INVITED

PAPER

Electrical Machines and Drives

for Electric, Hybrid, and Fuel
Cell Vehicles
Induction and switched-reluctance machines can provide the needed
characteristics, but permanent magnet brushless machines offer
a higher efficiency and torque density.
By Z. Q. Zhu, Senior Member IEEE, and David Howe

ABSTRACT | This paper reviews the relative merits of induction, • Wide speed range, with a constant power operating
switched reluctance, and permanent-magnet (PM) brushless range of around 3–4 times the base speed being a
machines and drives for application in electric, hybrid, and fuel good compromise between the peak torque
cell vehicles, with particular emphasis on PM brushless machines. requirement of the machine and the volt-ampere
The basic operational characteristics and design requirements, rating of the inverter.
viz. a high torque/power density, high efficiency over a wide • High efficiency over wide speed and torque ranges,
operating range, and a high maximum speed capability, as well as including low torque operation.
the latest developments, are described. Permanent-magnet • Intermittent overload capability, typically twice
brushless dc and ac machines and drives are compared in terms the rated torque for short durations.
of their constant torque and constant power capabilities, and • High reliability and robustness appropriate to the
various PM machine topologies and their performance are vehicle environment.
reviewed. Finally, methods for enhancing the PM excitation • Acceptable cost.
torque and reluctance torque components and, thereby, improv- In addition, low acoustic noise and low torque ripple
ing the torque and power capability, are described. are important design considerations. On an urban driving
cycle, a traction machine operates most frequently at light
KEYWORDS | Brushless drives; electric vehicles; electrical loads around the base speed. Therefore, in general, it
machines; hybrid vehicles; induction machines; permanent- should be designed to operate at maximum efficiency and
magnet machines; switched reluctance machines minimum acoustic noise in this region.
Typical torque/power-speed characteristics required
for traction machines are illustrated in Fig. 1. Induction
I. INTRODUCTION machines (IM), switched reluctance machines (SRMs),
Electrical machines and drives are a key enabling and permanent-magnet (PM) brushless machines (Fig. 2)
technology for electric, hybrid, and fuel cell vehicles. have all been employed in traction applications, and can be
The basic characteristics which are required of an designed to exhibit torque/power-speed characteristics
electrical machine for traction applications include the having the form shown in Fig. 3. In the constant torque
following [1]–[3]. region I, the maximum torque capability is determined by
• High torque density and power density. the current rating of the inverter, while in the constant
• High torque for starting, at low speeds and hill power region II, flux-weakening or commutation phase
climbing, and high power for high-speed cruising. advance has to be employed due to the inverter voltage and
current limits. In region III, the torque and power reduce
due to the increasing influence of the back-electromotive
Manuscript received June 10, 2006; revised November 11, 2006. force (back-EMF). However, the power capability and the
The authors are with the Department of Electronic and Electrical Engineering,
University of Sheffield, S1 3JD Sheffield, U.K. (e-mail: Z.Q.Zhu@sheffield.ac.uk;
maximum speed can be enhanced without sacrificing the
D.Howe@sheffield.ac.uk). low-speed torque capability by employing a dc–dc voltage
Digital Object Identifier: 10.1109/JPROC.2006.892482 booster [4], a technique which is employed in the Toyota

746 Proceedings of the IEEE | Vol. 95, No. 4, April 2007 0018-9219/$25.00  2007 IEEE
 Zhu and Howe: Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles

Fig. 3. Idealized torque/power-speed characteristics.

Fig. 1. Torque/power requirements for traction machines.

tions are highlighted. Optimal flux, maximum efficiency,

and low acoustic noise operation are then discussed.
hybrid system, or by employing series/parallel winding
connections, i.e., series connection at low speed and A. Basic Characteristics
parallel connection at high speed, as demonstrated in [5] IMs are robust, relatively low cost, and have well-
and [6]. In general, however, the design considerations established manufacturing techniques. Good dynamic
and control methods for the three machine technologies torque control performance can be achieved by either
are significantly different, as will be discussed in this vector control or direct torque control. For convention-
paper. Electrical machine design cannot be undertaken in al IMs, the constant power range typically extends to
isolation, but must account for the control strategy and the 2–3 times the base speed. However, for traction machines,
application requirements, both static and dynamic. Hence, this can be extended to 4–5 times the base speed, which is
a system-level design approach is essential. generally desirable [7].
This paper describes the basic operational character- The torque-speed characteristic of an IM is mainly
istics and design features associated with the foregoing characterized by the starting torque, the pull-out torque
electrical machine technologies for traction applications, and the associated speed, and the maximum speed. The
and reports the latest developments, with particular electromagnetic torque is given by
reference to PM brushless machines, for which there are
various topologies.
Rr0
mpVs2 s
T¼ 
2  (1)
II . INDUCTION MACHINES 2
fs
R0
Rs þ sr þXk2
Of the three electrical machine technologies under
consideration, induction machines are the most mature.
In this section, the basic characteristics of IMs are briefly and the maximum torque, i.e., pull-out torque, is
reviewed and specific design features for traction applica-

Vs2 2
Tmax / / s (2)
fs Xk Lk

while the starting torque and corresponding phase

current are given by

mpVs2 Rr0
Tst ¼ h 2 i (3)
2
fs Rs þ Rr0 þXk2

Fig. 2. Main traction machine technologies. (a) IMVinduction

Vs
Ist ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
 2 (4)
machine. (b) SRMVswitched reluctance machine.
Rs þ Rr0 þXk2
(c) PMMVpermanent-magnet machine.

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where Vs and fs are the supply voltage and frequency, s

is the stator flux-linkage, m is the number of phases, p is
the number of pole-pairs, s is the slip, Rs and Rr0 are the
stator winding resistance and effective rotor cage
resistance per phase, respectively, Xk ¼ Xs þ Xr0 ¼ 2
fs Lk
and Lk are the short-circuit reactance and the total stator
and effective rotor leakage inductance per phase,
respectively. The pull-out torque is independent of the
rotor resistance, approximately inversely proportional to
the total stator and rotor leakage reactance, proportional
to the square of the stator flux (or voltage), and inversely
proportional to the square of the supply frequency. The
starting torque is proportional to the square of the supply
Fig. 4. IM traction machine [10]. Rating: 120 Nm, 11.5 kW at
voltage, while the lower the stator and rotor leakage maximum speed of 7600 rpm, 26 kW at base-speed of 2020 rpm.
reactance and the lower the supply frequency, the higher
will be the starting torque.

