[PaulMakesThings]

Just hoping someone can clear up some confusion I'm having trying to select stepper motors, drives and power supplies that match.

Look at this one for example:

NEMA 23 Stepping Motor | 18.0 kg-cm | 6 Wire | 57BYGH317

1.8 Ohm 2.5A, so if I run this thing at 24V, I'll be drawing 13.3A per motor. The biggest motor driver listed on the same store can only take 6A, and it's 24 -70V. If I want to keep that motor under 2.5A, I would have to run it at 4.5V, and I don't see a lot of controllers with voltage that low. The lowest they sell is 12V. Can you see my confusion here? What do people normally do?

[john-100]

Hi Paul ,

when the motor is stopped
to pass a 2.5A current through the 1.8 ohn coil you would need a 4.5V DC voltage across the ohm coil

once you start to step the motor the initial current through the coil will be limited by the the 1.8 ohm DC resistance and the reactance of the coil
think of this reactance is an extra "resistance" that increases with both the inductance of the coil and the frequency of the AC voltage across the coil

to be able to step the motor a high speed you need to select a motor with a low inductance and force the current to rapidly reach the required current
by using a higher motor supply voltage and some form of current limiting

for low power motors you could use use "dropper" resistors as found in some old stepper controllers

with your motor for instance
you can use a 22.5V supply by connecting 7.2 ohm resistors in series with the motor coil { 22.5 / ( 1.8 + 7.2 ) = 2.5 }
the catch being the 45W being lost in the resistor !

thats why modern stepper controllers use a PWM or "chopper" current control
to set the average motor current


John

[blitz355]

Hi Paul ,
with your motor for instance
you can use a 22.5V supply by connecting 7.2 ohm resistors in series with the motor coil { 22.5 / ( 1.8 + 7.2 ) = 2.5 }
the catch being the 45W being lost in the resistor !

thats why modern stepper controllers use a PWM or "chopper" current control
to set the average motor current
John

What if your driver has a chopper current control such as the HobbyCnc EZ board, would the current limiting resistor connected to the coil be necessary?

[john-100]

with stepper controllers that have PWM (or chopper) current control
you would not use dropper resistors as well
you would reduce the efficiency of the controller and too large a dropper resistor could prevent the copper current control working

the SLA7078M used in the hobbycnc board has built in current sense resistors
when the current through the motor coil reaches the required level
the volt drop across the sence resistor switches off the IC output until
the oscillator in the IC resets the current control circuit

http://www.sanken-ele.co.jp/en/prod/..._sla7070me.pdf




John

[Crevice Reamer]

Hi Paul. Some of the info on this page may be helpful to you:
Page 2

CR.

[blitz355]

with stepper controllers that have PWM (or chopper) current control
you would not use dropper resistors as well
you would reduce the efficiency of the controller and too large a dropper resistor could prevent the copper current control working
John

Well said, thank you for your time

[PaulMakesThings]

Thanks for the help everyone,
I think I will try out something with parts I have then. I have a Nema 23 motor marked 2.4V 1.8A, a 24V power supply, and a TB6560 based parallel port driven controller. I had assumed that I had something wrong since I blew out a chip on the same setup earlier, but as they are all second hand parts there may have been a problem with one of them.

I have a budget sufficient enough to buy a gecko drive and some new steppers, I'd just like to practice on my junk drawer stuff first if I can.

[PaulMakesThings]

I set up the same thing that blew out a chip last time and it worked fine. I had tested the motor afterward and found a shorted coil and had assumed that it was damage resulting from a bad setup, but I just ran a matching motor which I checked for shorts ahead of time on an identical controller and it worked fine. I guess that's what you get when you buy surplus sometimes.