erimental results demonstrate these capability
improvements. Additional performance and functionality im-
provements of particular value for high-voltage (e.g., 42 V)
alternators are also demonstrated. Tight load-dump transient
suppression can be achieved using this new architecture. It is also
shown that the alternator system can be used to implement jump
charging (the charging of the high-voltage system battery from a
low-voltage source). Dual-output extensions of the technique (e.g.,
42/14 V) are also introduced. The new technology preserves the
simplicity and low cost of conventional alternator designs, and can
be implemented within the existing manufacturing infrastructure.
Index TermsAutomotive power generation, boost rectifier,
dual-output extensions, jump charging, load-dump transient
suppression, Lundell alternator, switched-mode rectifier.
I. I
NTRODUCTION
T
HE ELECTRICAL power requirements in automobiles
have been rising rapidly for many years and are expected
to continue to rise (Fig. 1). This trend is driven by the replace-
ment of engine-driven loads with electrically-powered versions,
and by the introduction of a wide range of new functionality
in vehicles. The continuous increase in power requirements is
pushing the limits of conventional automotive power generation
and control technology, and is motivating the development of
both higher-power and higher-voltage electrical systems and
components [1], [2].
One consequence of the dramatic rise in electrical power re-
quirements is that the inherent power limitations of the conven-
tional Lundell alternator are rapidly being approached. This is a
serious problem due to the large investment in manufacturing in-
frastructure for this type of alternator and the relatively high cost
of other machine types. The move toward dual- and high-voltage
electrical systems (e.g., 42-V systems) also poses a challenge
for future alternators. Specifically, practical implementation of
42-V electrical systems will require much tighter transient con-
trol (e.g., for load dump) than is presently achieved in the Lun-
dell alternator.
Here we introduce a new design for automotive alternators
that utilizes the conventional Lundell machine but incorpo-
Manuscript received March 6, 2002; revised November 13, 2003. Rec-
ommended by Associate Editor N. Femia. This work was supported by
the member companies of the MIT/Industry Consortium on Automotive
Electrical/Electronic Components and Systems.
The authors are with the Laboratory for Electromagnetic and Electronic
Systems, Massachusetts Institute of Technology, Cambridge, MA 02139 USA
(e-mail: djperrea@mit.edu; vahe@mit.edu).
Digital Object Identifier 10.1109/TPEL.2004.826432
Fig. 1.
Automobile electrical power requirements [1].
rates a simple switched mode rectifier along with a special
load-matching control technique. The new design allows much
higher levels of output power and efficiency to be achieved
as compared to conventional designs while retaining low cost
and simplicity of structure and control. It is ideal for supplying
anticipated new loads, such as electromechanical engine
valves, that have a large, speed-dependent power requirement.
Furthermore, the approach provides additional performance
and functionality improvements that are of central importance
for future 42-V automotive electrical systems, including tight
transient control and the ability to jump charge the 42-V battery
from a low-voltage source.
We first consider the characteristics and limitations of con-
ventional Lundell alternators. A new alternator design incor-
porating a simple switched-mode rectifier and load matching
control technique is then introduced. This leads to an experi-
mental evaluation of the new design, including power output,
losses, and efficiency. Next, the implications of the new design
for dual- and high-voltage alternators are considered, and the
transient control and jump-charging features of the new design
are validated. We conclude with a summary and overall assess-
ment of the new technology.
II. L
UNDELL
A
LTERNATOR
The Lundell, or ClawPole, alternator is a wound-field
synchronous machine in which the rotor comprises a pair of
stamped pole pieces (claw poles) secured around a cylin-
drical field winding. The field winding is driven from the stator
via a pair of slip rings. The stator is wound in a three-phase
configuration and a full-bridge diode rectifier is traditionally
used at the machine output. Alternator system output voltage
0885-8993/04$20.00 © 2004 IEEE PERREAULT AND CALISKAN: AUTOMOTIVE POWER GENERATION AND CONTROL
619
(or current) is controlled by regulating the field current. A rela-
tively long field time constant and a high armature synchronous
reactance are characteristic of this type of alternator, and tend
to dominate its electrical performance.
