Power electronic handbook

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25 Automotive Applications of Power Electronics 643

12 V

Ultrasonic

Motor

FIGURE 25.7 Drive circuit for an ultrasonic motor.

Solenoid

Coil

Solenoid

Coil

(a) (b)

FIGURE 25.8 Power electronic circuits for driving solenoids.

by electromechanically driven engine valves [25]. The opening

and closing of the intake and exhaust valves can be controlled

to achieve optimum engine performance and improved fuel

economy over a wide range of conditions determined by vari-

ables such as the speed, load, altitude, and temperature. The

present cam system provides a valve proﬁle that can give opti-

mum engine performance and improved fuel economy only

under certain conditions.

Two power electronic circuits suitable for driving the

solenoids for valve actuation are shown in Fig. 25.8. The

circuit of Fig. 25.8a is suitable for solenoids that require uni-

directional currents through their coils, while the circuit of

Fig. 25.8b is suitable for solenoids that require bidirectional

currents through their coils.

25.4.5 Electric Air Conditioner

It is desirable to replace some of the engine-driven functions

of a vehicle with electrically driven counterparts. The beneﬁts

of driving these functions electrically include the elimina-

tion of belts and pulleys, improved design and control due

to independence from engine speed, and resulting increased

efﬁciency and improved fuel economy. Furthermore, there is

the opportunity for operation of the function in the engine-off

condition.

The air conditioner is an example of an engine-driven func-

tion that could beneﬁt from electriﬁcation. The engine drives

the compressor of the air conditioner. Consequently, the speed

of the compressor varies over a wide range and the compres-

sor has to be over-sized to provide the desired performance at

engine idle. Also, since the compressor speed is dependent on

the engine speed, excessive cooling occurs at highway speeds

requiring the cool air to be blended with the hot air to keep

the temperature at the desired level. Furthermore, shaft seals

and rubber hoses can lead to the loss of refrigerant (CFC) and

pose an environmental challenge.

In an electric air conditioner, an electric motor is used to

drive the compressor [26]. The motor is usually a three-phase

brushless dc motor driven by a three-phase MOSFET bridge.

The speed of the compressor in an electric air conditioner is

independent of the engine speed. As a result, the compres-

sor does not have to be over-sized and excessive cooling does

not occur. Also, shaft seals and hoses can be replaced with a

hermetically sealed system. Another beneﬁt of an electric air

conditioner is the ﬂexibility in its location, since it does not

have to be driven by the engine.

25.4.6 Electric and Electrohydraulic Power

Steering Systems

The hydraulic power steering system of a vehicle is another

example of an engine-driven accessory. This system can be

replaced with an electric power steering (EPS) system in which

a brushless dc motor is used to provide the steering power

assist [27]. The electric power steering system is more efﬁ-

cient than the hydraulic power steering system because, unlike

the engine-driven hydraulic steering pump, which is driven by

the engine all the time, the motor operates only on demand.

Another system that can replace the hydraulic power steering

system is the electrohydraulic power steering (EHPS) system.

In this case, a brushless dc motor and inverter can be employed

to drive the hydraulic steering pump. The ability of the EPHS

system to drive the pump only on demand leads to energy

savings of as much as 80% as compared with the conven-

tional hydraulic system. Challenges in implementing EPS and

EPHS systems include meeting the required levels of cost and

reliability for this critical vehicle subsystem.

25.4.7 Motor Speed Control

Some of the motors used in a vehicle require variable speed

control. Consider, as an example, the blower motor used to

provide air ﬂow to the passenger compartment. This motor is

typically a permanent magnet dc motor with a squirrel-cage

fan. The speed of the motor is usually controlled by varying

the resistance connected in series with the motor winding.

This method of speed control leads to a signiﬁcant power

loss. A low-loss method of speed control employs semicon-

ductor devices as shown in Fig. 25.9. In this case, the speed

of the motor is controlled via PWM – that is, by switching

the MOSFET on and off with different duty-ratios for differ-

ent speed settings. An input ﬁlter is needed to reduce the EMI

generated by the switching of the MOSFET. This method of

speed control is equivalent to supplying power to the motor

644 D. J. Perreault et al.

Motor

FIGURE 25.9 Low-loss circuit to control the speed of a motor.

through a variable-output dc-to-dc converter. The converter

is located close to the motor and no ﬁlter is required between

the converter output and motor winding.

