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Fan / duct resonance with VFD

Fan / duct resonance with VFD

Fan / duct resonance with VFD

(OP)
Hi everyone,

I am trying to understand the mechanical side of an issue (spot the electrical engineer!).

I have heard of a case where fan blades have been broken off due to resonance. As I understand this at certain frequencies (fan speeds) the air flow can reflect back and excite the ducting and / or the fan assembly. If this is bad enough damage will occur to the blades that may even result in them cracking. Is this right? Apparently this can be made worse by too tight control of the motor.

This is where my interest is. When setting up the VFD a couple of options are;
1. Set skip frequencies to avoid resonance
2. Use a "looser" form of control.

Option one makes sense - no problems there.

Option two, is this because being loose the motor (fan) can be pushed around by the air saving stress on the blades? To my mind if running at resonance then damage will occur irrespective of the motor control system.

Discussion most welcome.

RE: Fan / duct resonance with VFD

Norske,

I've never run into option two before....  Even with "loose" control (whatever that is), you'll still run the risk of operating at resonance for some period of time.

Let's see if anyone else has a thought.

Good on ya,

Goober Dave

 

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