Stepper motor confusion

[mauritsw]

Hi,

I'm building a CNC plamsa cutter. I have 2 motors that should have 425 oz-in and 269 oz-in of torque. Wat I don't understand is that both motors have a current rating of 3A. If I take a 24V power supply, shouldnt I have 72W of max power on both motors? Where's the difference?

Another question. Is the current rating on the motor meant per coil? And what about the current rating on the driver, is this also per coil? Or should I just directly translate what's on the motors to the driver?

Thank you all, and best wishes for 2023!

[peteeng]

Hi Maur - A N23 425oz-in motor is usually 114mm long and a 269oz-in motor is 76mm long. The short one will be 2.8A and the long is 4.2A not 3A. So check the paperwork with those motors. I'd run them at 48V if you haven't sorted the PS yet. The quoted current is the total max current the motor could draw. Since there are 100's of coils in the motor if it where by coil you'd need a small power station to run these things So the 114mm motor will need a 4A supply and the short one a 3A supply say. or both can be run from a 7 or 8A supply. They can be run from lesser amp supplies with no harm done. Depending on the motion performance you need. Peter

[mauritsw]

Hi Peter, thanks for the reply.

Ah I meant phase instead of coil. In my understanding if I apply microstepping, both coils with be activated simultaneously at moments. Thats why I thought I might have to half the amperage on the driver. But I understand now that thats not the case.

But... To make it more confusing, if I look at the datasheet, it actually does say 4A per phase... Which is kind of contra of what I just learned...

This is the motor: https://www.reichelt.nl/nl/nl/stappe...29320.html?r=1
It's indeed 114mm long

And this is the other one: https://www.reichelt.nl/nl/nl/stappe...29323.html?r=1
76mm, correct.

[dazp1976]

Hi Peter, thanks for the reply.

Ah I meant phase instead of coil. In my understanding if I apply microstepping, both coils with be activated simultaneously at moments. Thats why I thought I might have to half the amperage on the driver. But I understand now that thats not the case.

But... To make it more confusing, if I look at the datasheet, it actually does say 4A per phase... Which is kind of contra of what I just learned...

This is the motor: https://www.reichelt.nl/nl/nl/stappe...29320.html?r=1
It's indeed 114mm long

And this is the other one: https://www.reichelt.nl/nl/nl/stappe...29323.html?r=1
76mm, correct.

6.8mh inductance?. Ouch!. You'll be lacking at high velocity. You will just constantly stall out.
I'd run these with a DM860 and at least 60vdc!. That will help overcome it.
This higher volt setup is proven, time and time again.

Mine are 3mh. I got only 300rpm with 24v on the mill (1500mm/min).
450rpm with 36v (2250mm).
650rpm with 48v (3250mm).
I then swapped to the DM860T drivers with 60v, I now get 800rpm (4000mm/min) and above without stalling and can accelerate flat out!.
I upped to 80vdc on the Z and can get even more.

It flies!.

[mauritsw]

Good to know there are some options if I need more speed. But since its for a plasma cutter I won't be using high speeds anyway. I think I'll just build it first and then see if I need more speed.

[joeavaerage]

Hi,

But since its for a plasma cutter I won't be using high speeds anyway.

Wrong, plasma cutters require high speed motion.

A stepper is a class of motor called a 'variable reluctance' motor. For such devices multiplying voltage and current is NOT applicable. It certainly is for DC motors and to a less accurate degree,
AC motors but falls short with steppers.

All stepper lose torque the faster they go, that's physics and cant be beat. The principle measure of how bad the torque reduction will be is inductance, the lower the better.

With 23/24 size steppers 1mH-2mH, 1 mH preferred and reject anything over 2mH.
With 34 size steppers 2mH-4mH, 2 mH preferred and reject anything over 4mH.

The classic method to overcome excessive inductance is to have the highest possible voltage driver and power supply. At the current time 80VDC drivers and 80VDC power supplies are
readily and cheaply available....use them. If not 80VDC then 100VDC or higher.

Craig