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Discussion in 'General Electronics Discussion' started by maxlds, Nov 4, 2010.

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  1. maxlds

    maxlds

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    Nov 4, 2010
    You'll have to forgive me. I'm new to the electronics scene, and I just want to make sure I don't do anything stupid. So, I want to basically create a big slot car track where the cart will sit on the electrified track and be power by a motor.

    I'll transform 120vac to 12 or 24vdc (thoughts?). I'd attach that to two aluminum rails, then have aluminum plates bring the power up to the motor in the cart. I've read that 12-24 vdc is completely safe. However, I've read that it's Amps that will kill you. Since the track won't be covered, I was wondering if it's safe.

    I know so little about amps and motors, but is seems pretty basic. Some motors tell me how many amps they draw under no load, then max load. If the threshold of perception is 3.5 to 5.2 mA (https://www.electronicspoint.com/ohms-law-again-t222434.html) and some motors say like 5 amps... of 5000mA (WAY above death). Other motors don't say anything.

    Considering these rails would be electrified, how can I guarantee safety, even if a person were to touch both rails at once?

    Please see my attached image for clarification.

    Please see my attached photo. Any help is appreciated.
     

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  2. shrtrnd

    shrtrnd

    3,712
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    Jan 15, 2010
    There are several things to consider for starters.
    Your voltage will be determined by the motor you're using, your amperage is going
    to be somewhat dependent on the length of your track. It sounds like this is not
    going to be a small project.
    You'll get more input from people as you add more information.
    Quick answer for a quick question.
     
  3. maxlds

    maxlds

    9
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    Nov 4, 2010
    Okay, so I guess I don't really care about the size of the motor. Let's say this one: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=260312243283#ht_1733wt_979 for example. It says input volts 24dc, current 330ma.

    If the motor says 330ma, then I imagine if I touched both sides of the tracks, I'd be toast, right?

    Now let's say I have 100 foot track (I don't know how big yet, but let's just suppose) with aluminum wire @ 18 gauge (http://www.homedepot.com/h_d1/N-5yc...splay?langId=-1&storeId=10051&catalogId=10053) . Would I get enough power around the track and not kill anyone?

    I could power opposite sides of the track with 2 sets of wires coming off the transformer if needs be, right?

    I just want to be safe, naturally, and want kiddos to be safe. Another option would be finding another way to cheaply propel a cart along a track.
     
  4. maxlds

    maxlds

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    Nov 4, 2010
    So, let's say 24vdc... 100ft track. 18 gauge aluminum wiring. Basically, I want to make a dark ride truck (like those at theme parks) that wouldn't carry people... so I'm guessing around 20 lbs or so.

    While I'm working on it (and taking it apart, putting it together, etc.) I just want to know that I won't be killed by the amps! I imagine if a motor has a 5amp draw, then I'd be in trouble, right? I mean, what determines the amps on the rails?
     
  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    24 Volts is not enough to push any hurtful currents through a person. While the statement you read is correct it fails to take physical values and Ohms law into consideration.
    24V DC can't be felt (by hands or feet). In my experience you have to go above 60V DC for any sensation. 24V AC on the other hand is on the verge of being unpleasant.
    Toddlers might be more sensitive of course, and they can get their hands just about anywhere, but still it would be nothing more than a strange sensation to them (1mA tops).
    You only need to worry about transferring sufficient power to the load, corrosion, and people shorting out the tracks with some metal, while staying at or below 36 Volts imho.
    Others may have actual law regulation info.
     
  6. shrtrnd

    shrtrnd

    3,712
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    Jan 15, 2010
    Moving 20 lbs. on a one-hundred foot track, is going to cost you some amperage. That'll mean you'll have to have a good circuit interrupt for Resqueline's reasons. (safety of people). Like Resqueline, I don't see a problem with low voltage and the amperage to the people, but somebody shorts your tracks with no circuit interrupt, and you've got the potential for overheating of the system itself, and fire hazard.
    PS, you're not going to be able to move 20 lbs with 330ma at 24VDC over a one-hundred foot track, unless somebody designed you one heck of an efficient motor.
    I'm also with Resqueline in that you'll probably have to meet some kind of legal electrical safety code requirements for this system to be used by the public.
     
  7. maxlds

    maxlds

    9
    0
    Nov 4, 2010
    Very helpful replies, thank you! I just want to make sure I'm clear:

    1. 24v dc will be just fine. Even though the motor reads 330 amps, I don't have to worry about the amps because...?

    2. The track would only be outside for a month, and I don't get much rain where I live. I'm sure there'll be corrosion problems, but I don't think I'm that worried.

    3. Shorting out the tracks with some metal... Can someone give me a brief overview of shorting and how that makes a fire hazard?

    4. "PS, you're not going to be able to move 20 lbs with 330ma at 24VDC over a one-hundred foot track, unless somebody designed you one heck of an efficient motor." - What would need to be bigger? mA? I could extend the power from the transformer to the other side of the track as well, right? Have 2 connections?

    5. Since this is just something for my yard and public will not be using it, I assume I'd be okay, legally?

    Thanks again for your help and responses.
     
