When you put your hand over a shop vac’s hose, or block the intake of a blower fan, it spins faster and the sound it makes gets higher pitched.
Why exactly?
My hypothesis is that by restricting the intake of air, a partial vacuum gets created inside. Vacuum = less air = less drag on the fan blades = faster spinning motor?
The usual description is that the fan blades “stall”, which means that they aren’t moving the air, which means that it takes no energy to make them spin.
In a “stall”, the air at the fan could be spinning around with the fan, or it could be flowing around the fan blades, but it’s not flowing through the fan blades.
However it happens, less air movement = less drag on the fan blades = faster spinning motor. The vacuum at the input disappears, the pressure at the output drops.
This is part of the complication of buying a vacuum cleaner: it doesn’t matter how powerful the motor is, if the air flow isn’t designed right, you won’t pick up the dirt.
I’ll go with that too.
When a vacuum is moving air normally, moving that air takes work, it’s a load on the motor (same idea as a propeller pushing air back to make the airplane go forward.
When the intake is blocked, the air is not moving past the fan. There will be a slightly lower pressure in front of the fan, in the hose, and normal pressure on the other side, but most likely the air reaches an equilibrium where air spins with the fan since the somewhat lower-pressure air spinning with the fan does not have enough pressure to push its way into the exhaust outlet.
Yes, it takes less energy to spin the fan through a less dense ‘atmosphere’ so it speeds up. Counter-intuitively, the motor is NOT straining harder even though it sounds like it is. In fact, if you put an amp meter on it you’ll see it’s actually drawing quite a bit less electricy, i.e. the load on it is significantly decreased. However, because the motor is also cooled by the airflow past it doing this for a prolonged time will cause it to overheat. Saw this on the very first episode of the great series Secret Life of Machines.
Depends on the vacuum. I have a Ridgid shop vac unit in which the vacuum air path is independent of the motor-cooling air path. You can cover the end of the hose all day long, and the motor will continue to move cooling air through itself without any trouble. YMMV.
No, it’s not the vacuum. The same thing happens if you block the exhaust, which makes the pressure inside higher.
Broadly, it’s because the blower is doing less work to the air. A similar situation happens when your wheels spin on ice. Most blowers run faster when flow is restricted. But this can vary with details of blower design.
It’s also true of a pool pump - if you block the intake, the current draw goes way down, and this feature is used to “tune” the energy used by the motor.
The speed increase has a lot to do with the speed / torque characteristic of this type of AC motor (Universal Motor). This is a typical speed torque characteristic of such a motor. As you can see in the curve, the speeds of this motor goes much higher compared to an induction motor (like a ceiling fan or pool water pump) at lower torques.
True. They use commutated motors because given 60 Hz line frequency, induction motors don’t spin nearly as fast, and spinning a blower faster gives it a higher pressure differential capability.
But if you’re interested in the blower per se, and imagine a simple and ideal experiment in which you gave it constant torque (a bit of an obscure setup I admit), the speeding up effect would be even bigger.
Those are all interesting answers but you are all wrong. When air flow is restricted the pressure in front of the blades increases slightly but not enough to make up for the fact that flow is restricted almost eliminating the fans ability to remove enough air from the intake to create any significant vacuum eliminating a large amount of the load because the blade pushes against pressure as it always does but no air flow means no suction which is a large part of the load. The pulling a vacuum portion of the work is almost gone the increase in pressure is minimal in front of the blades higher but not much
Actually Melbourne pretty much had it. Sorry I skimmed over the thread and missed your comment on drag
The part about pressure dropping at output with restricted flow is only true if output is not blocked. the air in front of blades will increase with restriction by a build up of restricted air as well as by the increase in rpm’s
I would be interested to know what sort of internet search led to turning up this old thread. And then our new poster went to the trouble of registering for the site, with a customized name to boot. Would you mind telling us a little bit about yourself?
Normally a centrifugal pump is moving air/liquid from the inside to the outside, speeding it up along the way. The acceleration of the mass of the medium is the work the pump is doing.
When there is no air/liquid (medium) coming from the inside, the medium stays in place. Now the pump is not doing any work (except fighting friction) So now it can spin freely: like a car with its wheels lifted from the ground.
Centrifugal pumps are weird, they have all kinds of properties that are completely counterintuitive for me. They spin freely when there is no flow;
When the speed of the pump stays the same, the pressure is a function of the density of the medium .
Perhaps a username change to ‘some_of_yours_wrong_is’
The ‘work’ explanation you and other provided seems to be a common answer to this question that I have seen several times before. After learning the explanation it seems intuitive that it must. If it was a water pump and the water source ran dry would you expect the pump to stay the same speed while only pumping air?
Good point seven years ago by @am77494 that this wouldn’t apply to a pump or fan driven by a synchronous motor.
Updated link: https://youtu.be/CJlrbMHLBd4
This is more a question of what kind of drive is powering the pump.
For practical purposes the (centrifugal) pump will behave the same pumping air as it would do “pumping” with closed shutters. (Except it will destroy bearings and seals without the water)
Many turbomolecular vaccuum pumps simply won’t work if the backing pressure is too high, because too many air molecules will start hitting each other instead of the fan blades.
As I pointed out, there would be no change of speed with a synchronous motor driving the pump. But if it were driven by a universal electric motor or an ungoverned gasoline engine wouldn’t you expect the impellor to speed up? Without the work of moving water why aren’t those motors speeding up?
