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Permanent magnet, probably not. 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. |
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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. So I would say go ahead and try swapping your stator with one from the ldo-42sth40-1004asc. It has 4ohms@11mH windings. Higher resistance and inductance indicate there are physically more winding turns in this stator, which results in higher torque for the same current. Since this motor is also 40mm long, the stator is the direct replacement of the default geared motor. In theory, it has about 38% more stall torque in the same form factor. It is very close to optimal in terms of torque density for 14.4V. One thing that may suffer with a high impedance motor like this, is peak power - meaning under load maximum rotation speed can be lower. However, for steering, it doesn't matter so much, as the quickest rotation is required when straightening the wheel (low load). I am curious if this stator would let you rotate a complete turn in your car at idle. |
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OxygenLiu, I converted the issue into a discussion and changed the title.
For now, I am still using rated torque (and not actual torque form measurements) when talking about motor torque. |
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I am wondering if you measured the actual current per phase? |
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It would be interesting to see the results of |
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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.
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