hybrid stepper motor - pwm control

Discussion in 'General Electronics Chat' started by Skeebopstop, Nov 16, 2012.

  1. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Dear all,

    I am proposing to use pwm control for a hybrid stepper motor application i am working on. My primary concern is with the soft iron teeth in hybrid designs struggling with the high frequencies. As they are only teeth i feel eddys are likely manageable and would hope the norm are laminated teeth but am not trusting the manufacturers replies.

    Anybody done this before in microstepping and can confirm whether it is really that big a concern? All the app notes i can find imply pwm control is fine but none specifically say but do speak of both hybrid and reluctance steppers alongside pwm so it would seem there is little concern. Hybrid stepper manufacturers also imply standard stepper drivers as suitable and most of those are pwm now a days.

    Thanks in advance
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Should not be a problem. The PWM is just to control the current given to the motor. The actual "step" signal is still a function of the driver logic. The internal capacitance of the driver chip and wiring to the motor will smooth out the ripple of the PWM.
     
  3. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Yes i know but my concern is with regards to induced eddys causing excessive losses in the ferromagnetic teeth. The rest i realise is just standard pwm current ripple. Have you done it before without any problems with hybrid steppers? The carrier freq ripple is still a concern for heating through induction heating.
     
  4. John P

    AAC Fanatic!

    Oct 14, 2008
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    I think it depends on the kind of PWM you're proposing. Does the voltage switch from forward to off, or from forward to reverse? If it's forward to off, then what you'd expect to do would be to use the motor's inductance to smooth out current flow, so it would be close to constant. That "internal capacitance of the driver chip and wiring to the motor" is microscopic by comparison.

    On the other hand, if you really go from voltage in one direction to the opposite and try to get current to do the same, you'll need a high voltage to overcome that inductive effect. I can't see any reason to do things that way. And maybe there would be eddy current losses.

    Here is a video of a stepper I set up last year during a conversation here on AllAboutCircuits, where someone asked if it's possible to use a 48 step/rev motor to drive the second hand of a clock, with 60 steps. You can clearly hear the PWM frequency on the sound track: the volume changes according to how close an individual step is to having 100% or 0% duty cycle for the phases that are on. It's silent 1 step out of 5 because then the duty cycle actually is 100%. You can also see that there are 4 LEDs which light up as the motor advances, and most of the time 2 of them are lit at once, but again, 1 step out of 5 only 1 LED is on.

    http://www.youtube.com/watch?v=EGwNYo69y2o
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Like John P said its the reversal of current that is the big driver of eddy currents. And PWM doesn't make current reversals, just on-off. Not to say there isn't a small amount of eddy current from PWM but not nearly as much as from AC.

    The biggest heat maker in a stepper comes from the winding resistance. Even when holding a position, like in a CNC machine, there is current through what ever winding that is needed to keep that position. And steppers are made to resist the heat that they create.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The inductance of the stepper motor winding is likely a couple (or a few) mH. That is a very high inductance by PWM standards, so with PWM of an adequately high frequency the magnetic field ripple made by the stator windings will be very low, so the induced eddy currents in the rotor "iron teeth" will be extremely low as the magnetic field is almost constant.

    Most commercial drivers use PWM of some type to drive the windings and set the desired average winding current.
     
  7. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    The eddies are from changes in magnetic field not current reversals. Yes the inductance is a few mH but the frequency is high and iron is not suitable above a few hindred hertz normally. I think i am just going to have to give it a go to find out and keep a.close eye on heating. Found a theais for the collinators for the lhc where they switched directly into hybrid stepper windi.gs. Yes bumping the frequency up might be an option but has its own complications for me.

    Thanks for the thread chat
     
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