Which Stepper Motor?

I am building a MP3DP from scratch and it’s time to select stepper motors. The ones that come in the parts bundle are spec’d at 76 oz-in @ 2 A/phase. Since there are two motors on each of the X and Y axes, it would require a 4A driver in order to fully drive these motors. The drivers on 3-D printer and GRBL control boards max out at 1.5-2.5A. This will only drive the 76 oz-in motor at around 50% of it’s capacity.

Is there a reason why the kit does not include a lower spec (lighter, cheaper) stepper motor, for instance a 1 amp / 40 oz-in for X and Y, which would seem to work just as well at the power levels that the Rambo boards are capable of producing?

  1. The motors are wired in series, so the current through one coil goes into the next.
  2. Running the motors are max current will create a ton of heat. Running a 2A motor at 900mA works better.
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According to the docs, the MPCNC actually only needs about a 45 oz-in motor, so you’re over-powered to start with. Things run cooler away from the redline…

Thanks for the responses.

After I posted this I ran across the “dual wiring” for boards that can support it. It looks like that uses one driver per motor, which is better (from 12 V, anyway) than driving them in series.

I’m not sure where that idea comes from, but driving them at 12V in series, at milling speeds, is still full torque with two motors. The drivers send a constant current, and as long as the motors aren’t going too fast, they have enough voltage to reach their full current. Separating them will just make the drivers turn off sooner.

Let me start by clarifying, this is mostly theoretical, and I am in NO WAY saying there is a problem with this design, which is terrific. But since you you asked… :slight_smile:

Steppers act similarly to inductors and since they are driven from a switched source, they do not provide anything like constant current. In a moving stepper motor the current in each coil is constantly changing before, during, and after each step, with a brief hold at the controller’s peak current limit during each step, except at high speed where it’s changing literally all the time. See any stepper controller data sheet (ie A4988) for more details about the ways various driver chip manufacturers deal with this. The only exception is when the motor is holding at a standstill. So yes, the voltage difference does not affect holding torque at standstill. But it does affect it the rest of the time.

From the inductor i-v equation, V/L = di/dt. So the higher the applied voltage, the faster the current rises, the faster the motor can get to its next step, the faster it can accelerate/go, and the greater torque it can generate at any high speed. This holds true for any given motor and controller combination. This is one reason why stepper drivers run at voltages so much higher than the motor nameplate voltage and why we drive 2.2V motors with drivers (A4988, drv8825, TB67S249, etc) that can handle 35-45 volts.

For this CNC router where the spec’d motors are oversized, have a very low 2.2V nameplate voltage, and fast moves are not the norm, this is a non-issue. But if I were using higher voltage (say 4V) motors, or I was building a 3D printer or laser engraver where I wanted the the fastest possible high speed moves and acceleration, I would run it at 24V. And, for that kind of application, series or not, I’d probably run it at 24V anyway.

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Right. 24V does give you more torque in the 50mm/s range. But we don’t usually go that fast and if we do, we’re travelling, so losing some torque is nbd. When we need the strength is at <20mm/s and usually <10mm/s, where 12V, even with series motors, will reach the max current each cycle.

We see this thought a lot. People assume that with series wiring, you need 24V, and it is not true.

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I would start with proper gearing. We use 16T pulleys for maximum power (12’s do not have enough grip on the belt to make it worth using). So instead of running everything hot and fast and expensive, just gear it correctly and use the steppers in the maximum torque range. The slower they turn the more power and control they have with less chance of oscillations and skipped steps.

Why use higher voltage steppers?

Speed is an odd thing…If your printer can go fast you still have massive acceleration curves, Laser, sure, maybe, but then you are fighting rigidity to combat oscillation and frame torque and twist. I can’t imagine a diode laser wanting to be used any fast than 100mm/s.

All that said, yes, 24V is more ideal, but also more expensive, what does that get you. A vast majority of the time, nothing, after that a few percent stronger and super high speeds. Then any other thing you want to use with the board will need top be 24V (more $$$$). I have some 24V power supplies if you want to run a real world test. I can put them in the shop in the next day or two. Everything has a trade off and cost is typically the first one.

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Damned armchair engineers… Squeezing every last milliamp or micro-Newton or μm out of a system that’s designed with tolerances an order of magnitude larger than the theoretical gains…

:wink:

That’s not really what is going on here, K.

Hey! I’m feelin’ cranky and under-caffeinated this morning… Dang whipper-snappers… :older_man:

I see that, upon re-reading the OP, and I see that they aren’t trying to squeeze theoretical efficiencies out of the system, they’re questioning the stepper motors on a well-known and tested system, which is normally a good thing. Always good to know why a design decision was made. I was being cynical in believing it was a precursor to trying to needlessly optimize the design. Mea culpa.

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Cranky fer sure…

You on mah lawn?! :anger:

:smile:

For sure, some boards don’t like 24 V, even some of them that say they do. Magic imported MOSFETs and all that.

It’s worse than that… some of us have actual degrees in this stuff. :stuck_out_tongue:

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Some probably even teach it… :face_vomiting:

FYI: My parents were in education for most of my youth, and my wife was a teacher before she became a school counselor. Good teachers are the salt of the earth, bad teachers are salt in the wounds.

One of my favorite profs in college taught control system loops. Making that fun and interesting is not an easy task.

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Is that even possible? Uhhh all I remember is the prof always talking about car’s cruise control…it wasn’t any fun until we got in the lab…oh wait, that was all of college for me.

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Was a chem professor, but the first problem he gave us was a stoichiometry problem based on the final step of producing LSD.

Better than the calc prof who told us if we’d done all the homework, understood all the lectures, etc., we’d get a ‘C’. Personally, I think he was annoyed he had to teach a bunch of freshmen calculus II instead of grad-level theory. Back when engineering graphics required a t-square, not a mouse.

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Spent an entire class designing a control loop for managing the temperature in a kegerator.

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