Placing a fan/fan motor in vacuum.. 1

[rjgoebel]

I am wondering what happens when you place a fan motor or air pump in a situation where it is trying continually to move air from a lower pressure to a higher pressure, ie, the exhaust fan on a microwave. Doesn't this cause problems for the motor? In terms of load, would this be a higher or lower load on the motor? And, what would this produce in terms of current through the motor?

I cannot remember my motor equations relating torque/speed to current. Anyone?

 

[quark]

It will be as good as running your fan in no load condition, but you do face problems with overheating of fan and the motor. Keep in mind that, the power consumed by a fluid moving device is proportional to the mass flow rate of moved fluid.

 

[ccw]

I assume this is the "induced draft" case you are referring to. It is used in various situations from incinerators to vacuum cleaners to turbo-molecular pumps. When the "vacuum" becomes significant and the fan/motor is integral, there are some special problems, no less than motors in space: 1. loss of convective cooling from the rarefied air stream, 2. advanced brush-commutator wear, 3. loss of bearing lubricant.

For the vacuum cleaner, it is easy to demonstrate that as the suction side air flow is throttled and the suction side pressure is reduced, the mass flow through the impeller is reduced, the power demand is reduced, so the motor speeds up.

There is a nice tutorial online on DC motor characteristics of which the serial wired DC motor or universal AC motor (drills, mixers, vacuum cleaners) is a special case.

For rudimentary motor theory see:

http://lancet.mit.edu/motors/motors3.html

For more theory and equations see:

http:/zone.ni.com and click on:
development library / motion control / servo motors / DC motor calculations pt. 1-4.

 

[amorrison]

Something to know about "vacuum".

A furnace(centrifugal) fan usually operates at less than one ince water pressure difference between suction and discharge pressure.

One standard atmosphere (14.7psi=101.3kPa) is equivalent to 406 inches of water height (called static pressure water gage).

So a fan decreases the absolute pressure on the "vacuum" side by < < 1% - which is not much of a "vacuum" at all.

 

[MintJulep]

I am wondering what happens when you place a fan motor or air pump in a situation where it is trying continually to move air from a lower pressure to a higher pressure

This is the normal situation for a fan or air pump.

Quark is quite correct regarding mass flow. For all fans the mass flow is a function of the delta pressure.

CCW is also correct regarding operation in a "real" vacuum and the general relation between mass flow and fan power.

 

[amorrison]

Power consumed = difference in pressure X volume moved per time.
When airflow is blocked the volume per time =zero
therefore no power is consumed (simple theory).
But there is still drag/turbulence inside the fan (real world)which causes some aerodynamic power consumption.