  8. shrtrnd

    shrtrnd

    3,712
    468
    Jan 15, 2010
    Nuts, either I think or type too slow for this site. Gone for the weekend, wrote-up a response last night, but timed-out on the site and it erased my reply. Will try a shorter, faster response here.
    I'm still assuming you're motor is 330ma not 330A (two different entries above).
    Shorting will be across the power supply to your set-up, the tracks will be fine.
    The nature of the beast is as your motor moves down the track, the further away from the power supply it gets, the resistance of the track adds-up, causing the need for more current.
    We can't know what that resistance will be by guessing. Build it, just be ready to look for a heftier power supply to meet your needs, if your original can't hack it.
    If your motor is drawing more current than the power supply can give, the power supply will start warming-up, you'll be able to tell by putting your hand on it, feeling for increased heat when you begin testing your system out.
    I don't know about the legality of what you build.
    Good luck.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,360
    2,755
    Jan 21, 2010
    The added resistance means that you need more voltage from the power supply in order to maintain the same motor current.

    If you ensure that the voltage drop along the tracks is less than a couple of percent of the total voltage (preferably less) then you may not even notice it.

    I can't imagine that 330A is right :D
     
  10. shrtrnd

    shrtrnd

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    Jan 15, 2010
    Any possibility of just putting a rechargeable battery WITH the motor? Are you just going to turn it on and let it run, or do you need to vary the speed, or start or stop it along the route of the track? Just another thought to consider. That way the motor power would remain constant.
    My experience with long electrical runs, is that with a set voltage, the current draw increases with the length of the run, causing heating in the power supply. I can't argue with *steve*'s assessment that an increase in voltage might be necessary. My assessment was based on the idea that you planned on using 12 or 24 volts, and I considered what the practical outcome would be over distance.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,360
    2,755
    Jan 21, 2010
    That's generally the opposite of what happens.

    It could happen if the device at the other end has a switchmode power supply, or if your wires have negative resistance.

    If it's the latter, can I have a couple of reels of it?
     
  12. shrtrnd

    shrtrnd

    3,712
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    Jan 15, 2010
    Uh-Oh,
    *steve* must be an engineer. I'm a tech.
    .... hole flow, or electron flow, .....
     
  13. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Ok, trying to answer OP's questions, brushing up on physics class at the same time (preferring SI units, the only Sensible & Intelligent choice):

    30.48m of circular 0.82mm2 Al track is going to have a total resistance of 0.525 Ohms at the most distant point. A 5% Voltage drop (=1.2V) allows a motor draw up to 2.3A.

    Btw.: is getting the car to 18 km/h in 20 seconds flat fast enough? Will the track be level?

    1. Because; what current the motor draws is not what current will pass through your body. The two are completely unrelated. Only the applied Voltage represents a risk.

    2. Aluminum develops a tough insulating oxide layer and (or) a high friction. You may have to experiment with different pickup brushes to find what works best.

    3. Say someone rests their metal wristwatch band on the tracks. If the PSU can deliver a sufficiently high current (say 10 Amps) then said person might get a burn.

    4. Accellerating 10kg to 5m/s will require 125J. If your motor is 24V * 0.33A = 7.9W (=6.25W @ 79% efficiency) then that accelleration will take 125J / 6.25W = 20 sec's.

    5. Ok, I'd assume so too. If in the US though, I'd not bet on anything involving common sense.. Put up some signs, just to cover your back; "keep off the track".
     
  14. maxlds

    maxlds

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    Nov 4, 2010
    Thanks - To be read/posted later!

    Thanks all for your responses. I don't have time right now to respond to questions, but I will as soon as I can. I just didn't want people to think I'd let the thread die. I appreciate your thoughts, and I will respond soon.
     
  15. maxlds

    maxlds

    9
    0
    Nov 4, 2010
    Resolved

    First off, I just wanted to thank you all for your thoughts. I ended up grabbing a windshield wiper motor from an old car, and took the battery as well. I figure that I'll just drop those in the cart and power it along a track made of PVC pipe. I imagine the battery will last for quite some time, and if not, I can certainly daisy-chain a few together. So, I'll run copper wire from the battery to a switch to my motor -- I'm sure it'll work fine... also nice to not have to worry about kiddos getting too close to electrified rail.

    I can't tell you how much I've learned looking through these forums. I appreciate all you've done to help me learn electronics. I'm sure the day will come that I'll be back on here trying to figure other things out. So, thanks again for your help!!
     
  16. shrtrnd

    shrtrnd

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    468
    Jan 15, 2010
    Recharge the battery overnight, and you should be happy with the results.
     
  17. barathbushan

    barathbushan

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    Sep 26, 2009
    i can say only two mechanical things about motors

    1) the voltage at which the motor operates corresponds to its speed , this is usually specified by the manufacturer , for my 45 rpm motor i use a 12v battery as specified

    2) the current is analogous to the torque that a motor produces , if you put heavy load
    onto the motor , the torque output must increase to keep the motor running , thereby
    drawing more current from the supply , remember the motor's windings must
    dissipate the power , hence if you overload the motor , there will be huge power dissipation at the windings , thereby ruining the motor
     
  18. maxlds

    maxlds

    9
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    Nov 4, 2010
    Thanks

    Got it! Thanks, guys! I just bought a 12v battery recharger, so I think I'm set! Thanks again for all the help!
     
  19. barathbushan

    barathbushan

    223
    0
    Sep 26, 2009
    hi max make sure your charger is of constant current or constant voltage charger type as specified by the battery manufacturer
     
  20. maxlds

    maxlds

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    Nov 4, 2010
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