Re: Industrial duty dimmer
From: Jaggy Taggy (outhouse_at_midcoast.com)
Date: 03/27/05
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Date: Sun, 27 Mar 2005 07:45:32 GMT
On 3/27/05 1:19 AM, in article
1111904355.990568.22580@z14g2000cwz.googlegroups.com, "Chris"
<cfoley1064@yahoo.com> wrote:
>
> Jaggy Taggy wrote:
>>
>> The air pump I will have to use is a 2 to 3 watt aquarium pump which
> indeed
>> uses a small solenoid which vibrates the bellows.
>> I tried to run one of these things off of a regular $5 dimmer and it
> worked
>> fine, but as I said 'momentary' performance does not impress me when
> failure
>> means having to fly to Europe to fix it, that is where my wont for a
>> reliable solution comes in.
>>
>> Using the aquarium pump continously and bleeding off the excess is an
>> interesting thought.
>>
>> Controls like this, either a valve or the pot of a dimmer, see an
> incredible
>> amount of abuse in a science museum environment and the final
> solution
>> should be the one which results in the most rugged 'visitor
> interface', and,
>> in spite of its perceived ruggedness, still the one the maintenance
> people
>> can easily fix.
>>
>> If needed I can get around the minimum load requirement by using an
>> additional dummy load resistor in conjunction with the regular
> dimmer.
>>
>> I am aware of PWM but that doesn't seem to be a solution in my case.
>>
>>
>> What is the variable frequency drive Chris is referring to??
>>
>>
>> Thanks for the postings so far, this is helpful to me
>>
>> Uwe
>
> Hi, Uwe. A couple of points:
>
> * Sorry -- the 20 watt variable frequency drive business was a bit of
> a joke (note the "winky" ;-) ). VFDs are frequently used for speed
> control of multi-horsepower AC motors.
>
> * You've got a point -- if the museum patrons are gooig to be tweaking
> this continuously, a small needle valve might not be a good idea. They
> wear out fairly quickly. In addition, they can be ruined easily by
> cranking them past full off.
>
> * I did a quick Google scan, and saw a couple of manufacturers of
> aquarium air pumps that have models with electronic air speed controls.
> These could possibly be the R+pot-C/DIAC/triac controls in the cheapie
> lamp dimmers, and one description mentioned that it had a bellows-type
> motor. So, in fact, what you're talking about with a dimmer-type
> controller might be possible.
>
> * One issue with these lamp dimmers I didn't mention has to do with
> the asymmetry of the DIAC triggering voltage. This results in a DC
> component in the load voltage. While this isn't a problem with lamps,
> it can cause issues with motors or inductive loads like solenoids. It
> heats the coil, and can result in damage. It's something that has to
> be taken into account.
>
> * I believe a power resistor in parallel with your pump is a good
> idea, but you shouldn't need more than a 680 ohm 25 watt resistor for
> 120V, or 1500 ohm, 50 watt for 240V. Since lamp dimmers are in series
> with the AC load, this should provide enough load current to make sure
> your dimmer works properly.
>
> * You mentioned that this business might be shipped to another
> country. You should be aware of voltage differences (if you get a
> 120VAC pump and send it to a country with 240VAC, the pump will have a
> very short, warm and exciting life. Also, if you're building it in a
> 60 Hz environment, and you ship it to a 50Hz environment, you'll have
> less air volume. Remember, bellows stroke frequency will be dependent
> on line frequency).
>
> * Given that this is going to a museum, you've got a couple of more
> issues, if the museum patrons are going to be playing with this
> control. In many countries, exhibits with electrical controls may be
> classified by the local regulatory agencies as machines. If so, you
> may have to deal with the requirement for low voltage (24V max.) panel
> controls. This would obviously mean a line voltage lamp dimmer with
> line voltage on the pot would be out.
>
> * You also have to deal with the potential for patrons to be downright
> malicious, at least by accident. Can you imagine what would happen if
> a 3rd grade urchin spills a Juicy Juice on a typical dimmer control?
> Bzzzt. Ouch.
>
> * From a practical standpoint, too, you have to look at the life cycle
> of the pot or dimmer control you're using. Most dimmers are basically
> made to be tweaked a couple of times a day for 10 years or so, meaning
> about 50,000 cycles. One busload of 3rd graders could run through that
> in one day, especially if your exhibit is fun and exciting to more than
> one of the little urchins. Imagine the delight of the museum tech
> having to change the dimmer every month. You might want to look at an
> AC control with remote pot, and use one of those Allen-Bradley machine
> panel potentiometers with washdown capability. The pot itself is
> mounted behind the panel mounted shaft, and joined with a flexible
> coupling, which eliminates the side-to-side torque that usually results
> in premature pot failure. Since the only moving part is on the front
> panel, it should be fairly easy for the museum tech to replace. I
> guarantee that this will be a predominant failure mode here. Get the
> one with the small terminal block under the pot for the three wires.
