A New Control Technique for Achieving Wide Constant Power Speed.pdf
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简介：本文档为《A New Control Technique for Achieving Wide Constant Power Speedpdf》，可适用于电子通讯领域，主题内容包含ANewControlTechniqueforAchievingWideConstantPowerSpeedOperationwithanInter符等。
A New Control Technique for Achieving Wide Constant Power Speed
Operation with an Interior PM Alternator Machine
Jackson Wai Thomas M. Jahns
University of WisconsinMadison
1415 Engineering Drive
Madison, WI 53706
Ph: 6082653821
Fax: 6082625559
EMail: waij@cae.wisc.edu
University of WisconsinMadison
1415 Engineering Drive
Madison, WI 53706
Ph: 6082625702
Fax: 6082625559
EMail: jahns@engr.wisc.edu
AbstractA new flux weakening control algorithm is
presented for achieving wide constant power operation with
an interior permanent magnet (IPM) synchronous machine
operating as an alternator. The algorithm is designed to
initiate flux weakening only when necessary by recognizing the
threshold conditions for current regulator saturation, making
the algorithm inherently adaptive to changes in the inverter
bus voltage. A 6kW 42V automotive integrated
starter/alternator (ISA) using a directdrive IPM machine
serves as the target application for this development.
Simulation results show that the new algorithm is capable of
delivering very good static and dynamic bus regulation
characteristics over a 10:1 engine operating speed range.
Tests are presently under way to confirm these performance
predictions using a prototype IPM starter/alternator system.
Keywords: Permanent magnet machines, AC motor drives,
road vehicle power systems, power control, synchronous
generators
I. INTRODUCTION
With proper design, an interior permanent magnet (IPM)
synchronous machine can achieve constant power operation
over a wide speed range in both the motoring and
generating regimes [1]. As a result, IPM machines are
attracting increasing attention for a variety of applications
ranging from electric vehicle propulsion [2] to machine
tools [3].
While the design of an IPM machine for extended speed
operation poses several technical challenges, the success of
the complete drive system depends on the availability of an
appropriate control algorithm that can extract the full
performance capabilities from the machine. In particular,
special flux weakening control techniques are necessary to
achieve constant power operation over a wide speed range.
Previous work reported in the literature has generally
focused on extended speed range applications for the IPM
machine operating as a motor [46]. This paper presents a
new control algorithm designed to extract nearconstant
power from an IPM machine over a wide speed range when
it is operating as an alternator.
The application that has motivated this work is a direct
drive integrated starter/alternator (ISA) for an automotive
vehicle [7]. The development of many new electrically
S/A rotor
acts as
flywheel
Torque converter
inside rotor hub Flywheel, alternator, starter, pulley, &
possibly a belt eliminated
New Parts Eliminated Parts
Fig. 1 Integrated starter/alternator (ISA) configuration.
powered accessories combined with wide interest in mild
hybrid concepts using electric machines for low speed
acceleration has drawn significant attention to ISA systems
[8]. As illustrated in Fig. 1, the new ISA replaces the
conventional engine flywheel and is driven directly by the
engine crankshaft without gearing. The machine is
designed to operate as both the engine starter and alternator,
replacing the two separate machines that perform these
functions in conventional vehicles, along with their
associated pulleys, belts, and gears.
Performance specifications developed in consultation with
several automotive manufacturers call for the ISA to supply
150Nm of starting torque at low speeds. In addition, the
ISA must deliver 42V power to the automotive accessories
over a 10:1 speed range extending from 4KW at 600rpm to
6KW at 6000rpm. It is this latter requirement that
motivated selection of the IPM machine for further
development as an attractive candidate for this application.
Significant progress has been made towards development of
an IPM machine that is capable of meeting these
challenging application requirements [9]. This effort has
culminated in construction of a prototype version of a 12
pole IPM machine designed for this directdrive ISA
application. Figure 2 shows the stator and rotor of this
machine, illustrating its use of two magnet cavities per pole
with saturating magnetic bridges that link the iron pole
pieces into unitary rotor laminations.
078037116X/01/$10.00 (C) 2001 IEEE
807
Fig. 2 Rotor and stator assemblies of prototype 6 kW IPM
machine for ISA application.
