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Thread: Lottery Wheel

  1. Question Lottery Wheel

    Hi I'm making an automatic lottery wheel for an art project.
    It's a wooden wheel about 1 meter diameter and one centimetre thick.
    It mounted to a mountain bike rear wheel (gives me the gears and the free wheel for free).

    Now I'm going to make this wheel spin for a couple of minutes and then stop for a while and then start to spin again. It will do this for a month straight around the clock.

    I'm a software programmer so the coding is not a problem and I have bought this 0/0/4 interface kit from Phidget (http://www.phidgets.com/products.php...roduct_id=1014)
    It will do the trick to turn on and off the motor in an interval.

    I have bought a motor from Micromotor called E192 (http://www.micromotorssrl.com/motori...riduttori.html). It's a 24 V DC motor. 300Ncm torque. Geared down 246:1. 430 mA (max load).
    See documentation: (https://www1.elfa.se/data1/wwwroot/w...A/05445507.pdf)

    I can power this from a transformer/adapter plugged in to the wall so no battery is needed.

    This is where the easy part ends. I have almost no knowledge of electrics but I'm a good googler. I guess this is not that hard if one knows but it's so basic no-one wants to write a tutorial about it.

    What do I need now to power this motor? I have read page after page of info about how to power a dc motor from batteries and making H-Bridges to switch direction... Regulators... capacitors and so on.
    But do I really need all this? The wheel is just going to spin in one direction and at a predefined speed. When the motor stops the wheel will keep on turning because of the freewheel in the mountain bike wheel so it will not start to generate current or motorbrake.

    Can't I just plug an AC/DC Adapter to the wall and then plug the out-cables into the motor?

    What happens if the motor is to weak and it takes some time to get the wheel spinning? Will it melt before I can say... damn? Will it start to burn?
    Since I have already built the wheel I know that when it has started to spin it will spin for many minutes without any new force added but its a bit heavy so getting it to spin takes some power (but I think 300Ncm will be enough).

    I think it would be good with a fuse. On what current should that burn? 430 mA? Should I use slow or quick burning fuses? I'm a bit worried about current spikes. When the wheel starts to spin, I think, there are higher current than when the wheel is in motion at top speed. Correct?. Is that the 430 mA stated in the documentation as current on max load?


    Many questions, I hope you guys can give me some of the answers.
    If any information is missing please tell me and I'll try to add it.

    Thousands of thanks in advance
    Nisse Bergman

  2. #2
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    Re: Lottery Wheel

    Welcome to the forum.

    That's current at max torque, not current at stall. Put a four or five amp power supply in there with a fast blow 1A fuse for startup and see what happens. You can use a relay instead of a motor controller, but preferably something solid state for longevity.

    You're not likely to find a "430mA fuse" anywhere.

    When the wheels is spinning the current draw will be lower. High current draw will be observed when you try to get the rotating mass started - moment of inertia. Max current draw will be observed when some idiot grabs the wheel and stalls the motor out. That's when the magic smoke will start to escape, and where you want it protected.

    You didn't give us much specifics on the wheel that I can see. Size, weight, etc, so calculating the torque isn't possible.

    For your own edification, a good place to start would be 0.5 * Mass * Radius2 or 1/2Mr2 That gives you the moment of inertia of a uniform disc. Then we probably need to go to τ = I α where τ = torque, I = moment of inertia, α =angular acceleration


    Sorry - I'm in the middle of writing a test for a statewide physics competition...
    Last edited by Adrenalynn; 04-13-2009 at 06:11 PM.
    I Void Warranties´┐Ż

  3. Re: Lottery Wheel

    Thanks for the quick answer.

    I expected that current at max torque and current at stall wasn't the same. It would have been to easy.
    Is it possible to calculate the current at stall or is that something one have to empirical tests on?

    No idiot will ever grab the wheel and stall it out since it will behind a class window (if its not a real idiot that crashes the window and grabs the wheel).