B. Design for Traction Applications

In addition to the general requirements cited in the out torque is proportional to the square of the flux-linkage
introduction for traction machines, essential design and inversely proportional to the stator and rotor leakage
parameters for IMs include the number of poles, the inductances. In the flux-weakening region, the flux
number of stator and rotor slots, the shape of the stator and reduces with increasing frequency, the consequent reduc-
rotor slots, and the winding disposition. The design tion in the pull-out torque being exacerbated by the fall in
process usually involves three stages: the voltage across the magnetizing reactance due to the
1) making appropriate choices for the pole number influence of the leakage reactance. Therefore, to order to
and stator/rotor slot numbers; obtain a wide speed range, it is again beneficial to
2) dimensioning the machine and designing the sta- minimize the leakage reactance. In [10], for example, this
tor winding to achieve a specified power at the was achieved by:
base-speed within a specified volume envelope;
3) simulating the machine performance over the full a) increasing the width of the stator slot openings to
operating speed range. reduce the stator slot leakage flux;
With an inverter fed machine, both a high starting b) increasing the air-gap length to reduce the
torque and a low starting current can be achieved, since the harmonic leakage flux;
supply voltage and frequency are variable. Thus, compared c) employing relatively wide, open rotor slots to
with machines designed for constant supply frequency reduce the rotor slot leakage flux;
operation certain restrictions, such as the need for a d) not employing skew so as to eliminate skew leakage;
specific rotor slot shape to achieve the required starting e) employing a copper cage (Fig. 4).
torque, are removed. By appropriate choice of supply A significant improvement in the available torque at
voltage and frequency, the starting torque can be almost as maximum speed was then achieved (Fig. 5).
high as the maximum torque, while a high efficiency can be
achieved by minimum slip control [8], [9].
The stator slot number and rotor slot number, and
their shape and size should be optimized to minimize
the total leakage inductance and resistance. Generally,
this favors the use of wide and relatively shallow rotor
slots and parallel sided teeth, as opposed to deep bars or
double cages in conventional IMs. This results in a lower
rotor leakage inductance, which, in turn, improves the
power factor and increases the peak torque. In addition,
the rotor slot area is more effectively utilized. When
combined with the reduced rotor resistance, a lower
leakage reactance is also beneficial in reducing the slip
frequency at rated torque, and the variation of the slip
frequency with load.
The speed range of an IM is limited by its pull-out
torque at high speed. As will be evident from (2), the pull- Fig. 5. Torque-speed curve of IM for traction drive [10].

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Over the maximum envelope of the torque/power-

speed characteristic, encompassing both constant torque
and constant power operating modes, the copper loss
only varies slightly with speed. In contrast, initially the
iron loss increases with speed and is a maximum at the
base speed, after which it gradually reduces as the degree
of flux-weakening is increased. It is well known that
when the iron loss and copper loss are similar the
efficiency will be maximized. Therefore, an IM for
traction applications should be designed such that the
iron loss is higher than the copper loss at around the base
speed, and vice versa at low and high speeds [9]. In this
way, a high efficiency can be maintained over the entire
operating speed range by incorporating optimal flux
control, i.e., by reducing the flux level at low torque, as Fig. 6. Variation of sound pressure level with flux and load [14].
will be discussed later, since the most frequent operating
condition generally demands low torque around base
speed.

C. Optimal Flux, Maximum Efficiency, and usually has to be determined experimentally, since no
Minimum Acoustic Noise general and simple analytical method is available [11].
High-efficiency operation is a very important issue for
traction drives. The optimal flux level for maximum
efficiency varies directly with the torque and inversely III . SWITCHED RELUCTANCE
with the speed [11]. Thus, at low torque it is advantageous MACHINES (SRMs)
to reduce the flux in an optimal manner in order to
reduce the iron loss and maximize the efficiency. A. Features of SRMs
However, as the torque level is increased the flux must The design and operational features of SRMs are
be simultaneously increased until the rated flux level is well-documented [15], [16], and may be summarized
attained; otherwise, the copper loss will increase exces- as follows.
sively due to the low torque per ampere. If optimal flux • Simple, robust rotor structure, without magnets or
control is employed, a significant efficiency improvement windings, which is desirable for high-temperature
is achieved at all loads in both constant torque and environment and high-speed operation. However,
constant power modes [11]–[13]. Above base speed, in the it can have a significant rotor iron loss.
constant power mode, the flux naturally reduces since it • Potentially low cost, although relatively high man-
is inversely proportional to the speed due to the limited ufacturing tolerances are required due to the need
inverter voltage. for a small air gap.
Optimal flux control for maximum efficiency also • Modest short-duration, peak torque capability as
results in lower acoustic noise [14], which, in general, the magnetic circuit tends to be relatively highly
increases with both the load and the flux. By way of saturated.
example, Fig. 6 shows the variation of the sound pressure • Smooth operation at low rotational speeds requires
level with flux and load, for a constant stator fundamental relatively complex profiling of phase current
frequency. It will be observed that: waveforms and accurate measurement of rotor
position.
1) under the same flux level, the sound pressure level
• Unipolar operation requires nonstandard power
increases with load;
electronic modules, but SR drives have an inherent
2) at light loads, a reduction in the flux can
degree of fault tolerance.
significantly reduce the acoustic noise; however,
• Since their operating is based on the sequential
as the load is increased the noise can increase as
excitation of diametrically opposite stator coils in
the flux is reduced;
machines having the basic 6/4 and 8/4 stator/
3) the optimal flux level for the lowest noise
rotor pole number combinations, the acoustic
emissions increases with the load.
noise, vibration, and torque ripple tend to be
Since both vector control and direct torque control, relatively high.
either indirectly or directly, control the flux and torque, The high-speed operating capability of SRMs, their
optimal flux control can be readily exercised. However, the relatively wide constant power capability, and the minimal
optimal flux level for each specific torque and speed effects of temperature variations offset, to some degree,

