How real electric motors work

John Storey


7. Stepper motors

Stepper motors are useful for positioning things. Unlike a conventional DC motor that just goes round and round when you apply power to it, a stepper motor requires that the current through its two or more windings changes in a specific sequence. Each time this sequence is carried out, the stepper motor makes one step, clockwise or anticlockwise. With an appropriate piece of digital logic (or a computer) providing an appropriate number of repetitions of the sequence, the motor will move through the required angle. Stepper motors are used to move the paper in printers, to position the read heads on disc drives, and in simple servo systems.

• Move in discrete, well defined steps
• Stay put when you remove the power

• Require complex drive electronics
• Not terribly efficient

This particular motor steps 1.8 degrees at a time. It is an example of a hybrid stepper motor, one that has a magnetised rotor that also has iron teeth.

The stator has a set of coils (in this case eight) each of which energises a single pole. Those poles, in turn, each have four teeth. The windings on poles 1, 3, 5 & 7 are connected together: let’s call this Phase A. Poles 2, 4, 6 & 8 are also connected together to form Phase B. There’s then an external connection to each end of the two phases and to the centre-tap of the coils, making six wires in all.

The rotor also has a series of teeth that exactly correspond to teeth on the poles of the housing. However, not all the rotor teeth line up with all the stator teeth at any one time! In fact, the rotor teeth are in two sets, a front set and a back set. These two sets are rotated by half a tooth with respect to each other. Between the two sets is a permanent magnet, and so the front set corresponds to a north pole, and the rear set a south pole. (All this seems unnecessarily complicated, and it is. The only reasons it’s done this way is so you can get nice small steps – in this case 1.8o at a time.)

If we energise the Phase A coils, the rotor will turn so its teeth line up with the teeth of that phase. Now, if we energise Phase B as well, the rotor will turn slightly because the teeth of the Phase B are not aligned with those of Phase A. Now we de-energise Phase A, and the rotor will align fully with the teeth of Phase B. Next we energise Phase A with the reverse polarity to before, de-energise Phase B, energise Phase B with the reverse polarity, de-energise Phase A, re-energise Phase A with the original polarity, and we’ve made one full step! If we were writing this down as series of dance steps for a barn dance it would probably look something like: A+ A+B+ B+ A-B+ A- A-B- B- A+B- A+.

There are many other varieties of stepper motors. Some have no magnets on the rotor (so-called switched reluctance stepper motors), and some have magnets but no iron.




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