GS001 Datasheet, PDF (1/8 Pages) Microchip Technology – Getting Started with BLDC Motors and dsPIC30F Devices

GS001
Getting Started with BLDC Motors and dsPIC30F Devices
Author: Stan D’Souza
Microchip Technology Inc.
INTRODUCTION
As a means of reducing high energy and maintenance
costs in motor control applications, BLDC motors are
seeing a resurgence in applications where efficiency
and reliability are important. The dsPIC30F motor con-
trol devices are ideally suited to drive and control a
wide range of BLDC motor types, in a large number of
applications. Microchip has developed a number of
solutions using the dsPIC30F and BLDC motors. This
document will help you select an appropriate solution
for your BLDC motor application.
BLDC MOTOR BASICS
DC brush motors have a permanent magnet on the
stator with the motor winding on the rotor. During rota-
tion, the current in the windings is reversed using
mechanical carbon brushes and a commutator located
on the rotor. The BLDC motor has permanent magnets
on the rotor with the electrical windings on the stator.
The first obvious advantage of the BLDC motor is the
elimination of the mechanical commutator and
brushes, which significantly improves mechanical
reliability. The commutator and brushes in DC motors
also give rise to sparking, so eliminating these
components means that BLDC motors can operate in a
harsh environment. The I²R heat losses in the windings
of a BLDC motor are now on the stator and can be
dissipated very easily. Consequently, efficiency of the
BLDC motor is vastly improved.
There are, however, some challenges when spinning a
BLDC motor. Firstly, a revolving electrical field has to
be created in the windings, which also has to be well
aligned with the magnetic field on the rotor. The
efficiency of the BLDC motor depends largely on the
alignment of the revolving electrical field to the
magnetic field on the rotor. To sense the magnetic field,
Hall sensors are normally used. Based on the signal
presented by the Hall sensors, the windings are appro-
priately excited. As the speed of the rotor increases,
however, there is a certain amount of lag between the
voltage excitation and the current effect on the
windings due to the inductance of the windings. To
overcome this lag, the voltage is initiated a little in
advance. This phenomenon is known as phase
advance and is implemented mainly in software at high
speeds of rotation. The result of phase advance is
better efficiency in the BLDC motor operation.
Sensored BLDC Motor Control
When driving a BLDC motor, it is important to know the
position of the magnetic rotor with reference to the
stator. Most commonly, Hall effect sensors are used to
generate feedback on the rotor position. This type of
control is called sensored BLDC motor control. Most
BLDC motors have three windings. Based on the
position of the magnetic rotor, two windings are ener-
gized at a given time with each phase conducting for
120 electrical revolution degrees, resulting in six
distinct combinations of energization. This type of drive
is called “trapezoidal” or “six-step commutation”.
SIX-STEP COMMUTATION
Figure 1 depicts a typical six-step commutation
scheme with the Hall sensor output overlay. Six-step
commutation offers a simple, yet efficient, method of
driving a BLDC motor. Hall A (HA), Hall B (HB) and Hall
C (HC) sense the position of the rotor with respect to
the windings, R, Y and B. Depending on the Hall sensor
reading from 1 to 6, an appropriate pair of windings is
driven high and low with the third winding not driven.
Each 360 degree electrical cycle is broken down to six
60 degree electrical sectors, in which one winding is
driven high, a second is driven low and the third is not
driven. Example: In Hall position 6 or sector 1, the R
winding is driven high while the B winding is driven low
and the Y winding is not driven. By reading the Hall
sensors, the six-step commutation algorithm can very
easily be implemented in software.
FIGURE 1:
TYPICAL SIX-STEP
COMMUTATION
HA
R
60°
HB
Y
HC
B
Sector 5 0 1 2 3 4 5 0 1
Hall
5 4 62 3 1546
© 2005 Microchip Technology Inc.
DS93001A-page 1