Testing Stepper Motor Holding Torque

Discussion in 'General Electronics Chat' started by BillB3857, Sep 16, 2009.

  1. BillB3857

    Thread Starter Senior Member

    Feb 28, 2009
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    I was trying to test a stepper motor for specified holding torque by applying the proper current (5 amps) to the windings from a constant current power supply. I found that it held within tolerance in one direction, but when attempting to break the hold in the opposite direction, the force required was about 30% less. Any ideas why it would have more holding torque in one direction than the other? My only thought was that going one direction, the generator effect was adding voltage to the source and in the other, it was subtracting voltage. Even with that theory, the constant current supply should have adjusted for the change....right?

    Any thoughts are appreciated.
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Stepper motors can get funky like that if they have been disassembled. The clearances inside are quite tight and a tiny misalignment can cause issues.

    To measure the holding torque you will need to average many different positions and for both rotational directions. And most motors (especially older ones) are specced for "2 phase on" holding torques, they are not designed to be used to hold anything with just one phase powered. So you should be testing it with 2 phases powered.

    This page shows some ways to measure stepper motors with no special equipment needed;
    http://www.romanblack.com/stepper.htm
     
  3. BillB3857

    Thread Starter Senior Member

    Feb 28, 2009
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    Thank you for the rapid response. Yes, the motor had been disassembled for bearing replacement. The power supply I was using was a dual unit with each unit capable of supplying about 4 amps. The spec for the motor stated 5 amp parallel which I took to mean 5 amps per phase. Since my supply would not do that, I put the power supplies in parallel and the windings in series. I then used an external meter to measure and adjust for the required 5 amps. From your response, It seems that micro-adjustments to the end bells may be required to balance the torques. I would have to assume that increasing the torque in on direction will necessarily decrease it in the other.

    On edit: The motor in question is 9 years old.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You MAY be able to re-align it. Generally in "pro" circles it's considered a destroyed motor if anyone has disassembled it (or loosened all the end cap screws).

    If you are going to try and save it, the first thing to check would be to make sure both the end caps have their original alignment to the stator, as each end cap has 4 ways it can be screwed on. If you are lucky there might be some paint fading or corrosion (from dust settling on the top of the motor) to show the "right" way the 3 pieces were always aligned.
     
  5. BillB3857

    Thread Starter Senior Member

    Feb 28, 2009
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    Again, thanks for the advice. I've learned a lot from the link you provided and other internet research. Looks like it takes some very specialized equipment (magnetizers, etc.) to do effective repair on steppers. Looks like a repair attempt to simply walk away from. Other PM motors I work with seem to have self shunting magnets. We always check Kv after a repair and they all seem to be well within spec even after full disassembly. Possibly the magnet orientation of the stepper variety does not provide the self shunting effect of the pure DC brush or the Brushless DC's we work with or, as you said, the alignment of the stepper is super critical.
     
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