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their relatively lower power factor. Thus, SRMs have 3) the degree of saturation in the magnetic circuit;
significant potential for use in vehicle propulsion systems 4) the allowable temperature rise.
[7], [17]–[19]. Thus, a high overload capability requires thicker stator
Typical SRMs are shown in Fig. 7, together with one and rotor back-iron and appropriate thermal management.
phase leg of the inverter. When a stator pole is aligned Above base speed in the constant power region, when the
with a rotor pole, the phase inductance is a maximum, inverter supply voltage is limited, commutation advance is
while in the unaligned position the inductance is a required. Thus, both the turn-on and turn-off angles are
minimum. When operated as a motor, the phase windings gradually advanced as the speed is increased, and the
are excited during the period when the inductance is machine eventually enters the single pulse mode of
increasing as the rotor rotates. When operated as a operation. When the machine is motoring, the peak
generator, the phases are commutated on and off during current is determined solely by the turn-on angle, while
the period when the inductance is reduced as the rotor when generating, both the turn-on and turn-off angles
rotates. The higher the ratio of the aligned inductance to influence the peak current [22]. At very high rotational
the unaligned inductance, the higher the torque/power speeds, i.e., region III of Fig. 3, further commutation
capability. In general, it requires the rotor pole arc to be advance is limited due to the influence of the back-EMF
slightly wider than that of the stator poles. Comparatively, and the winding inductance, since the phase current
SRMs have relatively few feasible stator/rotor pole waveforms become continuous. However, as will be
number combinations (6/4, 8/6, and integer multiples described later, by employing two-phase overlapping
thereof being the most common). Further, the stator excitation and continuous conduction the power capability
poles are generally parallel-sided and carry a concentrated at high rotational speeds can be enhanced. Clearly, the
coil, as illustrated in Fig. 7. However, several alternative foregoing operational characteristics of an SRM are
SRM topologies have been proposed, of which the long- appropriate for traction applications.
pitched winding SRM [20] which utilizes the variation of
the winding mutual inductances, rather than the variation C. Constant Power Operation
of the phase self-inductances, to produce torque, and the An SRM is capable of extended constant power
segmented rotor SRM [21] are arguably the most notable, operation, typically up to 3–7 times the base speed [23].
since they may produce a similar torque density to that This is usually achieved by phase advancing the excitation
of conventional SRMs. until overlap between successive phase currents occurs.
The high-speed performance of an SRM depends
B. Operational Characteristics heavily on the rotor pole design, and in general, requires
SRMs are usually operated in the discontinuous a compromise between the constant torque and constant
current mode, although continuous current operation power capabilities. For example, in [23] it was shown that
may be advantageous under certain operating conditions. when the leading dimensions of 6/4 and 8/6 SRMs were
As was shown in Fig. 3, three operational modes generally fixed, and the rotor pole arc was varied, the constant power
exist for traction drives. Thus, in the constant torque range was extended to 9 6 times base speed when the rotor
region I, the phase currents are controlled by PWM to pole arc was narrower than the stator pole arc and the
produce the desired output torque, the peak torque depth of the rotor pole was relatively large. However, the
capability depending on: machines under consideration had relatively low torque
1) the allowable maximum current from the inverter; densities. The constant power capability also depends on
2) the rate of rise of the current after a phase the number of stator and rotor poles. When the number is
winding has been commutated on; increased the constant power capability and the overload
capability are reduced, albeit the higher the torque/power
density and the higher the power factor and efficiency. By
way of example, [23] shows that a 6/4 machine exhibits a
much wider constant power range (viz. up to 7 times
base speed) than an 8/6 machine (viz. up to 4 times base
speed), which compares to a constant power operating
speed range of 2 times base speed for a 24/16 SRM [18].
Often, however, the number of stator and rotor poles is
dictated by the space envelope constraints. In summary,
not only is the ratio of the aligned to unaligned inductance
reduced as the number of stator and rotor poles is
increased, but the constant power operating speed range is
compromised due to the limited scope for phase advan-
Fig. 7. Typical SRMs and one phase leg. (a) Three-phase, 6/4 SRM. cing, and although the constant power performance could
(b) Four-phase, 8/6 SRM. (c) One-phase leg of inverter. be enhanced by reducing the number of turns per phase,

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Fig. 9. Overlap excitation techniques for extending constant power

operating range. (a) Conventional excitation at high speed with
phase advance. (b) Overlapping excitation with commutation
advanced. (c) Overlapping excitation with commutation retarded.

Fig. 8. SRM with integrated flywheel and clutch for mild-hybrid vehicle enhanced at both low and high speeds. However, the
[25]. Cranking: 45 Nm (0–300 rpm), continuous motoring: 200 Nm
(300–1000 rpm), transient motoring: 20 kW (1000–2500 rpm),
improvement in performance gradually reduces as the
continuous generating: 15 kW (600–2500 rpm), transient generating: excitation current is increased and the magnetic circuit
25 kW (800–2500 rpm). (a) Schematic. (b) Rotor/stator without becomes more highly saturated.
winding. (c) Assembled unit. Finally, a control strategy which employs freewheeling
diodes in parallel with the power switching devices in a
conventional half-H-bridge inverter together with an
appropriate zero-voltage period (Fig. 11) can also be used
to boost the power capability when an SRM is operated as a
this compromises the torque capability for a given inverter generator [22], [27].
voltage-ampere rating.
Alternatively, the extended high-speed constant power
operation can be improved with continuous phase current
excitation, by increasing the number of turns per phase.
The torque per ampere capability below base-speed is then
not significantly compromised, as has been demonstrated
for a 24/16 SRM [24] and an 18/12 SRM [25] (Fig. 8),
which shows an SRM which was developed for a mild
hybrid vehicle application.
The use of conduction overlap between two phases to
increase the torque and to reduce torque pulsations is
common practice [6]. Fig. 9 illustrates overlapping
conduction by advancing [6] or retarding [24] long-dwell
commutation [15], both also incorporating phase advance.
Bipolar excitation (Fig. 10) [6], [22], [26] can also be
employed to improve the torque density and reduce torque
pulsations, as well as to increase the efficiency. The long
flux paths that are associated with SRMs supplied from
conventional unipolar drives then become short flux
paths, and the torque and efficiency are significantly Fig. 10. (a) Conventional excitation. (b) Bipolar overlapping excitation.

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Fig. 12. Idealized back-EMF and phase current waveforms from PM

brushless machines. (a) BLDC. (b) BLAC.

