connection and has a secondary grounded connection. However, harmonics are not the issue - they were low - but there was the odd low-frequency 8 to 9 Hz.
It's an LCI drive with a DC-link converter - we are thinking that the step-up transformer is affecting the electric circuit. Has anyone found that the effective resonance frequency could be as low as 8-9 Hz? This is a large motor with about 10 MW operated on a stable grid.
I posted this in Electric Power Eng'r forum - no response:
We used a step-up voltage transformer for a test run with a variable-speed 60 Hz motor. There was an odd subsynchronous frequency near 9 Hz found in the converter output current signal. Could the frequency come from the step-up transformer?
We used a step-up voltage transformer for a test run with a variable-speed 60 Hz motor. There was an odd subsynchronous frequency near 9 Hz found in the converter output current signal. Could the frequency come from the step-up transformer?
A speed increasing gear has a shaft displacement probe on the pinion. During operation, there is subsynchronous vibration on the pinion that changes with load. Frequencies are at one times and two times bull gear frequency, with much smaller amplitudes at other harmonics of bull gear frequency...
Why are you trying to do it yourself? There are great instruments out there; one example:
http://www.endevco.com/main/news/2775b.php
Filter Integrator Module 35818 provides a capability for single or double integration of the input vibration signal.
Also contact: http://www.wilcoxon.com
Be...
My experience is that bearing temperature does not change much unless there are the two circumstances given above. If it's not an oil supply problem, the cause of the vibration would also have to simultaneously affect loading of the bearing. Also check oil drain sight glass before and after next...