>
> You can buy or borrow a lamp dimmer and actually try your idea.
> Measure the DC resistance of the solenoid coil. Then, after you hook
> up the parallel load resistor, run it straight off line voltage for a
> while with the cover off. Get an idea how hot the coil becomes at full
> voltage. An educated (but careful) finger is probably good enough
> here. Then hook up the lamp dimmer in series, and see what happens.
> Measure the DC component by putting a DVM across the load, set for
> highest range DC. Measure maximum DC voltage (this will usually occur
> between 5% and 40% of maximum control voltage). Figure out how much DC
> power is going to be dissipated. Try leaving the dimmer at that
> setting for twenty minutes, and then see if the coil gets hot. Try
> several power settings, and see. See if yuo can make the thing stall
> and get stuck at lower voltage. It's a start, anyway. After you've
> done this, you can look at different types of AC controls that will
> work for you. There are some which work with inductive loads, some
> which have forgiving minimum current requirements, and can be used with
> an external speed pot. There are also others which provide an isolated
> low voltage for a remote control pot which complies with EU strictures
> on low voltage controls. I'd need a lot more information before
> specifying something myself.
>
> <dreamland>Of course, if I wanted to nuke this problem, and money was
> no object, only reliability, I'd use a 24VDC motor driving a
> peristaltic pump, and use a PWM motor controller with the Allen Bradley
> washdown machine panel pot as the user control. You can get everything
> you need except the Alen Bradley pot, including the speed control, from
> the Cole-Parmer catalog. Make sure you use the most expensive, most
> durable tubing available.</dreamland> (This is very expensive, and
> should be viewed as humor, unless you've got a lot of cash to spend.)
>
> Where I live, the local science and industry museum has a tech
> department, and has people who are paid to do this kind of thing. I
> know if I had a problem of this type, I'd want to call them up and ask
> a few good questions. They also may be able to come up with more
> gotchas off the top of their heads. If you're a member of your local
> science museum patron support group and you ask nicely, you might be
> able to buy one of the techs or engineers a good lunch, and in
> exchange, be able to ask a few questions and pick their brain about
> your project.
>
> Good luck
> Chris
>
Chris, thank you for your very thoughtful answer, you hit on most of the
pertinent questions I can think of.
First a few points regarding the effort which often goes into these
controls. They easily cost hundreds of dollars since you can never let the
public handle a potentiometer directly, they are always isolated and
protected from radial and axial loads and from turning past their stops.
In addition to all these precautions I design the knobs in such a way that
the public does not really have much leverage on the whole assembly, for
example by using a flat knob with a shallow depression for your index
finger, hard to exert much force that way.
And then you obviously need the best pot money can buy.
And then you need to make it easy to change because it will fail anyways.
The pumps in our case are pretty much determined. I looked for the pump with
the right amount of flow and a very low noise level. I have been using this
particular one for years and will try not to change it. I supply my exhibit
with a big transformer so I can continue to use this type of pump even in
Germany with 220V!
The pump will last about a year under continuous use. I used an arrangement
to bleed of the excess in an earlier version and thought the electronic
control would allow to change the airflow similar to a bleeder arrangement
but prolong the life of the pump since it might not run continuously,
statistically speaking the visitors might leave it in the off position half
of the time.
Also, in a bleeder valve (I designed my own back then, not a needle valve
but a ported cylinder which does not need any stops, you can keep turning
it) it is not that easy to make a channel which will gradually and evenly
bleed off air over 180 or 270 degrees, we are talking a very small groove
here...
Early tests seemed to indicate that a dimmer arrangement with this pump
might work, but for how long??
The discussion here seemed to question the reliability of this arrangement
and since then I have been thinking again about the bleeder arrangement.
And indeed I think I will go with a modified bleeder valve, which I can
build as beefy as necessary, it can have a continuous turn, no stops
necessary, and I managed to find an arrangement which allows the entire
bleeding valve to be removed as a unit from the outside while no screws are
visible, another requirement of our particular set-up.
And Chris, finally, referring to the suggestions in your last paragraph, I
am one of those people one often calls for advise in these matters. I know
this must be a disappointment to you but this is how I earn my keep and it
is quite a challenge given all the other constraints and the publics
ingenuity to disassemble the exhibits to come up with good solutions.
We will see how long this one will last in Germany....
Anyways, thank you and the other posters for the input, I think I got a good
approach now.
Uwe
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