Fig. 11. Freewheel diode configuration and (a) ‘‘þ1’’; (b) ‘‘0’’;
and (c) ‘‘1’’ commutation. very effective when SRMs are operated in both single
pulse mode and PWM voltage control, it is much less
effective with PWM current control, since this results in a
varying PWM switching frequency. A fixed frequency
current controller can, however, alleviate the problem.
D. Acoustic Noise, Torque Ripple, and Further, the technique is less appropriate for application
Their Reduction to SRMs which exhibit multiple resonances. The vibration
The acoustic noise which is radiated from an SRM is and acoustic noise can also be reduced [38] by employing
often cited as a major disadvantage. At low rotational two-phase overlapping excitation, which, as stated earlier,
speeds the acoustic noise is due predominantly to is beneficial for extending the constant power operating
resonances that are induced by the torque ripples, and range. In general, however, the acoustic noise emissions
may be reduced by appropriate profiling of the phase from SRMs remain a significant issue.
current waveform. The key to obtaining the optimal
current profile is an effective method for estimating the
instantaneous torque. At high rotational speeds the I V. PERMANENT- MAGNET
acoustic noise is dominated by radial vibration resonances BRUSHLESS MACHINES
[28]. The acoustic noise becomes significantly higher at
high rotational speeds and loads. However, various A. Brushless DC and AC Machines and Drives
techniques have been proposed for reducing the vibration Due to the permanent-magnet excitation, PM brushless
and acoustic noise. The most effective method is to employ machines are inherently efficient [39]–[48]. They are
a relatively thick stator yoke [29], [30] since this increases generally classified as being either sinusoidal or trapezoi-
the mechanical stiffness and, thereby, reduces the vibra- dal back-EMF machines [48] (Fig. 12). The corresponding
tional response. However, the outer diameter is then in- control strategies are usually classified as being either
creased, but this, in general, is advantageous in improving brushless DC (BLDC), or brushless AC (BLAC). In a BLDC
the overload capability since the stator yoke becomes less drive, the phase current waveforms are essentially
saturated. Reducing the supply voltage is also usually rectangular, while in a BLAC drive the phase current
helpful in reducing the acoustic noise at light load. SRMs waveforms are essentially sinusoidal. Ideally, in order to
also generate significantly lower noise when operated maximize the torque density and minimize torque
under voltage control rather than current control, due to pulsations, it is desirable to operate a machine which has
the fact that random switching of the current controller a trapezoidal back-EMF waveform in BLDC mode, and a
results in a wide-band harmonic spectra, thereby increasing machine which has a sinusoidal back-EMF waveform in
the likelihood of inducing mechanical resonances [31], BLAC mode. In practice, however, the back-EMF wave-
[32]. In [33], the relationship between the vibration of the forms may depart significantly from the ideal, and, indeed,
stator and the rate of change of the phase currents at turn- irrespective of their back-EMF waveform PM brushless
off was highlighted, while a current shaping algorithm to machines may be operated in either BLDC or BLAC mode,
limit the rate of change of current at turn-off and, thereby, although the performance, in terms of efficiency and
achieve a smoother radial force waveform was described torque ripple, for example, may be compromised. Similar
in [34] and [35]. However, the method proposed in [36] is to induction machine drives, when operating at low torque
arguably the most effective, in that it introduced a zero- an optimal flux level exists for minimum iron and copper
voltage loop between two step changes in the applied loss, and hence, maximum efficiency.
voltage, such that, together with a knowledge of the stator Fig. 13 shows a schematic of a typical PM brushless
natural frequencies, anti-phase stator vibrations were drive. In both BLDC and BLAC drives, rotor position
induced. However, it has limitations [37], since, while it is information is necessary, although the required position

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ponent of armature reaction, which reduces the effective

back-EMF by flux-weakening.
Various design features may be employed to obtain a
sinusoidal back-EMF waveform. For example, the stator
slots and/or rotor magnets may be skewed, a distributed
stator winding might be employed, or the permanent
magnets could be appropriately shaped or magnetized, etc.
However, a distributed overlapping winding [Fig. 14(b)],
results in longer end-windings, which results in a higher
copper loss and a lower torque density, while skewing of
Fig. 13. Schematic of PM brushless drive. either the stator or rotor makes manufacture more
complicated. Hence, it is often preferable to either shape
the magnets or impart a sinusoidal magnetization distri-
bution [49], which results in an essentially sinusoidal air-
gap field distribution, which is conducive to a low cogging
resolution is different. For BLDC drives, in which the
torque and also a low iron loss. The rotor back-iron in a
phase currents only have to be commutated on and off,
self-shielding, sinusoidal magnetized PM machine [49] is
low-cost Hall sensors are often employed, while for BLAC
not essential since negligible flux flows within the inner
drives, in which the phase current waveforms have to be
bore of the magnet. This is, therefore, also conducive to a
precisely controlled, a relatively high-cost resolver or
low rotor inertia, which can be an important consider-
encoder would be generally used. In addition, however,
ation. Recently, however, there has been a trend to employ
numerous sensorless techniques have recently been
a fractional ratio of slot number to pole number and a
developed or are under development for both BLDC and
concentrated stator winding for BLAC motors so as to
BLAC drives.
achieve a sinusoidal back-EMF waveform and a low
Although various rotor topologies and stator winding
cogging torque. However, when the slot number per pole
dispositions may be employed, BLDC machines predom-
is fractional, the reluctance torque is usually relatively
inantly have surface-mounted magnets on the rotor, and a
small with a concentrated stator winding. In order to
concentrated nonoverlapping, fractional-slot, stator wind-
utilize the saliency, an overlapping stator winding is
ing [Fig. 14(a)]. This results in short end-windings and,
usually required [Fig. 14(b)], as will be discussed in
therefore, a low copper loss, and the potential for a high
Section IV-D.
torque density, while a six-step inverter can be employed
Dq-axis theory can be used to analyze the electromag-
with PWM current chopping. Two-phase, 120 elec.
netic performance of a BLAC machine, and the optimal
conduction is the most common operational mode for a
relationship between the d-axis and q-axis currents in
three-phase BLDC machine, while maximum torque per
vector control and flux-weakening control strategies being
ampere in the constant torque region and extended speed
determined analytically [50], BLAC motors are relatively
operation can realized by advancing the commutation
(Fig. 15). Similar operational characteristics can be
obtained in a BLAC drive by controlling the phase currents
in such a way that they produce a demagnetizing com-

Fig. 14. Stator winding dispositions. (a) Nonoverlapping winding.

(b) Overlapping winding. Fig. 15. Torque-speed characteristics of PM brushless machines.