II. CONTROL TECHNIQUE PRESENTATION
A. IPM Machine Operating Envelope
The principles for achieving wide constant power
operation using IPM synchronous machines have been
established in the technical literature [4,10]. The machine
equations governing the dynamic operation of the IPM
machine in the rotor dq reference frame are summarized
below. Note that the daxis is aligned with the rotor
permanent magnet flux.
vd = rs id + plds  wrlqs (1)
vq = rs iq + plqs + wrlds (2)
lds = Lds id + fpm (3)
lqs = Lqs iq (4)
where
id and iq are the d and qaxis stator currents [A]
vd and vq are the d and qaxis stator voltages [V]
lds and lqs are the d and qaxis stator flux linkages [Wb]
Lds and Lqs are the d and qaxis stator inductances [H]
(Note that Lqs> Lds in an IPM machine)
fpm is the daxis permanent magnet flux linkage [Wb]
wr is the rotor rotation frequency [elec. rad/s]
Magnetic saturation has a significant impact on the
operating characteristics of an IPM machine. Experience
with the automotive ISA machine [9,11] has indicated that
saturation can be modeled quite accurately by making Lqs a
function of the qaxis stator current iq while treating Lds as a
constant. Crosscoupling saturation effects [12] between
the q and daxes are not significant in this machine and are
not included in this model.
Equations (1) and (2) can be rewritten using (3) and (4) plus
chain differentiation to reflect the fact that Lqs is a function
of iq:
vd = rs id + Lds pid  wr Lqs iq s (5)
vq = rs iq + Lqs' piq + wr( Lds id + fpm) (6)
where
Lqs' =
q
qs
i
L
iq + Lqs (7)
Previous work [4,10] has demonstrated the value of the dq
current plane for evaluating the extended speed operating
characteristics of IPM machines. More specifically, such
plots provide a means of visually depicting the constraints
imposed by the inverter voltage and current limits and their
interactions. Figure 3 presents the dq current plane plot for
the ISA machine described above whose electrical
parameters are provided in the Appendix. The Appendix
includes a numerical expression for Lqs as a function iq that
has been fit to machine's inductance characteristics.
The current limit manifests itself in Fig. 3 as a circle of
amplitude 326A (peak) centered at the origin, while the
voltage limit takes the form of a family of nested ellipses all
centered at the IPM machine's characteristic current value,
fpm/Lds. The radii of the ellipses vary inversely with the
rotor speed. These ellipses are distorted in the vertical (iq)
direction because of Lqs saturation effects, and their longest
diameters exhibit a noticeable counterclockwise tilt from
the horizontal (id) axis because of stator resistance effects.
At any given speed, the IPM machine can operate at any
combination of iq and id values that falls within the
overlapping area of the current limit circle and the voltage
limit ellipse associated with that speed.
800 600 400 200 0 200 400 600
600
400
200
0
200
400
600
id [A]
iq
[A
]
Current Limit
Circle
Imax
Voltage
Ell ipses
Max T/A
without saturation
Max T/A with Lqs
saturation effect
Motoring
Generating
ff P M/Lds
ww r increasing
Generat ing Mode
Envelope Trajectory
Fig. 3 Plot of idiq current plane for the 6kW IPM machine showing
current limit circle, voltage limit ellipses, and current vector
trajectories.
808
Figure 3 identifies the maximum torqueperAmp current
vector trajectory for the ISA machine in the second
quadrant motoring regime for starting operation, with and
without the effects of magnetic saturation. The trajectory
without saturation forms an angle with the negative –id axis
that exceeds 45 degrees, while the angle of the
corresponding trajectory with saturation is less than 45
degrees.
During generating operation of the IPM machine at elevated
speed, the maximum output power point follows the
periphery of the current limit circle towards the negative id
axis as indicated in Fig. 3. This motion is forced by the
increasing speed that progressively shrinks the voltage limit
ellipse, preventing the machine from operating in the
vicinity of the maximum torqueperAmp trajectory (for
generating) identified by a dashed line in Fig. 3.
The amplitude of the ISA machine's characteristic current,
fpm/Lds (=135A) is less than the inverter current limit
(326A). This may seem surprising since optimum extended
speed operation occurs when the value of fpm/Lds equals the
inverter current limit [1]. However, the other ISA system
specifications, including a limit on the machine’s maximum
backemf amplitude, result in this lower value of
characteristic current. Given this situation, the maximum
generating output power trajectory eventually separates
from the current limit circle as the speed increases,
following a trajectory that leads to the fpm/Lds characteristic
current point on the negative id axis at infinite speed.
