A simple DC electric motor. When the
coil is powered, a magnetic field
is generated by the rotor, causing rotation.
When the rotor becomes horizontally
aligned, the torque becomes zero. At this point, the
commutator reverses the direction of current through the
coil, reversing the poles.
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Jeremy
Fielding gives a good tour of motor types, including AC
induction motors with capacitor start, which are heavy reliable
fixed-speed motors that are very common in appliances, but aren't
used as much in robotics because you normally want variable speed,
higher torque, and lower mass.