Another low-loss method that can be used to control the

speed of a motor employs a three-phase brushless dc motor.

The speed in this case is controlled by controlling the MOS-

FETs in the dc-to-three-phase-ac converter that drives the

motor.

25.5 Multiplexed Load Control

Another emerging application of power electronics in auto-

mobiles is in the area of load control. In the conventional

point-to-point wiring architecture, most of the loads are con-

trolled directly by the primary mechanical switches, as shown

in Fig. 25.1. In a point-to-point wiring architecture, each load

has a dedicated wire connecting it to the fuse box via the

primary switch. Consequently, fairly heavy wires have to be

routed all over the vehicle, as illustrated in Fig. 25.10a. The

situation is made worse when multiple switches control the

same load, as is the case with power windows and power

door locks. The complete harness of a 1994 C-class Mercedes-

Benz that uses point-to-point wiring has about 1000 wires,

with a total length of 2 km, over 300 connectors and weighs

36 kg. The process of assembling the wiring harness is difﬁcult

and time consuming, leading to high labor costs. Retroﬁtting,

fault tracing, and repairing are time consuming and expen-

sive. The bulky harness also places constraints on the vehicle

body design, and the large number of connectors compromise

system reliability.

An alternative wiring technique is to control the loads

remotely and multiplex the control signals over a communi-

cation bus, as shown in Fig. 25.10b and c. A control message is

sent on the communication bus to switch a particular load on

or off. This allows more ﬂexibility in the layout of the power

cables and could allow the pre-assembly of the harness to be

more automated. Furthermore, with communication between

the remote switches, it is practical to have a power manage-

ment system than can turn off non-essential loads when there

is a power shortage. One possibility is to group the remote

switches into strategically located distribution boxes, as shown

in Fig. 25.10b. A power and a communication bus connect

the distribution boxes. Another possibility is to integrate the

remote switches with the load, i.e. point-of-load switching,

as shown in Fig. 25.10c. In Fig. 25.10b the transceivers are

also built into the distribution boxes, while in Fig. 25.10c each

load and primary switch has an integrated transceiver. The

point-of-load switching topology is attractive because of its

simplicity, but raises cost and fusing challenges.

Multiplexed remote switching architectures have been

under consideration since at least the early 1970s, when

Ziomek investigated their application to various electrical sub-

systems [28]. The initial interest was dampened by cost and

reliability concerns and the non-availability of appropriate

remote switches. However, advances in semiconductor tech-

nology and rapid growth in the automotive electrical system

revived interest in multiplexed architectures. The SAE Mul-

tiplexing Standards Committee has partitioned automotive

communications into three classes: Class A for low data-rate

(1–10 kbit/s) communication for the control of body func-

tions, such as headlamps, windshield wipers, and power win-

dows, Class B for medium data-rate (10–100 kbit/s) parametric

data exchange, and Class C for high data-rate (1 Mbit/s) real-

time communication between safety critical functions, such

as between ABS sensors and brake actuators [29]. Although

load control is categorized as Class A, lack of any widely

accepted Class A communication protocols has lead to the

application of Class B and Class C communication IC’s to

load control. Class B has received the most attention due to

the California Air Resources Board mandated requirement for

on-board diagnostics (OBD II) and a large number of com-

peting protocols, including the French vehicle-area network

(VAN), the ISO 9141 and the SAE J1850, have been devel-

oped [30]. Of these, the SAE J1850 is the most popular in

the US. Another popular protocol is the controller area net-

work (CAN) developed by Bosch [31]. Although designed for

Class C with bit rates up to 1 Mbit/s, it is being applied for

Class A and Class B applications due to the availability of

inexpensive CAN ICs from a large number of semiconductor

manufacturers.

Remote switching systems require remote power switches.