It is important to note that the illustrated generating mode
trajectory in Fig. 3 for speeds above the corner point
actually represents an optimistic outer limit for the current
vector locus that can only be approached but never quite
reached for an actual current regulated drive. This is true
because the outer boundary of the voltage ellipse at any
speed corresponds to sixstep voltage operation,
representing a condition in which the current regulator
loops are completely saturated. Since the current regulator
loses control of the instantaneous machine phase currents
under such conditions [4], the current vector command
must be continually adjusted so that it always resides safely
inside the voltage ellipse. However, it is desirable to
approach the ellipse as closely as possible under heavy load
conditions in order to deliver maximum power from the
IPM machine, taking full advantage of the available
inverter dc bus voltage.
With this observation in mind, it is clear that angle between
the commanded current vector and the negative daxis in
Fig. 3 must be reduced as the shrinking voltage ellipse
progressively intrudes on the current limit circle for speeds
above the corner point. This control action, illustrated in
Fig. 4, provides the basis for the new generating mode
control algorithm described in the next section.
IS1
IS2
q2
q1
 Iq
 Id
Flux Weakening
Action
Fig. 4 Plot of current vectors illustrating flux weakening action
during generating mode operation.
Starter/
Alternator
Machine
Shaft Angle
Transducer
T
e
*
i
q
42V
+

DC Link
Capacitor
Vector
Rotator
V d c
V
dc
V dc
T Start
(For Starting)
V
dc
*
(For
Generating)
+  P I
S/A
Control
Module
id
V q
e*
V
d
e*
V q
s*
V
d
s*
qqr
V dc
Gate
Signals
Space
Vector
Modulator
Vector
Rotator
d/dt
ww r
qq r
qq r
Fig. 5 Block diagram of ISA controller configuration.
B. Controller Configuration
A toplevel block diagram of the ISA controller is provided
in Fig. 5. This figure includes many of the features that
typically appear in highperformance vector control
configurations utilizing synchronousframe current
regulation. That is, phase current information is combined
with rotor position feedback to develop measurements of
the instantaneous id and iq that are fed to the current
regulator imbedded in the block labeled S/A Control
Module. Another input to this module is the torque
command Te* that is derived from either the starting torque
command or the bus voltage regulator, depending on the
system's operating mode.
The outputs of this module are the voltage commands vde*
and vqe* in the synchronous reference frame. These voltage
commands are then converted back to the stationary
reference frame using a second vector rotator before being
applied to a space vector modulator. This modulator
develops the inverter switching commands, making use of
the measured bus voltage vdc to insure a smooth transition
from PWM operation into saturated sixstep voltage
operation under all bus voltage conditions [13].
809
Figure 6 shows the detailed structure of the S/A Control
Module that appears as a block in Fig. 5. The control
components enclosed by the inner dashedline box in the
lower half of Fig. 6 are responsible for flux weakening
operation and are only are activated at elevated speeds
when the current vector approaches the prevailing voltage
limit ellipse at any given speed. Under all other conditions,
such as lowspeed starting operation, the fluxweakening
controller is essentially dormant, having no effect on the
control action of the Fig. 6 S/A Control Module. The
control elements lying across the top half of Fig. 7
dominate its response under such conditions, as described
below.
Starting Operation
The vector control principles for controlling the ISA
machine to deliver high starting torque (up to 150 Nm) as a
motor are well established in the literature [14] and will be
summarized only briefly here. Since starting mode
operation occurs only at low speeds where the voltage limit
ellipse is not a factor, the maximum torqueperAmp
trajectory in the second quadrant provides the preferred
operating points for motoring torque production. The
maximum available starting torque from the ISA machine
occurs at the point in the second quadrant where the
maximum torqueperAmp trajectory intersects the current
limit circle.
The S/A Control Module in Fig. 6 executes this starting
mode control by converting a starting torque command Te*
into the corresponding values of d and qaxis current
commands, idm and iqm, using the function blocks f1 and f2
and the polartorectangular conversion block. These f
functions are preprogrammed with the maximum torque
perAmp trajectory that matches the IPM machine's
characteristics. The two ffunctions develop the desired
current vector trajectory in polar coordinates as current
amplitude I* and angle q* commands (see Fig. 5) for
convenient compatibility with the flux weakening control
algorithm that will be described in the next section. The q*
angle command passes through to the polartorectangular
converter unaffected by the flux weakening control
algorithm at low speeds during starting.
The upper right side of Fig. 6 represents the stator current
regulator in the synchronous reference frame. There are
many different ways in which such a regulator can be
implemented. The particular implementation shown in Fig.