    I don't really understand why I need a four or five amp power supply when the fuse blows at one amp. The reason why I'm a bit sceptical is that five amp power supplies is more expensive that a one amp. I also find it a bit hard to find those five amp power supplies.

    I'm a bit confused why the fuse should blow on one amp for one more reason. If the max amp on the motor is 430 mA then wouldn't it have broken long time ago when the current is one amp and the fuse blows?

    Anyway. Ill try with one amp powersupply and a quick fuse on one amp and see if it works.
    If that is stupid please tell me.

    /Nisse

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    Re: Lottery Wheel

    Hi Again,

    Current at Stall should be provided by the manufacturer, but frequently isn't. An adequately sized ammeter can deliver that number trivially - I prefer the over-design method.

    If you can be absolutely certain that no mechanical/physical/electrical/human-error failing will ever stall the motor, then you are free to take liberties. Personally, for a few dollars, my engineering experience has taught me the foolishness of my ways.

    You have a fifteen amp breaker on a typical socket in a house. The wire, 14 Gauge, is rated to fire-code at 15 amps. That means it won't burst into flame at 15, 20, even 25 amps. The fuse is always under-rated against the power source. It's meant as protection. Fusing a 1A power supply at 1A is just begging for Murphy to hand you your arse on a platter, well toasted. The general rule of thumb is to double it. Those of us with well toasted arses tend to double it, then double it again... I sell 5A @ 12vDC power supplies on my site, I believe, but no 24vDC supplies. I'm sure I could dig some links if you need. At no point should you be thinking about less than 3A, or I wash my hands of the whole thing.

    The fuse on the half amp motor blows at 1A for a few reasons. One: practicality in sourcing - it's a common fuse. Two: Noise, transients, spikes, startup current. A motor is a big power resistor when it's not moving. It sucks down a ton of power, well past the ratings for that period until it overcomes inertia. It doesn't like to stay there for long. Heat is additive. If it sits at even .5A for long enough the one amp fuse will get cranky and blow - especially a quick-blow fuse. The heat will just build and build and the fusable link will unfuse itself, which looks really cool on a highspeed camera - but I digress. Motors are also noisy, full of spikes and transients as the field experiences progression. Which also creates Ohmic heating.

    I certainly wouldn't say "stupid" - that'd be impolite. I'd say "inexperienced", and keep my tongue firmly in my cheek. Some things just have to be experienced, and I'm all for smiling knowingly and letting the person experience for themselves. I think a 1A/1A supply and fuse mated together is an unsafe combination that would challenge most fire codes, so that's the one place I would strongly advise reconsideration.
    I Void Warranties´┐Ż

  5. Re: Lottery Wheel

    Thanks this is very helpful. I have been searching so long for some hard facts.

    I have been looking around and there are a couple of transformers with 24V and 5Amps but they are breaking my fragile budget (they cost about 150USD). I'll maybe rethink this and make the wheel spin with a ordinary table-fan that blows air at the wheel. I have no idea if it will work but if it does it will be a lot cheaper.

    I'll be posting some images later when this is up and running.

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    Re: Lottery Wheel

    If you're going low budget, maybe you should go see the local scrapyard and look for an old washing machine. Get the motor, belt and drum out, attach to the wheel, hook up 110v (i guess that's what you have over there, right?) and voila.
    Switching on/off can be done with a simple off-the-shelf timer that costs maybe 5$.

    It's the programming that's involved that'll set you back a few bucks to interface with the motor.

    /edit; one more tip: back here halogen lighting is at 24V. You can get some cheap electronic halogen transformers with built-in overload/temperature protection (about 15€, that's uhm... 18$ or so?) in the local DIY store. Hook up some large current diode bridges, a few caps to filter out the transients, and you're good to go.
    Not the cleanest supply but it'll work.
    Last edited by ScuD; 04-14-2009 at 10:51 AM.
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