A. Simple Alternator Electrical Model
Fig. 2 shows a simple electrical model for a Lundell alter-
nator system. The field current
of the machine is determined
by the field current regulator which applies a pulse-width mod-
ulated voltage across the field winding. Average field current
is determined by the field winding resistance and the average
voltage applied by the regulator. Changes in field current occur
with an
/
field-winding time constant that is typically on the
order of 100 ms or more.
The armature is modeled as a Y-connected set of sinu-
soidal three-phase back emf voltages
, and
and
synchronous inductances
. The electrical frequency
of the
back emf voltages is proportional to the alternator mechanical
speed
and the number of machine poles
.
The magnitude of the back emf voltages is proportional to both
frequency and field current
(1)
A diode bridge rectifies the ac machine outputs into a constant
voltage
representing the battery and associated loads. As will
be seen, this simple model captures many of the important char-
acteristics of alternators while remaining analytically tractable.
Other effects, such as stator resistance and mutual coupling,
magnetic saturation, waveform harmonic content, etc., can also
be incorporated into the model at the expense of simplicity.
B. Alternator Electrical Behavior
To characterize alternator electrical behavior we turn to the
simple electrical model of Fig. 2. The constant-voltage load of
the rectifier makes the analysis of the system different from the
classic case of a rectifier system with a current-source (or induc-
tive) load. Nevertheless, with reasonable approximations the be-
havior of this system can be described analytically [3], [4]. For
example, alternator output power versus operating point can be
calculated as
(2)
where
is the output voltage,
is the back emf magnitude,
is the electrical frequency, and
is the armature synchronous
inductance.
Fig. 3 shows the calculated output power versus output
voltage of a conventional 14-V automotive alternator at con-
stant (full) field current, parameterized by the speed of the
alternator. As can be seen, for any given speed there is a sub-
stantial variation in output power capability with output voltage.
At each speed there is an output voltage above which the output
current (and hence output power) becomes zero. This voltage
corresponds to the peak of the line-to-line back emf voltage,
above which the diodes in Fig. 2 will not conduct. At each speed
there is also a single output voltage at which maximum output
power is achieved, and this output voltage is substantially below
the line-to-line back emf voltage magnitude. This behavior can
Fig. 2.
Simple Lundell alternator model.
be traced to the large armature synchronous inductances of
the Lundell machine. Significant voltage drops occur across
the synchronous inductances when current is drawn from the
machine, and these drops increase with increasing output cur-
rent and operating speed. Consequently, the Lundell alternator
exhibits heavy load regulation when used with a diode rectifier.
For example, in a typical automotive alternator, back voltages
in excess of 80 V may be needed to source rated current into a
14-V output at high speed. An appropriate dc-side model for
the system is a large open circuit voltage in series with a large
speed- and current-dependent output impedance. The output
power versus output voltage characteristics of Fig. 3 may
then be understood in terms of the maximum power transfer
theorem for a source with output impedance. In short, the high
armature synchronous reactance of the Lundell machine results
in a large dc-side output impedance, and necessitates the use
of large back-emf voltages to source rated current. This high
alternator output impedance results in the power deliverable by
the alternator at a given speed to be maximized only at a single
load matched output voltage.
Consider the operational characteristics of the automotive al-
ternator system described by Fig. 3. The output power versus
output voltage curves of Fig. 3 are calculated for constant (full)
field current and parameterized by the speed of the alternator,
with 1800 rpm corresponding to idle speed and 6000 rpm cor-
responding to cruising speed. At any given speed and output
voltage, output power can be reduced below the value shown by
reducing the field current, which in turn reduces the back-emf
voltage and output current. If the alternator is used at the de-
signed output voltage of 14 V, then the output power capability
across speed is represented by the vertical 14-V locus inter-
secting the curves of Fig. 3. The alternator delivers its maximum
idle speed power near the 14-V design voltage. At higher speeds
and 14-V output, the