VFD Incorrect RPM Setting

[nhcf]

What is the impact of an incorrect motor speed setting in a VFD? Today I noted that a 2 pole, 3555 rpm motor was connected to a VFD configured for 4 pole 1780 rpm. Application is a 75 HP centrifugal blower, open loop application. The motors / drives have probably been running this way for 3+ years.

What is the impact of running with the incorrect settings? I plan to correct the settings tomorrow, but am not sure if this has any impact on operation.

 

[Skogsgurra]

The main impact is that you have to set 50% of desired speed. And also get half the actual RPM in the display. If RPM is not shown, all the better. If voltage to frequency ratio is right, which it seems to be, there are no other problems. Not as far as I can see.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

You will get a whole lot more air flow. Far more than twice.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

It has been working like this for more than three years. So I think all the OP gains from changed settings is that it looks more "right" if anyone cares about the looks.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

The VFD is likely using frequency to set the V/Hz pattern and for the actual speed setting. If so, then nothing much really happens besides the rpm meter being wrong.

The rpm setting might be used in the vector controls or for slip compensation, but a fan isn't a difficult load so the VFD could have easily been left in V/Hz mode so it's not using any of that. Even if it is using the rpm setting for something internal, it would still work even though it might not be ideal since the fan isn't a difficult load to control.

 

[jraef]

Typically it would not be an RPM value used in the vector calcs anyway, it would be a Hz value. So if the centrifugal BLOWER was designed around a 2 pole motor, and the only error here was that the VFD programming was left at the factory default setting of 4 pole, then the only thing that would affect would be the RPM reading on the display, if any.

Had the blower been designed for a 4 pole motor and someone put in a 2 pole, then the HP required by the blower would have increased by a factor of 8 (cube of the speed difference), so the blower would have required 600HP and the 75HP drive would have tripped on Over Current almost immediately.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[nhcf]

Thanks for the replies.

I agree, from what I can tell the only impact of the incorrect setting is the rpm display - all other operation looks correct. Just wanted to check to make sure I was not missing something.