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Fig. 17. Comparison of torque-speed characteristics which result

Fig. 16. Comparison of torque-speed characteristics of BLAC and with two-phase, 120 elec. and three-phase, 180 elec. conduction
three-phase, 120 elec. BLDC drives. modes of operation.

relatively small and the stator windings have a low

easy to control and exhibit excellent performance, in terms inductance, since the magnet has a relative permeability
of maximum torque per ampere control and optimal which approximates to that of air, i.e., r  1, and the
extended speed operation [51]–[53]. In contrast, the effective air gap is the sum of the actual air gap length and
control strategy to realize constant power operation for a the radial thickness of the magnets. However, the magnets
BLDC drive is generally more complex. are exposed directly to the armature reaction field, and,
As was shown in [54] and [55], above the base-speed hence, are susceptible to partial irreversible demagneti-
the maximum achievable output power and torque when a zation. SPM machines are also generally to have a
machine is operated in the BLAC mode are higher than relatively limited flux-weakening capability. However,
that which can be achieved when the same machine is the flux-weakening capability, as well as the merits of PM
operated in two-phase, 120 elec. conduction BLDC mode, machines having a fractional number of slots per pole and
irrespective of whether it has a trapezoidal or sinusoidal a concentrated stator winding, will be discussed later.
back-EMF waveform (Fig. 16). At high speed, the phase Fig. 19(a-2) shows a schematic of a motor in which the
current waveform will approximate to a sinusoid even in a magnets are inset into the rotor surface. The magnet pole-
BLDC drive, due to the influence of the winding reactance, arc is, therefore, less than a full pole-pitch. However, since
while any harmonics in the back-EMF waveform will cause the q-axis inductance is now greater than the d-axis
the flux-weakening performance to deteriorate. However, inductance, a reluctance torque can be developed.
by employing a three-phase, 180 elec. conduction strategy, b) Interior Permanent-Magnet (IPM) Machines:
the high-speed power capability of a BLDC machine can be Fig. 19(b) shows examples of brushless machines in
improved, although below base-speed its torque capability
will be reduced [48], [55]–[58], as illustrated in Figs. 17
and 18.

B. Permanent-Magnet Brushless Machine Topologies

In this section, the basic topologies of PM brushless
machine, classified according to the location of the
permanent magnets, are described.

1) Radial-Field MachinesVPermanent Magnets on Rotor:

A radial-field PM brushless machine may have either an
internal rotor or an external rotor, while the PMs may be
located either on the surface or the interior of the rotor.
a) Surface-Mounted Permanent-Magnet (SPM)
Machines: This is the most widely used topology for
PM brushless machines [Fig. 19(a-1)]. However, since the
d-axis and q-axis stator winding inductances of such
machines are the same, they exhibit zero reluctance Fig. 18. Torque-speed characteristics for brushless BLAC and
torque. Further, in general, the armature reaction field is three-phase, 180 elec. BLDC operation.

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bonded ferrite or rare-earth, such a machine can ex-

hibit an extremely wide flux-weakening capability and a
high torque density, without the risk of generating an
excessive back-EMF should an inverter fault occur at high
rotational speeds. However, such a rotor structure is rel
atively complex and expensive to manufacture [61], [62].
A virtue of the rotor topology shown in Fig. 19(b-2) is
that, when the pole number is relatively high, flux focusing
can be exploited and the air-gap flux density can be
significantly higher than the magnet remanence. Hence,
low-cost, low-energy product magnets, such as sintered
ferrite, may be employed. By way of example, Fig. 21
shows a generator, which was developed for an electric
vehicle auxiliary power unit [63]. Flux-focusing enables an
air-gap flux density of 0.6 T to be achieved when sintered
ferrite magnets, having a remanence of 0.38 T are
employed. Such a machine topology also exhibits a higher
d-axis inductance since the armature reaction flux only
passes through a single magnet, rather than two magnets as
in the other machine topologies, making it very suitable for
extended constant power operation.

2) Radial-Field MachinesVPermanent Magnets on Stator:

Fig. 19. Alternative radial-field PM machine topologies with magnets When the permanent magnets are located on the stator,
on rotor. (a) Magnets on rotor surface. (b) Magnets inside the rotor. the rotor must have a salient pole geometry, similar to that
of an SR machine, which is simple and robust, and
suitable for high-speed operation. The stator carries a
nonoverlapping winding, with each tooth having a
which the magnets are accommodated within the rotor. concentrated coil. The permanent magnets can be placed
In Fig. 19(b-1) the magnets are radially magnetized, on the inner surface of the stator teeth, sandwiched in the
while in Fig. 19(b-2) they are circumferentially magne- stator teeth, or mounted in the stator back-iron.
tized. Generally speaking, however, leakage flux from the Irrespective of their location, however, the torque results
magnets is significantly greater than that in SPM predominantly from the permanent-magnet excitation
machines. However, since the magnets are buried inside torque, i.e., the reluctance torque is negligible, although
the rotor iron, the magnets are effectively shielded from the torque production mechanism relies on the rotor
the demagnetizing armature reaction field during flux- saliency. Compared with conventional permanent-magnet
weakening operation. Further, since the d-axis induc- brushless machine topologies, generally, it is easier to
tance is smaller than the q-axis inductance, a reluctance limit the temperature rise of the magnets as heat is
torque exists, while the d-axis inductance is high com- dissipated more effectively from the stator.
pared with that of an equivalent surface-mounted magnet a) Permanent Magnets in Stator Back-IronVDoubly
motor topology. Therefore, generally, such machine to- Salient PM Machine: The machine topology which is
pologies are eminently appropriate for extended speed, shown in Fig. 22(a) is referred to as a doubly salient
constant power operation in the flux-weakening mode
[48], [51]. Indeed, a variant of the topology illustrated in
Fig. 19(b-1) is employed in the Toyota hybrid vehicle [4],
Fig. 20. The V-shaped disposition of the permanent
magnets serves to increase the air gap flux and the dis-
tributed stator winding enables the reluctance torque to
be utilized.
Multiple layers of magnets may also be employed to
further increase the saliency ratio, although, in practice,
the number of layers is usually limited to  3. An extreme
case, however, is to employ an axially laminated PM rotor
in which permanent-magnet sheets are sandwiched
between the laminations [60]. In this way, a small volume Fig. 20. Open-circuit field distribution in PM BLAC machine in
of permanent-magnet material, which is generally a Toyota hybrid vehicle.