An ideal remote switch must have a low on-state voltage,

be easy to drive from a micro-controller, and incorporate

current sensing. A low on-state voltage helps minimize the

heatsinking requirements, while current sensing is needed for

the circuit protection function to be incorporated into the

switch. To withstand the harsh automotive environment the

switch must also be rugged. Furthermore, if PWM control is

required for the load, the switch must have short turn-on and

turn-off times and a high cycle-life. The traditional means of

remotely switching loads in an automobile is via electrome-

chanical relays. Although relays offer the lowest voltage drop

per unit cost, they require large drive current, are relatively

large, are difﬁcult to integrate with logic, and are not suit-

able for PWM applications [32–34]. Therefore, their use will

be limited to very high current, non-PWM applications. The

power levels of the individual loads in the automobile are too

low for IGBTs and MCTs to be competitive. Bipolar transistors

25 Automotive Applications of Power Electronics 645

(a)

(b)

Alternator

Battery

(c)

Communication Bus

Power Bus

Alternator

Battery

Alternator

Battery

Power Bus

FIGURE 25.10 Alternative control strategies illustrated for a simple automotive electrical system with six loads (L1-6) and six primary switches

(S1-6): (a) conventional direct switching architecture with a single fusebox (F1); (b) multiplexed remote switching architecture, with remote switches

and transceivers in three distribution boxes (D1-3); and (c) multiplexed point-of-load switching with electronics integrated into the loads and the

primary switches.

are also not very attractive because they are harder to drive

than a MOS-gated device. Because of its fast switching speed,

low voltage drop, relative immunity to thermal runaway, low

drive requirements, and ease of integration with logic, the

power MOSFET is the most attractive candidate for remote

switching. Smart-power MOSFET devices with integrated logic

interface and circuit protection have recently become available.

Use of these devices for power electronic control of individual

loads has become economically competitive in some subsys-

tems, and may be expected to become more so with the advent

of higher voltage electrical systems.

The beneﬁts of remote switching electrical distribution sys-

tems have been demonstrated by Furuichi et al. [35]. The

multiplexed architecture they implemented had 10 remote

units (two power units with fuses, power drivers and signal

inputs, ﬁve load control units with power drivers and signal

inputs but no fuses, and three signal input units with only sig-

nal inputs). To increase system reliability, each power unit was

connected to the battery via independently fused power cables.

Although wiring cost decreased, the authors report an increase

in overall system cost due to the additional cost of the remote

units. Intel’s CAN ICs with data rates of 20 kbit/s were used

646 D. J. Perreault et al.

TABLE 25.5 Comparison of a multiplexed and the conventional

system, as reported by Furuichi et al. for a compact vehicle [35].

In the multiplexed system, the function of nine electronic control

units (ECUs) was integrated into the remote units

Point-to-point Multiplexed Change (%)

Harness weight (kg) 14.0 9.8 −30

ECU weight (kg) 1.2 0.0 N/A

Remote unit weight (kg) 0.0 3.5 N/A

Total weight (kg) 15.2 13.3 −12.5

Number of wires 743 580 −21.9

Number of terminals 1195 915 −23.4

Number of splices 295 246 −16.6

Length of wire (m) 809 619 −23.5

for the transmission and reception of control signals over an

unshielded twisted-pair ring bus. Intelligent power MOSFETs

were used as the remote switches and fusing was done with

mini-fuses. The results of their work are shown in Table 25.5.

Although weight of the wiring harness was reduced by 30%,

the total system weight decreased by only 12.5% due to the

added weight of the remote units.

25.6 Electromechanical Power

Conversion

Power is generated in the automobile by an electrical machine

driven by the engine. In the early days of the automobile,

the electrical load was small and a dc generator was used for

this purpose. As the electrical loads grew, the dc generator

could not meet the growing demand of electrical power and

was displaced by a three-phase alternator and diode rectiﬁer.

Continuously increasing power and performance require-

ments are driving further evolution in automotive power

generation and control, and are motivating the introduction of

power electronics and improved electrical machines in auto-

mobiles. In addition to high-power alternators, future appli-

cations of electromechanical power conversion may include

integrated starter/alternators and propulsion systems. This

section describes some of the machine and power elec-

tronic technologies that are useful for meeting the increasing

challenges in the automobile.

