Driver for an unusual stepper motor

I recently bought a new step motor and I'm having a hard time finding a suitable driver.

The specifications are: Stepping degree: 0.9 (400 steps) Phase voltage: 6.8v Phase resistance: 4 Ohms Phase current: 1.7A Holding torque: 0.9Nm

It was one of the few 6.35mm (1/4 inch) shaft motors I could find in Europe that was both in stock and available for retail purchase.

Now, to potential drivers:

  • I have an EasyDriver, but it can handle only 0.75A max.
  • The BigEasyDriver can provide only 1.4A max.
  • The Adafruit Motor Shield can provide only 1.2A max.
  • The official Arduino Stepper Motor Shield can handle up to 2.0A, but according to some people, its not a particularly good stepper driver.
  • The only Pololu driver that can work with this 6.8v motor is the DRV8834. It has a continuous current per phase rating of 1.5A, which with a heatsink can be pushed to 2.0A

So it looks like I will have to choose between the Arduino Stepper Motor Shield or the Pololu DRV8834.

Would it be better to use the Arduino Stepper Motor Shield, to take advantage of the fact that it can handle up to 2.0A? Or, would a DRV8834 with a heatsink be a better option?

Is there another driver that would be a candidate to work with this motor?

The Arduino Stepper Motor Shield is overrated and probably will not deliver more than 1 amp/channel on a continuous basis. However, you can use a chopper style driver like the DRV8834 from Pololu and set the current limit to slightly less than the rated 1.7 A maximum for that motor. The DRV8834 is supposed to tolerate 1.5 amps on a steady state basis, but I have not used that particular one.

Edit: the DRV8825 will also work. http://www.pololu.com/product/2133 You are not constrained by the voltage rating of the stepper motor, only the current rating. In fact, the combination of stepper motors and chopper drivers works much better with high voltage supplies that with low voltage supplies.

JRemington, thanks for the quick feedback.

When I looked through the Pololu range, I considered the DRV8825 because of its higher current per phase (1.5A). But, because of its minimum voltage rating of 8.2, I figured it wouldn't work well with the motor.

Will using the DRV8825 shorten the life of the motor due to the fact that the motor is rated at 6.8 volts? Or is the Amp rating all that matters?

The DRV8825 has the followig Amp tolerances:

  • Continuous current per phase: 1.5A
  • Maximum current per phase: 2.2A

Am I right in believing that the higher Amp tolerances of the DRV8825 (when compared to the other Polou drivers) will help reduce the operating temperature of the driver?

Pololu notes that the 2.2A rating requires additional cooling. Do you have any recommendations for cooling?

Could one make an adequate heatsink out of some drawn aluminum channel attached with thermal paste to the driver IC?

The higher voltage won’t bother the motor at all. The important thing is not to exceed the current.

You may find that the motor works perfectly well with 1.5amps - certainly worth trying.

I don’t know if the higher current rating will reduce the temperature or if the driver is more tolerant of higher temperatures.

A home-made heat sink is better than nothing. Paint it black. In my limited experience commercial anodised black heat sinks with several fins perform better than home made ones.

Additional cooling probably means the biggest heat sink you can fit and a forced airflow. If you are very adventurous you could build a water cooled heatsink :).

…R

The voltage rating of a stepper motor is just the winding resistance times the steady state current limit, and has no independent significance.

Those high current, low resistance motors are not at all unusual. They are designed for industrial applications and chopper motor drivers. The combination is USUALLY used with power supplies that provide 5-10 times the voltage rating of the motor windings, as the higher voltage increases the motor speed and responsiveness.

These low impedance motors are designed for current control, not voltage control, so you always use a chopper drive.

The supply voltage is chosen to overcome the back-EMF at the maximum speed you want the motor to go, which for 4000rpm might be as high as 120V for instance.

1.7A is really into the realm of professional stepper drive units, as its pushing the limits of the single-chip chopper drivers which have DMOS MOSFETs of 0.2..0.5 ohm, so start to generate significant heat above 1A.

A finned heatsink with small fan is going to enough for the DRV8834, as the dissipation will be ~2W at 1.7A. The DRV8834 has the lowest dissipation of any of the single chip drivers I've seen with typical MOSFET resistances of 0.19 ohm at temperature (worst case could be 0.3 ohms though)

Thanks everyone for the feedback!

It's great to be able to get this level of insight, and it goes to show that there's no substitute for experience.

Thanks again!