supplying accurate output voltage < input voltage

[Laurav]

On Jun 16, 1:33 pm, Jamie
&lt;jamie_ka1lpa_not_valid_after_ka1l...@charter.net&gt; wrote:

The only way
to properly regulate that is to measure the air mass flow and use that
as the feed back.

I suggested that to the guy who makes them. It would be great if you
could set 5 CFM on the dial and be done with it.
Roughly how much more expensive would that be? The web link you gave
for the brushless DC motor is $1300. He charges $250 for this
motor.

RPM's is going to vary to maintain flow.
Not much in my experience, I'm adjusting the rheostat manually and if

I were standing by it, I could adjust the flow well enough.

 You seem to think what
you have is some high volume unit?
No, it's a low volume of air. However it pushes the air at high

pressure, because it has to go through many feet of hose. He said it
has high static pressure. Do you know how that changes the controller
picture?

  Seeing that this is a medical device you have
It's an industrial safety device actually.

You need to keep an eye on the brushes! They don't last
like you think they should.

Yes, the brushes have worn out twice on me already. The motor just
quits without warning and exposes me to allergens. That gives me
an idea though - if you open the motor housing, is it easy to tell how
worn the brushes are?

Laura

 

[Laurav]

On Jun 16, 1:50 pm, Bennett &lt;bjpr...@cal.berkeley.edu&gt; wrote:

I wonder whether a viable alternative is to control the air pressure rather than the voltage.

It's the motor itself that needs to be controlled. It gets too noisy
at high speed.
Laura

 

[Laurav]

On Jun 16, 1:33 pm, Jamie
&lt;jamie_ka1lpa_not_valid_after_ka1l...@charter.net&gt; wrote:

  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.

It's actually an industrial safety device. It's for use in
environments that aren't immediately dangerous to life or health.
Autobody workers use such respirators, because they work with
chemicals like isocyanates that won't immediately cause damage, but
you don't want to be breathing them long-term. If it fails on me I
get sick for several days.

Laura

 

On Sat, 16 Jun 2012 06:15:34 -0700 (PDT), Laurav &lt;pbbl@cyberspace.org&gt;
wrote:

On Jun 16, 12:57 am, e...@whidbey.com wrote:
Can you supply any info on the motor itself? Not the current draw but
the type of motor?

He said it's a universal motor. It has brushes.
It would be enough to control the input voltage, that's what I'm now
trying to do by hand. I think if I were standing by the rheostat all
the time, I could control the motor speed well enough.
It's in a metal box, I don't know what's inside the box.
What Jamie seemed to be saying is that there's some kind of feedback
loop that might make the motor very voltage-sensitive, that he said
would apply to a fan motor. This isn't a fan, it's an air turbine.
In any case, this is what I have, and what I'd like to get running
more evenly.
It's not like there are a huge variety of airline respirators on the
market. I'm not sure if the very expensive NIOSH-certified ones have
more sophisticated motor control or not. I somewhat doubt it - I
called one company and he said the innards were about the same for his
NIOSH-certified version vs the home use version.
These airline respirators are used by auto body workers or hobbyists,
sandblasters, etc. Precise volume control might not be necessary for
them.
The voltage at an outlet does seem to vary more than the utility co.
voltage, so I may have electrical problems that can be fixed and would
give more even airflow.
Laura
How close to maximum speed are you running the motor? If somewhere

around 1/2 then there are pretty simple circuits that use feedback
from the motor to regulate speed. I could email you a schematic of
one. But a universal motor is a bad choice for this application.
Cheap though. Another solution may be a CPAP machine. They are for
forcing air down a snoring person's nasal passages to keep them
breathing and stop the snoring. These devices are similar to your
machine but have much better air pressure regulation and are quieter.
I don't know if they put out as much air as you need because they are
made for sleeping people but the one I have is adjustable to pretty
high pressure and volume. They are available used for pretty cheap.
Call a local Senior Center. The one I use is over 6 years old and
still works great, especially considering it gets used every night.
Eric

 

[William Sommerwerck]

If it's a universal motor, it will run on DC, right?

So... How about using Zener diodes to regulate the voltage? Cheap and
simple -- in theory.

 

[Jamie]

Laurav wrote:

On Jun 16, 1:33 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net&gt; wrote:

The only way
to properly regulate that is to measure the air mass flow and use that
as the feed back.


I suggested that to the guy who makes them. It would be great if you
could set 5 CFM on the dial and be done with it.
Roughly how much more expensive would that be? The web link you gave
for the brushless DC motor is $1300. He charges $250 for this
motor.


RPM's is going to vary to maintain flow.

Not much in my experience, I'm adjusting the rheostat manually and if
I were standing by it, I could adjust the flow well enough.


You seem to think what
you have is some high volume unit?

No, it's a low volume of air. However it pushes the air at high
pressure, because it has to go through many feet of hose. He said it
has high static pressure. Do you know how that changes the controller
picture?


Seeing that this is a medical device you have

It's an industrial safety device actually.


You need to keep an eye on the brushes! They don't last
like you think they should.


Yes, the brushes have worn out twice on me already. The motor just
quits without warning and exposes me to allergens. That gives me
an idea though - if you open the motor housing, is it easy to tell how
worn the brushes are?

Laura

Yes, you can inspect the brushes. Normally they should still have enough
on them to have a good spring holding tension against the commutator..
Many of them put a mark on the side of the brush or crimp something on
the lead wire to indicate when they should be changed.

Also, if you have gone through that many brushes already, it's a good
chance the commutator is close to its life cycle. You need to inspect it
for uneven surface wear which may include deep grooves.


Jamie

 

[Laurav]

On Jun 17, 11:24 am, e...@whidbey.com wrote:

How close to maximum speed are you running the motor?
Much less than 1/2 max speed, I think. I measured the voltage the

rheostat is outputting at my usual setting, and I was surprised to see
it was 104 V, with ~ 120 V input.
Ideally I'd have an inline sensor to measure air speed in the hose,
and if the air speed is outside a preset range, the sensor would raise
or lower the voltage.

Another solution may be a CPAP machine.

An airline respirator has a powerful motor for pushing air through a
long length of hose, unlike a CPAP machine.

Laura