3-Phase AC Induction Motor Vector Control Using DSP56F80x
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Introduction
This application note describes the design of a 3-phase AC
induction vector control drive with position encoder coupled
to the motor shaft. It is based on Motorola’s DSP56F80x
dedicated motor control device. The software design takes
advantage of the SDK (Software Development Kit)
developed by Motorola.
AC induction motors, which contain a cage, are very popular
in variable speed drives. They are simple, rugged,
inexpensive and available at all power ratings. Progress in the
field of power electronics and microelectronics enables the
application of induction motors for high-performance drives,
where traditionally only DC motors were applied. Thanks to
sophisticated control methods, AC induction drives offer the
same control capabilities as high performance four-quadrant
DC drives.
The drive application concept presented is that of vector
control of the AC induction motor running in a closed-speed
loop with the speed/position sensor coupled to the shaft. The
application serves as an example of AC induction vector
control drive design using a Motorola DSP with SDK
support. It also illustrates the usage of dedicated motor
control libraries that are included in the SDK.
Motorola DSP Advantages and Features
The Motorola DSP56F80x family is well-suited for digital motor control, combining the DSP’s
calculation capability with an MCU’s controller features on a single chip. These DSPs offer many
dedicated peripherals, including a Pulse Width Modulation (PWM) unit, an Analog-to-Digital
Converter (ADC), timers, communication peripherals (SCI, SPI, CAN), on-board Flash and RAM.
Generally, all the family members are well-suited for AC induction motor control.
AC Induction Motor
The AC induction motor is a rotating electric machine designed to operate from a 3-phase source of
alternating voltage. For variable speed drives, the source is normally an inverter that uses power
switches to produce approximately sinusoidal voltages and currents of controllable magnitude and
frequency.
A cross-section of a two-pole induction motor is shown in Figure 3-1. Slots in the inner periphery of
the stator accommodate 3-phase winding a,b,c. The turns in each winding are distributed so that a
current in a stator winding produces an approximately sinusoidally-distributed flux density around the
periphery of the air gap. When three currents that are sinusoidally varying in time, but displaced in
phase by 120° from each other, flow through the three symmetrically-placed windings, a
radially-directed air gap flux density is produced that is also sinusoidally distributed around the gap
and rotates at an angular velocity equal to the angular frequency ωs of the stator currents.
The most common type of induction motor has a squirrel cage rotor in which aluminum conductors or
bars are cast into slots in the outer periphery of the rotor. These conductors or bars are shorted together
at both ends of the rotor by cast aluminum end rings, which also can be shaped to act as fans. In larger
induction motors, copper or copper-alloy bars are used to fabricate the rotor cage winding.
Mathematical Description of AC Induction Motors
There are a number of AC induction motor models. The model used for vector control design can be
obtained by utilization of the space vector theory. The 3-phase motor quantities (such as voltages,
currents, magnetic flux, etc.) are expressed in the term of complex space vectors. Such a model is valid
for any instantaneous variation of voltage and current and adequately describes the performance of the
machine under both steady-state and transient operation. Complex space vectors can be described
using only two orthogonal axes. We can look at the motor as a 2-phase machine. The utilization of the
2-phase motor model reduces the number of equations and simplifies the control design.
Vector Control of AC Induction Machines
Vector control is the most popular control technique of AC induction motors. In special reference
frames, the expression for the electromagnetic torque of the smooth-air-gap machine is similar to the
expression for the torque of the separately excited DC machine. In the case of induction machines, the
control is usually performed in the reference frame (d-q) attached to the rotor flux space vector. That’s
why the implementation of vector control requires information on the modulus and the space angle
(position) of the rotor flux space vector. The stator currents of the induction machine are separated into
flux- and torque-producing components by utilizing transformation to the d-q coordinate system,
whose direct axis (d) is aligned with the rotor flux space vector.
Rotor Flux Model
Knowledge of the rotor flux space vector magnitude and position is key information for the AC
induction motor vector control. With the rotor magnetic flux space vector, the rotational coordinate
system (d-q) can be established. There are several methods for obtaining the rotor magnetic flux space
vector. The implemented flux model utilizes monitored rotor speed and stator voltages and currents. It
is calculated in the stationary reference frame (α,β) attached to the stator. The error in the calculated
value of the rotor flux, influenced by the changes in temperature, is negligible for this rotor flux model.
Control Process
After reset, the drive is in the INIT state and in the manual operation mode. When the RUN/STOP
switch is detected in the stop position and there are no faults pending, the INIT state is changed to the
STOP state. Otherwise, the drive waits in the INIT state. If a fault occurs, it goes to the FAULT state.
In the INIT and STOP states, the operating mode can be changed from the PC master software. In the
manual operating mode, the application is controlled by the RUN/STOP switch and UP/DOWN push
buttons; in the PC remote-control mode, the application is controlled by the PC master software.