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Coming back from a nice camping trip, I noticed all air pumps made higher and higher pitch when the air bed were about to be fully inflated. It is heard like the the motor is speeding up when it is overloaded. I opened the pump but it seems very simple - electricity from the batteries are directly feed into a motor through a switch. However, my understanding about a electronic motor is that the speed should decrease when load increases. Is there anything I missed?

10/4/2012 Update

Finally, I made my poor man's power supply by a computer PSU and went through a test. Driven by 3.3V, the air pump draws 4.5A when it runs with a free airway; when the output nozzle is blocked, the current drops to 3.3A. The test supports Dave's answer and I agree that this question is more related to physics.

The picture is the test setup.enter image description here

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This question really belongs in physics, but I'll take a stab at it.

Increasing back pressure on a positive-displacement air pump does not increase the load on the motor; it actually decreases it. On each stroke of the pump, less air escapes through the exhaust valve, and more of it remains in the chamber to help push the piston (or diaphram) back down, returning energy to the motor.

In other words, the output airflow drops off faster than the inverse of the back pressure, and the total work (airflow × pressure) drops.

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  • \$\begingroup\$ The pump he is talking about isn't a positive-displacement pump. It's a centrifugal blower type. \$\endgroup\$ – Connor Wolf Sep 24 '12 at 18:14
  • \$\begingroup\$ Hmm. I suppose a similar argument might apply. In the limiting case, where the output pressure of the pump matches the back pressure, there's no new air entering the pump, so the motor doesn't need to do any work accelerating it. It just has to do enough work to keep the existing air in the pump spinning. \$\endgroup\$ – Dave Tweed Sep 24 '12 at 18:43
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    \$\begingroup\$ A centrifugal pump which is moving fluid has to do work because in the absence of added energy, the rotational speed of material which moves away from the center would decrease. Since any material whose rotational speed is less than the rotational speed of the pump is going to collide with the vanes, such material pushes on the vanes in the direction opposite the direction of rotation. If flow is blocked such that no fluid can move through the pump, then the fluid which is in the pump will reach an equilibrium radial distribution and spin around as a solid mass. \$\endgroup\$ – supercat Sep 24 '12 at 19:07
  • \$\begingroup\$ I tried to measure the current change when the air way is blocked but the power supply (MAX 1.3A) I have in hand cannot start the motor. I ordered an ATX PSU from amazon and it's coming this weekend. I will use the 5V output to do a test once it is arrived. \$\endgroup\$ – Codism Sep 27 '12 at 16:28
  • \$\begingroup\$ I have updated my question and accepting your answer based on my test. Thanks \$\endgroup\$ – Codism Oct 5 '12 at 3:32

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