How can reduce the speed of a 9.6 v toy motor

Discussion in 'The Projects Forum' started by LeeH, Dec 4, 2008.

  1. LeeH

    Thread Starter New Member

    Dec 4, 2008
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    I have a school project. I used an old toy truck 9.6 volt, made a homemade Pendulum switch, out of a clothes hanger wire (which is to shut the car down when it levels out, on top of a ramp). We can use only two 9 volt batteries, so I put two in parrell. The problem is the cars speed is to fast it needs to be cut about in half, or start out slower, because it jumps and the pendulum switch gets a bad contection, any suggestions.
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Use a resistor in series with the motor with a value of about 10 ohms and power rating of greater than 10W.
    Another option is to use take a long wire, wrap it around a small iron rod (many turns) as to create a coil and put it in series with the motor. This will cause the current to increase rapidly and thus the motor will start slower.
    Another option is to put a quite heavy piece of iron on the car to make it start slower. :p This is not a very good idea because the motor may be burned.
     
  3. LeeH

    Thread Starter New Member

    Dec 4, 2008
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    Mik3, thanks for your response, I think I will go with the resistor
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    Take care because it may get very hot! A coil solution would be better because it wouldn't dissipate power as the resistor and the motor would run at almost full speed after the slow start up. My opinion.:)
     
  5. floomdoggle

    Senior Member

    Sep 1, 2008
    217
    2
    Try one battery first, might save the need for a resistor. Also, what is the motor rated at? You will probably need to know why, or how this project works. Mik is one of the best, so listen to him.
    Dan
     
  6. IT_Guru

    Member

    Nov 20, 2008
    17
    0
    Use PWM (Pulse width modulation) to slow it down, yet retain 70-80% of the full-speed torque.
    I think there are ckts for this in the site
    http://www.allaboutcircuits.com/scr...j5vhl8&cof=FORID:9&q=motor+speed+control#1043

    The problem with brute force methods (in line resistor, choke it adding weight) is they will wear/run the battery down really fast and may burn the brushes and commutator ruining the motor. Thats what happens with a battery powered milk Frother when you use too much powdered milk ;-)

    hope that helps.
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    He will get rid of the resistor but he will put a bulky inductor. Depends if he needs full final speed. The idea of one battery is a good one and might work too.

    What's the rated voltage and current of the motor?
     
  8. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Be aware that there is a downside to using a power resistor in series with a motor as a means of slowing the motor down. That downside is a loss of torque. As the motor is loaded it attempts to draw more current. The additional current demand increases the voltage drop across the resistor resulting in less voltage available to power the motor.

    Two popular methods of reducing the speed to a motor are Pulse Width Modulation (PWM) and powering the motor from a lower voltage source. Of these two methods, PWM is the one I prefer.

    hgmjr
     
  9. floomdoggle

    Senior Member

    Sep 1, 2008
    217
    2
    Excuse me, HGMJR, using a resistor changes the output of a motor as easily as as electronic circuitry, and should be used more often. The simple is more elegant than the complicated. As I always say, I'm just the handyman, what do I know?
    Dan
     
  10. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Here are a couple of exerpts that I found on the web.

    It has been my experience that using resistors in series with DC brushed motors as a means of reducing the speed tends to make them unpredictable both in speed and torque.

    hgmjr
     
    Last edited: Dec 5, 2008
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Resistors are the cheap and highly inefficient approach. A good deal of power will be wasted as heat dissipation, and has already been mentioned, the results will be unpredictable.

    Using PWM is really a very efficient way to reduce the average power applied to a motor. It's certainly more complex, but the components are few and inexpensive for a project like this.
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,173
    397
    If we are voting on your project then I go for the 10Ω @ 5-10W. All else depends on how deep your junk box is, what test equipment you have ,how much to spend. Finish your project and when you have time experiment with PWM. Do not be too hard on series resistors. they have been used for many years,sewing machines,fan control, and yes I used it on starter motors. It would be nice to know more about your motor.
     
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