# Fan-motor system resonance problem

• ArnoVonck
In summary: I will look into it. How have you determined the 18.1 Hz impeller frequency?Should not this value be equal to the control frequency if motor and fan are directly linked?We've measured the motor and impeller speed at different frequencies. How would you describe the resonance of the system?What is the direction of the greatest oscillation?The fan is operating on an oscilating or surging zone of its operational curve. Could it be that the fan is working on an oscilating or surging zone of its operational curve?Does its perfomance vary while the resonance lasts?Yes, it does. Could you
When you run it a rated design rpm (1470 rpm), are you getting rated static pressure?

Are there other systems tied into the duct? I assume it's going to a scrubber?

russ_watters and ArnoVonck
Based on Video #2 in Post #17, it appears that the blower housing has a mechanical vibration at 18.2 Hz. Mechanical vibrations are analyzed as spring - mass systems. The mass in this system is the blower housing, the spring is the side of the housing in the photos, and the motor base is fixed. That's why the request for the test in Post #27. Sometimes, your fingers are all the test equipment that you need. This might be one of those times, but you need to run the test to find out.

If so, the remedy is simple. Weld on a pair of stiffeners as shown by the heavy black lines in the image below.

You can leave the angled braces where they are, or make new ones that connect the same location on the base to the top of the stiffeners.

The theory is as follows: Natural frequency = ##\sqrt{\left( \frac K M \right)}##, where:
K = Stiffness
M = Mass
Since we want to increase natural frequency from 18 Hz to a value above the maximum operating speed of 30(?) Hz, it is necessary to increase the ratio of K to M by about a factor of 4. Reducing the vibrating mass that much is not practical, but it is practical to increase the stiffness. The stiffness of a thin plate is very low, and it is easy to increase that stiffness by a factor of several times by adding the stiffeners shown.

russ_watters, ArnoVonck and DaveE
A thought: Those diagonal stiffeners ? Do they resonate ?? If so, can you add masses to 'quench' them ??

jrmichler said:
That's why the request for the test in Post #27.
The 3rd of May we'll do another run-up test. I'll do as you said in Post #27

erobz said:
When you run it a rated design rpm (1470 rpm), are you getting rated static pressure?

Are there other systems tied into the duct? I assume it's going to a scrubber?
No we don't get rated static pressure. Indeed, it's going to a scrubber.

ArnoVonck said:
No we don't get rated static pressure. Indeed, it's going to a scrubber.
I'm assuming your s.p. was too high, which was why it is currently being run at 1086 rpm? Or is it the vibrations just passing through 1086 rpm on the way to 1470 rpm the issue?