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Determine speed of DC motor

Discussion in 'Electronic Basics' started by [email protected], Mar 9, 2009.

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  1. Guest

    I have a DC motor with the following specifications:

    RPM @ 0 Torque: 3500
    Stall Torque : 0.01 Nm
    V_ss : 8V

    Background information:
    The motor will be used to drive a miniature monorail in a Tug-of-War
    type competition.

    Since this is the only motor I can use, I want to ensure it is running
    at it's maximum power. How do I determine what speed the motor will
    actually rotate at? Do I have any control over this?
  2. Guest

    A tachometer (either photo-reflective or mechanical) can be used to
    measure the motor's speed.

    The speed will be determined by the capabilities of the power supply
    (voltage & current available) and the load applied. You COULD run the
    motor on 12 or 24 volts for more speed, but a the cost of shortening
    the motor's life - possibly to a matter of seconds. Since the motor
    is specified, do the competition rules also limit the capacity of the
    power source?

    In a Tug-of-War, the winner is determined by strength, not speed. You
    can gear down the motor to provide less speed and greater torque.

    You need to specify what rules apply to power supply,, gearing, etc.

  3. Guest

    The power source is fixed, I'll have no control over that.
    My intention was to gear down for as much torque as I can get with the
    materials provided, but I want to ensure that the motor runs at it's
    maximum efficiency and I'm not sure how to determine that.
  4. Guest

    First of all, there are no competition rules. We're given a set of a
    materials and told to build a monorail. That's really all there is to
    I was the one asking for help, so wouldn't I disclose as much
    information as possible?
  5. Rich Webb

    Rich Webb Guest

    Don't take Phil's attitude personally. He seems to enjoy hanging around
    s.e.b and ranting at people who ask, well, basic questions -- although
    there are frequently good bits of advice amongst the flying spittle.
  6. Nobody

    Nobody Guest

    The speed depends upon the applied voltage and the load. If the voltage is
    fixed, then the speed will decrease as the load increases. The above
    figures state that, for an 8V supply, the speed will be 3500 RPM at no
    load, reducing to zero (stalled) at a load of 0.01Nm.

    For tug-of-war, you don't need power, just torque, which means that you
    want the lowest possible gear ratio. IOW, this is a mechanical problem,
    not an electronic one.

    OTOH, if you're allowed to introduce some electronics between the power
    source and the motor, then it would be an issue of finding the maximum
    current which the windings can handle and constructing a constant-current
    switching regulator to provide that current regardless of speed.
  7. Guest

    So what is the mathematical relationship between load and motor speed?

    I want to be able to calculate the speed that the wheels will turn.
    My gear ratio has not been finalized.
  8. There are really 2 limits for motors: a maximum speed above which things
    start to come apart & a maximum current above which the windings start
    to smoke.

    Your motor is spec'ed at 3500 rpm, yet it's a small motor (small diam,
    small centrifugal forces) so it'll probably do more. More is better:
    power = speed x torque. If you have more than one of these, you could
    experiment with higher speeds. Just increase the voltage.

    Torque is proportional to current. So maximum current is the current at
    maximum torque. Lock the rotor & adjust the current until maximum
    torque is reached. That is the maximum current. Motors are designed
    for a certain temperature rise. I.e., the maximum current is determined
    by the temperature limit. Excessive current => excessive temp => smoke.
    If your contest is short, you could run higher currents. Rated
    maximum current is for continuous running.

    First you need to know how much torque you can put to use. I.e., before
    your driven wheel(s) start to slip. Then gear down to that torque.
    In use, control the voltage to give the maximum current.

    I now see that speed hasn't come into this, so maybe I'm missing
    something. Oh, wait ... here's how it comes in: when your tractor is
    /moving/, the gearing-down for torque & the voltage used to get maximum
    current/torque could cause the motor to exceed it's speed rating. So
    when it moves, you would have to monitor speed.

  9. Phil Allison

    Phil Allison Guest

    "Phil Allison"

    First of all, there are no competition rules.

    ** Blatant lie.

    We're given a set of a
    materials and told to build a monorail.

    ** Then the limited resource of materials ARE the rules.


    And you have kept THAT secret.

    I was the one asking for help, so wouldn't I disclose as much
    information as possible?

    ** No Google Groper ever fucking does.

    A simple matter of fact - you included.

    ....... Phil
  10. Phil Allison

    Phil Allison Guest


    So what is the mathematical relationship between load and motor speed?

    ** Too obvious to even mention.

    I want to be able to calculate the speed that the wheels will turn.

    ** I want $1M.

    My gear ratio has not been finalized.

    ** You are one, totally arrogant little prick

    - aren't you.

    ...... Phil
  11. Guest

    In other words, you have no idea.
  12. Nobody

    Nobody Guest

    The simplest theoretical model suggests it should be linear: speed is
    proportional to voltage, current is proportional to load. As the load
    increases, so does the current, so does the voltage lost due to winding
    resistance. IOW, draw a straight line from [email protected] to [email protected]

    This assumes that either the impedance of the power supply is low (i.e.
    you still get 8V at stall current) or that any voltage drop has been
    accounted for in the quoted stall torque figure.

    But there are enough other factors that you would be better off
    measuring it.
  13. bw

    bw Guest

    Nice response, esp the temperature limits. I suspect the OP is at the Power
    = torque x RPM level.
    He should just google dynamometer.
  14. Yeah, I had a similar feeling - that most of what I said would go right by.

  15. Jasen Betts

    Jasen Betts Guest

    speed is proportional to the voltage across the motor without its

    (treat the motor as a zero resistance ideal motor in series with a
    real resistor)

    constant current will give you constant torque (if you can ignore
    friction and sources of drag)
  16. Jasen Betts

    Jasen Betts Guest

    you don't have sufficient information.

    stall current (or DC resistance) would help.

    also are you allowed to overdrive the motor?

    what are the rules? is building a "crawler" (with extremely low gear
    ratio) all that's needed to win?
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