Brushless DC Motors

[Magneto]

Brushless Motor Construction

DC brushless motors are similar in
performance and application to
brush-type DC motors. Both have
a speed vs. torque curve which is
linear or nearly linear. The motors
differ, however, in construction
and method of commutation. A
brush-type permanent magnet
DC motor usually consists of an
outer permanent magnet field
and an inner rotating armature. A
mechanical arrangement of
commutator bars and brushes
switches the current in the
armature windings to maintain
rotation. A DC brushless motor
has a wound stator, a permanent
magnet rotor assembly, and
internal or external devices to
sense rotor position. The sensing
devices provide signals for
electronically switching
(commutating) the stator
windings in the proper sequence
to maintain rotation of the
magnet assembly.
The rotor assembly may be
internal or external to the stator
in a DC brushless motor. The
combination of an inner
permanent magnet rotor and
outer windings offers the
advantages of lower rotor
inertia and more efficient heat
dissipation than DC brush-type
construction. The elimination of
brushes reduces maintenance,
increases life and reliability and
reduces noise and EMI
generation.

Brushless Motor Commutation

The possible number of phases
and winding arrangements for
the DC brushless stator are quite
varied. As in the case of brushtype
DC motors, increasing the
number of phases reduces torque
ripple. However, an important
practical consideration for DC
brushless motors is the number of
electronic switches required to
commutate the phases. Three
phase motors provide a
compromise in this regard and
are popular in many applications.
The winding arrangement for a
three phase motor may be either
a Y or a Æ configuration. The
most efficient operation of the
motor requires current flow in
more than one phase at any
instant and current reversal in
each of the phases at some point
during 360 electrical degrees of
rotation. This is turn requires a
minimum of two electronic
switches per phase. It may be
noted that the Y configuration
with a lead common to the three
phases can be commutated in a
unipolar mode with only three
electronic switches. However,
the motor torque is reduced
with this scheme.

Hall Effect Commutation

Rotor position sensing is essential
for proper commutation of DC
brushless motors. Magnetic
sensing with inexpensive Hall
effect switches is frequently
adequate. The devises require
little space and can easily be
placed within the motor. Optical
encoders or resolvers may also be
used. Cost, operating
environment of the motor,
intended application and
performance all influence the
choice.
As an example of Hall effect
commutation, consider the
typical commutation scheme
shown in the table for the Hurst
motors. Commutation of a three
phase motor with current reversal
requires that the windings be
switched every 60 electrical
degrees. If three sensing devices
are spaced 120 electrical degrees
apart, and if each device has a
50% duty cycle, six discrete 3-bit
signal states are produced at 60
degree intervals as the rotor
turns. Each change of state
triggers switching of the stator
windings to a particular terminal
pair and polarity.

Reversing

The sensors are located so that
switching occurs 30 electrical
degrees before the peak in the
torque vs. angle curve for that
terminal pair. Operation of the
motor in the reverse direction
requires only a change in the
switching sequence. A number of
manufacturers offer an integrated
circuit to perform the sensor
signal decoding and provide
signals to sequence the power
switches for the motor phases.

Brushless Motor Control

DC brushless motors are used in
the same types of applications as
DC brush-type motors, e.g.,
servo, constant speed, variable
speed, controlled torque, etc. The
methods of control are similar to
those for brush-type motors.
Most will involve some type of
current control whether in an
open loop mode or a closed loop
mode with position and perhaps
velocity sensing. A description of
a typical drive circuit will illustrate
the possibilities.
The power switches for a three
phase motor are usually arranged
in three half bridges as shown in
the figure. Power MOSFETs of
bipolar devices may be used.
Provision is normally made in the
control circuit to delay turn on of
one device in each leg until turn
off of the other device is
complete to prevent shorting the
power supply. Motor current may
be sensed with a single resistor in
the common lead. For more
information about controls,
please consult the factory.

Current Limiting

In current limiting or speed
control applications the sensed
current may then be compared
with a fixed or variable reference
and the resulting signal used to
control motor current (speed,
torque, etc.). Current control
schemes include pulse width
modulation or linear operation of
the power switches. Dynamic
braking is accomplished with this
circuit by turning off the top
switch and turning on the
bottom switch in each leg of the
bridge. The winding currents are
then short circuited through the
bottom switches. As DC brushless
motor applications have
increased, many of these control
features have been incorporated
into integrated circuit controllers.