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Motor control help

Discussion in 'Electronic Basics' started by Mark Monson, Jan 24, 2004.

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  1. Mark Monson

    Mark Monson Guest

    I'm an inventor with little electronics knowledge. What I want to do is
    reduce the speed of a woodworking router from 30,000 RPM to about 7,000 RPM.
    I need a compact circuit that can be mass produced for OEM. The router
    draws about 10 amps full load at 120V AC. The brush type motor will
    always operate at the same reduced speed.

    I've used a commercially made external speed control but it's noisy and the
    speed kind of hunts around. Any help is appreciated.

    MM
     
  2. Router motors are generally universal wound types (universal, because
    they can operate from AC or DC). This kind of motor has very poor
    speed regulation but can handle a wide range of load torques by
    varying its speed. No simple voltage or current regulating circuit
    will provide very good speed control (especially under varying torque
    load) for this kind of motor without some independent speed feedback
    mechanism.
     
  3. Mark Monson

    Mark Monson Guest

    OK. My question is still what specific circuit I need to knock the speed
    down to about 7,000 RPM. This is application doesn't require precision
    speed. It's a power tool. I don't need feedback loop accuracy.

    MM
     
  4. You don't take a hint very well. I don't think you are going to find
    a circuit that satisfies you, but you might try something like the
    second one on:

    http://www.st.com/stonline/products/support/motor/uni.htm

    I don't have component values for it, though.
     
  5. Mark Monson

    Mark Monson Guest

    You don't answer questions very well. Maybe because you don't know the
    answers. In any case, universal motors are commonly controlled by simple
    trigger type speed switches as found on VSR drills, and high/low switches as
    found on Milwaukee's Sawsall. All I need is a similar circuit that has a
    fixed rather than variable speed reduction built into it. Maybe somebody
    else out there is better qualified to help. Thanks for your time.

    MM

    I don't think you are going to find
     
  6. Bill Vajk

    Bill Vajk Guest

    Actually his answer(s) was supurb.
    You failed to understand that there is no fixed
    answer to your question.
    Trigger speed switched rely on a feedback loop, the user. The sawzall
    2 speed switch does not give fixed speeds.
    You are clarly in over your depth.
     
  7. I explained the reason that a fixed speed control is very difficult
    for universal motors. And that it is possible to produce such a
    thing, but that it requires a speed feedback mechanism. I admitted my
    limitations in not being able to do what you need.
    If they are and respond, I will learn something. Good luck.
     
  8. Tim Auton

    Tim Auton Guest

    The killer here was your requirement of "about 7000rpm". That implies
    a (roughly) constant speed is important to you. Speed control is easy,
    but relatively constant speed under varying load is much harder.

    You seem to think that "fixed rather than variable speed reduction" is
    a trivial distinction. You are very, very wrong. It is an important
    distinction in terms of electronics design - the distinction being the
    requirement for feedback.

    A simple speed controller controls only the power to the motor. Power
    and bit speed (rpm) have no direct relationship. To include rpm in the
    equation you need feedback (of the rpm).

    Your question was phrased in terms of rpm. You got a good answer. If
    you're not happy with it I suggest you learn why a speed controller
    which controls the power to a motor is different to one which controls
    its rpm, then re-phrase the question. I think that's what John was
    hinting at. Actually, his first answer wasn't a hint: It was a lesson,
    which you failed to take on board.


    Tim
     
  9. JeffM

    JeffM Guest

    You don't take a hint very well.
    Actually, around these parts
    Popelish is known as patient, even-tempered, and informative.

    If John tells you that you're a bozo
    (though he never would never say so in so many words),
    chances are you're a bozo.
     
  10. Ross Mac

    Ross Mac Guest

    Well said!
     
  11. krikkit

    krikkit Guest

    I have a motor speed control circuit that works well. I found it in an
    electronics mag many years ago, built several of them and they work very
    well.One was installed in the foot peddle of a sewing machine and I use one
    on a woodworking router with excellent results.It will run any AC motor with
    brushes and maintain a constant speed regardless of loading.
    Most motor speed controllers work much like lamp dimmers in that they
    reduce the RMS voltage applied to the motor, with no feedback the actual RPM
    of the motor will vary dependant with the load applied. (A variable speed
    drill has feedback, the operator will squeeze the trigger more as the drill
    is loaded and slows down.)
    This particular circuit applies power to the motor during the positive half
    cycle of the AC sine wave, during the negative half of the cycle it measures
    the back EMF of the motor. As the motor is loaded and slows down, the back
    EMF will drop, a comparator senses the change and increases the voltage
    applied to maintain the selected speed.
    I'm sure this circuit is exactly what you're looking for Mark.
    Anyone wants the mag article, I will scan it and send it by email.

    Kevin Wile
    Halifax NS
    Canada
     
  12. Mark Monson

    Mark Monson Guest

    OK. You're saying that reducing the voltage will reduce the speed. I know
    this works with DC motors on DC power, but I didn't know this would work
    with Universal motors on AC power. So I could add a resister in series or
    electronically clip the AC waveform to reduce the voltage.

    This particular application may do fine with a simple voltage reduction
    because the working load on the motor is relatively constant and far below
    the power reserve even at half of the full speed rating.

    Thanks for your help.

    MM
     
  13. JeffM

    JeffM Guest

    OK. You're saying that reducing the voltage will reduce the speed.
    No. That's not what he's saying at all.
    Changing the voltage level (amplitude) is NOT the way to do this.
    He's talking about a "phase controller" as Popelish already has.
    The technique is akin to "pulse-width modulation" on DC motors.
    The group has already told you this will allow you to CHANGE the
    speed,
    is not going to achieve REGULATION.


    To the group:
    Has anyone got experience with using
    a Rotational Speed Governor on an electric motor?
    http://images.google.com/images?hl=...q=governor+trainweb+diesel&btnG=Google+Search
     
  14. Sure. My wife's Kitchen Aid mixer has one in it. I added some spark
    suppression components to it to make the contacts last longer.
     
  15. krikkit

    krikkit Guest

    What I am saying is that this circuit that I have built, tested and use
    regularly does achieve regulation of the motor's speed. It applies power to
    the motor during the positive half of the AC sine wave, measures the back
    EMF of the motor during the negative half of the cycle. It will increase the
    power being applied if the speed has fallen below the desired RPM's because
    of increased loading. When the load decreases, it senses the increase in
    back EMF voltage and compensates by lowering the voltage being applied . It
    acts as a govenor, and does achieve regulation over a wide range of loads.
    The motor's speed can be slowed to almost zero RPM and it will still develop
    the same torque as when running full speed.
    Simply adding a resistor or clipping the AC voltage will slow the motor
    down but will not regulate the speed.
    I've tried to find this circuit on the web but am unable to. It was
    published in Electronics Today's Project Book #3 1986.
    I don't have a webpage to post it to, but perhaps that would be the easiest
    way as seeing it would make it much easier to understand.

    Kevin Wile
    Halifax NS
    Canada


    http://images.google.com/images?hl=...q=governor+trainweb+diesel&btnG=Google+Search
     
  16. Mark Monson

    Mark Monson Guest

    Kevin

    I would like to see this circuit. Can you send to it to


    Thank you

    MM
     
  17. JeffM

    JeffM Guest

    I don't have a webpage to post it to
    Post it to alt.binaries.schematics.electronics
    and put a pointer to it here.
     
  18. krikkit

    krikkit Guest

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