Specifications

119Motor Control
user adjusts the speed control or when
the microcontroller receives a command
through the electronic interface, the
microcontroller then instructs the current
sink/source DAC (e.g., DS4432) to change
its output current value. This forces the
regulator to change the output voltage
to the motor up or down, respectively, to
keep the feedback pin’s voltage constant.
Alternatively, if the motor can handle
the high-DC voltage, one can convert
the input control to a pulse-width-
modulated (PWM) duty cycle applied
to a power switch between the power
supply and the motor. By varying the
duty cycle, the average power to the
motor is adjusted, as is its output power
and speed. If constant speed is needed
under a varying load, then motor
speed detection is needed. This motor
speed signal (usually a pulse frequency
proportional to the motor rotation rate)
must be fed into a controller that will
respond by either adjusting the motor
voltage or the PWM duty cycle. With
sucient switching frequency, the
inductance of the motor windings act
as a lowpass lter that keeps the motor
current close to constant with only minor
ripple, thus producing low torque ripple.
To reverse the direction of the BDC
motor, current must ow through the
motor in the opposite direction. This
can be accomplished using power
MOSFETs or IGBTs in an H-bridge
conguration (Figure 2). These MOSFETs
can be either voltage controlled or
PWM controlled for speed control.
Brushless DC (BLDC) Motors
A BLDC motor spins the magnets
instead of the windingsthe inverse of
a BDC motor. This has advantages and
disadvantages. A BLDC motor has neither
commutator nor brushes, so it requires
less maintenance than a BDC motor. The
BLDC motor’s rotor can take dierent
forms, but all are permanent magnets.
The armature is xed and holds the stator
windings; the rotor carries the magnets
and can either be an “inrunner” or an
“outrunner.” Inrunners have the rotor
inside the stator and outrunners have
the rotor outside the stator (Figure3).
Either approach eliminates the problem
of connecting the power source to a
rotating part through a commutator. The
brushes and mechanical commutator are
replaced by electronic commutation of
the stator windings. This increases motor
life signicantly. The initial cost of a BLDC
motor is higher than an equivalent BDC
motor, although the cost of permanent
magnets has decreased signicantly over
the past years. With precise commutation
and rotor position sensing, eciency is
generally higher than equivalent BDC
motors. They also produce more torque
per unit weight. Another signicant
advantage for industrial applications is
that since there are no brushes, there are
no sparks generated, so the BLDC motors
can be used in explosive environments.
Due to their higher eciency over
a wide range of speeds and loads,
BLDC motors are seeing wider use in
heating, ventilation, air conditioning,
and refrigeration (HVAC&R) systems.
MOTOR
Q4Q3
Q2Q1
V
DC
Figure 2. H-bridge for driving a BDC motor in both directions.
When Q1 and Q4 are on, the motor moves one direction. When
Q2 and Q3 are on, the motor moves in the opposite direction.
Figure 3. Disassembled outrunner BLDC motor. Fixed armature
carries the stator windings. The rotor carries the permanent
magnets.