Dust collector under loaded?

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MD84

Senior Member
Location
Stow, Ohio, USA
I have installed a 5hp dust collector. The customer wishes to have better performance on a profiler machine.

They expected better performance comparable to installations in other locations.

I believe the dust collector fan is not fully loading the motor. Do you agree? What options do I have?

This is a 5hp 3Phase 460v nominal. Nameplate efficiency states 87.5% and power factor 0.92. The actual voltage is 492. I get 2A on each leg. I expected 5.4A fully loaded.

My work:

(5*746)/(492*0.875*0.92*1.73)=5.44

Please check my work. Why such a difference? Is it just the fan blade mechanics do not load motor down enough? Is this engineered head room?
 
ActionDave said:
Do you have a motor issue, or a ducting issue, or even a make up air issue?

If the fan is moving less air because of restriction somewhere it will draw lower amps.
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Exactly.

Though "make up air" on such a unit would really be the discharge side of the blower.

Don't forget plugged filters would also reduce air flow and lessen motor load.
 
I think there is a ducting issue. The hose is necked down where it attaches to the equipment once it hits the equipment there is more restriction.

The filters were just cleaned thoroughly. I suppose I could disconnect the ducting at the collector and retake current measurements. This would isolate the problem hopefully.
 
I removed the ducting at the collector inlet and doubled my current. Thanks for the help here. Now I can show the customer where the improvements need to be made.

I have a ways to go with my motor knowledge. I was thinking that the restricted airflow would present an increased load to the motor and would therefore increase the current. This could possibly be true for other motor types?
 
MD84 said:
I suppose I could disconnect the ducting at the collector and retake current measurements. This would isolate the problem hopefully.
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The dust collector I installed warned in the manual not to run with the intake wide open. It would overload the motor.

Cheers, Wayne
 
MD84 said:
I removed the ducting at the collector inlet and doubled my current. Thanks for the help here. Now I can show the customer where the improvements need to be made.

I have a ways to go with my motor knowledge. I was thinking that the restricted airflow would present an increased load to the motor and would therefore increase the current. This could possibly be true for other motor types?
Click to expand...
This has more to do with the physics of the blower then the knowledge of how whatever is driving it works.

Centrifugal fans and pumps are loaded by the amount of media they are moving not by pressure alone. If put a restriction in the flow (inlet or outlet side doesn't matter) you reduce the amount of media that can flow and it takes less energy to move less media.

Positive displacement methods of moving the media (say for air movement a piston in a cylinder) the pressure does have much greater impact on the power needed, but for a centrifugal fan or pump when there is flow blockage the impeller is able to slip in the media where a positive displacement method has to compress the media if it is to keep moving.
 
MD84 said:
I was thinking that the restricted airflow would present an increased load to the motor and would therefore increase the current. This could possibly be true for other motor types?
Click to expand...

In short the less air you are moving the less work is being done so the current drops.

This is true with items like fans, blowers, centrifugal pumps etc.

On the other hand if you slow the flow down on a positive placement type pump, like a hydrulic pump, or a pressure washer pump the current will go up and if you stop the flow altogether you will stall the motor if there is no relief valve.
 
I'm going to give you the same advice I give many others.

Read the manual and then call the manufacturer. You're not a mechanical engineer; you're an electrician. Manufacturers want these calls. If you have this question, so do many others. If they get the same call enough, they'll revise their manual. They want you to have a positive experience with their product and they will go to great lengths to do everything they can for you. They're likely to ask what size duct, how long, how many bends, etc and also what's being cut on what type machine, etc. The manual should give some info on this but if not, call them.
 
mgookin said:
I'm going to give you the same advice I give many others.

Read the manual and then call the manufacturer. You're not a mechanical engineer; you're an electrician. Manufacturers want these calls. If you have this question, so do many others. If they get the same call enough, they'll revise their manual. They want you to have a positive experience with their product and they will go to great lengths to do everything they can for you. They're likely to ask what size duct, how long, how many bends, etc and also what's being cut on what type machine, etc. The manual should give some info on this but if not, call them.
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Likely true for top of line models. For cheap junk made elsewhere you are lucky to understand the instructions - if you even get any.
 
kwired said:
Likely true for top of line models. For cheap junk made elsewhere you are lucky to understand the instructions - if you even get any.
Click to expand...

Buy off Alibaba or anything that ships from China and get what you pay for.
 
MD84 said:
I removed the ducting at the collector inlet and doubled my current. Thanks for the help here. Now I can show the customer where the improvements need to be made.

I have a ways to go with my motor knowledge. I was thinking that the restricted airflow would present an increased load to the motor and would therefore increase the current. This could possibly be true for other motor types?
Click to expand...

generally the work/power/current a motor performs/requires is proportional to the 'substance' it moves
air (either direction), fluid, conveyor, etc.
if you put a restriction on the inlet/suction of a pump/blower, less fluid/air can be move, hence lower power/current

a curve(s) can illustrate this
http://www.pdblowers.com/admin/uploads/59_urai_pressure_performance_curve.pdf

top is flow (y axis) vs blower speed (x axis)
middle is temp rise/heat of compression, ignore for our purposes
bottom is HP (y) vs speed (x)

let's use a slightly overdriven speed of 2000 rpm to start with
think of the pressures as restrictions
at 2 psi it can move 590 cfm
at 7.3 psi 540 cfm
the lower the restriction, the more it moves

from the curves:
to move 590 CFM at 2 psi requires 7 HP
to move 540 CFM at 7.3 psi requires 22 HP

to move only 540 cfm at 2 psi 6 HP (and slowing the blower to 1800 since it a positive displacement machine) less HP for less flow: 590/7 vs 540/6
to increase to 590 cfm at 7.3 psi 26 HP (and speeding the blower up to 2200) more HP for more flow: 590/26 vs 540/22

therefore power is proportional to flow
 
Great advice here and very good insight. I have learned something new today. It does make sense to me now with the various explanations. Thank you
 
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