25.6.1 The Lundell Alternator

The Lundell, or claw-pole, alternator is a three-phase wound-

ﬁeld synchronous machine that is almost universally used for

power generation in present-day vehicles [1]. As illustrated in

Fig. 25.11, the rotor is made of a pair of stamped pole pieces

(“claw poles”) ﬁxed around a cylindrical ﬁeld coil. The ﬁeld

winding is driven from the stator via a pair of slip rings and

FIGURE 25.11 Structure and circuitry of the conventional Lundell

alternator.

brushes, and causes the two pole pieces to become opposing

magnetic poles. A full-bridge diode rectiﬁer is traditionally

used at the machine output, and a fan mounted on the rotor

is typically used to cool the whole assembly.

The dc output voltage of the alternator system is regulated

by controlling the ﬁeld current. A switching ﬁeld regulator

applies a pulse width modulated voltage across the ﬁeld. The

steady-state ﬁeld current is determined by the ﬁeld-winding

resistance and the average voltage applied by the regulator.

Changes in the ﬁeld current occur with an L/R ﬁeld-winding

time constant in the order of 100 ms or more. This long ﬁeld-

winding time constant and a large stator leakage reactance are

characteristic of this type of alternator and tend to dominate

its performance.

The alternator is driven by means of a belt, and is designed

to operate over a wide speed ratio of about 10:1, though much

of its operating lifetime is spent within a narrower 3:1 or 4:1

range. The gearing ratio provided by the belt is a design vari-

able for the alternator; an alternator mechanical speed range

from 1800 to 18,000 rpm for a 12-pole machine is typical.

A simple electrical model for the Lundell alternator is shown

in Fig. 25.12. The armature of the alternator is modeled as a

Y-connected set of leakage inductances L

and back voltages

, v

, and v

. The fundamental electrical frequency ω of

Battery

Field Current

Regulator

field

FIGURE 25.12 A simple Lundell alternator model.

25 Automotive Applications of Power Electronics 647

the back-emfs is one-half of the product of the number of

machine poles p and the mechanical speed ω

. The magnitude

of the back-emfs is proportional to the electrical frequency and

the ﬁeld current. For the sinusoidal case, the line-to-neutral

voltage back-emf magnitude can be calculated as:

= kωi

(25.1)

where k is the machine constant and i

is the ﬁeld current.

The diode bridge feeds a constant voltage V

representing the

battery and other loads. This simple model captures many of

the important characteristics of the Lundell alternator, while

remaining analytically tractable. Other effects, such as stator

resistance, mutual coupling, magnetic saturation, and wave-

form harmonic content, can be incorporated into this model

at the expense of simplicity. The constant-voltage battery load

on the alternator makes the analysis of this system different

from the classic case of a diode rectiﬁer with a current-

source load. Nevertheless, with reasonable approximations,

the behavior of this system can be described analytically [8].

Using the results presented in [8], alternator output power vs

operating point can be calculated as:



−(4V

/π

)

πωL

(25.2)

where V

is the output voltage, V

is the back-emf magnitude,

ω is the electrical frequency, and L

is the armature leakage

inductance. Extensions of Eq. (25.2) that also include the effect

of the stator resistance are given in [8].

As can be inferred from Eq. (25.2), alternator output power

varies with speed, and is maximized when the back-emf mag-

nitude of the machine is substantially larger than the output

voltage. In a typical Lundell alternator, back voltages in excess

of 80 V may be necessary to source-rated current into a 14 V

output at high speed. Furthermore, as can be seen from

Eq. (25.2), the armature leakage reactance limits the output

power capability of the alternator. These characteristics are a

result of the fact that signiﬁcant voltage drops occur across the

leakage reactances when current is drawn from the machine.

