# Stepper Motor Selection - Conflicting Torque Curves

I am selecting a stepper motor in order to open a valve. The stepper motor shaft will be coupled to a valve shaft.

We would like to open the valve in 0.2-0.3 seconds. The output shaft must turn 4 revolutions in order to completely open the valve. This comes out to 800-1200 rpm (5300-8000pps for a 200 step motor). I will also need about 8 oz-in of torque to turn the valve shaft.

I’m currently looking at stepper motor #1209 (pololu.com/product/1209) and believe that it should serve my requirements. I would run the motor in full-stepping mode in order to get a little more torque out.

I was looking at the torque speed curves, but there seem to be two different conflicting torque speed curves.

One is on the description page of the motor (a.pololu-files.com/picture/0J229 … 88e1f0d242)

The other is in the spec sheet (pololu.com/file/0J689/SY35ST36-1004A.pdf)

I realize the units for torque in each graph are different but I converted a couple separate values and they aren’t the same. In any case the shape of the curves are not the same. As far as I can tell, the parameters of each curve are the same (24VDC, Constant Current 1.0A and half-stepping mode).

What gives?

A couple other questions:
Is there a maximum voltage I can use to drive the stepper motor? At first I thought the rated voltage was that maximum but research tells me this is not the case.

On the other hand, I believe that the rated current (1A) is the maximum current. Can anyone confirm? So I will need to use active current limiting on my stepper motor driver to keep the current under 1A, especially if I go up to high voltages.

I was planning on using an arduino to drive the stepper motor. I also believe that I may have to use acceleration limiting. So instead of stepping right at 800-1200 rpm I should start at a safe speed and work my way up to my top speed. How do I figure out a safe speed to start the stepper at, and how can I determine how fast I can accelerate up to 800/1200rpm?

I don’t know why the torque curves are different. Perhaps one is “guaranteed by manufacturer” and one is “measured on an example by Pololu”?

It is very common to use active current limiting when driving stepper motors. The reason is that with a higher voltage, you can “force” the current limit to be reached faster, and thus get more “usable” torque out of the motor.

Also, if you only run this motor intermittently, you may be able to drive more current through it. As long as it doesn’t overheat the coils during activation, the motor is likely to work OK.

Hello.

I have moved your thread from the Pololu Motor Controllers and Motor Drivers section to the Other Pololu Products section.

The pull-out torque curve that is on the #1209 stepper motor product page matches the one from the manufacturer’s website, which as you have pointed out, does not match the one from the manufacturer’s datasheet. In general, I recommend only using pull-out torque curves like this as a general guideline of what the motor can do, since your results will likely vary depending on your setup. However, we are aware of the discrepancy and are currently looking into the matter.

As jwatte pointed out, you can run stepper motors at voltages that are higher than their rated voltage as long as you limit the current to at or below your stepper motor’s rated current. This prevents the stepper motor from overheating and becoming damaged. The #1209 stepper motor is rated for 1A per phase, so that is where you would want to set your limit. As for acceleration ramping, I do not know of any sure way to determine how fast you can accelerate aside from trial and error. It will probably depend on your setup and what kind of load you have attached to the output shaft of your stepper motor.

-Brandon