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permanent magnets is essentially invariant with the

rotor position. Therefore, the cogging torque is not sig-
nificant. However, a major disadvantage is that, due to
the unipolar flux-linkage, the torque density is relatively
poor compared to that of other PM brushless machines
[65], although, as was reported in [66], it can still be
higher than that of an induction machine.
b) Permanent Magnets on Surface of Stator TeethV-
Flux-Reversal Permanent Magnet Machine: This machine
topology is also commonly referred to as a flux-reversal PM
machine [Fig. 22(a)] [67], [68]. Each stator tooth has a pair
of magnets of different polarity mounted at its surface.
When a coil is excited, the field under one magnet is
reduced while that under the other is increased, and the
salient rotor pole rotates towards the stronger magnetic
field. The flux-linkage with each coil reverses polarity as
the rotor rotates. Thus, the phase flux-linkage variation is

Fig. 21. Generator for EV auxiliary power unit [63]. 9 kW at 4200 rpm,
sintered ferrite magnets (remanence = 0. 38 T), max. air-gap flux
density: 0.6 T. (a) Stator. (b) Rotor. (c) Flux distribution.

permanent-magnet machine. For a three-phase machine

a magnet is required in the stator back-iron for every
three teeth, while for a four-phase machine a magnet is
required for every four teeth. The variation of the flux-
linkage with each coil as the rotor rotates is unipolar,
while the back-EMF waveform tends to be trapezoidal
[64]. Thus, this topology is more suitable for BLDC
operation. However, the rotor may be skewed in order to
Fig. 22. Alternative radial-field PM machine topologies with magnets
obtain a more sinusoidal back-EMF waveform to make it on stator. (a) Magnets in stator back-ironVdoubly salient PM machine.
more appropriate for BLAC operation. Further, it will be (b) Magnets on surface of stator teethVflux-reversal PM machine.
noted that the air-gap reluctance as seen by the (c) Magnets in stator teethVflux-switching PM machine.

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bipolar, while the phase back-EMF waveform is, again, power factor [79], [80]. This impacts significantly on the
essentially trapezoidal. Such a machine topology exhibits a associated VA rating of the power electronics converter,
low winding inductance, while the magnets are more which has inhibited its application.
vulnerable to partial irreversible demagnetization. In
addition, significant induced eddy-current loss may be C. Design and Control Issues for PM Brushless
induced in the magnets, which also experience a Traction Machines
significant radial magnetic force. Further, since the air- As stated earlier, traction machines are required to
gap flux density is limited by the magnet remanence, the have a high torque density, a high overload capability, a
torque density may be compromised. wide operating speed range, and a high efficiency, while it
c) Permanent Magnets in Stator TeethVFlux-Switching is desirable that they have a degree of a high fault tolerance
PM Machine: This machine topology is also referred to as a and are low cost. In this section, design considerations
flux-switching permanent-magnet machine [Fig. 22(c)] related to the above issues are discussed. However, they
[69]–[71]. The stator consists of BU[-shaped laminated often contradict each other. For example, reduction of the
segments between which circumferentially magnetized cross-coupling magnetic saturation may also reduce the
permanent magnets are sandwiched, the direction of saliency ratio and consequently the reluctance torque;
magnetization being reversed from one magnet to the the selection of the base-speed is usually a compromise
next. Each stator tooth comprises two adjacent laminated between the constant torque performance at low speed
segments and a permanent magnet. Thus, flux-focusing may and the constant power performance at high speed.
be readily incorporated, so that low-cost ferrite magnets can
be employed [70]. In addition, in contrast to conventional 1) Torque Density and Overload Capability: The general
PM brushless machines, the influence of the armature torque equation for a PM brushless machine, which has
reaction field on the working point of the magnets is both excitation torque and reluctance torque components,
minimal. As a consequence, the electric loading of flux- is given by
switching PM machines can be very high. Therefore, since
the phase flux-linkage waveform is bipolar, the torque
3  
capability is significantly higher than that of a doubly salient T¼ p m Iq  ðLq  Ld ÞId Iq (5)
PM machine [65]. The back-EMF waveform of flux- 2
switching PM machines is essentially sinusoidal, which
makes them more appropriate for BLAC operation. In where p is the number of pole-pairs, m is the stator
addition, since a high per-unit winding inductance can winding flux-linkage due to the permanent magnets, and
readily be achieved, such machines are eminently suitable Ld , Lq and Id , Iq are the d- and q-axis inductances and
for constant power operation over a wide speed range. currents, respectively. In order to maximize the torque
density, it is desirable to increase m by reducing the
3) Other PM Brushless Machine Topologies leakage flux. This can be achieved by introducing airspace
a) Axial-Field Machines: Axial-field PM machines have flux barriers or interpole magnets, as illustrated in Fig. 23.
an axially directed air-gap flux [72], [73] and can comprise a m can also be increased by utilizing flux focusing
single-sided stator and a single rotor, a double-sided stator [4], [63], as illustrated in Fig. 24. The torque density can
and a single rotor, or a single stator and a double-sided rotor. also be enhanced by increasing the saliency ratio [3], [81],
In each case, a large axial force exists between the stator and as illustrated in Fig. 25. Further, since the short-duration
the rotor. As with conventional radial-field PM brushless torque capability is determined primarily by the demag-
machines, the stator can be slotted or slotless, although it is netization withstand capability of the magnets and the
more difficult to manufacture a slotted stator for axial-field level of magnetic saturation, reducing the d- and q-axis
machines. Thus, slotless designs are more common. cross-coupling magnetic saturation by incorporating air
However, while this eliminates cogging, it exposes the flux barriers, as illustrated in Fig. 26, can enhance the
winding air-gap flux. Hence, a multistranded conductor or overload capability.
Litz wire may be required to minimize the eddy-current
loss. Further, since the effective air gap is large, the winding 2) Flux-Weakening Capability: It is well known [62], [82]
inductance is generally relatively small, which may limit the that the maximum flux-weakening capability, defined as
constant power speed range. the ratio of the maximum speed to the base-speed, under
b) Transverse-Flux Machine: Generally, transverse supply inverter voltage and current limitations, can be
flux machines have a relatively large number of poles, all achieved when a PM brushless machine is designed to have
of which interact with the total ampere-conductors of 1.0 per-unit d-axis inductance such that
each phase. This enables very high electric loadings and,
hence, high torque densities to be achieved [74]–[78].
m Ld Ir
However, they have a significant leakage flux and a Ld ¼ or ¼1 (6)
relatively high winding inductance, as well as a poor Ir m

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Fig. 24. Flux focusing by appropriate disposition of magnets.

ratio is 1.0. However, the higher the flux-linkage m to

achieve a high low-speed torque capability, the more
difficult it is to realize wide-speed operation (Fig. 27) [83].
In [62], it was shown that it was possible to
design any PM brushless machine to achieve Binfinite[
flux-weakening capability. Clearly, however, if the rated
current is high (e.g., the machine is liquid cooled), it is

Fig. 23. Reduction of leakage flux by introducing airspace flux

barriers or interpole magnets.

where m is the stator flux-linkage due to the magnets, Ld

is the d-axis inductance, and Ir is the rated current.
Although it is possible to design a PM brushless
machine which satisfies the foregoing requirement, gene-
rally, for most PM machines Ld Ir = m G 1, since the d-axis
inductance is relatively low as a consequence of the
recoil permeability of the magnets being approximately
equal to 1.0. Nevertheless, the higher the ratio of
Ld Ir = m the higher will be the flux-weakening capability Fig. 25. Improvement of saliency ratio. (a) Lower reluctance q-axis
[Fig. 27(a)], which, theoretically, is Binfinite[ when the armature reaction flux path. (b) Multilayer and (c) Axially laminated.

