Problem with a fan controling by the inverter ;/

Micha³ wrote:

I once lent a colleague a power supply that had both an adjustable
output voltage and an adjustable current limit. He connected it to a
fixed resister, and concluded from his inability to adjust voltage and
current independently that the power supply was defective.

He was a physicist, not an engineer, but it I was nevertheless surprised
that it took 15 minutes of equations and graphs on the blackboard to
show him why the supply necessarily behaved as it did.

Where is the pressure measured? Flow? Once you leave the fan chamber,
the relation between pressure and flow depends only on down-stream
conditions.

Jerry
--
Engineering is the art of making what you want from things you can get.
???????????????????????????????????????????????????????????????????????

Jerry Avins wrote:

Perhaps he was a _theoretical_ physicist?

--

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

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"Applied Control Theory for Embedded Systems" came out in April.
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Osoba nazywajaca sie Jerry Avins w liscie z dnia  2006-06-26 21:04
napisala nastepujace slowa:

(...)

I measure only pressure after the fan...

Hmm.. I read your text about a physicist.. And.. And I see some
connection to my problem but.. but it's very far :P

I need to keep constant flow.. I control a fan speed.. And I need to
keep a pressure about 20-30% of the nominal pressure fan. I know from
the characteristic of the fan that it isn't possible. But.. Maybe you
have an idea how to change some part of the system to reach this parameters?

--
Best regards,
Michal

Micha³ wrote:

The flow you want may not be possible with the pressure you need to
keep. Think of the outlet duct as a resistor, with pressure analogous to
voltage and flow analogous to current. Although the flow through it is
not linearly proportional to pressure, they rise and fall together.
Maybe if you tell us more about your constraints and the reasons for
them, we'll be better able to offer specific suggestions.

I congratulate you on your command of English. It certainly is up to the
task.

Jerry
--
Engineering is the art of making what you want from things you can get.

Osoba nazywajaca sie Jerry Avins w liscie z dnia  2006-06-26 23:21
napisala nastepujace slowa:


Yes. I know that. But if we make an assumption that it is possible, what
I need to control my pressure with keeping stable air flow? Maybe some
chamber before the fan (in place where is already a guide ring)? Then I
can try to keep constant air flow.



Ok. I'll try to describe my problem more precisely :)

Already my system looks like this:

--- oven -----> guide ring ---> fan ---
|                                     |
------<-----------burner---<-----------

I measure a pressure drop and a temperature (but it's not the case in
this situation) in the oven. To keep needful parameters in oven I must
control an air flow in a fan.  Nowadays I control it by opening or
closing a guide ring. It's not ease to control and very not optimized
system.


Future:
I need to dismount a guide ring and replace it by the inverter connected
to the fan. But I know if I do it I won't be able to keep constant air
flow, because if I decrease a fan speed to some level of pressure
(pressure necessary to continue all technological process in the oven)
then my air flow also decrease.




Pardon? I think that I don't understand you.. Well my English -- I still
learning that language and I know that I make many mistakes but I
believe that you'll have understanding for me and nevertheless you won't
leave me alone with my question :)


--
Michal

Micha³ wrote:

   ...


It seems that the air is circulating in a loop.


I don't know what you mean by "inverter". Is the pressure in the oven
too high, or too low when the flow is right? If it is too low, throttle
the oven's outlet. If it is too high, throttle the oven's inlet. Either
way, adjust the fan speed to maintain the needed flow.


I mean that you write English better than merely well enough to
communicate your problem. I salute you!

Jerry
--
Engineering is the art of making what you want from things you can get.

Osoba nazywajaca sie Jerry Avins w liscie z dnia  2006-06-27 15:18
napisala nastepujace slowa:


Right. About 80-90% of the air is still the same. 10-20% is a 'fresh'
air, which contain oxygen (burner needs it).


[...]

Well... Word 'inverter' I found in a dictionary. In polish this element
is called as 'falownik'. In english, inverter is a electric device that
can control for an example a rotary speed of the engiine, by the
controlling frequency of the current - ehh.. it's difficult to explain
that - but I believe that you will understand me ;)

Right. I must adjust a pressure in the oven when the flow is still
constant. So - solve of the problem could be mounting a throttle after
and before the oven? Throttles can be connected with the inverter (which
control speed of the fan) by the feedback loop. Am I right?


[...]

Thanks you ;)

--
Michal

Micha³ wrote:

It's a good word, but it has too many meanings. I understand now what
your device is.


At any given flow and pressure combination, one of the throttles can be
wide open. (You may never need it at all.) The relation between throttle
setting and fan speed won't be simple. You may need to experiment and
create a table. Moving one of the controls (throttle, speed) slowly and
the other more rapidly is a way likely to succeed.

I hope that someone who has actually built a system very much like yours
can offer you his/her experience.

Jerry
--
Engineering is the art of making what you want from things you can get.

Osoba nazywajaca sie Jerry Avins w liscie z dnia  2006-06-27 21:13
napisala nastepujace slowa:

[...]

Well, maybe this is a solve of my problem, but - it's similar to the
situation that I've already: a manual guide ring (it may be called as
throttle) installed befor a fan. So, here an inverter isn't as much
needful ;/



Anyway - thanks you for your time and help ;)

--
Michal

Micha³ wrote:

   ...

Assuming that the flow is held constant by varying the speed of the
blower -- that's what the inverter is for -- the pressure in the furnace
can be raised by throttling its outlet, or lowered by throttling its
inlet. There's not much else available to be controlled.

Jerry
--
Engineering is the art of making what you want from things you can get.

