Fan Motor Current

[infinity]

I was having a discussion with a group of electricians the other day about what would happen to the current on a belted fan motor if the supply air to the fan was cut off (damper closed). My response was that the current would go down because the fan would be doing less work (moving less air). No one agreed. I used this example:

I have a shop vacuum running and measure the current,
I put my have tightly over the end of the hose and the motor starts to whine loudly because the fan motor is now running faster. When the motor runs faster it is because it's doing less work therefore the current will go down.

These were my arguments:

> A motor will work harder under load
> Motor draws more current the harder it works
> For a fan, the amount of work is defined by the amount of air that is being moved

Most disagreed, two wanted to think about it, one thought that the logic was correct. Any opinions?

 

[iwire]

Rob I agree with you.

 

[GMc]

I'm going to guess the current will go up. Cutting off the air supply on the vacuum would create more of a vacuum causing the motor to work harder.

 

[chris kennedy]

I just tried it with a blow dryer, cutting off the supply air cut the current in half.

 

[iwire]

I'm going to guess the current will go up. Cutting off the air supply on the vacuum would create more of a vacuum causing the motor to work harder.

So you are saying the more air the fan moves the less current will be required? :smile:

If there is vacuum in the intake that means the resistance of spining the fan is less, that is why just like Rob pointed out that the motor in a vacuum cleaner will speed up when you block the intake.

 

[daleuger]

Sounds pretty reasonable.

 

[GMc]

So you are saying the more air the fan moves the less current will be required? :smile:

If there is vacuum in the intake that means the resistance of spining the fan is less, that is why just like Rob pointed out that the motor in a vacuum cleaner will speed up when you block the intake.

Well first off I thought Rob was asking a trick question then I thought the fan would be working harder because it's trying to move the same amount of air.

I should have done like Chris and tried it before answering. Or maybe I should have had another cup of coffee before answering..:smile:

 

[infinity]

Well first off I thought Rob was asking a trick question then I thought the fan would be working harder because it's trying to move the same amount of air.

:smile:

No trick question just an exercise in counterintuitive thinking. We want to think that the fan works harder because its' air flow is restricted but it's actually the opposite that's true because it does less work.

 

[GMc]

No trick question just an exercise in counterintuitive thinking. We want to think that the fan works harder because its' air flow is restricted but it's actually the opposite that's true because it does less work.

Great question, you got me thinking and thats why I love this forum.

 

[stickelec]

Shut-off the discharge and the current will drop as well. Same holds true for a centrifugal pump.

 

[Besoeker]

Shut-off the discharge and the current will drop as well. Same holds true for a centrifugal pump.

Given that Power is head x flow, it will of course.
Not generally a good idea, though. The pump will churn around the water in it and get hot.
Many, possibly all, of the pumping schemes I have worked on, have had flow sensors to shut down the drive motor in the absence of adequate flow.

 

[winnie]

If the fan were instead a positive displacement device (such as a bellows), then I'm pretty sure you'd get a rather different result.

Question: with an ordinary fan does the current go up when the damper is partially closed, because air is still moving but now there is more resistance to work against?

-Jon

 

[iwire]

If the fan were instead a positive displacement device (such as a bellows), then I'm pretty sure you'd get a rather different result.

Of course, but I don't see many bellows style air pumps around. :smile:

Question: with an ordinary fan does the current go up when the damper is partially closed, because air is still moving but now there is more resistance to work against?

As I understand the current will drop, less work being done.

 

[stickelec]

I fully agree that heat and cavitation are both concerns when operating a centrifugal pump against a closed valve. However, it is often done when "switching pumps". ie: two pumps in same service but only one runs at a time.

Also operating a fan against a partially closed inlet can be an issue if that "method of closure" is not symmetrical (might be a better word than that). What I'm getting at is - I have seen people put plastic-sheeting on large industrial fans (10-12 ft dia) to partially restrict flow.

That will cause the Blades to load/unload as they pass over the restriction and can lead to blade or hub failure.

 

[infinity]

Here's a quick test I ran on a vacuum today, sorry for the somewhat poor quality. I was trying to hold the hose and camera at the same time.

 

[Besoeker]

I fully agree that heat and cavitation are both concerns when operating a centrifugal pump against a closed valve. However, it is often done when "switching pumps". ie: two pumps in same service but only one runs at a time.

Yes, during the changeover period.
Many large pumps I have dealt with have a reflux valve - a non-return valve that opens when the pump produces enough head to overcome the system pressure. In this case, the pump will run against closed valve for a short time during run up.

 

[GeorgeB]

I was having a discussion with a group of electricians the other day about what would happen to the current on a belted fan motor if the supply air to the fan was cut off (damper closed). My response was that the current would go down because the fan would be doing less work (moving less air).

I agree.

It takes pressure (deltaP) and flow to do work. If we use electrical terms, it takes volts and (in phase) amps to do work. 120 volts on a receptacle ... no work. 1 amp through a #10 wire ... no work IN THE WIRE ... no "deltaP" or voltage drop.

If one has a positive displacement pump, blocking the outlet will cause the pressure to (theoretically) approach infinity ... the big number, not you <g> ... with an "nonocompressible" fluid while the fluid compresses with current increasing until something breaks.

Blocking the inlet ... hydraulic ... will cause cavitation of the pump inlet lowering load to friction until again, something breaks.

How does it go ... been there, done that, got the tee shirt ... many times.

George

 

[chris kennedy]

And the blow dryer test.

Supply open.

Supply partially blocked.

And fully blocked.

 

[rattus]

Electrical Analog:

Electrical Analog:

Think of a simple circuit. Raise the resistance, and the current goes down. Less work is being done.

Blowers are designed to work against a head of pressure. Some blowers will overheat if operated with no air restrictions. Of course, restricting all air might deprive the motor of cooling air, and it will overheat for a different reason.

 

[GMc]

And the blow dryer test.

Supply open.

So it does go up when just partially blocked? If just partially blocked, less air flow, why does the current go up?