6 applies feedforward decoupling compensation to the
output voltage commands using the machine's steadystate
voltage relations, so that:
vqss = rs iqm + wr (Lds idm + fPM) (9)
vdss = rs idm  wr (Lqs iqm) (10)
Polar
To
Rect
Voltage
Compensation
Block
iq
m
id
+ 
+

+
+
+
+
vd
ss
vq
e*
qq *'
 I *T e
*
Modulation
Index
Calculator+

Mth
bb **
Flux Weakening
Control Module
S/A
Control
Modlule
A
S
vdc
G
G
Integrator
Maximum
Torque/Amp Functions
00 << b b ** << 1 1
id
m
iq
DD iq
DD id
vq
ss
vd
e*
vq
e'
vd
e'
f2(T)
f1(T)
vq
e* vd
e*
qq
**
ww r
M
Fig. 6 Block diagram of S/A Control Module.
Generating Operation
Development of an effective fluxweakening control
algorithm provides the key to achieving extended constant
power operation to meet the demanding generating mode
requirements of the automotive ISA application. The new
algorithm implemented in this system works by directly
reducing the angle q of the commanded current vector with
respect to the negative daxis as illustrated earlier in Fig. 4.
The control elements that implement this flux weakening
action are enclosed in the subblock labeled Flux
Weakening Control Module in the lower half of Fig. 6.
This module executes the current vector angle reduction by
multiplying the angle command q* from maximum torque
perAmp function f2 by a scaling factor b* that varies
between 0 and 1. According to this approach, only the
angle of the current vector, and not its amplitude, is directly
controlled by the flux weakening algorithm. As long as the
value of b* is 1, the Flux Weakening Control Module exerts
no influence on the current vector trajectory defined by the
maximum torqueperAmp trajectory.
The value of b* is determined by how closely the required
stator voltage approaches the saturated sixstep voltage
value at any instant in time. Since the ISA drive is
designed for high frequency PWM operation, the
instantaneous PWM modulation index, M, can be used as a
convenient and sensitive indicator of the current regulator’s
proximity to total saturation (i.e., M = 1). The modulation
index value can be conveniently calculated using the
synchronous frame dq voltage commands, vqse* and vdse*, as
follows:
M =
dc
2*e
qs
2*e
ds
V2
)v()v(
p
+
(11)
where Vdc is the measured value of the dc link voltage at the
converter input terminals.
810
Any excess value of M above a preset threshold level, Mth
(less than but close to 1) is integrated to reduce the value of
b* (0<b*<1) that scales the current vector command angle
according to q*’ = b* q* . This feedback loop achieves the
desired flux weakening action in Fig. 4 by swinging the
current vector in towards the negative daxis in order to
retain it within the prevailing voltage ellipse at that time
instant. By this action, the value of the modulation index
naturally decreases as the current vector operating point
moves away from the voltage ellipse boundary, establishing
a stable operating point loop in the flux weakening regime.
When the torque command and/or rotor speed drop
sufficiently so that flux weakening is no longer required,
the feedback loop naturally responds accordingly to reduce
its effect on the current vector angle q. That is, the value of
b* quickly integrates back to 1 when the modulation index
drops below its threshold value M*. Thus, the feedback
loop becomes passive with no effect on the vector control
operation until flux weakening action is required again.
Use of the measured value of dc link voltage Vdc in the
modulation index calculation (11) makes it possible for the
controller to adapt immediately to any changes in the bus
voltage. This feature is important in applications such as an
automotive ISA where the dc bus voltage can be expected
to change significantly depending on the bus loading and
the vehicle battery state of charge. On the other hand, this
voltage measurement can be replaced by a constant value in
applications where the dc bus voltage is known and well
regulated within a narrow range, eliminating the need for
the bus voltage sensor.
The flux weakening algorithm introduced above is naturally
bipolar, working as well for motoring operation as for
generating where it is primarily needed in the ISA
application. The resulting motoring torque envelope along
the voltage ellipse boundary for the IPM machine in this
ISA application is shown in Fig. 7. This figure confirms
that the required 150 Nm for starting will be available at
low speeds during operation on the maximum torqueper
Amp trajectory.
IV. CONTROLLER PERFORMANCE
A. Drive System Simulation
Steadystate and dynamic performance characteristics of the
complete ISA drive system including the machine, PWM
inverter and the controller have been investigated using
computer simulation. Matlab/SimulinkTM was chosen as the
preferred simulation software tool for carrying out this
analysis.
The switching behavior of the inverter is modeled using six
ideal switches. The IPM machine is modeled in the dq
synchronous