January 1999
INTRODUCTION
Brushless DC motors (BLDC) are becoming widely used in the field of control motors. These kind of syn-
chronous motors are used as servo drives in applications such as computer peripherals equipment, ro-
botics, and as adjustable-speed drives in load-proportional capacity-modulated heat pumps, large fans ,
compressors and so on.
Brushless DC motors are referred to by many aliases as brushless permanent magnet, permanent ma-
gnet AC motors, permanent magnet synchronous motors, etc. The confusion arises because a brush-
less motor does not directly operate off a dc voltage source. It is generally driven (supplied) from an in-
verter which converts a constant voltage to a 3-phase voltage with a frequency corresponding
instantaneously to the rotor speed.
One of the advantages of BLDC motor is the sparks absence. The brushes of a DC motor have several
problems as regards to brushes’ life and dust residues, maximum speed and electrical noise. BLDC mo-
tors are potentially cleaner, faster, more efficient, less noisy and more reliable. However, BLDC motors
require a more complex electronic control.
This application note will show how this complexity can be reduced by using ST52x301 Fuzzy controller.
AN OUTLINE OF BRUSHLESS MOTORS
The Brushless motor has the physical appearance of a 3-phase permanent magnet synchronous machi-
ne.
The brushes and commutator have been eliminated and the windings are connected to the control elec-
tronics. Electronics replaces the function of the commutator and energizes the proper winding. The ener-
gized stator winding leads the rotor magnet and switches just as the rotor aligns with the stator.
In synchronous motor drives, the stator is supplied with a set of balanced three-phase currents, whose
frequency is f.
If p is the number of the poles in the motor, then:
f =
p
4
pi
ω
s
(1)
where ω
s
(rad/s) is the flux synchronous speed or, that is the same, the rotor speed. This equation links
the rotor speed to the phases switching frequency of the electronic drive.
The above currents produce a constant amplitude flux φs in the air gap, which rotates at the synchro-
nous speed ω
s
. Since the flux amplitude is proportional to the current amplitude, it is enough to manage
winding current level to control the rotor torque.
From Brushless theory [3-4] it is possible to demonstrate that
T
em
=K
t
Φ
f
I
ph
sin (δ)(2)
where k
t
is a constant, φ
f
is the field-flux, δ is called torque angle. δ represents the angle between the
phase linked flux φ
fph1
and the relative stator current I
ph1
.
AN1113
APPLICATION NOTE
Brushless Motor Fuzzy Control by using ST52x301
Authors: G. Grasso, M. Di Guardo
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