working out the current draw of a stepper motor

[geekinesis]

Hi
I am working out the current of the steppers inside my laser cutter using a formula - current draw in amps = voltage divided by resistance across the coil

now I have bipolar steppers with 4 wires and am measuring a resistance of 3.3 across each pair.

the voltage supply is 24 volts to the existing controller board (newlydraw)

so do I assume the current draw is 24/3.5 = 6.8 amp

or should it be 24/7 = 3.4 amp (adding the resistance of the coils together)

or something completely different

and if I have a 3.5 amp controller board with 3 axis. Will this be ok to drive the two steppers?

[geekinesis]

ok another thought

If the 3 axis controller is powered by 24 volts, then is each stepper receiving 24 volts? or a fraction of the 24 volts?

[geekinesis]

i am just going to buy a couple of new steppers. At least then I can order exactly what I need rather than risk damaging the existing steppers.

[zax15uk]

The steppers are most likely 12V 1A, running at 24V for improved speed (this is standard for steppers, usually 2-4X the rated voltage is 'normal').

If you do run them from your 3.5A then just set the current limit to 0.5A and run the motor back and forth a few times, then check the temp. If it's cold, increase to 0.75A and test again. It is normal for them to get slightly warm once at maximum current. Over current will eventually result in failure of the motor, although does improve torque and most motors can be pushed above their rating providing your controller reduces the holding current at idle.

Keeping it simple, your calculation isn't accurate as the coils are an inductive load and resistance will increase as they are energised.

Zax.

[doorknob]

ok another thought

If the 3 axis controller is powered by 24 volts, then is each stepper receiving 24 volts? or a fraction of the 24 volts?

Normally the motors would be wired so that each motor is receiving 24 volts.

As the other poster mentioned, it is normal to run such steppers at a higher voltage than their "rated" voltage.

[geekinesis]

I finally decided to pull out the motors so I could get to the specs.

axis (left to right as you look at machine)

42BYG112LS dual shaft

1.8 v

0.60 a


Axis (front to back)

42BYG114

12 v

0.40 a

So it looks like the 24 volts is running each individually as you say. 1.8v up to 24 volts seems like a big step.

[SkipW]

I finally decided to pull out the motors so I could get to the specs.

axis (left to right as you look at machine)

42BYG112LS dual shaft

1.8 v

0.60 a


Axis (front to back)

42BYG114

12 v

0.40 a

So it looks like the 24 volts is running each individually as you say. 1.8v up to 24 volts seems like a big step.

geekinesis, will this help any?

[geekinesis]

geekinesis, will this help any?

thanks it does

Also...If I run a 12v 0.4 amp motor at 24v

Will it be pulling 0.8 amp?

[zax15uk]

24V for a 1.8V stepper is high, but providing current is well controlled it will allow high speeds. There could be some inline resistors.

Zax.

[geekinesis]

Ok I have my tb6560 driver board and have used the spindle relay to switch the laser on and off.

The board itself is a 3.5 a board which allows the option for 25% current (.9amp) so I have ordered some 0.33 ohm resistors to replace the .15 ohm resistors that limit the current to 0.4 amp

That should give me the options to run the existing (0.4amp) motors at the correct current, and at a higher voltage.

(someone with better maths could probably correct me on the resistor choice before I blow things up

So nearly there. I almost have a laser running from linux emc at a total cost of £30 ($45) so far

My backup plan is to just order 2 new steppers to run at 1.6 amp and then I dont need to worry about changing the resistors.

It took me a while to work out how to configure emc as the manual that comes with the driver board is for mach3.

Also I read that some people have had problems with the tb6560 driver boards. I am not going to go above 12v and 1a, well below its limits.