Stalling DC motors Question

Discussion in 'General Electronics Chat' started by Bermbuster, Aug 9, 2008.

  1. Bermbuster

    Thread Starter New Member

    Aug 9, 2008
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    Hi Everybody
    I have a problem that I am trying to reduce or eliminate in a parallel circuirt. I have an electronic speed control for an RC truck.
    My truck has 2 dc motors (one for each axle) when I run the truck and the front axle unloads from climbing (wheels start spinning) the rear wheels stop moving.
    My problem is the power is not being distributed between the motors evenly. I think I understand what is happening. The motor with the least resistance isgetting/using all the power while the motor which is under load is starving for power. I did try to make this a series circuirt as well but I had even worse results....
    thanks for any and all help.....
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Yep, current is like that. It flows most where resistance is leat, meaning the unloaded motor will hog it.

    There is a possibility you can rewire the motors in series. That would increase power to the loaded motor when the other axle unloads, but you would need to double the driving voltage to get the same power. this would mean more batteries and could cause problems with the speed controller if it can't handle the extra voltage.


    Someone else might have some ideas, too.
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I would expect that in a series circuit, drive would shift to the unloaded motor but in a parallel circuit the loaded motor should receive most of the power since the loaded motor will have the lowest electrical resistance.

    Both motors see the same voltage source, so providing the voltage doesn't sag significantly under load, there is no reason why the rear motor should be losing out.

    Is the rear motor stalling simply because it cannot provide enough torque or is the battery not able to supply the current required?
     
    Last edited: Aug 9, 2008
  4. Bermbuster

    Thread Starter New Member

    Aug 9, 2008
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    The motor may be stalling for lack of torque (motors and gearing are identical) Im using a 20c lipo which is rated at 20amps cont. and Im using small 150watt motors.
    I retraced my steps and I decided to go back to a series circuit.
    When the truck is in the air (no load) only one axle spins.....
    Once I put it on the ground both spin and when I start to climb it works better.....The best thing I can do to minimize this stalling will be to now volt up and gear down. Right now Im using a 2 cell lipo 7.4v but i will try a 3 cell 11.1... Unfortunately I cant gear down now due to the motor is not adjustable (yet). Thanks for your help.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    You understand the theory about an electric motor backwards.
    1) A stalled motor is working as hard as it can and has a very high current.
    2) A motor running without a load has a very low current.

    Think about the physics of a 4-wheel drive vehicle going up a hill. When accellerating then the weight moves toward the rear as a load on the rear motors and the motors at the front are unloaded and begin to slip.
     
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    A series circuit will be worse. As the load increases on one motor, the voltage across it will decrease while the voltage across the unloaded motor will increase by the same amount. In series you will lose virtually all drive to the loaded motor.
     
  7. Bermbuster

    Thread Starter New Member

    Aug 9, 2008
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    The motors being used are dc brushed motors and I can tell (you would see this too) that the stalled (potentially loaded motor) is not getting the power...(motor stays cool to the touch)
    Theory is great but this is reality.....
    All the power goes to the unloaded motor and the axle spins like mad.....
    Electrons are like water they follow the path of least resistance.....
    Im just tring to figure if I can alter this path to my advantage....
    thanks for your reply
     
  8. Bermbuster

    Thread Starter New Member

    Aug 9, 2008
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    What I see is with the series circuit is a lower overall speed but I am not seeing the stall effect as bad. My next step is to increase my overall voltage to the motors ...currently 7.4v I will go up to 11.1v and Im sure I will get some more speed. I am using this vehicle to rock crawl. A high speed is not needed just the ability to climb over obstacles.....
    thanks for your reply
     
  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    The path of least resistance IS the stalled motor, it has the lowest electrical resistance.
     
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