Work = Force times distance.
The fan blades move through a distance in a circle.
The blades do work against a force. Normally, that is pushing air to make it move.
In the case of a plugged filter, there is a pressure difference on the two sides of the fan blade, due to the partial vacuum. In order to move the blades, the motor must do work against this force.
Nope. Sorry, I give up.
One more try. If it’s doing more ‘work’, then why does it speed up?
Kieth, many cars do rely on air from the blower to cool the motor.
Prove it speeds up.
Have you never put your hand over the end of the hose on a vacuum cleaner? It speeds up, right? Just like a heater fan might do if you blocked the air inlet completely.
In an earlier post I mentioned “windage” loss.
That’s work done to spin air around (with the resulting turbulence) in the immediate vicinity of the fan blades, even when there is no flow through the inlet/outlet.
Squirrel cage fans and the turbine-like fans do speed up when the flow is blocked, propeller-screw fans (see below) slow down.
I think @melott means that this isn’t a fan spinning in a vacuum situation. If the fan were spinning in outer space, in true vacuum, then the blades wouldn’t have any force on them. But even though the filter is blocked, and no air moves through it, there is still air in the chamber, and the fan blades are meeting resistance as they spin through the air. It isn’t necessary for the air to move through the filter for work to be done. Like if you poke your arm out the window of a moving car, there’s plenty of force felt on your arm, even though you aren’t moving the air much to speak of. Contrast that to putting your arm out of a spaceship in outer space, there, you wouldn’t feel any force. That’s the difference I think melott is speaking of.
Texases has a good point, if the motor speeds up, that would tend to indicate there is less load on the motor. But less load isn’t the same as no load. Whether the motor would heat up or cool down in that situation would depend in part on how much the motor’s cooling come from the air flow past the motor. But it would also depend on how much the current through the motor changed, as the back-emf changes when the load changes. It’s not such a simple thing to calculate I expect, and would depend on the motor design and how much cooling it gets from the wind flow.
insightful, name one.
Most HVAC blower motors have a air vent hose connected to the HVAC housing. Later model vehicles have a vent tube molded into the motor housing.
However, I have only replaced one failed blower motor on a vehicle with a plugged cabin filter @100,000 miles. Most blower motors failures that I see have nothing to do with the cabin filter.
I replaced many leaking/corroded evaporator cores in 1990’s vehicles and they usually had a build up of damp leaves on the inlet side. The cabin filters are keeping the debis out so now I have less work. Throw away your cabin air filters.
Its a good feature,even construction equipment has it(in enclosed cabs) but the tight owners oft times dont see the need for it or AC for that matter(the old timers didnt have it,etc,blah blah-Kevin
Cheapskates are free to remove the filter and breathe dirty air
Just don’t complain about a plugged/rusted/leaking evaporator later on
Kieth, 2007 Honda Civic among many others. Look at the pics at this link:
The two square holes you see next to the white impeller lead to a tube cast into the backing plate (also clearly seen). This tube takes air from the discharge of the blower (high pressure zone) and routes it to the end of the motor. The air flows through the motor, thus cooling it, and discharges back into the blower behind the impeller (low pressure zone).
You’re welcome.
Yes. But if the fan is pumping little or no air, and the motor isn’t doing any work, thus the motor isn’t drawing any amperage so the motor isn’t getting hot.
Tester
Sorry if I’ve posted twice. Anyone else having trouble posting today?
No problems here
I might have had too many links.
Kieth, here’s another:
http://www.alibaba.com/product-gs/582807405/Car_heater_blower_NISSAN_ATLAS_blower.html
Note the cooling tube.
Tester, Even a blocked blower creates a lot of turbulence and the amps will still be significantly above zero.
I don’t know if any of you can read a simple graph, but if you look at Fan/Power Consumption on this page, you can see the power consumption drops as the air flow drops. And if you interpolate the graph futher to left to where there’s no airflow, what do you think the power consumption is?
http://www.engineeringtoolbox.com/fans-efficiency-power-consumption-d_197.html
Tester
Tester, note the descriptor for the graph: " The ideal power consumptions for fans at different air volumes and pressure increases are expressed in the chart below:". Each line on this graph represents multiple fans operating at their normal air flows, not an individual fan operating over its airflow range. In other news, I put a wattmeter on a 120V squirrel cage space heater and with fan only, got 15 watts. Blocking the suction, the fan audibly sped up and power went down 2 watts to 13 watts. This is probably a cheap shaded-pole AC motor with very low efficiency.
I wish I had a cabin filter, I taste salt on my lips after 1/2 inch of snow followed by 2 inches of salt.