One other word of advice for the student who wants to build a motor:
Start with a DC motor or "universal" AC/DC motor ... as opposed
to an AC motor, i.e. one that requires AC input.
The rationale is simple: A DC motor makes torque whether it is
running or stopped. A two-phase AC motor doesn't work very well
unless it is already running, and this increases the conceptual
workload when you try to analyze it. Forsooth an AC motor might
well have _zero_ starting torque unless you do something tricky,
and first-timers generally don't benefit from trickery.
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OTON if you've got your heart set on an AC motor let me know;
that's a whole different discussion.