Author Topic: Running a NEMA 8 stepper at 6500rpm (and maybe kill yourself in the process.)  (Read 772 times)

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Offline BreakingOhmsLawTopic starter

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Just a friendly reminder that you can run a stepper at stupid speeds, if you just slowly ramp it up. >:D

I just ran a tiny NEMA 8 at 22kHz step speed (corresponds to 6600 rpm @200 steps per turn).
I have no doubt i could go a lot faster, but I am scared the FR4 will snap and I will end up bleeding to to death on my workshop floor with a piece of PCB sticking out of my neck.
 
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Offline amyk

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At those speeds it's no longer "stepping", but just running like a brushless motor with an insanely high number of poles.
 

Offline ebastler

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Just a friendly reminder that you can run a stepper at stupid speeds, if you just slowly ramp it up. >:D

Provided you don't need any torque, of course.  :)
 

Offline james_s

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I knew someone who did that about 20 years ago with one of those little steppers like the sort that used to be in floppy drives and printers. I think he had to give the 12V motor over 100V to overcome the inductance once it got up to speed.
 

Offline BreakingOhmsLawTopic starter

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Actually, the speed signal correlates perfectly with the stepping speed. The challenge is to increase the speed at a rate where the motor can follow.
If the axle goes out of sync for more than 0,5 degrees, you end up braking the motor instead of accelerating it. 
The secret sauce here is: you need a good speed signal. The trick I used here: The rotor (this is a field mill if you haven't guessed already) is made of PCB with a black solder mask. Underneath are two small strips without solder mask, narrow enough that it only triggers the speed signal in a single step position (see image below). That way the software can easily tell the phase shift of the rotor versus the step speed. You can even "catch" it again after losing sync without needing to start from zero. It's all software in the end.
Steppers are usually not made for these speeds, they have the wrong type of bearings. They are made for torque, not speed. But if you need it to go fast, this is the best approach. Doesn't even get hot, i run it with just 9V / 85mA.
I will probably make the motor current software controllable in the final board, to get more torque at low speeds and decrease current once it hits target speed to decrease noise.
Have a nice day!

EDIT: inserted image

IMG-20210206-112058" border="0
« Last Edit: February 06, 2021, 10:28:03 am by BreakingOhmsLaw »
 


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