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Questions about servo motors

Discussion in 'Electronic Basics' started by Eric R Snow, Jun 6, 2004.

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  1. Eric R Snow

    Eric R Snow Guest

    To the motor folks,
    I have a CNC mill from which the original motors were (by me) removed.
    Now, because of control changes, these motors are going to be
    re-mounted. I know which motor came off which axis but I am thinking
    that the motors were put on the wrong axes by the previous owner. This
    is because the axis which has the heaviest load had the smallest
    motor. So, testing torque, and not physical size, seems the best way
    to determine which motor goes where. Is the torque per watt output on
    brushed linear motors linear? Can a lower voltage power supply be used
    to determine which motor is strongest? If the same power supply is
    used to power each motor and the torque measured is it safe to assume
    that the motor with the highest torque at a low voltage will be the
    one with the highest torque at the higher operating voltage?
    Thank You,
    Eric R Snow
  2. There is a factor you are missing. THe power output of the motor is
    roughly proportional ot the mass of the motor, but the torque required
    by a given axis is also related ot the gear ratio between the motor
    and the load. So if the axis that you say is the heaviest load also
    has a bigger gear down, it might do fine with the lower torque motor.
    The power the motor has to provide is related to both the load forces
    and the load speed (i.e. inches per second of travel). The motor must
    match both the torque and speed requirements to be a good fit. The
    whole system has to be taken into account.
  3. Eric R Snow

    Eric R Snow Guest

    The motors are all geared to the screws the same and the lead screws
    all have the same pitch. I should have included this.
  4. Then proceeding to your last question, yes, it is safe to assume that
    a motor that produces more torque at low voltage will also provide
    more torque at higher voltage. In general, the torque is proportional
    to the current and the speed is proportional to the voltage.
  5. Eric R Snow

    Eric R Snow Guest

    Thanks John,
    I just finished testing the motors and I get three different torque
    and amp readings. However, the stall amps were the same on two motors
    while the running amps was high on one. But there is a big difference
    in torque between the lowest torque and the mid torque. And a small
    difference between the mid torque and the highest torque. I'm gonna
    chalk the small difference up to my measuring technique.
  6. Were you regulating the current during stall, or just taking what ever
    some supply could deliver?

    To see if the two motors are designed for different speed torque
    capabilities (assuming they are permanent magnet DC motors, not
    stepper motors) you need to measure the torque they produce during
    stall with a fixed current (or at least, a measured current) passing
    through that is well below rated current, so you don't over heat them,
    or degauss their magnets. That gives you their inch pounds (or some
    other torque units) per amp. Then you spin them at a known RPM and
    measure how much voltage they generate. This gives you the RPM per
    volt rating. As I said, their power capability (amps times volts)
    will be roughly proportional to their mass if they are of similar
    construction but different sizes. But these two tests will tell you
    if one is designed for more speed and the other for more torque.
  7. Eric R Snow

    Eric R Snow Guest

    I used a lead acid lawnmower battery as the power source. I measured
    the amps while testing. The highest, at stall, was 12 amps. I stalled
    the motors for only a secnd or so. Probably less. I did this maybe 5
    times on each motor. I sure hope I didn't harm the magnets. That would
    really fry me if I did something that stupid.
  8. Rich Grise

    Rich Grise Guest

    If they're geared the same, put the big motor where it moves the
    more stuff.

    Have Fun!
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