Possible satuation of permanent magnet with 3.3A peak current?

[OxygenLiu]

The rate current of my NEMA17 stepper motor is 1.3A, and rated torque 0.45Nm. It is assumed a linear torque/amp ratio of 0.45Nm/1.3A. With 3.3A peak phase current, the expected torque is about 1.14Nm.

I am wondering if the max phase current 3.3A will satuate the permanent magnet, leading to a much lower torque than expected. It would be ideal to have a dynometer like this to evaluate the controller and motor performance in detail.

[dzid26]

Permanent magnet, probably not. The steel core, possibly. But from my experience, these motors do produce more torque with higher currents, though the increase may not be exactly linear as you are suggesting. Another limitation is thermal at this kind of current - short term for the PCB and quickly after for the motor. Additionally hot motor has higher copper resistance so it naturally starts to limit max torque. The value could be estimated, but in practice, we have close-loop torque control that takes care of those small differences.

If you need more torque, you may have to either use a motor with a bit more windings (but less than 4ohm per phase) or a bigger motor. I found some Nema17 stepper of the same length with 7.5ohm (link, link link) windings (which is probably too much for 12V, so stall torque will probably be less than for your motor, but much cooler 14V/7 = 2amps max), and I am going to test what are limitations of going too many winding. For one, with this amount of winding and inductance max speed will be lower. If that "extreme" test is somewhat successful, that means that another 4ohm motor LDO-42STH40-1004ASC (of the same size so it should be possible to simply swap the stator) should also work fine - and this could be an ultimate motor for 14.4V that is in the car.

The dynamometer would be the best to characterize the motor precisely. For stall torque really just a scale (load cell) is needed at the known length of the arm. For spinning torque, I was also thinking of a simpler eddy current copper disk brake device with an electromagnet varying the load and a load cell. It would provide a very smooth load but perhaps would be useless at low speed. Everything takes time though. :)

[dzid26]

OK, I quickly tried the 7.5ohm/15mH motor. No surprise, it has good torque at a low current. But top stall torque is limited by the max current possible (ohms law). - Additionally, torque ripple is very bad at high current, since the control is not trying to predict this limitation - so two phases produce more current than a single one. The no-load speed is lower, but still is very high: 75rev/s (vs 125rev/s on ~5mH motor), so no issue there.

So I would say go ahead and try swapping your stator with one from the 4ohms@11mH motor. This is the optimal stator of this size for the 14V system. - It should provide 38% more torque and so I am very curious if that stator would let you rotate a complete turn in your car at idle.