Connecting Multiple DC Motors to a Single Driveshaft

Discussion in 'The Projects Forum' started by Debaser, Jul 14, 2013.

  1. Debaser

    Thread Starter New Member

    Jul 14, 2013
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    0
    I am looking to build a cheap electric motorcycle and am wondering about the possibility of using multiple dc motors to drive a single shaft. The reason I want to use multiple motors instead of one big one is that I can readily obtain 2.5hp motors for cheap, and sourcing a large motor in New Zealand is hard/ expensive
    However, the motors I can get hold of are likely to vary somewhat in their exact specifications. While most will have the same hp rating, they have different manufacturers, and are likely to vary somewhat in RPM and amps.
    My question is, if I connect 2 (or more) of these motors up to a single driveshaft and power them from the same controller, what will the effects be?
    I am guessing that the slightly faster one will in effect be slowed down by the other, and the slower one will made to spin at a higher RPM. Providing I didn’t go over the maximum rated power and RPM of the weaker motor, would this be ok?
    Thanks in advance!
     
  2. Ramussons

    Active Member

    May 3, 2013
    557
    92
    You are right, in Theory. The "lazy eye" principle is stronger.:D
    You may end up with a burnt out motor. You will need to design a circuit to balance the loading of the motors - by monitoring the power input to the motor.

    Ramesh
     
  3. donpetru

    Active Member

    Nov 14, 2008
    186
    25
    More DC motors requires the use of several DC-DC converters. Or you can use a higher wattage motor with a single DC-DC converter. But as you said, it's a very expensive solution. However, both solutions have a cost very close. So, it is pointless to complicate things more than they are now.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    A 2.5hp motor is equivalent to a 1800W. The motor on my ebike is only 500W.
    How are you going to power a 2.5hp or 5hp motor?
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
    If the motors aren't quite identical, it may still be possible to run them off the same power source. You'd need at look at the torque/speed curves and see how much torque each one would be putting out for a given speed, and if they're roughly the same, it might be good enough. But is there enough room on your bike for two motors? And Mr Chips made a good point--where's the electrical power to run this rig going to come from?
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    A long extension cord?
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Welcome Debaser.

    What voltage are these motors?
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Or maybe a Flux Capacitor?
     
  9. Debaser

    Thread Starter New Member

    Jul 14, 2013
    2
    0
    Thanks for the replies everyone.

    Just to clarify, I am hoping to build a motorcycle, rather than ebike. I agree that 1.8kw would be more than enough for an ebike, but I am thinking something more like 9kw will be the minimum I need for a motorbike.

    The motors are being sourced from used treadmills. I can get these for next to nothing. I will do my best to make sure that the motors have the same voltage, RPM, and draw the same current. However, I might not be able to get them perfectly matched. 120vdc is probably going to be the voltage of the motors I end up using. I realise the idea of having a motorcycle powered by treadmill motors might sound a little odd, but I like the idea of using recycled parts and doing something a little different.

    I am still deciding on the energy source. 18650 lithium ion batteries are probably going to be the way to go. The reason for this is that I will need high a high voltage, and getting this high voltage from larger batteries is going to be prohibitively heavy/expensive. Before people scoff at the idea of using laptop batteries to power a motorbike, 18650 are what Tesla use in their electric cars. However, if tubeguy can offer me some working drawing for a flux capacitor I would seriously consider changing my energy source ;-)

    People have suggested that I need to design a circuit to balance the load to the motors, has anyone got any suggestions of how this could be done? All (constructive) suggestions welcome!
     
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    A bit of silliness got the better of me. I wish I had a design for a working flux capacitor. I'd be laying on some tropical, sandy beach right now. :)
     
  11. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Years ago I did work on a project where two 10 hp 3-phase AC motors were used to power a long serpentine overhead conveyor system.The motors where spaced evenly along the conveyor. Speed control was required.

    The working solution was to connect both motors to a single AC VFD. Only one motor used a tach for speed feedback.The load was shared evenly between both motors with no problems. I would think DC motors would behave similarly, but again they would need close matching.

    The other thought is a very small resistance in series with each motor to aid balancing. (Maybe even carefully sized connecting cables)

    Here's a link on electric car conversions:
    http://www.diyelectriccar.com/forums/showthread.php?t=669&redir_from=668
    You may not need as much DC motor HP as you think. I seem to recall something as high as an 8:1 ratio between gas engines to electric power. Check out the "How much power do I need section"
     
    Last edited: Jul 14, 2013
  12. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    Treadmill motors are likely to be permanent magnet motors, which follow a straight voltage-rpm curve. Trying to get these motors to load share is going to be a big pain. If you can find some series wound motors (like automotive starters) they will share load more readily.

    Ps. Automotive mods are taboo here @ AAC so don't get bitter when your thread is closed.
     
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