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What are the stepper motors in the A5?! (Working out correct TMC21xx voltages)

cs2000cs2000 Posts: 56🌟 Super Member 🌟
edited April 2019 in Modifications & Upgrades

Hi all, trying to do some work on the wiki, figuring out the correct current recommendations for the TMC2130 stepper drivers, but im stuck at figuring out what the hell motors this machine uses.

The etching on my E motor for example says "42shdc4026-15b", that yields NOTHING on google, aside from photos of a Nema17 stepper motor, so....is that just code for a standard  Nema17?

From my findings, if this is the case, the Nema 17 has a "Rated Current" of 2 Amps. According to the internet, the TMC21xx series of stepper drivers should be set at the same rated continuous current as the motor, so...2 amps on the stepper. But then again, the continuous current rating of the 21xx  drivers is 2.0 amps, with 2.5 amp peak so lets say 1.75 amps at max to stop it from being driven super hard against its limits.

The standard A498 drivers should be set using this formula (lets assume 2 amps so...) 2 * 8 * 0.05 = 0.80V (which they are)

The TMC21xx series of drivers should be set at the same rated current as the motors as ive previously said.

Im just wanting to check this as (and sam will agree) the TMC21xx ratings are kind of guesswork currently based off of one other website where someone suggesting starting at these voltages and probably are wrong, so I wanna get some actual proper numbers for the community.

This would also mean that technically, every other motor on the A5 should be driven at 2 amps (1.75 max for safety), assuming these actually ARE Nema 17's, but I cant see the numbers on the other motors at this time, im assuming they're all the same.

Im suggesting that they should be set at;

X = 0.85V (standard, as its not under much load)

Y = 1.25V (slightly higher than normal as it has to overcome resistance and is dragging a heavy-ish load)

Z = 1.70V (much higher than normal, the driver is controlling two steppers, so 2x the 0.85v that a single stepper should need) E = 1.00V (slightly higher then normal as it has to overcome the resistance of very long Bowden tube) Any comments on this? Im not an electrical engineer, so I kinda feel like I must be missing something here, not sure how to check that those voltages ive suggested are under the 2 amps the drivers can handle.
Post edited by cs2000 on

Comments

  • cs2000cs2000 Posts: 56🌟 Super Member 🌟

    OK, done some poking and prodding today with the little info I have. Turns out, Nema 17 is a standard of stepper driver, not a manufacturer or anything. Im stull turning up nothing at all on that exact model, so it must be some china market special stepper motor.

    Anyway. The TMC21xx and the motors have a max Amps rating of 2 Amps, but when we set the potentiometer on the stepper drivers, we're measuring in volts, so, how do we check things? Well, you need the steppers phase resistances.

    Resistance in the stepper motors is measured on the two phases. To measure the resistance, disconnect the motor at the connectors on the motherboard. There are four wires to each motor. Starting from either side of the connector, the first two wires are one phase and the remaining two wires are the second phase. Measuring the motors on the A5 using my multimeter in ohms mode, I get 3.8ohms in one phase and 3.9 in the other, so basically the same.

    We can then use the formula:

    Voltage / Resistance = Current (amps)

    So, plugging in my newer slightly higher values, we get these numbers

    X = 0.85V - 0.236 amps
    Y = 1.30V - 0.34 amps
    Z = 1.70V - 0.44 amps

    WELL under the 2A rating of the drivers and steppers themselves.

    The critical thing is temperature of course. The motors are rated up to 100c, but in some places, they're fixed to the A5 with plastic brackets, which would clearly melt before 100c.

    Ive not done a long print yet due to a potentially faulty stepper motor, but the short while I had it running, the motors were stone cold still (as they should be, running (at most) 1/4 of their rated currents.

    Il update with longer term results and if these prove good, update the wiki too in the future.

  • Samuel PinchesSamuel Pinches Posts: 2,997Administrator
    edited May 2019
    I don't think your calculations are correct. Don't forget about the difference between peak and RMS values. 
    I = current
    I_rms * sqrt(2) = I_peak

    For TMC2100 the max current is 1.25 amps RMS.
    The formula for the TMC21XX is: Vref = I_rms * 1.412
    so at 0.44 amps RMS, you would have a vref of 0.62V
    That means at 1.70V you are actually setting it to 1.2A RMS, and remember the max is 1.25A RMS!

    For the default included A5 steppers (A4988), they have a max of 2A RMS,  which is actually more than the TMC.
    Important: the A4988 vref formula is different: Vref = I_rms * 0.4
    Thanked by 1cs2000
    Post edited by Samuel Pinches on
  • cs2000cs2000 Posts: 56🌟 Super Member 🌟

    Ahh yes, I did forget the difference there. Like I said, this isn't what im good at, so im working with what little info and knowledge I have haha.

    Id best back that off a bit then ;)

    Regardless, didn't fix my stepper grinding issue anyway, so I suspect the proposed values on the wiki are actually pretty much in the ballpark.

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