07-08-2012, 03:47 PM
Single Stage Brushless DC Motor Drive with High Input Power Factor
for Single Phase Applications
SingleStagePaperV2.pdf (Size: 1.26 MB / Downloads: 100)
INTRODUCTION
Traditionally, single phase input brushless DC motor
drives feature two cascaded closed-loop switching converter
stages in the power path as shown in Fig. 1. First, a diode
bridge supplies rectified line voltage to a boost converter,
which is closed-loop controlled to provide input power factor
correction. The boost converter charges a bulk energy storage
capacitor to a voltage higher than the peak line voltage.
Finally a voltage-fed, step-down inverter is used to drive the
brushless DC motor.
SINGLE STAGE APPROACH
The power section of the brushless DC drive presented
herein consists of three distinct elements as shown in Fig. 2.
A passive full-wave bridge rectifier is used to rectify
incoming single phase line voltage. A small high frequency
bus capacitor, which is used to reduce input current ripple at
the switching frequency, stores negligible line frequency
energy. Therefore, the rectified voltage is basically a rectified
sine wave. Finally, a three-phase voltage source inverter unit
modulates the bus voltage applied to the motor. A six-step
commutation sequence is used with two active phases and one
idle phase at any given time. As a result of phase
commutation, the motor presents an approximately constant
back-emf at the terminals corresponding to the active phases.
This system is equivalent to a Buck converter with a constant
voltage load as shown in Fig. 3, where the inductor L
represents the motor equivalent inductance.
DESIGN ISSUES FOR SINGLE-STAGE DRIVE
In this section we discuss design issues related to the
proposed single-stage drive. All design quantities calculated
in the previous section are functions of only three variables:
peak input voltage s V , output power P , and conduction
angle J . Typically the design requirements specify a
nominal desired operating point: peak input voltage s V ,
output power P , and motor speed w . The main design
decision is to choose the motor torque constant T K , which
determines the motor back-emf
COMPARISON TO CONVENTIONAL TWO-STAGE
APPROACH
It is not easy to compare the proposed single-stage
approach to a conventional two-stage solution using a Boost
power factor preregulator. Reference [1] attempts such a
comparison in terms of cost and efficiency for a specific
design, but acknowledges the difficulty in making a fair
comparison. In the proposed solution the Boost stage with its
large electrolytic output capacitor is eliminated, but at the cost
of higher stresses in the inverter switches, which result in the
use of higher-rating, more expensive switches. Quantifying
the cost connected to the higher switch stresses is difficult.
The conclusion was that there is a reduction in cost,
corresponding to approximately 20% of the cost of the Boost
stage.
DISCUSSION AND CONCLUSION
The viability of the proposed approach has been
demonstrated through simulation and experimental results. A
number of observations can be made.
Regarding the speed control loop, the achievable
bandwidth is ultimately limited by the 120Hz torque ripple
present on the motor. However, for the proposed target
applications in low-cost home appliances with high inertia
loads, this limitation does not appear to be a serious problem.
The charge (one-cycle) modulation method appears
superior to conventional peak current mode control, because
it compensates for the energy stored in the motor inductance
providing a more ideal input current waveform.