2558 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007
A Transient Cosimulation Approach to Performance Analysis of Hybrid
Excited Doubly Salient Machine Considering Indirect
Field-Circuit Coupling
Xiaoyong Zhu
1
, Ming Cheng
1
, Wenxiang Zhao
1
, Chunhua Liu
2
, and K. T. Chau
2
School of Electrical Engineering, Southeast University, Nanjing 210096, China
Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China
This paper presents a hybrid excited doubly salient (HEDS) machine, which can be used as the integrated starter–generator (ISG)
for modern automobile and hybrid electric vehicles (HEVs). The key of the proposed machine is to incorporate both direct current (dc)
field windings and permanent magnets (PMs) in the stator, hence, offering a compact arrangement of hybrid field excitations, while the
rotor is simply composed of salient poles without windings or PMs. The air-gap flux can be strengthened or weakened with a reasonable
dc field current so that the electromagnetic torque and induced electromotive force (EMF) can be effectively regulated. To predict the
electromagnetic performances of the machine more accurately, a new transient cosimulation approach considering indirect field-circuit
coupling is proposed. Experimental results on a prototype machine have been given to verify the technique. The method is not only more
efficient and flexible, but also more accurate and stable, which can also be used in analyzing other electric machine and drive systems.
Index Terms—Cosimulation, doubly salient machine, finite element analysis (FEA), hybrid excited machine, integrated starter–
generator (ISG).
I. INTRODUCTION
I
NSTEAD of using separate machines for cold cranking and
battery charging, the concept of integrated starter–generator
(ISG) is becoming attractive for modern automobiles and hy-
brid electric vehicles (HEVs). Because of the requirements of
very high starting torque (up to four times the rated torque) for
cold cranking, as well as constant output voltage over a very
wide speed range (25% to four times the base speed) for battery
charging, the design of this ISG is challenging [1]. Recently,
a permanent magnet (PM) synchronous machine has been uti-
lized for ISG application, since it inherently offers high torque
density and high efficiency. However, because of uncontrol-
lable PM flux, it cannot maintain high efficiency or constant
output voltage over the wide speed range. To enable flux con-
trol, the idea of stator-doubly-fed doubly-salient (SDFDS) ma-
chine has been proposed [2], in which a dc field winding re-
places the PMs to facilitate flux control and online efficiency op-
timization. However, this topology inevitably needs high-field
winding magnetomotive force (MMF) to realize the desired flux
linkage, hence degrading the machine efficiency.
In this paper, a hybrid excited doubly salient (HEDS) machine
is proposed for automotive engines. The machine topology and
flux control principle are introduced in Section II. In order to
predict the electromagnetic performances of the HEDS machine
more accurately, a new transient cosimulation approach is pro-
posed, in which a transient 2-D finite element analysis (FEA)
is coupled with circuit simulation. The principle and coupling
mechanism are introduced in details in Section III. By using the
new cosimulation technique, the corresponding electromagnetic
performances are predicted in Section IV. Moreover, a 12/8-
pole HEDS machine has been designed and built for evalua-
tion. Experimental results of the prototype machine are given to
Digital Object Identifier 10.1109/TMAG.2007.893318
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Fig. 1. Cross-section of the proposed HEDS machine.
verify the proposed new cosimulation approach. Finally, some
conclusions are drawn.
II. M
ACHINE DESCRIPTION AND FLUX CONTROL PRINCIPLE
Fig. 1 shows the structure of a three-phase 12/8-pole HEDS
machine. There are four pieces of PMs in the stator. The
machine consists of two types of stator windings: a three-phase
concentrated armature winding and a dc field winding. The
function of the armature winding is the same as that for an
SDFDS machine, whereas the dc field winding works as a tool
for flux control and efficiency optimization.
III. C
OUPLING PRINCIPLE OF THE COSIMULATION METHOD
In this paper, the proposed new cosimulation of indirect cou-
pling method provides the possibility of system level simulation,
where integrating the transient magnetic solver of Maxwell2D
®
into the system circuit simulator, Simplorer
®
.
Fig. 2 illustrates the general flow-chart of the cosimulation
method. On the FEA side, at each time step, the solutions of
the HEDS machine model are extracted from Maxwell2D at
first, and then the coupling inductance matrix and induced volt-
ages together with the winding currents are sent to the circuit
0018-9464/$25.00 © 2007 IEEE
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ZHU et al.: TRANSIENT COSIMULATION APPROACH TO PERFORMANCE ANALYSIS OF HEDS MACHINE 2559
Fig. 2. General flow-chart of the cosimulation method.
Fig. 3. Integrated finite element/circuit model.
simulator of Simplorer. After FEA at each time step, the FEA
system’s coefficient matrix is frozen. On the circuit simulation
side, these coupling coefficients are used to perform circuit sim-
ulation in terms of a smaller circuit time step, hence extracting
the Norton equivalent conductance matrix and the source cur-
rents flowing into the windings through coupling nodes from
the circuit simulator of Simplorer. Maxwell2D converts these
Norton circuit coupling parameters to a loop matrix and solves
the finite element equations for the next Simplorer time step. A
reasonable time step
is very important to keep the accuracy
of the cosimulation method. When the
is set to 1 s in the
HEDS machine system, the cosimulation can be to preserve the
required convergence.
The coupled-field analysis [3] is becoming attractive since
it can study the interaction between different fields, including
electromagnetics, thermal, and mechanics. As this coupled-field
analysis is essentially based on FEA, it is possible to incorpo-
rate it into the proposed cosimulation approach so that the per-
formance analysis will become more comprehensive.
