Quote Originally Posted by printbus View Post
Well, you are correct that inductance is resistant to current change, much like capacitance is resistant to voltage change.

It would be helpful to know whether the mH value you have is right. The type numbers on the stickers differ - ours apparently being a newer part number than the type number on the CS page. What appears to be the motor manufacturer doesn't cross-reference an old part number to our new one (42BHH8-050-24A) in their new part number listing. Assuming 32 mH is correct, and applying the math from this reprap thread, the impedance from the inductance at a 125 Hz step rate for 100mm/sec movement on an X/Y motor would be 25 ohms. That is considerable. EDIT: Yeah, the numbers for the X/Y motor may not apply, but the other thread gave those to work with. I retired from engineering so I didn't have to fret over this kind of stuff.
IIRC, the steps/mm on the extruder is around 860, which is 1/16 microsteps. This means it's about 54 full steps/mm. If you're trying to retract at 25mm/s, this is 1350Hz.

That seems like a lot. Let's try calculating it another way. The effective diameter of the hobbed bolt is about 6mm, so the circumference is about 19mm. 200 steps per motor revolution times 5.2:1 gear ratio divided by 19mm effective circumference equals 55 steps/mm. That agrees, so it's likely about right.

How does this compare to an X/Y motor? A typical 20-tooth pulley has a 40mm circumference, which means 100mm/sec would be 2.5rev/sec. Time 200 steps/rev = 500Hz. So the extruder is moving almost three times as fast during a 25mm/s retract.

Check my math.

We still have the question about whether I have identified the correct motor.