Stepper current - per phase and M906 setting

jhalewood

Hi,

I'm confused about stepper currents and M906

Using a Duet 3 6HC

I have these steppers: https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Robotics/42BYGHM809.PDF
which are rated at 1.7A/per phase

When you look here: https://duet3d.dozuki.com/Wiki/Choosing_and_connecting_stepper_motors#Section_Rated_current
It says "Rated current
This is the maximum current you may pass through both windings at the same time. "

After lots of googling, I'm just more confused. There's so many disparate answers everywhere.

I don't understand if this means I should set M906 to 1.7A or 3.4A --- it seems if it 1.7A per phase, and the max current is thru both windings, then 2 windings by 1.7A each = 3.4A?
(both of these would be multiplied by 80%, but for sake of brevity, just listed 100% values)

Using the 1.7A value as max, I get skipping in some situations on the extruder. I'm just looking at amps as one workaround amongst the usual actions you take to address skipping.

jumpedwithbothfeet

@jhalewood,1.7A and use 80% of that, if your extruder is skipping that maybe a heat related issue and something else to look into!

engikeneer

@jhalewood usually the max rated current is based on a thermal limit (ie the motor gets too hot if you put more current through the phases).
The both-phases rating will probably be higher, but I doubt it will be anywhere near double, as its still the same motor you're putting the heat in. Id guess 20% higher, but that's a bit finger in the air... There's also a question of whether the stated values are peak or RMS. Someone who understands this better might be along to explain it
Having said that, I currently run a couple of my steppers slightly above their rated current without issue (never checked if it was per phase or not). You need some good airflow around them for the heat to dissipate. I'd try just turning up the current a bit until the motors get too hot to touch after running for a bit (I think about 60C is a typical limit). Make sure you're mounts aren't PLA though!
Of course everything else you do to help the situation is also a good shout