LEARNING TO DRIVE:
THE BLDC MOTOR By Fred Eady
If you were to go back and survey past SERVO articles that I have written, you would probably conclude
that I have this thing about motors and motor drivers. For instance, we recently tackled Universal
Motors and constructed a Universal Motor controller. I also presented more than one SERVO stepper
motor controller project. In these pages, we’ve driven linear actuators, rotated hobby servo rotors, and
built circuitry to oversee the direction and speed of simple brushed DC motors.
This month, we’ll add yet another motor and motor
driver project to the pages of SERVO. The motor in
the spotlight this time around is a direct descendent
of common universal brushed DC motors. However, this
motor type contains no brushes as its brushes have been
replaced by Hall effect sensors and electronic circuitry. A
brushed DC motor contains a set of brushes, a stator
assembly, and a rotor assembly. Commutation — as it
relates to brushed DC motors — is the process of switching
current and thus, magnetic fields between the stator
assembly and rotor assembly of the motor. The motor
brushes play a large part in the process of commutation as
the position of the brushes (like the stator) is constant with
respect to the rotor’s magnetic fields. Thus, the brushes are
actually part of the motor’s stator assembly.
The magnetic fields of the stator always react with the
commutating magnetic fields of the rotor in such a way
as to coerce rotary motion via the motor’s rotor shaft. Our
new motor type has no
stationary brushes that
provide a reference point
for commutation.
Instead, the motor type
we’re about to discuss
contains Hall effect
sensors, which are
positioned for use as
commutation reference
points by the brush
replacement circuitry.
This type of motor is
Photo 1. This BLDC motor comes called a brushless DC
ready to work with drive circuitry motor or just BLDC.
that employs the use of its After we go to school on
built-in Hall effect sensors. These this new motor, we’re
sensors are ignored when the going to design a
motor is spun using sensorless microcontroller-based
BLDC motor drive techniques. BLDC motor driver circuit
30 SERVO 12.2008
that takes the place of the Hall effect sensors.
BLDC 101
As you would imagine, the absence of brushes in a
BLDC motor does have physical and electrical advantages.
On the whole, BLDC motors last longer and run quieter
acoustically and electrically than comparable universal DC
motors. Pound for pound, a BLDC motor can deliver higher
torque than its brushed counterpart. In addition to being
more reliable, quieter, and stronger, BLDC motors have the
ability to run faster than universal brushed DC motors while
requiring less maintenance. BLDC motors are turning in
automobiles, kitchen appliances, medical equipment, and
aircraft. If you have one of those new front-loading washing
machines, it’s a good bet that a BLDC motor is spinning the
tub. These motors can also be found spinning diskettes and
hard drive platters. The power-to-weight ratio of BLDC
motors make them very popular for use in model aircraft. If
BLDC motor technology is soaring around in flying robots,
you can bet there’s an application for them in land-roving
robotic manifestations.
I attempted to disassemble the BLDC motor you see
in Photo 1 with no joy. I suspect that its components are
sealed to preserve the integrity of the positioning of the
motor’s integral Hall sensors. I didn’t have a second motor
on hand. So, I didn’t work too hard at pulling it apart. The
model BLY171S-24V-4000 BLDC motor shown in Photo 1 is
manufactured by Anaheim Automation. This particular
motor has a permanent magnet rotor, a three-phase stator,
and a built-in trio of Hall sensors. Picking apart the model
number tells us that this motor is a NEMA size 17 type
BLDC motor. The 1S denotes a single shaft motor with 11
oz-in of continuous stall torque. The BLY171S-24V-4000’s
motor windings are rated for 24 volts and are able to spin
the rotor shaft at 4,000 RPM.
The BLY (for short) connects to the outside world using
a standard eight-wire BLDC motor scheme. Three of the