These drops increase with speed and current, and cause the

alternator to exhibit signiﬁcant drop in output voltage with

increasing current. Thus, an appropriate dc-side model for a

Lundell alternator is a large open-circuit voltage (related to

the back-emf magnitude) in series with a large current- and

speed-dependent output impedance. This characteristic, cou-

pled with the long ﬁeld time constant, is the source of the

undesirable load-dump transient characteristic of the Lundell

alternator. In this transient, the large open-circuit voltage is

transiently impressed across the alternator output when the

load is suddenly reduced.

The efﬁciency of the conventional Lundell alternator is

relatively poor. Typical efﬁciency values are in the order of

40–60%, depending on the operating point [1, 36, 37]. At low

and medium speeds, losses tend to be dominated by stator

copper losses. Iron losses become dominant only at very high

speeds [1].

25.6.2 Advanced Lundell Alternator Design

Techniques

The conventional diode-rectiﬁed Lundell alternator, though

inefﬁcient, has so far met vehicle electrical power requirements

in a cost-effective manner. However, continuing increase in

electrical power demand and growing interest in improved

fuel economy is pushing the limits of conventional Lundell

alternator technology. This section describes some established

and emerging technologies that can be used to improve the

performance of the Lundell alternator.

25.6.2.1 Third-harmonic Booster Diodes

One widely used approach for improving the high-speed out-

put power capability of Lundell alternators is the introduction

of third-harmonic booster diodes [1]. In this technique, the

neutral point of the Y-connected stator winding is coupled to

the output via a fourth diode leg, as illustrated in Fig. 25.13.

While the fundamental components of the line-to-neutral

back voltages are displaced by 120

◦

in phase, any third-

harmonic components will be exactly in phase. As a result,

third-harmonic energy can be drawn from the alternator and

transferred to the output by inducing and rectifying common-

mode third-harmonic currents through the three windings.

The booster diodes provide a means for achieving this. At high

speed, the combination of the third-harmonic voltages at the

main rectiﬁer bridge (at nodes a, b, and c in Fig. 25.13), com-

bined with the third-harmonic of the back voltages are large

enough to forward bias the booster diodes and deliver third-

harmonic energy to the output. In systems with signiﬁcant

(e.g. 10%) third-harmonic voltage content, up to 10% addi-

tional output power can be delivered at high speed. Additional

power is not achieved at low speed, or in cases where the

third-harmonic of the back voltage is small.

25.6.2.2 Lundell Alternator with Permanent Magnets

The structure of the rotor of the claw-pole alternator is such

that the leakage ﬂux is high. This reduces the output current

capability of the alternator. The leakage ﬂux can be reduced by

placing permanent magnets on the pole faces or in the spaces

between the adjacent poles of the rotor. This modiﬁcation

allows the alternator to deliver more output current. Placing

the magnets in the spaces between adjacent poles is a better

approach because it is simpler to implement and leads to a

higher output current at engine idle [38].

648 D. J. Perreault et al.

Battery

Field Current

Regulator

field

FIGURE 25.13 Lundell alternator with booster diodes.

25.6.2.3 Twin-rotor Lundell Alternator

The maximum power capability of the Lundell alternator is

limited in part by the limit on its length-to-diameter ratio

imposed by mechanical stresses on the stamped pole pieces.

This prevents the Lundell alternator from being arbitrarily

scaled up in size. The power capability of conventional designs

is probably limited to 3 kW, which is likely to be unacceptable

in the foreseeable future [39]. One way to retain the cost-

effectiveness of the claw-pole alternator while achieving higher

output power is to place two claw-pole rotors back-to-back on

a common shaft inside a common stator [40]. This effectively

increases the length of the claw-pole alternator without chang-

ing its diameter. This design allows higher power alternators

to be built while retaining most of the cost beneﬁts of the

claw-pole design.

