AN2388
Application note
Sensor field oriented control (IFOC)
of three-phase AC induction motors using ST10F276
Introduction
AC Induction motors are the most widely used motors in industrial motion control systems,
as well as in home appliances thanks to their reliability, robustness and simplicity of control.
Until a few years ago the AC motor could either be plugged directly into the mains supply or
controlled by means of the well-known scalar V/f method. When power is supplied to an
induction motor at the recommended specifications, it runs at its rated speed. With this
method, even simple speed variation is impossible and its system integration is highly
dependent on the motor design (starting torque vs maximum torque, torque vs inertia,
number of pole pairs). However many applications need variable speed operation. The
scalar V/f method is able to provide speed variation but does not handle transient condition
control and is valid only during a steady state. This method is most suitable for applications
without position control requirements or the need for high accuracy of speed control and
leads to over-currents and over-heating, which necessitate a drive which is then oversized
and no longer cost effective. Examples of these applications include heating, air
conditioning, fans and blowers.
During the last few years the field of electrical drives has increased rapidly due mainly to the
advantages of semiconductors in both power and signal electronics and culminating in
powerful microcontrollers and DSPs. These technological improvements have allowed the
development of very effective AC drive control with lower power dissipation hardware and
increasingly accurate control structures. The electrical drive controls become more accurate
with the use of three-phase currents and voltage sensing.
This application note describes the most efficient scheme of vector control: the Indirect Field
Oriented Control (IFOC). Thanks to this control structure, the AC machine, with a
speed/position sensor coupled to the shaft, acquires every advantage of a DC machine
control structure, by achieving a very accurate steady state and transient control, but with
higher dynamic performance.
In this document we will look at the complete software integration and also the theoretical
and practical aspects of the application.
October 2006
Rev 1
www.st.com
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Contents
AN2388
Contents
1
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
AC induction motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1
1.1.2
Stator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2
Three-phase induction motor and classical AC drives . . . . . . . . . . . . . . . . 6
2
Vector control of AC induction machines . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1
2.2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Theory on vector control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1
2.2.2
2.2.3
2.2.4
Space vector definition and projection . . . . . . . . . . . . . . . . . . . . . . . . . . 10
The (a,b,c)(α,β) projection (Clark transformation) . . . . . . . . . . . . . . . . . 11
The (α,β)(d,q) projection (Park transformation) . . . . . . . . . . . . . . . . . . . 12
The (d,q)(α,β) projection (inverse Park transformation) . . . . . . . . . . . . . 13
2.3
2.4
2.5
Block diagram of the vector control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
The current model (rotor flux estimator) . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Space vector modulation (SVPWM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3
Hardware design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1
3.2
3.3
System configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
MDK-ST10 control board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Three-phase high voltage power stage (powerBD-1000) . . . . . . . . . . . . . 23
3.3.1
3.3.2
Power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Auxiliary supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.4
3.5
3.6
Gate driver board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Current sensing board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
The 3-phase AC induction motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4
Software design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.1
4.2
4.3
4.4
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Software organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Software variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Base values and PU model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.4.1
Magnetizing current
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
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AN2388
4.4.2
4.4.3
Contents
Numerical considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
ST10-DSP features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.5
Analog value scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.5.1
4.5.2
4.5.3
Current sensing and scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Rotor mechanical position sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
The PI regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.6
Clark and Park transformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.6.1
4.6.2
4.6.3
4.6.4
4.6.5
(a,b)->(α,β) projection (Clark transformation) . . . . . . . . . . . . . . . . . . . . 44
The (α,β)-> (d,q) projection (Park transformation) . . . . . . . . . . . . . . . . . 45
The Current model implementation (rotor flux estimator) . . . . . . . . . . . 45
Generation of sine and cosine values . . . . . . . . . . . . . . . . . . . . . . . . . . 46
The space vector modulation module . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5
6
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3/54
Background
AN2388
1
1.1
Background
AC induction motor
The AC induction motor is a rotating electric machine designed to operate from a 3-phase
source of alternating voltage. Asynchronous motors are based on induction. The cheapest
and most widely used is the squirrel cage motor 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. For variable speed drives, the
source is normally an inverter that uses power switches to produce approximately sinusoidal
voltages and currents controllable in terms of frequency and magnitude.
Like most motors, an AC induction motor has a fixed outer portion, called the stator and a
rotor that spins inside with a well-optimized air gap between the two.
Virtually all electrical motors use magnetic field rotation to spin their rotors. A three-phase
AC induction motor is the only type where the rotating magnetic field is generated naturally
in the stator because of the nature of the supply.
In an AC induction motor, one set of electromagnets is formed in the stator because the AC
supply is connected to the stator windings. The alternating nature of the supply voltage
induces an Electromagnetic Force (EMF) in the rotor (just like the voltage is induced in the
secondary transformer) as per Lenz’s law, thus generating another set of electromagnets;
hence the name “induction motors”.
Interaction between the magnetic field of these electromagnets generates a revolving force,
or
torque.
As a result, the motor rotates in the direction of the resultant torque.
1.1.1
Stator
The stator is made up of several thin laminations of aluminum or cast iron. They are
punched and clamped together to form a hollow cylinder (stator core) with slots as shown in
Figure 1.
Coils of insulated wires are inserted into these slots. Each grouping of coils,
together with the core it surrounds, forms an electromagnet (a polar pair) on the application
of AC supply.
The number of poles of an AC induction motor depends on the internal connection of the
stator windings. Internally they are connected in such a way, that when an AC supply is
applied, a rotating magnetic field is created.
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AN2388
Figure 1.
Stator core and windings
Background
1.1.2
Rotor
The rotor is made up of several thin steel laminations with spaced bars, which are made up
of aluminum or copper, along the periphery. In the most popular type of rotor (squirrel cage
rotor), these bars are connected mechanically at the ends and electrically by the use of
rings. The rotor consists of a cylindrical laminated core with an axially placed parallel slot for
carrying the conductors. Each slot carries a copper, aluminum or alloy bar. These rotor bars
are permanently short-circuited at both ends by means of the end rings. The rotor slots are
not exactly parallel to the shaft in order to decrease magnetic hum and slot harmonics.
Moreover this reduces the locking tendency of the rotor. In fact, the rotor teeth tend to
remain locked under the stator teeth due to direct magnetic attraction between the two. This
happens when the number of stator teeth are equal to the number of rotor teeth.
The rotor is mounted on the shaft using bearings on each end. One end of the shaft is
usually kept longer than the other for driving the load. Some motors may have
position/speed sensing devices. Between the stator and the rotor exists an air-gap, through
which, due to induction, the energy is transferred from the stator to the rotor like a
transformer. The generated torque forces the rotor and then the load to rotate.
The magnetic field created in the stator rotates at a synchronous speed (N
s
).
N
s
=
60
×
where:
N
s
= synchronous speed in RPM
p
p
= the number of pole pairs
f =
the supply frequency in Hertz
f
p
p
The magnetic field produced in the rotor is alternating in nature because of the induced
voltage. The frequency of the induced EMF is the same as the supply frequency. Its
magnitude is proportional to the relative velocity between synchronous speed (stator
frequency) and rotor speed. Since the rotor bars are shorted at the ends, the EMF induced
produces a current in the rotor conductors.
When the magnetic field is generated the rotor starts to run in the same direction trying to
reach the same speed. The rotor revolves slower than the speed of the stator field. This
difference is called
slip
(s). The slip varies with the load so that an increasing of the load
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