If this fan is on a space heater it to be made of metal.
If you blocked the inlet to the fan, and power cosumption only dropped 2 watts, that means that cheap low frequency motor is using that 13 watts to turn the mass of the fan.
But blower motors are high frequency motors turning light weght fans.
Tester
Good plot. Now note that if the filter is blocked, there is a partial vacuum between the filter and the fan, but not downstream from the fan. Read off the plot from the curve for very large pressure increases.
Secondly, you don’t need power to turn the mass of the fan. Only to accelerate it, or to overcome internal resistance in the fan bearings, or to do work against the air. Objects do not need power to keep moving unless there is friction or some force acting in the opposite direction. In the absence of these things, they coast.
Isn’t this horse dead yet??? Melott, you are incorrect. The fan is operating in decreased pressure air , which has lower resistance on the fan because it’s less dense. The fan then spins faster. You erroneously thinking that the higher pressure air will push back against the fan, causing increased resistance, but this simply doesn’t happen. The fan reaches an equilibrium, with the fan itself being in an area of decreased pressure/density. The hand-over-the vacuum-cleaner-hose example demonstrates this phenomenon quite nicely.
Tester, all the big blowers I’ve specified (up to 100 hp) were 60Hz, same frequency as the space heater.
Consider this thought experiment. If you set up a fan to blow through a long box say, and did an experiment where you varied the fan speed while measuring the air flow out the end of the long box, you’d get a graph exactly as Tester has posted. The faster the fan blows, the more more air would flow, and the more the more electrical power the fan would draw. And Tester’s conclusion is valid, that if you extrapolated off to the left, with the fan not turning, there would be zero wind flow, and its a no-brainer, no power consumption if the fan is turned off.
Likewise, if you used different fans (some bigger, some smaller, some with different types of blades) and did the same experiement with the fan speed in all the fans the same rpm, you’d still get the same type of graph. The bigger fans would push more air, and draw more power. Same graph. More flow produced corresponds to more power consumption.
But the in both cases there’s nothing blocking the flow of air though the box. The question seems to be “what happens if the box is totally blocked off”? By def’n there is no wind flow through the box. But does that mean there is no power consumption in the fan then?
I’m a wee bit skeptical of that conclusion.
GeorgeSJ, when I did real engineering, we would use this type of calculated data to allow one to specify an airflow and total pressure rise and then estimate ideal power consumed not including motor and fan inefficiencies. We would then throw in these efficiencies to get real HP and rough- size the blower. Final fan selection was always in the hands of the blower vendor.
Also, per the following:
a small shaded-pole motor like mine is at most 20% efficient. That and “windage” losses makes my observations totally reasonable.
And now, we, the Car talk community, will definitively determine how many angels can dance on the head of a pin!
5000 . . . ?
Will you guys get a room and finish this already?
6000 . . . ?
Insightful, I stand corrected, but your links did provide some definitive evidence to answer the original question, I changed the links to show.
But Tester is still right about reduced power consumption in a vacuum, although it only goes to zero in theory, not in practice because of other friction components as well as the back draft from air trying to get back into the chamber through the fan.
7000 . . . ?
Said fan is not IN a vacuum. It has partial vacuum on one side of it, more or less normal air pressure on the other side.
137
8000 . . . ?
Said fan is IN a low pressure area.
Okay, let me try this: All modern car blowers produce a pressure rise by centrifugal force created by the rotating squirrel cage vanes. With no airflow, this blower is essentially spinning a cylinder of air captured between the vanes. The pressure at the inside of the vanes (say partial vacuum) is, of course, less than the pressure at the outside of the vanes (say atmospheric). Ready? Drum roll…this means that the average pressure is the same on both sides of each vane! Therefore, no work is being done because of the pressure rise. Turbulence (“windage”) and motor losses remain as the only power consuming mechanisms.
The never ending story . . .
Baaaaahhhhhhh…
right, you have just demonstrated that there is different air pressure on the two sides of the fan blades (vanes). this produces a force in the opposite direction that the fan is turning. the motor must do work against this force to keep the fan turning.
Melott: no, the force vector created by the different air pressure is radial while the fan blades travel tangentially (at 90 degrees to the force vector) and therefore cannot impart a tangential force to the blower wheel.