Motor Current Decreasing Under Load???

[JayMan07]

I always assumed that electric motor current would increase under load, but every time I test this motor I find that it decreases.

It's a DC blower motor I have.  It's 12V and draws 635 mA normally.  If I put it under load (by blocking some the air flow) the current drops to about 450 mA.

I tried to research this online, but everything I found said that current should increase under load.  Am I missing something simple or what?

[ignator]

Te blower is a 'pump'. By blocking the output, you reduced the power needed to move the air.
So a decrease in input power is the correct observation. This is true for most pumps that are not positive displacement.
I know it seems non-intuitive. The fan blades aerodynamically are in a stall, by blocking the air flow.

[IanB]

What ignator said. Blocking the air flow through a fan decreases the load on the motor. You can often tell this is happening from the way that the fan speeds up. Try it with a hairdryer (on cold): block up the inlet holes and the whine will increase as the blower runs faster.

[JayMan07]

Ok thanks for clearing that up!  That does make sense to me now.  And I tried measuring the current on a regular DC motor I had laying around and found that it did rise a lot under load.

[AlfBaz]

Try it with a hairdryer (on cold): block up the inlet holes and the whine will increase as the blower runs faster.

It is a little counter intuitive but most of us would see this with a vacuum cleaner on a day to day basis.
Years ago at a workshop we would start massive auxiliary fans after an overhaul with no load by blocking the intake

[max_torque]

For fans / pumps etc Blocked airflow = less mass of air moved = less power required from motor driving fan.

If you carefully use your finger (or better, a plastic biro or bit of wood) to "slow the fan down" by pressing it against the spinning fan hub, current should go up!

[David Hess]

I know it seems non-intuitive. The fan blades aerodynamically are in a stall, by blocking the air flow.

This is a great description of what is going on.  I wish I had said it.