Page 291 - Anatomy of a Robot

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Speed The number of windings and the frequency of the power fed to the coils
fix the speed of the motor. The speed of AC motors is basically constant. As such,
they may not be the best for robots. Let’s consider just 60 Hz of power for these
examples. If just three windings form a single rotating field (one pole), the motor
spins at 60 Hz or 3,600 revolutions per minute (RPM). As three more winding
coils are added, the number of poles goes to 2 and the RPMs go down to 1800.
The following equation is used to determine the RPM, where p is the number of
three winding coils (poles), f is the frequency of the power, and s is the speed of
the motor in 4 RPM:

s 60 f>p

Cooling The windings are on the outside case, where they can be cooled easier.
Furthermore, with no brushes, the casing can be wide open to admit air for
cooling.
Controls AC motors are not easy to control, in either speed or position. It is pos-
sible to build an electronic controller to trim the speed and power consumption of
an AC motor, but it is best used in situations where only gross mechanical power
is needed, especially for constant speed applications.
Portability Given that a portable robot probably is running off batteries, AC
motors may not be the right choice. Along with the difficulties of controlling the
speed and position of an AC motor, it’s fair to conclude they may not be a good
choice in a robot.

DC Motors

DC motors come in many different styles. AC motors only have fewer styles because
their architecture attempts to take advantage of the existing movement (waveform) of
the AC power.
Like most motors, DC motors generate movement by creating magnetic fields within
the motor that attract one another. By and large, DC motors have permanent magnets
in the stator and the rotor has the coils (the reverse of AC motors). But since DC power
has no movement (waveform) of its own, the motor electronics must create a change in
the DC waveform as the motor rotates. This can happen in several ways.