Choosing correct power supply for Stepper Motors

I am building a machine and need 2 Stepper Motor for that. The motors are driven using by a 3.3v arm device.

I have made the following selections regarding the stepper motor, stepper motor driver and the power supply.

Power Supply 12 Volt Power Supply - 3.5 Amp Single Output

Stepper Motors Stepper Motor: Unipolar/Bipolar, 200 Steps/Rev, 42×48mm, 4V, 1.2 A/Phase

Stepper Motor Driver DRV8825 Stepper Motor Driver Carrier, High Current

I tried my best to research the compatibility and came up with these.

Is this a good selection considering the fact that the Power Supply will be driving 2 of these motors.

I will be running the motors at 1/16 step for high resolution.As far as the speed is concerned,it's going to be pretty slow but they will be running continuously for hours in end.Basically what I am trying to do here is make a V-Plotter.As far I can tell, there will be loads of start stop motion in the motors though.

• How fast are you planning to run the motors? What sort of motion profiles are you running on these motors? Are both constantly running? Nov 11, 2013 at 1:55
• edited the question! Nov 11, 2013 at 2:45

As long as the steppers are running relatively slowly, let's say a few hundred steps per second, you should be fine.

Your driver uses PWM to control the current through the motor windings. In microstepping mode the driving waveform at the motor terminals looks like a sine and cosine (sines offset by 90 degrees) and the driver modulates the outputs to maintain the current to the windings.

The 1.2A appears to be peak current. So at standstill we're talking 1.2A*4V = 4.8W max per phase (though a microstepping driver will never drive those two phases "fully" simultaneously). At any rate, at very slow speeds/standing we're talking about ~13W for the two motors vs. your 40W power supply. So no problem.

As you start speeding things up two things happen:

1. The motors starts acting as a generator producing counter-EMF (and we don't know the Ke constant for those motors). Now to maintain the same current your driver has to overcome this counter-EMF. This is why you want the highest voltage you can get if you're planning to run your motors at speed.
2. We are going through the microstepping waveform and really start caring more about the RMS current (So we can effectively divide by ~1.4 since the power supply doesn't care about peak current as much over very short durations). So about 0.86A RMS per phase. That adds up to ~3.42A which is still lower (though not by much, than your 3.5A supply).

So as long as you're well below a speed where your back-EMF is ~8V you're fine. If you plan to go to this point or above it you'll run out of voltage and you're also left with relatively small margins on power. It also depends on how much power you need for the rest of your system. Unfortunately I'm not quite sure how to calculate the back-EMF constant (Ke) from your motor parameters. Looking at the pulling torque curve (given with a 24V supply) I would guess that up to 1000-2000pps you'll still be OK and it looks like the torque is dropping by ~25% over ~1500pps so it's probably in the range of 6V/1500pps (very rough guess). Since we know the inductance maybe someone can show us how to calculate back-EMF constant (I'd be interested in learning)...

• Amazing answer.I was planning on including some acceleration and deceleration to the motors to speed up the drawing .Based on your information,I think using a 24V 4.5Amp power supply would be a better bet for quick action.What do you think? Nov 11, 2013 at 10:23
• @SteveIrwin As the vendor curves are shown for 24V you can get a pretty good idea of where you'll be with 24V. Nov 11, 2013 at 20:06

using a correct power supply for stepper motors, or servos, is difficult. You just cannot select based on voltage and amperage alone. biggest criteria is the so-called over-voltage protection circuitry in most power supplies. over-voltage protection circuitry is marketing/sales hype, and misleading. over-voltage protection circuitry does not sense over-voltage, it senses fast current change, which generates an internal voltage, that turns off the power supply. turning off the power supply does protect the power supply, but causes any motor to stop/stall/miss steps. This turnoff lasts 2-3 seconds, devastating for any machine operations.

Best power supply is one without over-voltage protection circuitry. Remember, it's not over-voltage protection circuitry. This is actually the greatest cause of poor motor performance than any other criteria.

Example: connect a 24v 10A power supply to a nema34 stepper motor. if you set the torque high, thus high amperage, say power supply 3 amps total (2 windings on at same time), and start the motor, the power supply voltage will drop in 200 msec to around 4V, then rise again after 3 seconds. LED blinks. You will never see over-voltage, voltage never is over 24.5V. Yet the power supply shuts off. At Excitron, we have analyzed this situation for over 18 years. One solution is set the brake for about 20% of load, then the power supply is slightly loaded at about .5A. Most power supplies cannot handle a 0 to 3A load without shutting off, but will not shutoff if loaded say, from .5A to 3A. Note that 3A is much less than 1/3 of the supply output of 10A, which has a surge capacity of about 13A. Depends on the power supply, but you get the idea.

CUI makes a robust enclosed 24V 72 watt supply that has no over-voltage protection circuitry. This supply is amazing, never shuts down due to rapid amperage change.

Therefore, almost all power supplies do not work well with high amperage motors.