IV. P
ERFORMANCE ANALYSIS AND VERIFICATION
The newly designed and built HEDS machine is analyzed by
using the transient cosimulation method. For comparing the ac-
curacy of the new method and traditional methods, results of
both the equivalent magnetic circuit model and a common static
FEA are also given.
Fig. 3 shows the proposed 12/8-pole HEDS machine, which
is cosimulated by Maxwell2D with Simplorer. From the figure,
the HEDS machine model created by Maxwell2D is effectively
Fig. 4. Predicted back-EMF waveforms and the measured one.
Fig. 5. Radial air-gap flux density distributions with different field excitations.
Fig. 6. Flux control capability at different dc field ampere turns.
linked and integrated in the Simplorer project. It should be noted
that the electrical pins of the machine, including three-phase ar-
mature windings and dc field winding, are available, which are
very conveniently connected with external circuit components.
On the mechanical side, a velocity source or load can be con-
nected to the machine directly, where inertia and/or damping
can be considered in a simple way.
By using the cosimulation method, when a velocity source is
given to the machine, it works as a generator, and then the tran-
sient back-electromotive force (back-EMF) of no-load at dif-
ferent speeds can be obtained through a voltage meter easily.
Fig. 4 shows the cosimulated and measured back-EMFs at the
speed of 1000 r/min. For comparison, the result of common
FEA is also shown in the figure. In general, the two compu-
tational methods agree with the measurement. Because the end
resistance and end leakage inductances have been considered in
the external circuit of the cosimulation model, the cosimulation
method takes advantages of higher accuracy.
To assess the level of the flux control capability, a flux control
coefficient
can be defined as follows:
% (1)
where
is the flux density of the air-gap without dc field cur-
rent, and
is the flux density at different dc field currents.
Fig. 5 shows the flux density distributions under different dc
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2560 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007
Fig. 7. Phase current and voltage at 1000 r/min. (a) Simulated. (b) Measured.
TABLE I
T
HE DC OUTPUT VOLTAGE
U
AND DC FIELD CURRENT
I
VERSUS SPEED
field excitations. Fig. 6 illustrates the flux control capability
versus dc field MMF.
Because the back-EMF of phase winding is proportional to
the flux density (or flux), hence, (1) is equivalent to
% (2)
where
is the root-mean-square (rms) value of the output
voltage, and
is the one with no dc field current. To vali-
date experimentally the cosimulated flux control capability, the
back-EMFs at different dc field currents at 1000 r/min are mea-
sured and the flux control capability is obtained by using (2),
which is plotted in Fig. 6. Moreover, for comparison, the flux
control capability from the equivalent magnetic circuit analysis
is also shown in Fig. 6 [4]. It can bee seen that the cosimulated
result is much closer with the measurement than the magnetic
circuit analysis results due to the fact that the eddy current ef-
fects, saturation, motion effects, and losses are considered in the
cosimulation.
By using the integrated finite element/circuit model shown in
Fig. 3, the HEDS machine is cosimulated as a generator over
a wide speed range. The voltage of the capacitance can be con-
trolled as a constant at the rated conditions by a proportional and
integral (PI) regulation. By online-tuning the dc field current, the
dc output voltage can be maintained constant over a wide range
of rotor speeds. Fig. 7 shows the waveforms of the simulated
and measured phase current and voltage at 1000 r/min. It can
be found that the waveforms using cosimulated method closely
match the measured ones. Table I lists the dc output voltage and
dc field currents at different speeds. Both cosimulation and mea-
surement show that the proposed machine can output a constant
voltage over a wide speed range with the use of flux control.
Fig. 8. Efficiency comparison between HEDS and SDFDS machines.
Based on the cosimulation method, the efficiency of both the
HEDS machine and SDFDS machines over the whole speed re-
gion is obtained as shown in Fig. 8. In the low-speed and rated
region, the HEDS machine inevitably needs less field winding
MMF to realize the desired flux linkage, resulting in higher ef-
ficiency than the SDFDS machine. In high-speed region, how-
ever, the HEDS machine needs relatively higher field winding
MMF to realize flux weakening, resulting in lower efficiency
than the SDFDS machine.
V. C
ONCLUSION
In this paper, a HEDS machine for automotive engines has
been analyzed. Because of its distinct capability of flux con-
trol, the proposed machine can realize a very wide range (four
times) of flux regulation, hence, achieving a high starting torque
for cold cranking and maintaining a constant output voltage for
battery charging over a wide speed range. These two features are
highly desirable for the machine to work as an ISG for modern
automobiles and HEVs.
The newly proposed cosimulation approach allows FEA
and the circuit simulator to work simultaneously, and provides
a tight integration and seamless data exchange capability in
system level. Experimental results on the prototype machine
have been given to verify the technique. The method is not only
more efficient and flexible, but also more accurate and stable. It
can readily be extended to other machine and drive systems.
A
CKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (NSFC) under Grants 50337030 and
50377004.
R
EFERENCES
[1] C. Williams, “Comparison and review of electric machines for in-
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[3] A. Monnier, B. Froidurot, C. Jarrige, R. Meyer, and P. Teste, “Ame-
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[4] X. Zhu and M. Cheng, “Design and analysis of a new hybrid excited
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Manuscript received October 30, 2006; revised February 5, 2007 (e-mail:
mcheng@seu.edu.cn).
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