25.6.2.4 Power Electronic Control

Another approach for improving the output power and

efﬁciency of the Lundell alternator is through the use

Battery

Field Current

Regulator

field

FIGURE 25.14 Lundell alternator with a switched-mode rectiﬁer.

of more sophisticated power electronics. Power electronics

technology offers tremendous value in this application. For

example, replacing the conventional diode rectiﬁer with a

switched-mode rectiﬁer provides an additional degree of

design and control freedom, and allows substantially higher

levels of power and efﬁciency to be attained from a given

machine. One such design is shown in Fig. 25.14. It employs

a simple switched-mode rectiﬁer along with a special load-

matching control technique to achieve dramatic improvement

in alternator output power, efﬁciency, and transient perfor-

mance [37]. The switched-mode rectiﬁer provides improved

control without the cost and complexity of a full active

converter bridge. By controlling the duty ratio of the switched-

mode rectiﬁer based on available signals such as alternator

speed, the alternator output power characteristic Eq. (25.2)

can be altered and improved, particularly for speeds above

idle [37]. Improvements in average power capability of a fac-

tor of two and average efﬁciency improvements on the order

of 20% are possible with this technology. Furthermore, the

25 Automotive Applications of Power Electronics 649

switched-mode rectiﬁer can be employed to achieve greatly

improved load-dump transient control.

25.6.3 Alternative Machines and Power

Electronics

The demand for increased alternator power levels, efﬁciency,

and performance also motivates the consideration of alter-

native electrical machines, power electronics, and design

approaches. While no alternative machine has yet displaced

the Lundell alternator in production vehicles, primarily due to

cost considerations, some potential candidates are reviewed

in this section. These include machines that are mounted

directly on the engine rather than driven from a belt. These

direct-driven machines become important as power levels

rise. This section also addresses the more general case of

the combined starter/alternators. While the use of a sin-

gle machine to do both starting and generation functions is

clearly possible, a separate (transient-rated) dc machine is

presently used for starting. This is because the large mismatch

in starting and generating requirements has made the com-

bined starter/alternator approach unattractive. However, as

alternator power ratings increase, the mismatch is reduced,

and a single starter/alternator system becomes more practical.

A combined system has the potential to eliminate the need for

a separate ﬂywheel, starter, solenoid switch, and pinion engag-

ing drive. It also has the potential to allow regenerative braking

and “light hybrid” operation, and to provide idle-stop capa-

bility (i.e. the ability to turn off the engine when the vehicle is

stopped and seamlessly restart when the vehicle needs to move)

for reduced fuel consumption. A move to this more sophisti-

cated approach relies upon advanced electrical machines and

power electronics.

25.6.3.1 Synchronous Machine with a Cylindrical

Wound Rotor

The claw-pole rotor can be replaced with a cylindrical rotor

to achieve better coupling between the stator and rotor.

FIGURE 25.15 Model of an alternator with full-bridge converter.

The cylindrical rotor is made from steel laminations and the

ﬁeld winding is placed in the rotor slots. The cylindrical rotor

is similar to the armature of a dc machine except that the

connection of the ﬁeld winding to the external circuit is made

through slip rings instead of a commutator. The cylindrical

rotor structure leads to quiet operation and increased out-

put power and efﬁciency. Unlike the claw-pole alternator, the

length of the machine can be increased to get higher output

power at a higher efﬁciency. The efﬁciency is higher since the

effect of the end windings on the machine performance is less

in a machine with a long length. It is also possible to build

the machine with a salient-pole rotor instead of a cylindrical

rotor. However, a machine with a salient-pole rotor is likely to

produce more noise than a machine with a cylindrical rotor.

A machine with a cylindrical wound rotor has similar power

electronics and control options as a claw-pole machine. If

generation-only operation is required, a diode bridge and ﬁeld

current control is sufﬁcient to regulate the output voltage.

Better performance can be achieved by using a switched-mode

rectiﬁer in conjunction with ﬁeld control [37]. If motoring

operation is desired (e.g. for starting), or even better perfor-

mance is desired, a full-bridge (active-switch) converter can

be used, as shown in Fig. 25.15. Since this is a synchronous

machine, some form of rotor position sensing or estimation

is typically necessary. The full-bridge converter allows maxi-

mum performance and ﬂexibility but carries a signiﬁcant cost

penalty.

25.6.3.2 Induction Machine

The stator of a three-phase induction machine is similar to that

of a three-phase synchronous machine. The rotor is either a

squirrel-cage or wound rotor. The machine with the squirrel-

cage rotor is simpler in construction and more robust than the

machine with a wound rotor in which the three-phase rotor

winding is brought outside the rotor through slip rings. The

rotor is cylindrical and is constructed from steel laminations.