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 Zhu and Howe: Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles

theless, it has been shown [86] that, by careful design,

the magnet working point in an SPM machine can
remain reasonably high up the magnet demagnetization
characteristic, even when the machine has Binfinite[
flux-weakening capability, due to the fact that 1.0 per-
unit d-axis inductance results primarily from stator slot
leakage and end leakage fluxes.

4) Rotor Eddy-Current Loss: PM BLAC and BLDC

machines are usually considered to have negligible rotor
loss. However, the rotor loss may be important in
machines equipped with surface-mounted magnets, in
terms of the resulting temperature rise. Eddy currents may
be induced in the permanent magnets, the rotor back-iron,
and any conducting sleeve which may be employed to
retain the magnets, by time and space harmonics in the air-
gap field. More specifically, they result from [87]: a) stator
slotting; b) stator MMF harmonics which do not rotate in
synchronism with the rotor; and c) nonsinusoidal phase

Fig. 26. Reduction of d- and q-axis cross-coupling magnetic saturation.

(a) SPM. (b) IPM.

much easier to satisfy (6), even for surface-mounted

magnet machines, which have a high m and a relatively
low Ld . For example, in [84] Binfinite[ flux-weakening
capability was achieved with an SPM machine equipped
with a self-shielding, sinusoidal magnetized rotor having no
back-iron, and in [85] with an SPM machine in which only
alternate stator teeth carried a coil. However, in general, it
is much easier to achieve a wide operating speed range with
machines equipped with an interior permanent magnet
rotor, since generally m is lower, while Ld is higher.

3) Demagnetization Withstand Capability: Operation in

the flux-weakening mode is a necessary requirement for
traction applications, while NdFeB is the most commonly
employed permanent-magnet material for PM brushless
machines. However, the magnets are required to have an
adequate demagnetization withstand capability at the
maximum operating temperature, when they are most
vulnerable to partial irreversible demagnetization. In
addition to effective thermal management, one means of
enhancing the demagnetization withstand capability is to
provide a low reluctance path for the demagnetizing d-axis
armature reaction flux such that it does not pass through
the magnets. One example of achieving this is to employ
narrower stator slot openings and thick tooth-tips, as
illustrated in Fig. 28(a), or thick rotor slot bridges in an
IPM machine, as illustrated in Fig. 28(b). However, such
features will also have an influence on m and Ld . In Fig. 27. Variation of torque and power capability with machine
general, however, it is easier to realize a high demagne- design parameters, when Ld Ir G m . (a) Variation with Ld Ir = m,
tization withstand capability for IPM machines. Never- when m and Lq =Ld are constant.

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Fig. 28. Improvement of demagnetization withstand capability by

introducing d-axis armature reaction demagnetization flux path.
(a) SPM stator design. (b) IPM rotor design.

5) Stator Iron Loss: Due to the fixed PM excitation, the

no-load iron loss increases with the rotational speed,
while the full-load iron loss in the constant torque
Fig. 27. (Continued) Variation of torque and power capability with
operating range is generally around 20%–50% higher.
machine design parameters, when Ld Ir G m. (b) Variation with m,
when Lq and Ld are constant. However, the iron loss which results on load in the flux-
weakening mode depends on the machine topology, as
illustrated in Fig. 29. In general, SPM machines have the
lowest full-load iron loss, and despite the increase in
fundamental frequency it usually becomes much lower
current waveforms, which result from six-step commuta- than the no-load iron loss as the degree of flux-weakening
tion and PWM.
In general, however, the rotor eddy-current loss is
relatively small compared with the stator copper and iron
losses. Nevertheless, it may cause significant heating of the
magnets, due to the relatively poor heat dissipation from
the rotor. In turn, this may result in partial irreversible
demagnetization, particularly of sintered NdFeB magnets,
which have relatively high temperature coefficients of
remanence and coercivity and a moderately high electrical
conductivity. It is particularly important to consider the
rotor eddy-current loss in a) machines with a high
fundamental frequency, e.g. high-speed and/or high-pole
number; b) machines with large stator slot openings, e.g.,
transverse flux machines; c) high power density brushless
dc machines, e.g., force-cooled traction machines with a
high electric loading; and d) machines whose windings
span a fractional pole-pitch and which have nearly equal
pole and slot numbers [88]. If the eddy-current loss is
unacceptable, the magnets may be segmented, axially and/ Fig. 29. Variation of iron loss in SPM and IPM machines when their
or circumferentially [89]. open-circuit stator iron loss are designed to be the same.

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is increased [90]. IPM machines generally have a sig-

nificantly higher full-load iron loss, which may be com-
parable to or higher than the no-load iron loss, since the
armature reaction field has a higher harmonic content due
to the small effective air gap [90], [91]. However, when
the magnets are simply inset into the rotor surface the
harmonic content in the armature reaction field increases
further, and generally results in the highest full-load iron
loss [86].

6) Fault-Tolerance: An important consideration when

operating in the extended speed, flux-weakening mode is
the consequence of an inverter fault which results in the
loss of the demagnetizing armature reaction field and an
excessively high back-EMF [92], [93]. In this regard, IPM
machines may be advantageous, since, for a given output
torque, the PM excitation torque, and, hence, the volume
of magnet material and the maximum back-EMF are lesser.
However, the consequences of an inverter fault occurring
when a PM brushless machine is operating in the flux-
weakening region remains a challenging issue.

