Overheating AA battery on spin project

Discussion in 'The Projects Forum' started by shawnh, Jan 3, 2009.

  1. shawnh

    Thread Starter New Member

    Jan 3, 2009
    2
    0
    HElp! My electronics education ended with my 7th grade electronics kit-but I was SURE this was within my abilities!!
    I built a spin art machine for my kids and the AA battery keeps getting redhot with use.
    I have a DC motor originally powered by 8 AA cells, a toggle switch and one AA battery powering it now, all wire with CAT 5 stranded wire.
    The motor spins a 4"x6" piece of 1/8 inch plywood which paint is dropped onto, making a swirl or starburst design.
    After a few minutes, the battery is almost too hot to hold. I think that as a motor spins, a current is generated and sent back toward the power source. Is that correct, is that the problem, and more importantly how do I fix it?
    Thanks for your input- I have six little hands with paint bottles ready to squirt any second!!!- Shawn
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hi Shawn,
    First, edit your 2nd post and remove your E-mail address before the "spambots" pick it up and your inbox gets jammed with junk. :eek:

    The problem is that your motor is designed to run at a much higher speed with more voltage applied, but right now it's running at a very low speed due to the heavy load that the 4"x6" plywood represents; actually near "stall" speed.

    As a motor increases in speed, it actually generates what is known as "back-EMF", or it's own internal voltage. If there is no load, the motor reaches top speed and generates the maximum back-EMF that balances out with the applied power.

    As the load increases, the speed of the motor drops, which decreases the internal back-EMF, and increases the draw from the external supply in an attempt to keep the motor spinning.

    Since your motor is turning so slowly, it's hardly generating any back-EMF, so the motor looks like nearly a dead short to the battery. As a result, the battery is expending a lot of power across it's own internal resistance, which is making it very hot.

    What you need is a 1.5v to 3v motor with a speed reduction unit; it could be as simple as a rubber band on the motor shaft looped around a jar lid screwed to the underside of the plywood, with some sort of bearing.

    But since you're in a jam, just pull out that old phonograph/turntable you don't use anymore, and let the kids have at it. ;)
     
  3. shawnh

    Thread Starter New Member

    Jan 3, 2009
    2
    0
    Thanks for the info! What if I hooked up 6vDC to it and a potentiometer?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    There is a drawback to placing a variable resistor in series with a motor in an attempt to control the speed. It turns out that when a motor is loaded it draws additional current from the power source. If a resistor is placed in series with the motor, the additional current demand will result in an increased voltage drop across the series resistor. The impact of this voltage drop is to rob the motor of voltage and thus make is more prone to stall. In effect, the addition of a resistor compromises the ability of the motor to develop the torque needed to respond to a varying load.

    There are two fairly straightfoward ways to manage the speed of a motor.

    The best way is to use PWM (pulse width modulation). A less complicated way is to power it from an adjustable power supply.

    hgmjr
     
  5. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    As was mentioned your stalled motor looks like a short to the battery.

    I just saw this "VCR Cat Feeder" on makezine.tv that solves a stalling problem
    using pulleys. Could be entertaining for an art project.

    http://tinyurl.com/8esyk9

    (* jcl *)
     
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