It is also possible to use a solid rotor instead of a laminated

rotor. However, a solid rotor leads to higher losses as compared

with a laminated rotor. The losses in a solid rotor can be

650 D. J. Perreault et al.

reduced by cutting slots in the rotor surface, ﬁlling the stator

slot openings with magnetic wedges to reduce the ﬁeld ripple,

and placing a copper cage on the rotor.

An induction machine requires a source that can provide

the leading reactive power to magnetize the airgap. This means

that a three-phase induction generator cannot supply power to

a load through a three-phase diode bridge. Capacitor supply of

the reactive energy is impractical because of the wide operating

speed range. In the most general case (in which both motoring

and generating operation can be achieved) a three-phase active

bridge can be used. If only generating operation is desired,

the power to the load can be supplied through a three-phase

diode bridge and the reactive power can be obtained from a

small three-phase active bridge provided for this purpose. This

design requires a large number of devices and complex control.

25.6.3.3 Reluctance Machines

The switched reluctance machine is a doubly salient machine.

Both the stator and rotor of the machine are made from steel

laminations to reduce the iron losses. Only the stator carries

windings; the rotor is constructed of steel laminations with a

salient shape. The structure of a three-phase switched reluc-

tance machine with six stator poles and four rotor poles is

shown in Fig. 25.16a. A winding placed on diametrically oppo-

site stator poles forms a phase winding. When a phase of

the machine is excited, a pair of rotor poles tends to align

with the excited stator poles to provide a path of minimum

reluctance. If the rotor is moving towards alignment with the

excited pair of stator poles, then the machine develops a pos-

itive torque and acts as a motor. If the rotor is moving away

from the excited pair of stator poles, then the machine devel-

ops a negative torque and acts as a generator. The advantages

of the switched reluctance machine include simple construc-

tion, fault-tolerant power electronic circuit, high reliability,

unidirectional phase currents, and low cost. The drawbacks of

the machine include high levels of torque ripple, vibration and

acoustic noise, and a relatively high power electronics cost.

The synchronous reluctance machine is a singly salient

machine. The stator of the machine is similar to that of

′

(a) (b)

FIGURE 25.16 Structures of: (a) switched reluctance and (b) synchro-

nous reluctance machines. AA



represents phase A winding.

a synchronous or induction machine. The rotor has a seg-

mented structure with each segment consisting of a stack of

axially laminated steel sheets sandwiched with a non-magnetic

material. The structure of a four-pole synchronous reluctance

machine is shown in Fig. 25.16b. A synchronous reluctance

machine has less torque ripple, lower losses, and higher power

density than a comparable switched reluctance machine. Inclu-

sion of permanent magnets in the rotor structure allows both

reluctance and magnet torque to be achieved. Such interior

permanent magnet (IPM) machines can achieve very high

performance and power density. When permanent magnets

are included, however, careful attention must be paid to the

effects of shutdown of the power electronics as an uncontrolled

back-emf component will exist in this case [41].

The switched reluctance machine, like the induction

machine, requires an external source to magnetize the airgap.

Several circuits are available to excite the switched reluctance

machine. A circuit that is suitable for the automotive appli-

cation of this machine is shown in Fig. 25.17. A phase leg

is needed for each stator phase of the machine. In this case,

the switched reluctance machine obtains its excitation from

the same bus that it generates into. Unlike the synchronous

and induction machines in which the number of wires needed

to connect the machines to the power converters is usually

equal to the number of phases, the number of wires needed

to connect the switched reluctance machine to a converter is

equal to twice the number of phases. This is of no particular

concern in a switched reluctance machine in which the power

converter is integrated with the machine in the same housing.

The synchronous reluctance machine also requires an external

source to magnetize the airgap. The machine usually employs

an active bridge similar to the one used with an induction

machine for the desired power conversion. The machine can

also employ the converters used with the switched reluctance

machine. In this case, the currents through the stator wind-

ings are unidirectional. The relative complexity of the power

electronics is a disadvantage of these machine types in the case

where only generator operation is necessary.

