Unipolar stepper motor problem

capercaper Posts: 9
edited 2004-11-01 - 14:19:25 in Robotics
hey guys..i have a jameco 237711 unipolar stepper motor with
tech stats:
[*]4 phases [*]9 VDC [*]7.5° step angle [*]Phase resistance: 20 Ohms [*]Current: 450 mA [*]Phase inductance: 18.5 mH [*]Detent torque: 130 g-cm [*]Holding torque: 800 g-cm

but im having a problem...it seems like it doesnt have enough power to move the robot....if i hold the robot up, it seems like crazy, but just·a little force of my hand will cause it to stop....is there any way to increase the torque? is 800g-cm not alot? is there a possibility that it is hooked up wrong eventhough its moving forward?


  • Dave PatonDave Paton Posts: 285
    edited 2004-11-01 - 01:01:23
    That holding torques is only when the stepper is stopped. Steppers have a torque curve that is opposite of DC motors. They have the most torque when stalled, while permenant magnet DC motors have the most torque at their highest RPM. Since your stepper is 'spinning like crazy' it's operating in a part of the curve where the torque is much smaller, and obviously insufficient.

    My first suggestion is to step the motors much more slowly. A few RPM or so. Also, try energizing two adjacent pahses instead of using a single-phase wave-drive scheme. The dual-phase version almost doubles the torque, but at the expense of power and heat.


    This is not a sig. This is a duck. Quack.

    Post Edited (Dave Paton) : 11/1/2004 1:03:29 AM GMT
  • Ken GraceyKen Gracey Posts: 6,593
    edited 2004-11-01 - 04:37:37

    I think I know what is wrong. Steppers draw lots of current and need a really good power supply. Wal-warts usually won't do the trick. The startup current can be well over an amp even though these motors are rated at 450 mA. If you are using a 9V battery it could be drained really quickly, too. I'd recommend trying 12V with C cells.

    For my CNC mill, I use 48-60V on 36V motors. It might be a waste of energy, but beefing up the power supply prevented me from having other problems along the way.

    This is just my experience, which is limited anyway.


    Ken Gracey
    Parallax, Inc.
  • Dave PatonDave Paton Posts: 285
    edited 2004-11-01 - 14:19:25
    Nice catch Ken, I didn't even think of the power supply issues. For the record, I did try out my 9V/500mA steppers with a simple 9V battery powering the L293 and the motor didn't even wiggle. Switching back to my NiMH packs, it spins just fine. High current AA, or C or D cells are your friend here.


    This is not a sig. This is a duck. Quack.
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