Secrets of Driverless Stepper Motor Operation

About: Was it you or I who stumbled first? It does not matter, the one of us who soonest finds the strength to rise must help the other. - Vera Nazarian, The Perpetual Calendar of Inspiration

I have a cabinet full of stepper motors of various configurations, both unipolar and bipolar. I wanted a quick, easy, reliable, and cheap way of testing them in both directions of rotation for acceptability since they are salvaged during roadside recovery sweeps of my 'hood from printers, scanners, copiers, etc.. The following method shows how I achieved this goal using the absolute minimum of components from my junque box.

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Step 1: Stepper Motor Theory, and Why This Works.

A stepper motor is a brushless, synchronous electric motor that converts digital pulses into mechanical shaft rotation. The speed of rotation is directly proportional to the frequency of the pulses. It being a synchronous motor, it will readily accept low A.C. voltage as well as D.C., but it's speed will be regulated by the voltage's frequency, typically 50- 60 Hz, thus output rpm will be fixed. Just as with a pulsed D.C. control, the direction of rotation can also be reversed- in this case simply by using a capacitor to induce a phase shift to the opposite coil(s), much like what a typical start capacitor does in an A.C. motor. Using a regular low voltage a.c. transformer as the power source, I tried varying the voltage with a triac controller on the primary, but it caused very erratic behavior so I abandoned that approach and settled for a middle of the road value of about 10 vac since the objective was to simply weed out any defective motors, not perform a task.

Step 2: Make a Start Capacitor

A non- polarized electrolytic capacitor is desirable for this job, however for the low voltages/ high capacitance used, they are rare, expensive, and hard to come by. Fortunately there is a method which allows us to convert the standard polarized d.c. capacitor to a non- polarized type. By connecting the – ends of two capacitors in series, a value of one half the rated microfarads is obtained at both + ends. This gimmick, when connected across the two coils, will initiate clockwise or counter clockwise rotation depending on which coil the electricity is applied to. This technique is sometimes used in audio circuits for signal processing and in other instrumentation circuitry as well, so no new ground is being plowed here, it's just another application. Keep the values the same for both C1 and C2 as the total capacitance needed will vary with motor impedance, and can best be determined by trial and error- when the motor under test no longer displays random rotation when toggled, you've got it.

Step 3: The Control Station

A salvaged pair of momentary action snap switches from an old food processor worked well for this fixture, allowing for clockwise and anti- clockwise motor operation. Breakout to the downstream adapter network is facilitated by using classic Fahnstock clips, and all features are combined on a scrap board. A typical protoboard setup would no doubt serve the same function using momentary push button switches for actuation.

Step 4: Parting Thoughts

This methodology was so successful that were I teaching stepper motor technology, this would probably be the first tutorial & project I'd have a student make; it's reliable, has the absolute minimum of parts needed to successfully achieve controlled movement and being so easy to understand, would be a good building block for more advanced circuit designs.

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    7 Discussions


    4 years ago

    Thanks, that's a handy tip. Never covered steeper motors when I was at uni doing engineering, and now you've told me all I need to know in about a minute!

    1 reply

    Reply 4 years ago on Introduction

    Thanks for the comment. I still can't pass by a junked printer on the roadside, being a hoarder isn't really that bad after all.

    To Beachside Hank,

    Thanks for this simple and understandable Instructable. I'm just a few years older than you and remember building a crystal radio receiver in 3rd grade. Remember Heathkits? As to the doofus instructor who didn't pick up free motors for his robotics club or even reply, it sounds like he needs to be sent to the Principal's Office. Maybe have his knuckles rapped a few times with a ruler.

    1 reply

    My dream radio build project was a Heathkit GR-64 Shortwave Receiver. This was a low-end model made from 1964 to 1971 and originally selling for $39.95. It covers AM and shortwave in four bands and utilizes four tubes. I even bought the manual for $.50 to study it, but as fate would have it, I never got enough money to buy it. In 1965 I was making $1.50/ hour as a junior electronics tech, $40 was very nearly a whole weeks pay for me then- ;-)

    Yeah, the steppers, I think I'll put them on Craigslist for any public school teacher who is interested, but I'll want to see their school i.d. before just handing them over.

    Thanks for your comments Steve.

    FREE steppers are even better, checked about 30 of 'em, only 1 questionable one so far, got about a dozen left to get back to, now all I need are some worthy projects. Unbelievably, I contacted our local H.S. about donating a bunch to their robotics club students, twice emailed the teacher, never even got a reply, go figure.