Info or opinion on stepper motors, please.

Discussion in 'General Electronics Chat' started by electrojim, Feb 1, 2015.

  1. electrojim

    Thread Starter Member

    Jun 3, 2009
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    0
    Hi, I'm newbie to stepper motors, but I find them particularly attractive because of their general availability and price point. My question has to do with the applicability of a stepper motor for driving an inertial load at a constant speed. It's essentially a crowned flywheel that would be coupled to the stepper with a fabric belt. Intuition tells me that a brushed or brushless DC motor in a servo loop might be the obvious and better choice, but again steppers are so handy, plentiful and easy to power. A friend points out that steppers are meant to "advance and stop," and my application is for fixed-speed rotation at at constant rate. In this case the motor would be spinning at about 300 r.p.m. If the load can be accelerated to speed gently, once it's there, will the motor be happy enough to keep it going, or is this going so much against the concept of a stepper that it would moan and groan? Thanks!
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
    2,364
    As you say they are cheap and available, if this is a constant speed application that does not have any feature for feedback then depending on what load maximum rpm and the degree of accel/decel, then steppers should be ok.
    Are you building your own stepper drive or thinking of something like the Gecko drives?
    For these you would just need some kind of step/dir generator.
    Max.
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    782
    One of the big advantages of a stepper is; you can count the pulses and work out where the load ended up - if the load has high inertia, this may not work out so well unless you include magnetic braking.

    One of the big disadvantages; Torque tails off with increasing RPM.
     
  4. electrojim

    Thread Starter Member

    Jun 3, 2009
    11
    0
    Thanks, guys. I'll be using one of the stepper driver modules that are ubiquitous on eBay. The motor is a NEMA 23, and the torque curve looks decent in the speed range for this application: about a 30% loss from max at running speed. As far as I can tell, all I need do is tie the direction input either high or low and run the output of my benchtop squarewave generator into the 'pulses' input. I'm not considering anything fancy like 'microstepping,' but the controller does provide selectable maximum current limiting and some semblance of wave-shaping for what they say is a 'smoother and quieter' drive. As for the concern over braking, this project doesn't need to stop, it just runs at a specified speed constantly. What I don't know is this: will output loading be reflected in the current that the motor draws, or do steppers simply respond to I=E/R input current and deliver a fixed torque? With a DC motor with brushes, like a 'pancake' motor I have, the current draw is nominal with no load, and then really starts pulls some amps as the output is loaded. I wonder is steppers do this too.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
    2,364
    Stepper motors are rated for a certain voltage, so the current is directly dependent on the resistance of the winding at the rated voltage, and this current should be constantly maintained for optimum stepper performance, the problem is that as the rpm increases so does the Inductive reactance which has the effect of course, of decreasing the current, at one time on older drives, a series resistor was used to maintain the correct current, now PWM drives are common and a much higher DC supply is used and the drive maintains a mean current level via the PWM that attempts to achieve the rated motor current at all times.
    Servo's use a variable current dependent on load.
    Max.
     
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