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we can continually make its field attract the next magnet in the rotor. As the rotor
rotates, electrical controls switch the field on the stator coils. This structure has
some clear advantages:
Electrical noise Much less electrical noise exists than with brushed DC
motors.
Fire hazard No sparks are made.
Reliability No brushes are used that could wear out. Further, far less mass
takes place on the rotor.
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 easy since the coils are on the casing, but because the speed
is controlled by linearly varying the power to the coils, the dissipation in the power
supply can become a problem.
Controls Brushless DC motors can be controlled with a similar type of chopped
waveform control that the brushed DC motors use (with accommodations for the
interference from the brushes). Since no brushes are used, the controller must also
sense the motor position. This makes the controller much more expensive.
Portability Brushless DC motors are fairly lightweight, but the controller can
be complex. Further, make sure the motor does not have delicate sensing wires (to
sense position). Try to get the kind where the controller senses the motor position
automatically. It makes the controller more expensive, but the motor will be more
mechanically reliable.

DC STEPPER MOTORS
Construction Stepper motors have much the same construction as AC motors
and DC brushless 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, we can continually make its field attract the next magnet in the
rotor. As the rotor rotates, electrical controls switch the field on the stator coils.
Some clear differences exist between steppers and DC brushless motors:
Stepping speed Stepping motors are designed with more rotational positions
and tend to step from position to position faster. They’re more like a digital sys-
tem and the DC brushless motors are more like an analog system.
Stopping Steppers are designed to stop on a dime and hold their position. For
this reason, they tend to have less rotational mass. DC motors can perform the
same feat but must have carefully designed servo systems to sense and hold
their position. Steppers hold the position that is defined by the motor geometry.
Speed Steppers are not necessarily designed for speed. If they go too fast, they
may lose their position by slipping over one too many poles. They have to move