D. Recent Developments
Fig. 30. Three-phase, 12-slot, 10-pole, fractional slot PM machines
1) Fractional Slot Machines: SPM brushless machines [97]. (a) All teeth wound. (b) Alternate teeth wound.
which have a fractional number of slots per pole and a
concentrated winding have been the subject of recent
research. They have an inherently low cogging torque,
short end-windings and, hence, a low copper loss, a high skew. However, the reluctance torque component is neg-
efficiency, and a high power density, as well as excellent ligible even when an IPM rotor is employed.
flux-weakening performance [85], [94]–[100]. The stator
coils may be wound either on all the teeth or only on 2) Hybrid PM and Current Excitation: Since the PM ex-
alternate teeth (Fig. 30) [95], [97]. In the latter case, the citation is fixed in a PM brushless machine, the current
phase windings are effectively isolated, both magnetically phase angle has to be progressively advanced as the speed
and physically, and a high per-unit self-inductance can is increased above base-speed so that a demagnetizing
readily be achieved to limit the prospective short-circuit d-axis current component is produced which reduces the
current, by utilizing the relatively high air-gap inductance flux-linkage m with the stator winding. Ultimately,
and the leakage flux at the slot openings. Due to the however, this may cause partial irreversible demagnetiza-
physical separation of the coils and the negligible mutual tion of the magnets. At the same time, due to the inverter
inductance between phases, the possibility of a phase-to- voltage and current limits, the torque-producing q-axis
phase fault is minimized. Therefore, the fault tolerance current component has to be reduced correspondingly.
and flux-weakening capability of such machines can be Consequently, the torque and power capability are limited.
significantly higher than for more conventional machine Thus, a compromise has to be made between the low-speed
designs. Fig. 31 shows a three-phase, 24-slot, 22-pole, PM torque capability and high-speed power capability.
BLAC machine which was developed for a supercapacitor- Hybrid permanent magnet and field current excitation
based electrical torque boost system for vehicles equipped has been shown to be beneficial in improving the power
with down-sized IC engines [99]. However, since the capability in the extended speed range, enhancing the low-
torque is developed by the interaction of a stator space- speed torque capability, and improving the overall
harmonic MMF with the permanent magnets, a relatively operational efficiency. There are several ways of realizing
high rotor eddy-current loss can result from the funda- such hybrid excitation. For example, dc winding may be
mental and low-order space-harmonic MMFs which rotate located on the rotor [101] or the stator [102]–[107], which
relative to the rotor [88], [89]. As stated earlier, however, is preferable since it does not require slip-rings. The
the magnets can be segmented to reduce the eddy-current magnetic circuit associated with the dc excitation may be
loss. A further advantage of such machines is that, due to either in series or in parallel with the magnetic circuit
the fractional number of slots per pole, the cogging torque associated with the PM excitation. However, although
is very small without employing design features such as series excitation is simple it requires a higher excitation

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Zhu and Howe: Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles

machines equipped with hybrid excitation, based on

doubly salient pole [102], consequent-pole [103]–[105],
and claw-pole [106], [107] machine topologies. The dc
excitation winding enables the air-gap flux, and, hence, the
torque capability, to be enhanced at low speed, to be re-
duced at high speed to facilitate extended speed operation,
and to be optimized over the entire speed range to improve
the efficiency. It also reduces the likelihood of an
excessively high back-EMF being induced at high speed
in the event of an inverter fault.

Fig. 31. Three-phase, 24-slot, 22-pole, PM BLAC machine with modular

stator winding and IPM rotor [99]. Rated output power ¼ 18.5 kW,
rated speed ¼ 1700 rpm, rated torque ¼ 105 Nm. (a) Cross section of
three-phase, 24-slot, 22-pole IPM BLAC machine. (b) Machine test rig.

MMF due to the low recoil permeability of the magnets.

Fig. 32. Hybrid excited PM machines. (a) Hybrid excitation based on
On the other hand, parallel excitation is more effective doubly salient pole structure [102]. (b) Hybrid excitation based on
electromagnetically but leads to a more complex machine consequent pole structure [105]. (c) Hybrid excitation based on
structure. Fig. 32 shows three examples of PM brushless claw-pole structure [106].

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V. CONCLUSION nologies can meet the performance requirements of

The operational characteristics, design features, and traction drives, and each machine technology has merits.
control requirements for induction machines, switched h
reluctance machines, and permanent-magnet brushless
machines for vehicle propulsion systems have been
reviewed, with emphasis on their low-speed torque and Acknowledgment
high-speed power capability. Given that they offer a higher The authors acknowledge the contributions of collea-
efficiency and torque density, particular emphasis has gues in the Electrical Machines and Drives Group,
been given to permanent-magnet brushless machines. University of Sheffield, Sheffield, U.K., and also the
Various PM brushless machine topologies have been support of industrial organizations, in particular, IMRA
highlighted, and their relative merits have been briefly U.K. Research Centre, Centro Ricerche Fiat, Volvo Tech-
described. In general, however, all three machine tech- nology Corporation, and FEV Motorentechnik GmbH.

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ABOUT THE AUTHORS

Z. Q. Zhu (Senior Member, IEEE) received the David Howe received the B.Tech and M.Sc.
B.Eng. and M.Sc. degrees from Zhejiang University, degrees from the University of Bradford, Bradford,
Hangzhou, China, in 1982 and 1984, respectively, U.K., in 1966 and 1967, respectively, and the Ph.D.
and the Ph.D. degree from the University of degree from the University of Southampton,
Sheffield, Sheffield, U.K., in 1991, all in electrical Southampton, U.K., in 1974, all in electrical power
and electronic engineering. engineering.
From 1984 to 1988, he lectured in the He has held academic posts at Brunel and
Department of Electrical Engineering, Zhejiang Southampton universities, and spent a period
University. In 1988, he joined the University of in industry with NEI Parsons Ltd. working on
Sheffield, initially as a Research Associate. He was electromagnetic problems related to turbo-
subsequently appointed to an established post as a Senior Research generators. He is currently Professor of electrical engineering at the
Officer/Senior Research Scientist. Since 2000, he has been Professor of University of Sheffield, where he heads the Electrical Machines and
electrical machines and control systems. His current major research Drives Research Group. His research activities span all facets of con-
interests include the application, control, and design of permanent- trolled electrical drive systems, with particular emphasis on permanent-
magnet machines and drives. magnet excited machines.
Prof. Howe is a Fellow of the Royal Academy of Engineering and a
Fellow of the IEE, U.K.

Vol. 95, No. 4, April 2007 | Proceedings of the IEEE 765