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'[EE] Clever ways of slowing down an AC totalizer?'
2007\06\08@181644 by Peter Todd

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I need to make an AC totalizer hours meter count time 10 times slower.
See, on the surplus market there are plenty of 6 digit models that will
take just over one year to roll over, I need one that will take 10 years
to roll over.

Now of course the easy way to do this is to just make a PIC chip turn
the totalizer on for one minute, off for nine etc. But then I'd have to
get a little transformer, mount it in my box, do up a circuit board, get
a solid state relay etc...

I want to be a bit more clever, if possible. If I were to run the motor
on 6hz, would it go a tenth as fast? Is this like a stepper motor, or
are there more complex issues at play? I know that a transformer that
works great at 60hz, would have to be much larger at 6hz...

Assuming the above was true, how would you go about generating 120V 6hz
anyway?


And before anyone suggests it... Mechanical solutions are right out.
The totalizers I want to use are very tamper resistant; the gears are in a
hermetically sealed welded box and looks are important for what I'm
doing.

- --
http://petertodd.ca
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2007\06\08@200847 by Dwayne Reid

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At 02:53 AM 6/8/2007, Peter Todd wrote:

>I need to make an AC totalizer hours meter count time 10 times slower.
>See, on the surplus market there are plenty of 6 digit models that will
>take just over one year to roll over, I need one that will take 10 years
>to roll over.
>
>I want to be a bit more clever, if possible. If I were to run the motor
>on 6hz, would it go a tenth as fast? Is this like a stepper motor, or
>are there more complex issues at play? I know that a transformer that
>works great at 60hz, would have to be much larger at 6hz...
>
>Assuming the above was true, how would you go about generating 120V 6hz
>anyway?

I think that you will find that trying to use a 6 Hz sine or square
wave signal to not work very well because of core saturation problems.

However, you might try using narrow pulses (a few ms wide) pulses
alternating at a 6 Hz rate.

Half-wave rectifier running directly from power line, small reservoir
cap, H-bridge driving motor.

For that matter, I've envisioning the possibility of a really simple
H-bridge with 2 small reservoir caps (one for each side of the motor
winding) and treating the whole thing like a capacitive discharge
supply.  Maybe . . .  Have to think it over.

Bottom line: think in terms of narrow pulses alternating at your
desired rate.  I think its got a good chance of working just fine.

dwayne

--
Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2007\06\08@205016 by Peter Todd

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On Fri, Jun 08, 2007 at 06:08:33PM -0600, Dwayne Reid wrote:
{Quote hidden}

Thanks for the input. Sounds like your idea could work, but to generate
those pulses I might as well have a microprocessor and at that point you
might as well just use the on/off scheme.

Still, I can see pulses being cheaper and smaller in some cases, a pulse
generating circuit would likely take up less space than a solid state
relay module. A key advantage would be that the circuit could run off a
low voltage wall wart, and therefore avoid having AC line voltage in the
project. (not that ms pulses are completely safe either)

- --
http://petertodd.org
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2007\06\09@000307 by Hector Martin

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If you go with the on/off idea, you might want to have the PIC track the
AC wave and turn it on/off at the zero-crossing, and doing timing based
on pulses. If you start chopping pulses in half it might end up causing
some drift (I guess it depends on how accurate you want to be).

You could also just have a fast switch and do per-pulse on/off. Same
idea, but instead of generating the pulses yourself you just chop the AC
wave.

--
Hector Martin (.....hectorKILLspamspam@spam@marcansoft.com)
Public Key: http://www.marcansoft.com/marcan.asc

2007\06\09@011726 by Robert Rolf

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Hector Martin wrote:

> If you go with the on/off idea, you might want to have the PIC track the
> AC wave and turn it on/off at the zero-crossing,

You also have to worry about inertia of the motor flywheel.

> and doing timing based
> on pulses. If you start chopping pulses in half it might end up causing
> some drift (I guess it depends on how accurate you want to be).
>
> You could also just have a fast switch and do per-pulse on/off. Same
> idea, but instead of generating the pulses yourself you just chop the AC
> wave.

Why not just divide the AC by 10 and use capacitor coupling to drive the motor
at a lower voltage (to prevent coil saturation. After all, the point is to reach
a certain current (flux) so 1/10th the frequency means 1/10th the voltage needed.
The problem with induction motors (as used on most totalizers, is that you need a
certain DI/Dt to get the needed back EMF off the rotor, so Dwayne's suggestion of
using pulses is probably correct.

Robert

2007\06\09@055931 by Peter P.

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As others have said, there is a problem with the minimum usable frequency to
make the motor turn, and there is a problem with inertia, which precludes using
separate sine periods to effect small turns. So the best answer is to run the
motor using normal AC, for 6 minutes every hour (or for 1 minute every 10
minutes). The infrequent starts and stops should minimize errors due to slip and
inertia. You may have to program some kind of offset into the code to account
for 'slip' when starting and stopping. The device may count too much or too
little depending on the mechanical damping of the inner parts (and this may
change over time as oil dries out etc).

I saw on your site that you implemented a clock using d'Arsonval analog meters,
and the wandering laser spot clock. Your web page does not say if you used a
stepper or something else as a motor for the spot. I have these references from
that time:

 http://www.piclist.com/techref/postbot.asp?
 by=time&id=piclist/2005/12/20/142136a&tgt=_top&key=25uA+and+pwm+&from=

 http://petertodd.ca/art/meter-clock/

(interestingly, your meter clock predates my idea in that thread by almost a
year ...)

Anyway, the source code for these projects is available on your site. Do you
plan to make the totalizer code available too ?

Peter P.


2007\06\10@105041 by Peter Todd

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On Sat, Jun 09, 2007 at 09:58:04AM +0000, Peter P. wrote:
> I saw on your site that you implemented a clock using d'Arsonval analog meters,
> and the wandering laser spot clock. Your web page does not say if you used a
> stepper or something else as a motor for the spot. I have these references from
> that time:

If you take a look at the source code and electrical schematics for the
laser dot clock you'll see I went with the stepper solution. The
mechanics were a lot more simple and robust than any sort of moving
mirror. Interestingly the disadvantages of the stepper, slow slew rate
and visible steps, have turned out to be advantages. The owner actually
rather likes the way if you look real closely you can make out visible
steps, and the 10 second "march" back to the starting point at midnight.

Then again, the fragility and delicacy of a mirror would be a plus too
for the same reasons. Except, then I'd have to fix it every time it
broke or was damaged.

>   http://www.piclist.com/techref/postbot.asp?
>   by=time&id=piclist/2005/12/20/142136a&tgt=_top&key=25uA+and+pwm+&from=
>
>   http://petertodd.ca/art/meter-clock/
>
> (interestingly, your meter clock predates my idea in that thread by almost a
> year ...)

If you look into all the "Meter Clock" websites that have popped up,
they all popped up after I posted my meter clock to the web... I know
for sure the guy behind http://www.meterclock.com was inspired by me for
instance.

> Anyway, the source code for these projects is available on your site. Do you
> plan to make the totalizer code available too ?

Of course. Schematics and hardware drawings too. Just don't expect it
anytime soon, I was hoping it'd be a quick project, but it looks like
it'll be just complex enough that I'll have to put it off for a month or
two.  I've got a major art show coming up July 6th, and I got a major
grant to make art for it. The totalizer (actually going to be called
"Two Choices Hour Meter") is on the end of the list of art I promised
to make for it...

- --
http://petertodd.org
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