# Bipolar Steppers: Rated Voltage Versus Supplied V

Hi,

I am considering reconfiguring my setup which was as follows:

Arduino Micro
Pololu A4988
Bipolar, 200 Steps/Rev, 35×28mm, 10V, 0.5 A/Phase
12 Volt DC Supply

With:
Arduino Leo (or) Pololu A-Star 32U
Pololu A4988
Bipolar, 200 Steps/Rev, 35×28mm, 10V, 0.5 A/Phase
9 Volt DC Supply

What would happen to the stepper if it gets 1 volt less than the rated voltage? I am assuming torque might change, but would it be that much and would the lower voltage affect the speed of the motor?

Thanks

So generally the voltage of a stepper is rated is the voltage that will draw its max amps at no current limiting.

So you are cutting your top current by a proportional rate to your voltage difference.

9V/12V = 75% max current. Current is proportional to the torque, so 75% max torque is available.

The much more significant effect is that you lose max speed that you can achieve. Steppers have inductance that limits how fast you can step them. The lower your voltage, the slower you can step a stepper before it stalls out.

At 9V, for a 12V motor, you will likely not be able to rotate fast at all.

Typically, I’ve heard numbers like “8 times the rated voltage” is what you should drive it at.
Another number I’ve heard, is 32*squareRoot(inductance in mH).

Those numbers are basically if you’re trying to get a fast RPM out of the stepper.

Are you rotating very slowly? Like 1 rev/s? Maybe you will be ok. I do not know exactly the correlation between voltage and the max step rate, but I suspect it’s not particularly complicated.

EDIT: The clarify the resounding point: YES, you normally run at a much higher voltage than the rating. A 3V motor may be run with a 12V power supply, or even, a 42V power supply if you’re really pushing it fast (not with the A4988 though! They have a 35V limit.) The key spec is you set the current-limiting pot of the stepper motor driver to the current limit (in this case, 0.5A)

I’d google “Stepper motor tutorial” or “driving a stepper motor voltage limits”

I should add, only 2-3 times the rated voltage also work O-K.

Most reprap 3D printers get decent speeds, and run with a 12-14V power supply and 2-3V rated motors.

Thanks, Tom.

The stepper I am using is rated at 10V and it is going to be running pretty slowly anyways.

You might be ok!

If you’re driving the 10V motor with 9V, you might expect 10% less torque in general from the motor. not too bad.

How to tell if you’re driving it too fast:
Requirements: 1 voltmeter with ‘current’ measurement mode for AC current*, or just a current measuring device (called anmeter.) Typically it’s a voltmeter with current measuring capability that people have, though.

Set current limit to whatever you want it (probably 0.5A I guess, though it will not reach quite that much as previously described.)

Take your anmeter, and wire it in series with 1 of the 4 wires to the motors. All power should be off whenever connecting or disconnecting the motor wires to and from the stepper driver!!

Now, slowly step your motor (very slow) and measure the current [this amounts to just watching the meter]. Then, adjust your program to incrementally increase the speed, until you notice the original measurement you got of a current is dropping off. Typically when I see the motor current dropping to 70-80% of its original value, I know that’s the functional limit of the stepper motor speed for that motor and driving voltage. It’s somewhat arbitrary since if you can deal with getting less than 1/2 the rated torque, then you can deal with a current about 50-60% less than the rated current.

*Why AC current measurement? Because the bipolar stepper motors are driven in a fashion that alternates the current through the windings, so it’s “alternating current.” I’m pretty sure the DC measurement won’t be useful I haven’t run this test in a while though so if I’m wrong is amounts to adjusting your voltmeter to be on the DC current mode not the AC mode.]

ALSO NOTE:
It is my understanding you might not be able to get good “microstepping” out of your motor at below the rated voltage, since there is little differential current limiting. But I could be wrong about this…I don’t know if you get “bad” microstepping or “nonexistent.” Start off your system with “full step” mode [I *think* that’s default when nothing is connected to the MS1,2,3 pins.]