Page 292 - Anatomy of a Robot

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MOTORS AND ACTUATORS 277
DC MOTORS WITH BRUSHES
Construction The rotor would stop spinning if the DC field in the rotor coils
never changed. By altering the polarity of the DC voltage on the coil as it rotates,
we can continually make its field attract the next magnet in the stator. As the rotor
rotates, a set of position-dependent switches in the rotor switch the field on the
rotor coils. The switches are implemented with a stationary, partitioned slip ring
on the rotor bearing (for incoming power) and brushes that drag around the ring
to power the coils. After the rotor rotates enough, the brushes move to the other
part of the slip ring and reverse the polarity on the coils. It’s a little like keeping a
carrot in front of a horse. This structure, however, has some clear disadvantages:
Electrical noise The brushes create sparks, which emit a great deal of elec-
trical radiation. Further, since the voltages change abruptly, the power supply
noise can be severe.
Fire hazard Sparks can touch off explosions.
Reliability Brushes can wear out and get clogged with dirt. After a while,
motors may need replacement brushes.
Speed DC motors are controlled by varying the voltage on the DC power sup-
ply. Higher-voltage motors are generally more powerful.
Cooling Cooling is a little more of a problem with DC brushed motors since the
electrical coils are inside on the rotor. Furthermore, since the speed is controlled
by linearly varying the power to the coils, the dissipation in the power supply can
become a problem.
Controls By controlling the voltage and current through the coils, both speed
and torque can be controlled. By and large, most DC motor controllers use a chop-
ping waveform to control the average DC voltage (as opposed to a linear regula-
tor). By turning the DC coil voltage off and on (to full voltage) very rapidly, the
average DC voltage on the coil can be controlled by means of a duty cycle. Such
motor drives are more efficient.
Portability DC motors tend to take up more room than AC motors of similar
power because of the brushes and coils on the rotor. Further, since the coils are on
the rotor, they have a considerable gyroscopic effect. A lot of spinning mass exists
on the rotor.

BRUSHLESS DC MOTORS

Construction Brushless DC motors have much the same construction as AC
motors. The rotor has permanent magnets, and the coils are on the case (stator).
By altering the polarity of the DC voltage on the stator coils as the rotor rotates,