Stepper motor driver, max speed problem

Discussion in 'General Electronics Chat' started by vortex3, Mar 15, 2013.

  1. vortex3

    Thread Starter New Member

    Jun 5, 2012
    5
    0
    Hi everyone,

    I'm designing a stepper motor driver using the LMD18245T 3A, 55V DMOS Full-Bridge Motor. My circuit is very similar to this one http://www.avrstmd.com/. I'm using a 8 lead motor connected in parallel (phase R = 1,1hom; phase L=1,7,mH). I'm using microstepping control (2, 4, 8 and 16 microsteps/step). I'm powering the circuit with a 50V, 5A power supply, and the LMD18245T is configured in order to reach a maximum of 3A/phase.

    My problem is that I can not reach speed over about 350rpm. But I have seen projects using the same configuration (for example this one: http://www.embeddedtronics.com/microstep.html) that claim to reach much higher speeds, around 1500 rpm.

    Does anybody have any clue what could be the problem?

    Thanks!
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
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    specs/datasheet for the stepper?
    is there a load attached to it?
    driver max frequency/step rate?
     
  3. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    This is due to the inductance of the windings. All stepper motors lose torque as the speed increases. Because the peak current in the windings reduce as the time they are energised reduces.

    I use stepper motors with a bought-in micro-stepping drivers (Parker) and speeds up to 3000RPM are possible.

    To get high speeds choose motors with the lowest inductance windings you can get. This means high current, low resistance. The driver needs to run from from the highest possible voltage. Here we use 80V.

    As you are building the driver yourself you need to look at getting the current to rise and fall as quick as possible. This means high voltages across the windings in both directions. How does your circuit do this? Can you look at your circuit with a 'scope while operating?
     
  4. vortex3

    Thread Starter New Member

    Jun 5, 2012
    5
    0
    Thanks for the quick responses:

    mcgyvr:
    I'm using a MAE HS200 2231 0300 stepper motor. The datasheet is here: https://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CDQQFjAA&url=http%3A%2F%2Fwww.sodimatel.fr%2Fimage_base%2FImg_1637.pdf&ei=30xDUZzaD4jXPJ78gbgI&usg=AFQjCNHZ6m3lO-PPq8UD5es3S-PA_nQQww&sig2=ewzeJxP0-KQFZWvXY0MSCg, page 15. It is a 8-lead motor with 1,1ohm and 1,7mH per phase, 3Amax. Currently I'm testing the motor without any load. The driver is this one http://www.ti.com/product/LMD18245. There is not specify its maximum step rate but I have seen project using this driver claiming that 1500 rpm is reach (links on my previous post).

    JDT:
    The motors that I have seen on the market usually have higher inductance values that the one that I'm using. My motor has an inductance of 1,7mH per phase (and 1,1ohm). According to you experience is this a high inductance value? how much inductance/resistance have your motors per phase?
    Unfortunately, the driver that I'm using support until 55V, but the motor supports until 84V. I'm using a 50V power supply (the one that I have available). However, at this level I would aspect a high speed that only 350rpm , don't you think?.
    My circuit use a H bridge power stage that apply the voltage to the winds using DMOS power switches on a H bridge configuration (bipolar control). Yes, I can see the signals with a scope while operating...
     
  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
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    To reach a high speed you need gradual not abrupt acceleration, what is you acceleration curve?
     
  6. vortex3

    Thread Starter New Member

    Jun 5, 2012
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    Hmmm... Interesting. I don't have a acceleration curve. I just move the motor at constant speed from the start. Should I gradually speed up instead? Thanks!
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yes you absolutely must ramp up speed to get very high RPMs from a stepper motor.

    It can also help if your motor is properly bolted to something big and heavy, and helps too if you have some flywheel object on the motor shaft.
     
  8. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Speed up and down gradually, I generally make this table driven in a uprocessor. Let us know if it helps, it should. Your car can go 60 mph but not 0 to 60 in a second.
     
  9. vortex3

    Thread Starter New Member

    Jun 5, 2012
    5
    0
    I made some test speeding up gradually at the start of the motor, and it helps. Now I'm able to reach around 470rpm. But still if i try to go to high speeds than that, the motor stalls.

    I'm making a constant acceleration, from 0 to 470rpm in about 2,5 seconds. But even if I try 5 seconds, I can not reach higher speeds.

    Any ideas? Thanks you all for your comments!
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Already said on post #7; bolted down, flywheel mass on the shaft. You might also need a higher drive voltage.
     
  11. vortex3

    Thread Starter New Member

    Jun 5, 2012
    5
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    Thanks for the information THE_RB, I will try to bolt it down. The flywheel mass on the shaft will be a little difficult to find for me, but I have attached to the shaft an incremental encoder, could it be sufficient? Thanks!
     
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