Laurav wrote:

Here's what the guy who makes the respirator said, relevant to voltage
sensitivity:
"the motor speed change should be proportional to the voltage change,
but that effect is amplified by the lowering of static pressure and
the head loss in the air line, so the air flow reduction at free flow
(at the air pump with no hose) would be reduced even more if measured
at the end of the air hose."

If you remove the load from a series wound motor (universal), it'll
increase in RPM's, greatly in many cases.

That type of motor is very sensitive to supply and exhaust flow..

essentially, you have a vacuum loader/cleaner motor.. The only way
to properly regulate that is to measure the air mass flow and use that
as the feed back. RPM's is going to vary to maintain flow.

In a series wound motor, like you have, it is almost a natural.

P.S.
They make Brushless vacuum/blower motors that produce high volumes
with integrated electronics for speed control of a simple 0..10 volts.

They just cost more... Are you getting the image now?

http://www.grainger.com/Grainger/DOM...VY4?Pid=search

That is the idea of what I am talking about, however, that unit there
is far to large for what you're talking about. You seem to think what
you have is some high volume unit? It is actually very small compared to
what I deal with. In your case, I can understand why. You don't need to
be over inflated!

Seeing that this is a medical device you have, I don't see you getting
much help from those that cold make that unit more suitable for your
needs, even though there are many that can do it for you. I think you
understand what I am talking about there, it's to bad but that has what
the world has turned to.

Personally, I can't believe they put in a universal motor, in the
first place? You need to keep an eye on the brushes! They don't last
like you think they should. It's called cheap, cheap!...

Jamie