Alternator as stepper: Thoughts invited

Discussion in 'The Projects Forum' started by margrove, Sep 29, 2009.

  1. margrove

    Thread Starter New Member

    Sep 27, 2009
    I'm interested in running an auto alternator as a stepping motor. From what I can glean from others who have done so is that with full stepping about 18 steps per revolution is possible without modification to the alternator, other than dispensing with the electronics.
    The problem though is that the stator windings are quite beefy and hungry for current which is a disadvantage. Rewinding the stator with more windings of smaller gauge wire is on the cards but I'm inexperienced in the field and would appreciate some ideas.
    Firstly, would it be advisable to rewind it in its original configuration, ie 3-phase or is there an advantage in other methods bearing in mind that I would like higher resolution with less current albeit at a higher voltage?
    Secondly, do I have to stick to the wave winding as on the original or can I do it in another way? I would like to go no further than half stepping, if at all possible.
    The alternator (100A) that I have to play with has 12 interleaved pole pieces on the rotor and 36 on the stator. I would prefer not to modify the rotor (no hydraulic press)!
    For control I can use a microprocessor with a Mosfet H-bridge for polarity reversal but other than that I would like the electronics as simple as possible.
    The max speed should be about 5000 rpm and about 100 steps per rev would be nice.
    Ultimately it would be used to drive a small lathe, for instance.
    Perhaps I'm overambitious but would really like to know what you gurus think.
    Grateful thanks,
  2. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    With the high number of poles in that "motor", the three phase drive frequency would be out of sight!
  3. margrove

    Thread Starter New Member

    Sep 27, 2009
    Sorry, not too clear there!
    Above 1000-2000 rpm the "motor" would run conventionally.
    Incidentally, would 1700 to 3400 steps/sec be difficult for a motor that size?
  4. Papabravo


    Feb 24, 2006
    Experience with conventional stepper motors suggests that you will face two similar problems.

    1. High inertia of the rotor requires the velocity to be ramped up to that speed. You can't just start running at such a speed.
    2. There are two resonances that you have to be careful of. There is a mechanical resonance in the 200-500 steps per second range. The motor will sit there and vibrate if you try to run at that speed. there is also an electrical resonance in the 2000-3000 steps per second range based on the RL time constant of the motor windings.
    If you look at speed-torque curves you will see that maximum torque occurs at low speed and as the speed increases the torque decreases. Top speed occurs when you run out of torque. Erratic behavior is quite possible in this region. Small changes in load inertia will cause the motor to stall and lose its mind as it sits there and...well...quivers!
  5. margrove

    Thread Starter New Member

    Sep 27, 2009
    Since the construction of an alternator is much different to that of a purpose designed stepper the mechanical resonance may be wildly different. Thanks for reminding me. I'll just have to wait & see because I don't have the skills to calculate likely values.