Micha³ wrote:


I'm not sure what you mean here by 'guide ring'.

If you have a fixed mechanical arrangement with a fan blowing into a
plenum, and your only degree of freedom is to adjust the fan speed, then
you will, of necessity, adjust the pressure inside the plenum and the
flow simultaneously -- there's no other way.

Is the 'guide ring' you're talking about some movable device that
changes the airflow somehow?  If so, and if you want to adjust things
for an airflow and pressure, then you'll need to adjust the fan and ring
independently, and have some fun making a responsive, stable system.

--

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

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Osoba nazywajaca sie Tim Wescott w liscie z dnia  2006-06-27 00:00
napisala nastepujace slowa:


Guide ring is a fixed mechanical element that can control an airflow.


Ok. I understand.



Yes. You right. But my boss want's to dismount a guide ring, and control
a fan only through a inverter.

If we mount a big chamber, before a fan, which can keep constant flow, a
decrease a pressure, what will be?



--
Michal

Micha³ wrote:


I assume that by "inverter" you mean drive?  "Guide ring" sounds like
something that, well, guides, where you must be talking about a variable
orifice.

Whether you have a fan running at fixed drive with the airflow
controlled by an orifice or a fixed orifice with the airflow controlled
by the fan drive you're still controlling pressure and flow
simultaneously.  It seems that what your concern should be (and perhaps
is) is whether you can adequately control your airflow by controlling
drive to the fan.

Ultimately you'll have to answer this question on your own, because it
depends a lot on the type of fan, the type of motor, the type of drive
you select ('inverter' covers a lot of territory) and your performance
requirements.

How are you controlling your orifice now?  Does your temperature control
loop generate a position command for a position loop around the orifice,
or does the temperature control generate a speed command for the
orifice, which then runs open loop?  How much fluctuation can you stand
in your flow?

Assuming that your fan will have a well behaved flow vs. drive
characteristic, it seems like you'd have a pretty good chance of putting
it in and having it work.  So the real question you need to ask is what
fan and motor combination do you need to achieve a flow vs. drive
characteristic that's as good as the fan, motor and variable orifice
combination you have now.

If it were _me_, I'd be saying how big the motor is, what kind of fan it
is (squirrel cage, axial, waving palm leaves, whatever), and what kind
of motor/drive technology (brushed DC with amplifier, brushless DC with
brushless amplifier, induction motor with variable frequency drive).
Then I'd sit back and hope some of the folks on this group who know more
about this sort of thing than I do tell me what's right or wrong about
the proposed choices.

--

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

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Osoba nazywajaca sie Tim Wescott w liscie z dnia  2006-06-27 16:37
napisala nastepujace slowa:


Inverter - electrical element which can control a drive by changing
current frequency :)

Guide ring - let's tell that's someting similar to orifice.



Yes. And this situation was. Now, I'll control a fan speed (by changing
his current frequency) and then I'll be able to control only flow. But I
need to control also a pressure. But how? It this is the main point of
the problem.




Correct. That's right.


Manualy. And I need to change it. I need to control flow and pressure
automaticaly - so I want to dismount an orifice and mount an 'inverter'
(connected to the drive of the fan).



Already I only see temperature on the digital indicator. When the
temperature is too high, worker needs to go down and more turn an orifice.




I determin that flow must be about 2 m^3/s, and pressure drop in the
oven form about 800 Pa to 1500 Pa.

Parameters of the fan: V = 10 m^3/s, p = 5kPa.

Now a fan works in his 100% of power. And my parameters (V=2m^3/s and
p€0-1500Pa) are setting up by the orifice.  Changing this all I want
to save an electric power (decreasing supply of power to the motor of
the fan) and controll my parametres directly from the fan.



Fan parameters:
n60 rpm
V,8 m^3/s
PS00 Pa

type: radial

control: now: motor connected directly to the fan; future: motor
controled by the inverter (electric element which is able to change the
frequency of the current :-) connected to the fan.



I know that motor and fan are redimensioned. As I sad earlier, I only
about 20-30% power of the fan. I can't replace fan to the new - so I
must apply some way to decrease fan power keeping air flow at the level
from the 100% fan power.


--
Michal

Micha³ wrote:


Doesnt seem to be that you are too concerned with pressure, just
temperature control via air flow regulation?

A good inverter (VF drive), eg Invertek, Hitachi, will offer the option
of PID control to a given set point - usually motor speed fed back from
a shaft encoder

But if the drive were fed a set point in a form it understands - perhaps
via a temperature-frequency converter or something - surely this would
act to control temperature via fan speed

Just how many groups are you posting this to?  Typically one active group
will get you an answer.

Michael

Osoba nazywajaca sie Herman Family w liscie z dnia  2006-06-27 06:05
napisala nastepujace slowa:


I'm looking for the many points of view at my problem. I'm very sorry if
I made something wrong ;/


--
Michal

Micha³ wrote:

Generally if you're going to post a question to multiple groups you
should cross-post it -- that is, post it all in one message so that
replies go to all the groups.  This lets everyone see all the answers,
getting you a better quality of answer and keeping me from giving you
duplicate answers.  Even so, you should limit yourself to just a few
newsgroups.

--

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

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Osoba nazywajaca sie Tim Wescott w liscie z dnia  2006-06-27 16:23
napisala nastepujace slowa:


Thanks you for information. Next time I will do that.


 > Even so, you should limit yourself to just a few

I send my message to one polish group and 4 english language.. So.. - in
my opinion - I send it 'to just a few newsgroups' :)))


--
Michal