# restaurant exhaust fan



## farlsincharge (Dec 31, 2010)

Got a call today, hood exhaust fan is down. Went and checked it out, motor tripped integral overload. Got it cooled down and started it up, draws 11.5 amps. It is only supposed to draw 8.8. Fan spins freely and motor draws 8.5 unloaded.

Seems to me that it should draw less running free on it's own, and a 3/4 horse motor should be able to handle the fan load without running over FLA. What could happen that could cause a motor to become "weak"?

It is quite possible that the motor used to be larger than 3/4 hp as it has been changed and it's a Baldor farm duty motor that was put in. It could be too much load for a 3/4 but I don't think so. The only markings on the fan is that it is rated at 5000 cfm.


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## MechanicalDVR (Dec 29, 2007)

1. What is the incoming voltage?
2. What voltage is the FLA rated at?
3. Are the filters in the hood clogged?


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## BBQ (Nov 16, 2010)

MechanicalDVR said:


> 3. Are the filters in the hood clogged?


That would reduce the current not increase it.



> *It is only supposed to draw 8.8.* Fan spins freely *and motor draws 8.5 unloaded*


.



This suggests to me that the motor is toast, I would not expect the unloaded current to be that close to the loaded current.


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## Aussielec (Apr 1, 2012)

What's you're supply voltage when the motor's running?

If it's 120 volt motor, 11.5 amps sounds about right for FLA on 3/4 hp motor.

Anyhow if the motor's overheating then it's probably time for a new motor.


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## erics37 (May 7, 2009)

Those restaurant fans take some abuse. Sucking grease all day and kinda sorta subject to the weather (significant here). I've replaced three this year that were simply toasted motors. One I didn't even have to megger, it toned out to the frame with a continuity tester :laughing:

They're fairly cheap and easy enough to replace.


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## chicken steve (Mar 22, 2011)

BBQ said:


> That would reduce the current not increase it.
> 
> why do mechanical loads decrease draw ?
> 
> ~CS~


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## BBQ (Nov 16, 2010)

chicken steve said:


> why do mechanical loads decrease draw ?


They don't unless the mechanical load drops which is the case here.

It is counter intuitive but when you block a fan or a centrifugal pump on either the inlet or outlet the work being done reduces so the current drops.

For a quick example block the inlet of your vacuum and listen to the motor speed up. 

For my own example, here is a post of mine from Holt's 

I happen to have this squirrel cage fan in my basement.











With it operating as unrestricted as possible at full speed. 4.8 amps

With it operating with only the the inlet blocked at full speed 3.8 amps

With it operating with only the outlet blocked at full speed 3.7 amps



Now on the other hand if you blocked a positive displacement pump, such as a hydraulic pump the current would increase and without a relief valve would stall the motor.


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## chicken steve (Mar 22, 2011)

BBQ, i'm aware of the phenomenon, but am not clear as to _why_ it happens

is there anything in the American Electricians HB you're aware of addressing this?

~CS~


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## BBQ (Nov 16, 2010)

chicken steve said:


> BBQ, i'm aware of the phenomenon, but am not clear as to _why_ it happens
> 
> is there anything in the American Electricians HB you're aware of addressing this?
> 
> ~CS~


Oh, you where not clear, go figure. :jester:

http://en.wikipedia.org/wiki/Conservation_of_energy

Moving air / water is work, when you stop moving air / water less work is done.


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## chicken steve (Mar 22, 2011)

what in this link applies BBQ?

~CS~


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## BBQ (Nov 16, 2010)

chicken steve said:


> what in this link applies BBQ?
> 
> ~CS~


I think most of it. 

With the fan blocked the only work being done is stirring up the air around and in the fan, that is less work than moving air through the fan.


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## chicken steve (Mar 22, 2011)

BBQ said:


> With the fan blocked the only work being done is stirring up the air around and in the fan, that is less work than moving air through the fan.


perhaps that is the easiest answer

~CS~


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## farlsincharge (Dec 31, 2010)

I had an epiphany after reading all the responses (Thanks). THE RUN CAPACITOR!
Pulled the motor today, and sure enough it was completely toast. 
I love being able to bounce ideas around with you guys, thanks again.


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## electrictim510 (Sep 9, 2008)

Well dargonnit, I just learned something today! Thanks guys.


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## TOOL_5150 (Aug 27, 2007)

electrictim510 said:


> Well dargonnit, I just learned something today! Thanks guys.


me too :thumbsup: This place occasionally is very helpful if you can sort through the trolling.


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## sparky970 (Mar 19, 2008)

I thought it was typical to use a motor without Klixons for a restaurant hood fan. Is this incorrect?


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## MechanicalDVR (Dec 29, 2007)

BBQ said:


> That would reduce the current not increase it.
> 
> .


 

I know the amps would go down, I was in the midst of a 1:28 am brainfart of some kind or other.


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## farlsincharge (Dec 31, 2010)

sparky970 said:


> I thought it was typical to use a motor without Klixons for a restaurant hood fan. Is this incorrect?


First I have seen, but it isn't the original motor. Most of the ones I have done new are not internally protected and get a motor starting switch with O\L.


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