31-08-2012, 03:05 PM
A Low Cost Drive for Three-Phase Operation of Brushless DC Motors
1A Low Cost Drive.pdf (Size: 826.57 KB / Downloads: 62)
Abstract
This paper presents a novel drive topology that
allows three-phase operation of Brushless DC (BLDC) motors
without an intermediate DC link capacitor. The proposed motor
drive uses a unique six-step Pulse Width Modulated (PWM)
switching algorithm to facilitate three-phase operation of the
BLDC motors. The proposed drive system is modelled using
Matlab/Simulink with SimPower Systems toolbox for a 320 W
BLDC motor. With the proposed unique switching algorithm, the
motor can be operated either in speed control or electromagnetic
torque control mode. The paper explains the derivation of the
switching algorithm, and illustrates the Matlab/Simulink model,
which is used to validate the drive system by simulations. Simulated
results indicate that the theoretical performance of the
proposed drive system is comparable to existing techniques. In
addition, the proposed drive system is simple to implement and
low in cost without an intermediate DC link capacitor. Thus the
proposed topology would be an attractive alternative for low cost
BLDC motor drives, where the torque ripple is not a concern.
INTRODUCTION
Permanent Magnet Synchronous (PMS) and BLDC motors
are becoming popular in industry applications. They do not
use brushes for commutation, which is performed electronically
by using an array of power electronic switches and rotor
position information. As a result, PMS and BLDC motors
have inherent advantages such as low maintenance, long operating
life, high efficiency and noiseless operation. Excitation
for both PMS and BLDC motors is provided by permanent
magnets, which are located in the rotor. Therefore, these
motors exhibit high power densities and superior speed torque
characteristics [1], [2].
A power electronic converter with rotor position feedback is
used to drive PMS and BLDC motors in practice. In general,
the power electronic converter can be one of the following two
types.
PROPOSED NOVEL TOPOLOGY
The proposed novel BLDC motor drive topology is illustrated
in Fig. 2. It is similar to the indirect converter topology
presented in Fig. 1, but without the intermediate DC link
capacitor. The intermediate DC link capacitor acts as a sink
to the stored inductive energy in BLDC motor windings. The
switching algorithm is designed in such a way that the stored
energy in the BLDC motor windings is dissipated within the
BLDC motor itself.
There are three devices in the current conduction path at any
particular instant in time during the three-phase operation of a
BLDC motor. Two of the three switches are controlled, either
using PWM or hysteresis technique, while the other switch
remains in “On” state for that entire switching interval. The
switch, which remains in “On” state provides a freewheeling
path to circulate the stored energy in winding inductances
of the BLDC motor. The proposed switching algorithm is
described in detail in the Section III.
SWITCHING ALGORITHM FOR A BLDC MOTOR
The switching algorithm produces square-wave currents that
approximate a three-phase sinusoidal input for the three-phase
operation of a BLDC motor. The proposed six-step switching
algorithm for the three-phase operation of a BLDC motor is
tabulated in Table I. The three Hall Effect sensor signals
are named as Ha, Hb and Hc, and all possible outcomes
of them under simulated conditions are tabulated in Table
I. The power electronic switches connected to phase A of
the BLDC motor are named as A1 and A2, where A1 denotes
the switch connected between the phase A terminal of the
BLDC motor and the positive DC rail of the rectified single
phase supply. A2 denotes the switch connected between the
Phase A terminal of the BLDC motor and the negative DC rail
of the rectified single phase supply. The same convention is
used for the other two phases for the ease of explanation.
SIMULATION AND RESULTS
The proposed BLDC motor drive topology is modelled
in Matlab/Simulink environment with the SimPower Systems
toolbox, to investigate its performance. In-built device models
of SimPower Systems toolbox are used to build the simulation
model. Using the developed Simulink model, both the speed
and the electromagnetic torque control techniques are validated
through simulations.
The signals, which represent the outputs of Hall Effect
sensors, are obtained from the output bus of the in-built BLDC
motor model [15]. They are fed to a logic subsystem developed
in Simulink in order to determine the appropriate power
electronic switches, which have to be controlled, according to
the switching algorithm tabulated in Table I. The BLDC motor
parameters, which were used for the simulations, are given in
Appendix I.
CONCLUSION
A novel and low cost topology, which facilitates three-phase
operation of a BLDC motor, has been described. The novelty
of this topology is due to the absence of the intermediate
DC link capacitor, which is usually a large electrolytic capacitor.
In addition, the use of the proposed unique PWM
switching algorithm permits the control of the rotor speed and
electromagnetic torque of the BLDC motor.
A model for the proposed drive system has been developed
and simulated in Matlab/Simulink environment with Sim-
Power Systems toolbox. The simulation results indicate that
the performance of the drive system is comparable to existing
techniques and would be an attractive alternative for low cost
motor drives, where the torque ripple is not a concern. In
addition, the proposed BLDC motor drive system would be
more reliable in long term operation due to the absence of the
intermediate DC link capacitor.