Introduction: Secrets of Driverless Stepper Motor Operation

By
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.

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.