On Tue, 28 Apr 2015 21:46:38 -0400, Bob Engelhardt wrote:

On 4/28/2015 8:47 PM, Lloyd E. Sponenburgh wrote:

The speed [current?] is dependent upon the mass of air being moved.
It's pretty large when sucking and blowing freely. The motor does the
most work moving the largest volume. When the inlet is blocked, the fan
just recirculates the air in the volute, and it's (essentially)
unloaded.
...

Oh, yeah ... I remember now, about the current increasing as restriction
decreases. Seems paradoxical until you think about it as a matter of
mass of air being moved. Thanks for that "model".

Now, back to the original problem: with the "extractor" operational, it
was overheating. The mass-of-air model would suggest that the cause of
that would be an air _leak_, not a blockage. I'll look into that & be
back.

Either that or the excess friction in the motor -- it really ought to spin
more or less freely (taking bearing seals into account, which I doubt are
there).

I recall grabbing the shaft of a 10-horse compressor motor and (slowly)
giving it a spin -- I think that would be my baseline.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com