Make Electronics

What Next? 289

Experiment 33: Moving in Steps

theory

Inside a stepper motor (continued)

In reality, the magnets are not separate from each other.
The edge of a rotor is magnetized in zones that alternate
between south and north polarity. And instead of multiple
coils, there are just four windings that go around all the
magnetic cores. But the principle is exactly the same. The 3D
rendering gives a general idea, and the photograph shows
what I found when I cracked open a typical stepper motor.

Now bear in mind that when this device is driven by a set of
555 timers, we don’t just connect negative to one wire at a
time on the left, leaving the others floating. In reality, at any
given moment, three of the timers have a negative output
and the fourth has a positive output. The last diagram in
Figure 5-112 shows this situation.

Suppose the top wire is positive while the other three are
negative, as shown in Figure 5-113. The positive output
does nothing, because it is balanced by the positive power
on the other end of the coils. The two negatives attached to
the bottom set of coils create equal and opposite forces that
cancel each other out (while wasting some power). So the
net result is the same as in Step 3.

In fact, you should find that you can disconnect the com-
mon wire completely while using the stepper motor with
555 timers, and the motor will still turn, because one of
the timers is providing positive power while the others are
negative. In fact, you’ll be running them more efficiently this
way.

S N S N S N S N

Figure 5-113. When the motor is driven by four 555 timers, they
are activating it by sinking positive voltage from it. The interior
workings of the motor look something like this. It’s not the
most efficient way to do the job.

Figures 5-114 and 5-115 may help to give you a clearer idea

of what the motor actually looks like inside.

Figure 5-114. This 3D rendering gives a better idea of what a

typical stepper motor looks like inside. The copper coils and

gray cylinders are stationary, while the black disc rotates

between them.

Figure 5-115. When a stepper motor is broken open, this is

what you’re likely to find. On the left, the rotor of the motor,

which has a magnetized band around its circumference, is still

attached to the lower half of the casing. On the right, the upper

half of the casing has been opened, and the coil has been re-

moved (actually the winding you can see consists of two coils,

wound in opposite directions). The spikes are the magnetic

cores that exert force on the rotor.