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DC motor info/help please

  1. Dec 18, 2004 #1
    Hi - can someone please help me with the following - I am mechanically minded not elecrically so please do not pick on me - I purchased a DC motor and gearbox (worm and wheel) from ebay (item 3858047682 if you want to look at it) - now it is supposed to work at up to 20 volts DC - now I was going to use a standard 4 amp 12 volt car battery charger to power it - what I do not know is -

    (1) if I used a 20 volt charger instead would it make the motor more powerfull as regards torque or just make the motor spin quicker thus making the gearbox output shaft spin quicker -

    (2) as I do not particularly want the output shaft turning faster would using a 12 volt 12 amp rather than a 4 amp battery charger produce more torque or would this not be of any benefit.

    I know I can increase the torque by feeding the output shaft into another gearbox or pair of pulleys to increase it but this is going to take up more room than I want to in the unit

    Thanking you in anticipation of your help - legin
  2. jcsd
  3. Dec 18, 2004 #2


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    Amps are drawn. If you hook a load to a 12 volt 10 amp source and it only draws 1 amp it doesn't matter how many amps the source is rated at as long as it is 1 or above.
  4. Dec 19, 2004 #3
    Averagesupernova - thank you 4 replying - I realise what you say is true - i.e. my light bulb only draws what it needs not the whole 5 amp supply it is being fed by - but can you answer the rest of the question please relating to the torque - if a dc motor is supplied by 12 or 20 volts - does applying the increased voltage to the motor make it a more powerful motor or just make the ouput shaft turn faster? thanks - legin
  5. Dec 19, 2004 #4


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    It does both, depending of course on how you load it. If you need more torque, use more voltage (as long as your source can handle the extra current). Also note that the motor will require much more current if it is exerting a torque on something rather than just freely spinning.
  6. Dec 19, 2004 #5


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    I'll try to convert this over to a physical metaphor. You build a small water wheel with some soup ladels arranged like a kids pinwheel. You spin the water wheel with a nozzle on a garden hose and it can spin quite fast. The more pressure you apply the faster the wheel spins.

    Now you take the axle spun by the water wheel and attach a load to it, maybe a string pulling up a weight. It cannot spin as fast anymore, but you find the more pressure you apply the faster it can spin. But if you lower the pressure it quickly reaches the point where it can't spin at all. If you remove the load and apply the same pressure it will spin at a speed that seems to fit the pressure. Even light loads are important to find the RPM based on the pressure.

    Now lets say you attach a very large load to the axle spun by the water wheel. You cannot make it spin with your garden hose so you get another. Even 2 can't so 3, 4, 5, 6 - they're so big a stream of water it can't even all hit the soup ladels anymore and it still cannot spin. If you have a firehose you have so much pressure and volume you could make it spin but might damage the whole setup if not careful.

    Ok, the pressure=voltage and the quantity of water is current. And from the above example, at a given pressure once you have reached a certain quantity of flow, any additional quantity does not change anything and is not used. At a higher rate of pressure that additional flow could easily be used to produce more work.


    Quick rules-of-thumb on electric motors that are true enough to illustrate the relationships.

    No-load full RPM is limited by voltage. More voltage means faster no-load RPM.

    power input = current times voltage

    Max current flows at stall (zero RPM) and can quickly burn up motors and supplies/batteries

    Roughly the same torque is produced from stall to 1/2 max RPM and then falls to zero torque at max RPM.

    power output = torque times rpm

    1/2 no load RPM is max efficiency (least amount of power input for power output)

    1/2 voltage = 1/4 power


    Its difficult to manage no-load RPM and loaded RPM to be the same without fancy circuitry. In between min/max it could be the voltage OR current limiting it from spinning as fast as you'd like. It changes at different RPMs since at full speed you need more voltage to go faster and near stall you need more current to go faster.

    You might find an old light dimmer or variable speed electric drill trigger that could be used as a variable resistor to wire in series to your motor from the supply. You could then limit the voltage to control your RPM. But this will also limit the current and is a catch 22 and you can see the available power drops quickly.

    The best electronic speed controllers limit the current sent to a motor to avoid burning up the supply (at stall the current can go into the hundreds and burn up everything) and then control the "average" voltage seen by the motor through very fast switching.

  7. Dec 20, 2004 #6
    Thank you both

    Stingray and Cliff J - thank you both for answering this thread - especially as you did not take the P--S - both of your answers have helped but Cliff J has put it in such a way that it is very easy for me to understand and I am very pleased he took the time to write all that out for me - because of this I can now close this thread - thank you both very much indeed - grandad_legin
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