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Motor Current Measurments — Parallax Forums

Motor Current Measurments

Danny22Danny22 Posts: 29
edited 2006-12-05 08:12 in Robotics
I took some current measurments on a new motor with the following results:

At startup the motor drew 7.2 Amps; Running, no load it drew 4 Amps continuous.

I measured the stall current by measuring the resistance of the motor leads and using Ohms Law and the result was 1.2 Amps!

The resistance of the coil was 9.7 Ohms and at 12 Volts = 1.237 Amps. This seems counterintuative to me as I would have

expected the stall current to be so much higher. Is my thinking wrong?

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-12-04 22:26
    Danny,
    ·
    ·· If the Motor has brushes you may need to slowly turn the motor shaft until you get the lowest reading.· BTW, how were you measuring the current during startup and run?

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    Chris Savage
    Parallax Tech Support
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-12-04 22:43
    "Stall Current" is the current when the shaft is being held still -- which can damage the motor, so measure it fast. The reason it's higher than the mere 'resistance' is because the motor is generating a magnetic field, which is trying to turn the shaft.

    I expect the 'stall current' would be slightly more than the 'startup' current. So yes, your thinking is off -- you haven't allowed for the mechanical load which resists the motor enough to make it 'stall'.
  • slamerslamer Posts: 52
    edited 2006-12-04 23:33
    Yep Motor current draw is directly proportional to the load applied to the motor. You might find these web pages useful.

    Team KISS Tips (ME)
    http://www.teamkiss.com/newkiss1220.html

    Check out the Tools link on this page for a motor torque calculator as well.

    http://www.architeuthis-dux.net/tcr-home.asp

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  • Danny22Danny22 Posts: 29
    edited 2006-12-05 01:55
    Thanks Chris et al for your replies. Let me explain:

    The motors are a matched set of Invacare wheel chair motors normally run at 24V. but I'm running them at 12V.

    To answer Chris, I used a DC ammeter and shunt rig that I put together years ago. but I also double checked by

    measuring the voltage across a .1 ohm power resistor in series with the circuit. The results were comparable.

    Now to the stall current. These motors are so geared down that it has been impossible for me to stop them while the're

    running. I've tried jamming the wheels with 2x4's, doorstops, inclined planes and other assorted objects. I even tried to

    run it into a wall, but the wheels just spun in place. As I was tightening the motor shaft into my large vice I remembered

    several articles in "Nuts and Volts", "Servo" and other robot books that said you could approximate stall current by the

    method that I described earlier. so that's what I did. I completely understand that true and accurate stall current can

    only be measured with the motor running and a stoppage applied. But since I couldn't do that I used the other method

    expecting it to be at least close, but it's not. Let me ask you all, should I just figure that the stall current is probably

    something more or around the value of the startup current. or what?
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-12-05 05:59
    Danny,
    ·
    ·· The thing with measuring the resistance is that as you move the motor slowly the resistance changes…You want the lowest reading, which may be hard to do if your system is geared so low.

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    Chris Savage
    Parallax Tech Support
  • Danny22Danny22 Posts: 29
    edited 2006-12-05 06:53
    Chris,

    You were right as usual. I rotated the wheels slowly while taking resistance measurments and the lowest was 1.4 Ohms

    which translates to 8.57 Amps. Much more realistic! Thanx a bunch.

    Danny
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-12-05 08:12
    Danny -

    I wouldn't doubt for an instant that the stall current could be as much at 10X the starting current. That's why you need to take a nearly instantaneous currrent measurement.

    Regards,

    Bruce Bates

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