Squirrel Cage Blower Speeds Up When Airflow is Blocked

Exactly! I noticed as a kid that the vacuum cleaner increased in pitch when the hose was plugged. I thought at the time that the motor was working harder. Only later did I realize that if it were working harder it would slow down, not speed up. Good answer!

"The power used by any fluid mover is the double-partial-integral of the change in pressure and the change in volumetric flowrate; in the simplified analogy of a pump working with incompressible liquids it is the pressure rise between outlet and inlet, multiplied by the volumetric flowrate, as simple quantities."

Almost - but not quite. Your statement suggests that if flow rate is zero, then any power consumed is the pressure rise multiplied by zero, which will equal zero, and while that is correct for fans and centrifugal fluid movers, try closing the discharge of a positive displacement pump and you will get an entirely different result.

Flow rate will be zero - but pressure and load will increase exponentially until either the overload pops, the shaft breaks, or the housing ruptures - the pump will definitely not increase in speed until something breaks.

I would assume "blower" implies gas, not liquid, and in any case a centrifugal pump is not positive displacement (like a piston pump), so I would expect the motor would speed up like my vacuum cleaner (without breaking anything ).

What is really happening is that the impeller moves around gas that is already moving rather than having to accelerate slow moving gas from the inlet, thus doing less work.

If you rupture your piping downstream of your positive displacement pump, then your pressure differential drops to zero at all flow rates. So no work is done. Your ball, because now you've got to mend it and prevent recurrence.

In the fan/blower at no flow, that just means there is no flow at the outlet. It does not mean that there is no bypass air flow, so there still is work being done on the air, and released, so the air might should heat up (slowly, depends on heat dissipation rate compared to actual power expended).

I agree with your statement about positive displacement pumps. Amazing how all that work can show up as black smoke.

Blocking the inlet or outlet of a centrifugal blower/pump will allow the speed to increase as the work being done by the impeller/fan is reduced.

I doubt that the air flow (Volume) is increased when partially restricted. Velocity, maybe if the outlet is blocked. Blocking the inlet will produce less flow but won't increase the outlet velocity.

pull a boat motor our of water while its running.......you'e removed the Load....drop it back in the water and the motor RPM drops, same thing, Slack explained it

Sorry, but no cigar.

In the case of the boat, you are removing the pump from the fluid. (changing the density of the fluid from that of water to only air)

That is NOT the same as restricting the flow of the fluid through the volute of the pump.

Do not pass Go. Do not collect $200.00.

....and now you have an overheated motor since you removed the source of it's cooling as well!

That's a 230v 3-speed motor...You should have 1 leg to yellow and the other to black would be high speed, the blue would be medium and the red low speed...You should have 2 brown wires for the capacitor the other wires are for the rotation direction....

https://www.grainger.com/product/GENERAL-ELECTRIC-1-3-HP-3M816

This motor is wired to 110V?

That would be a problem...but I'm hoping he meant 110v to each leg...

Since it is a 230v motor running at 115v, it is running way under powered and when unloaded it then speeds up 3 times faster. When I plugged it into 230v, it only sped up a small amount when I closed off half the port which is what I have seen before. I guess I need to find a 115v motor for the blower. Thanks all!

A sail sensor on a squirrel cage blower???????????

This motor is meant to run with a certain amount of resistance...If you remove it the motor will draw more amps and overheat, the same thing happens if you restrict the air too much....The motor runs at 1100 rpm regardless...

Sorry, no cirag youeither. (Surprise)

The motor is happy running with no load/resistance. It will use much less power and run cooler because it is not doing any work beyond turning the armature.

I have to disagree. this is a blower motor running with the squirrel cage intact....Although it seems logical that as you open it to the free air that the amps would decrease, but that is not the case, they actually increase....I've seen this many times, and it's why you can't get a true amp reading with the air handler open...

What you are saying is logical for any centrifugal blower, pumping any fluid. If it moves more air/fluid, it will draw more current. "This motor is meant to run with a certain amount of resistance" is a foreign concept to me.

Resistance to air flow....Carry a certain load? Do a certain amount of work...Well it doesn't matter anyway he was trying to run it on 110v to begin with....

..."ECM blower motors behave differently because they are programmed to adjust their output according to the resistance they are working against, but that is an entirely new discussion which I will save for another article."...

http://hvacrfundamentals.blogspot.com/2009/08/understanding-centrifugal-fan-motor.html

OK, I defer to you.

You obviously don't understand electric motors or centrifugal fans. The higher the volume produced the more power is required. Conversly lowering volume will reduce power required. Reducing volume therefor load will DECREASE power required ie. Amps x volts and motor will run at no load speed. Most motors have little difference between no load and full load speed, definitely not 20% or more. Overload speeds vary but not for long as fusion follows.

A blower that has the air flow blocked will draw less amps initially , that's true, but it will overheat after a while and eventually draw more amps...I don't think fusion is possible here....

The thermal OL will solve the overheating problem, if ever there is one.

Anyway, if this is a squirrel cage blower motor, depending on the design, there is no airflow over or around the motor from the fan because the motor is outside the airbox.

Any fan will decrease power required as flow decreases so your motor is probably over loaded at full volume and speeds up to correct speed as load is reduced. I suggest getting hold of a potable tachometer to check motor speeds at different air flows (loads).

That is totally normal.

Nothing new.

So many side tracks...

The motor is designed for 230V, the OP applied 115V to it. At 1/2 voltage, the motor will only develop 1/4 of it's normal PEAK and STARTING torque, which is used to accelerate it and 1/2 running torque if it does manage to get to full speed. At 1/2 torque and with dampers wide open allowing possible full low, the motor slip will increase drastically. So the load on the motor causes it to slow down and you do NOT get full flow. But because the torque is artificially hampered by the wrong voltage, the current is also kept in check (current follows torque), so it's possible that the motor will NOT overload, at least not immediately. That motor has built-in thermal protection that automatically resets. So I would think that either the load is so low on it that the motor is NOT overloading, or maybe it is, then the motor cools off and resets itself and the OP never noticed.

As the dampers are closed, the flow is restricted so the load on the motor becomes less and the motor is able to accelerate more it with the artificially low torque it is able to produce. It will still not allow full speed from the motor until maybe the dampers are closed all the way and there is no flow at all.

A SC motor cannot go faster than mains frequency (depending on no of poles) so if it speeds up 3 x time, then it suggests it was running at 1/3rd mains frequency to start with - which I find a bit odd?.

But I don't much about blower motor design however - so it might be normal practice - but it struck me as a bit odd.