Parallel stepper problem

Basic Information:

Printer Model: Mendel90
MCU / Printerboard: BTT Manta/CB1/TMC2209
klippy.log
klippy.log (629.9 KB)

For my Z axis on a Mendel90 I have 2 Nema 17 steppers wired in parallell. The OEM configuration was series to a 4898 Melzi board. Although the new arrangment seemed to work at first it now has one stepper motor frequently turning the wrong way. I changed the motor out but the problem remains.

Should I return to the original scheme or is it something else ?

Stepper motors have 2 key parameters:

  • Inductance - To keep it simple, we reduce the theory to say: The lower the inductance the more dynamic / fast a stepper is in movement but the lower is its holding torque, i.e. its ability to keep a load stationary in place
  • Resistance - Again in simple terms: The lower the resistance, the higher the current is, the faster the motor reaches its nominal torque in motion. But: Due to the higher current the motor gets hotter and again low resistance has a negative effect on holding torque

What are the differences between series and parallel wiring?

  • Compared to series, parallel wiring will reduce overall inductance and resistance to 1/4
    • One stepper 4 mH
    • Two in series: L = 4 mH + 4 mH = 8 mH
    • Two in parallel: L = (4 mH * 4 mH) / (4 mH + 4 mH) = 2 mH
  • Parallel: Each stepper will get the full supply voltage (e.g. 24V) → Positive effect of the dynamic / top speed properties but due to the overall lower resistance the current is higher and the motors / stepper driver will run hotter
  • Series: Each stepper will only get half the supply voltage → Bad for high speed / high dynamic application
  • Quite some more aspects could be considered, like effect of one defect stepper, etc

Where are series / parallel steppers used in 3D printing and what do we need?

  • Typically only on the Z-Axis
  • Z-axes are neither high dynamic, nor high speed axes
  • Z-axes should keep the bed in place → holding torque is relevant

Conclusion (at least mine and according to my understanding):
For Z-axes the series wiring is better than parallel

IMO only possible if you made a mistake in physically wiring the steppers, e.g. mixing up the coils between the steppers.

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Side note:
If someone explained a stepper motor in the middle ages, he probably would end up on a stake and be burned for witchcraft. So if anyone knows better, I happily stand corrected.

It worked initially and the wiring is correct. I only switched it when rewiring for the new board. The original had only one Z-axis connection whereas the new board had 2 in parallel. The TMC2209 driver has more than sufficient current capacity for the ‘ll’ configuration. Maybe the series wiring will be helpful in keping tthe 2 in ‘phase’. I agree that ‘stepper motors and drivers’ seem like witchcraft or a black art.

I am concluding that a friction issue on the affected side is pushing the motor out of phase, causing it to reverse. I have new threaded rod coming as there is some slight damage to one. Machine was left unsupervised for 6 years so maybe one of the linear bearings has an issue too.

Stepper motors are not “witchcraft or a black art”.

They are very well understood and there is a lot of good information out there to explain their operation along with that of their drivers. The basis of it is quite technical but the basics are quite easy to understand as long as you have some rudimentary electrical and mechanical knowledge.

I’m just putting this out here because people stop researching or trying to understand if they think that things are arbitray and, going along with that, capricious. The result of this attitude is thrashing around and trying things at random.

I’m really not an expert in this field but I have a hard time imagining that friction would cause one motor to reverse its polarity. One motor loosing steps and lagging behind is totally valid (but then the lead-screw doesn’t have a slight friction but is outright scrap)

What would lead to a stepper switching direction in a parallel circuit is beyond me (given it is correctly wired).

A less than stellar crimp on one pin was the cause of the motor direction irregularity ! Mea culpa ! Never assume anything.

The new lead screws are a week away still so I now have time to read up on stepper motors ! The basics of their operation is understood but ‘stealth chop’ and ‘spread cycle’ are new to to me. ‘Black art and Witchcraft’ comments were the result of my frustration and impatience at the time.

I am going to stay with the ‘ll’ wiring for now as there isn’t a reason to change it back.

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