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Can anyone explain this method of dc motor control?

Discussion in 'Electronic Basics' started by [email protected], Mar 27, 2005.

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  1. Guest

    These paragraphs are from an article about a homebuilt hybrid car in
    Mother Earth News.

    "Any project fresh off the drawing board has its share of problems, and
    the Opel hybrid was no exception. When David pressed the accelerator
    for the first time, he got a 300-amp surge which melted his relays. So
    he searched his graduate texts for the answer ... and finally found it
    in-of all places-an old high school physics book: A pulser was
    necessary to "chop" the current flow and prevent a heavy initial draw
    to the drive motor.

    As Dave explains it, "The motor will always have full voltage and full
    current, but the pulser makes it 'think' the voltage and amperage are
    cut down to about 1/4 of what's actually available. With this
    gadget-which is simply a combination of a reworked car generator and an
    old fan motor-I can keep the draw within limits and effectively control
    the car's acceleration . . . without sacrificing the maximum current or
    voltage that's necessary for high-speed driving. I could have achieved
    the same results with a commercially available FCR control ... but one
    of those units would have cut my power slightly, and cost in the
    neighborhood of $800! I can build my own device for about $25, and I
    can fix it myself if it breaks!"


    Boyntonstu
     
  2. Tom Biasi

    Tom Biasi Guest

    The article is talking about controlling the motor speed my adjusting the
    duty cycle of the pulses being sent to it.
    Look up DC motor speed controllers and this type will be discussed
    somewhere.
    It seems the person made his own version with some simple parts but as you
    can see he goes into no detail.
    Tom
     
  3. Chris

    Chris Guest

    Hi, Boyntonstu. You're talking about "pulse width modulation", or PWM.
    It's a method to ramp up motor speed by rapidly (hundreds to thousands
    of times a second) switching the motor voltage on and off. The motor
    reacts to the percentage of time the switch is on. It has a lot of
    advantages over many other kinds of motor control, particularly for
    bringing high torque DC motors up to speed slowly.

    Google this exact phrase:

    "Pulse Width Modulation" +"motor control"

    to get some information. I found the third hit helpful

    http://mechatronics.mech.northwestern.edu/design_ref/actuators/motor_PWM.html

    Good luck
    Chris
     
  4. Guest

    Hey guys, you don't get the question.

    I know all about electronic PWM controllers like Curtis and Alltrax.
    The article is a PULSER speed controller that is made up of electrical
    and not electronic components; namely a generator and a fan motor.

    Let me rephrase the question:

    How can you control the speed of a dc motor using a generator and a fan
    motor?


    Boyntonstu
     
  5. Hey. Crosspost properly. Not only did you crosspost improperly, but you
    also made subtle changes to the title. Grrr!
     
  6. Jamie

    Jamie Guest

    Pulse Width Modulation?
     
  7. Jamie

    Jamie Guest

    Hmm.
    sounds like a backyard hack job to me.
    lets see.
    Generator is attached to the shaft so it generates
    more power as the RPM's increase, this in turn drives
    the Fan Motor or Helps in increasing the FAN motor
    speed.
    then the blades on the fan simply hit a mechanical weighted
    switched that will pulse the contacts and at some point when
    going fast enough stay closed..
    sounds like hack to me.
    or it could be using some kind of optical switch via the fan
    blades that engages mercury relay.
    who knows..
    i guess the centrifugal would take over at some point and
    keep the contacts closed.
     
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