© 2009 Microchip Technology Inc. DS01299A-page 1
AN1299
INTRODUCTION
A large number of motor control applications are
consistently and continuously looking for methods to
improve efficiency while reducing system cost. These
are the two main factors that are driving the efforts to
improve existing motor control techniques, such as
trapezoidal control, scalar control and Field-Oriented
Control (FOC).
FOC has become more popular in recent years due to
the fact that the cost required to implement this
technique is no longer a constraint. The available
technology and manufacturing process now make it
possible to implement this control technique in a 16-bit
fixed-point machine such as the dsPIC
®
Digital Signal
Controller (DSC).
Efficiency is another reason that has allowed FOC to
gain ground over scalar and trapezoidal control
techniques on low-cost and mid-cost applications. It is
also well suited in applications in which hard
requirements are low noise, low torque ripple and good
torque control over a vast speed range.
Field-oriented control can be implemented using
position sensors such as encoders, resolvers or Hall
sensors. However, not all motor control applications
require such granularity given by a resolver or encoder;
and, in many cases, they do not require control at zero
speed.
These applications are a perfect target for using
sensorless techniques in which the motor position can
be estimated using the information provided by the
currents flowing through the motor coils. There are two
popular approaches to this sensing technique: the
dual-shunt resistor and the single-shunt resistor.
The dual-shunt resistor technique utilizes the
information contained in the current flowing through
two motor coils in order to estimate the motor position.
The single-shunt resistor technique utilizes only the
information contained in the current flowing through the
DC bus to reconstruct the three-phase currents, and
then estimate motor position.
In this application note, the single-shunt approach is
discussed. For information on the dual-shunt resistor
approach, please refer to the application note, AN1078
“Sensorless Field Oriented Control of PMSM Motors”.
CURRENT MEASUREMENT
The information contained in the current flowing
through the motor coils allows a motor control algorithm
to operate the motor in a region where the motor
produces the maximum torque, or to operate the motor
at certain performance, or even to be able to
approximate or estimate internal motor variables such
as position.
Three-phase AC Induction Motors (ACIMs),
Permanent Magnet Synchronous Motors (PMSMs) and
Brushless Direct Current (BLDC) motors in particular
use a three-phase inverter as the topology of
preference. This topology, which is shown in Figure 1,
allows individual control of the energy applied to each
coil, which enables the motor to be efficiently operated.
FIGURE 1: THREE-PHASE INVERTER TOPOLOGY
Authors: Daniel Torres and Jorge Zambada
Microchip Technology Inc.
3-Phase
InverterRectifier
DC Bus
Three-Phase AC
Motor
Single-Shunt Three-Phase Current Reconstruction
Algorithm for Sensorless FOC of a PMSM
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