Motor current limits & wiring resistance

engikeneer

I have recently switched from a bowden setup to a direct drive, so my beefy extruder motor has been replaced with the E3D pancake stepper running a Bondtech BMG. I'm quite aware that I am now hitting the limits of motor torque if I push the extruder too hard so am trying to dial in what my actual limits are.

https://e3d-online.dozuki.com/Document/DwqNPVTK2ss2V5LB/MT-1701HSM140AE.pdf

I know that the standard recommendation is 50-85% rated motor current. The E3D pancake is rated at 1.4A/phase, so I've initially set my M906 to 1200mA. At this current, I find that the motor is warm to the touch but definitely not hot, so I'm wondering if I can push it higher.

It may just be that moving around on the print head (coreXY without an enclosure) throws enough ambient air over the motor to keep it cool (as opposed to just being in stagnant air), but I'm wondering if it is also because I'm getting some losses in my motor cables.

I measure the phase resistance of the motor at 1.9ohm (matching the datasheet), but if I measure at the Duet-end of the cable (while unplugged from the Duet!) I get more like 2.2-2.3ohm. The cables are quite long (~1.2m) so this is isn't crazy, but I'm thinking am I effectively running the motor at ~15% lower ~~current~~ power/voltage than what the Duet outputs? I don't understand steppers/drivers well enough to know if that means I could/should bump up the currents though...

Any thoughts/suggestions welcome

PS, already running at 24V, and am running a chimera (so 2x motors & extruders), so can't really afford the size/weight of a bigger stepper...

JoergS5

@engikeneer my thoughts are: Bondtech uses a gear ratio of 1:3, so torque should be more than enough. A second thought, if you suspect the resistance of the wire is too high, you could use thicker wires (eg AWG 28 => 20). In case you use Duet 3, there are recommendation which wires and connectors to use, they are thicker than Duet 2's.

dc42

@engikeneer, yes if the motor is not hot then you can increase the current to 1400mA, maybe a little higher. Stepper motor current ratings are normally specified with both phases energised, whereas when using microstepping only one phase is ever given the specified current.

engikeneer

@JoergS5 According to this calculation: https://duet3d.dozuki.com/Wiki/Choosing_an_extruder_drive, I have ~7.7N of extrusion force at rated current so a bit lower than the minimum target of 10N. I knew this before I upgraded, and to be honest so far it hasn't been a major restriction, I'm just aware I'm hitting a limit I didn't before.
Thanks for the tip on the Duet 3 docs - I'll get reading! My cables are currently a 1m lead that came with the motor, then duponts to a 20cm lead to the Duet. Agree it's not ideal - I plan on replacing with a single lead with a bit more copper at some point, I just need to get better at JST crimps.... I have some old shorter motor cables hanging around somewhere so might try them as a tester to see the impact

@dc42 thanks the input - I think you've corrected me about the microstepping on another post before so I really should pay more attention For now I'll bump up the currents and see how we go with a few longer prints.