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.
Advantages:
• Move in discrete, well defined steps
• Stay put when you remove the power
Disadvantages:
• 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.
