Microchip Technology AN908 Using Manual

Using the dsPIC30F for Vector Control of an ACIM
Author: Dave Ross, John Theys
Diversified Engineering Inc.
Co-Author: Steve Bowling
Microchip Technology Inc.
INTRODUCTION
This application note describes a vector control
application that is written for the dsPIC30F family of
devices. Except for a brief discussion on control theory,
the information presented assumes you have a basic
understanding of AC induction motor characteristics.
References are included in some instances to provide
background information.
SOFTWARE FEATURES
The Vector Control software has the following features:
• The software implements vector control of an AC
induction motor using the indirect flux control
method.
• With a 50 µsec control loop period, the software
requires approximately 9 MIPS of CPU overhead
(less than 1/3 of the total available CPU).
• The application requires 258 bytes of data
memory storage and 256 bytes of constant
storage. With the user interface, approximately 8
Kbytes of program memory are required.
• The memory requirements of the application allow
it to be run on the dsPIC30F2010, which is the
smallest and least expensive dsPIC30F device at
the time of this writing.
• An optional diagnostics mode can be enabled to
allow real-time observation of internal program
variables on an oscilloscope. This feature
facilitates control loop adjustment.
VECTOR CONTROL THEORY
Background
The AC induction motor (ACIM) is the workhorse of
industrial and residential motor applications due to its
simple construction and durability. These motors have
no brushes to wear out or magnets to add to the cost.
The rotor assembly is a simple steel cage.
 2004 Microchip Technology Inc.
AN908
ACIM's are designed to operate at a constant input volt-
age and frequency, but you can effectively control an
ACIM in an open loop variable speed application if the
frequency of the motor input voltage is varied. If the
motor is not mechanically overloaded, the motor will
operate at a speed that is roughly proportional to the
input frequency. As you decrease the frequency of the
drive voltage, you also need to decrease the amplitude
by a proportional amount. Otherwise, the motor will
consume excessive current at low input frequencies.
This control method is called Volts-Hertz control.
In practice, a custom Volts-Hertz profile is developed
that ensures the motor operates correctly at any speed
setting. This profile can take the form of a look-up table
or can be calculated during run time. Often, a slope
variable is used in the application that defines a linear
relationship between drive frequency and voltage at
any operating point. The Volts-Hertz control method
can be used in conjunction with speed and current sen-
sors to operate the motor in a closed-loop fashion.
The Volts-Hertz method works very well for slowly
changing loads such as fans or pumps. But, it is less
effective when fast dynamic response is required. In
particular, high current transients can occur during
rapid speed or torque changes. The high currents are a
result of the high slip factor that occurs during the
change. Fast dynamic response can be realized with-
out these high currents if both the torque and flux of the
motor are controlled in a closed loop manner. This is
accomplished using Vector Control techniques. Vector
control is also commonly referred to as Field Oriented
Control (FOC).
The benefits of vector control can be directly realized
as lower energy consumption. This provides higher
efficiency, lower operating costs and reduces the cost
of drive components.
Vector Control
Traditional control methods, such as the Volts-Hertz
control method described above, control the frequency
and amplitude of the motor drive voltage. In contrast,
vector control methods control the frequency, ampli-
tude and phase of the motor drive voltage. The key to
vector control is to generate a 3-phase voltage as a
phasor to control the 3-phase stator current as a
phasor that controls the rotor flux vector and finally the
rotor current phasor.
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