25.6.3.4 Permanent Magnet and Hybrid

Synchronous Machines

The permanent magnet synchronous machine designed with

high-energy rare-earth magnets operates with high efﬁciency,

high power density, low rotor inertia, and low acoustic noise.

The excitation from the permanent magnets is ﬁxed and, there-

fore, the regulation of the output voltage of the machine is

not as straightforward as in a synchronous machine with a

wound rotor. For generator operation, machines of this type

can use switched-mode rectiﬁers to regulate the output volt-

age [42, 43]. The boost rectiﬁer of Fig. 25.14 is one possible

implementation of this approach. Alternatively, a diode rec-

tiﬁer followed by a dc/dc converter can be used to regulate

the generator system output [44]. Another method proposed

25 Automotive Applications of Power Electronics 651

Phase

FIGURE 25.17 Circuit for a switched reluctance machine.

for this type of system involves the use of tapped windings

and two three-phase SCR bridges [45]. The taps on the phase

windings are connected to one bridge, while full phase wind-

ings are connected to the other bridge. The bridge connected

to the full phase windings is used to supply power to the dc

bus at low engine speeds, while the converter connected to the

taps is used at high speed. The use of a tapped winding and

dual bridges helps the system cope with the wide speed range

of the alternator and limit the losses associated with the pul-

sating output currents. In the case when both motoring and

generating modes are desired, a full-bridge converter can be

used. Again, as this is a synchronous machine, some form of

position sensing or estimation is necessary. Also, in all of these

systems the effects of failure of the power electronics must be

carefully considered as there is no possibility of regulating the

back voltages by ﬁeld control.

Attempts to develop a simpler voltage regulation scheme for

permanent magnet synchronous machines have led to a per-

manent magnet/wound-rotor hybrid synchronous machine in

which the rotor consists of two parts: a part with permanent

magnets and a part with a ﬁeld winding [46]. The two parts

are placed next to each other on a common shaft. The rotor

with the ﬁeld winding can employ claw-pole, salient-pole, or

cylindrical structure. The ﬁeld current generates a ﬂux that is

used to either aid or oppose the permanent magnet ﬂux and

regulate the output voltage of the machine. One possible fail-

ure mode of this approach that can lead to catastrophic failure

is if the ﬁeld winding breaks while the machine is operating

at high speed. In this case, the generated output voltage will

become large and uncontrolled. Some means of mechanically

disconnecting the alternator at the input or electrically discon-

necting it at the output may be necessary to limit the impact

of this failure mode.

25.6.3.5 Axial-airgap Machines

The principle of operation of an axial-airgap, or axial-ﬂux,

machine is the same as that of a radial-airgap machine. An

axial-airgap machine is characterized by a short axial length

and large diameter. The structure of an axial-airgap per-

manent magnet machine with surface magnets is shown in

Stator

Magnets

Rotor

Shaft

FIGURE 25.18 Structure of an axial-airgap permanent magnet

machine.

Fig. 25.18 [47]. The stator of the machine can be slotless or

slotted. Two different magnetic circuit conﬁgurations are pos-

sible. In the NN conﬁguration, the magnetic polarities in one

pole pitch on both sides of stator are the same so that there

are two main ﬂuxes with symmetrical distribution through

the stator. In this case, the conductors can be wound into two

back-to-back stator slots to make one coil. The machine has

a large stator yoke dimension because the ﬂux passes through

the yoke, but less copper loss because of short end windings.

In the NS conﬁguration, the magnetic polarities in one pole

pitch on the opposite sides of stator are the opposite of each

other so that there is only one main axial ﬂux through the

stator. In this case, the stator yoke dimension is small, but

the end windings are long because the direction of current in

the back-to-back stator slots is the same. The iron losses are

small due to small yoke dimension and the copper losses are

high because of long end windings. Heat removal is more chal-

lenging due to small stator dimensions. The structure shown in

Fig. 25.18 is that of an axial-airgap permanent magnet machine

with surface magnets. In an axial-airgap machine with interior

permanent magnets, the magnets are embedded in the steel of

the rotor.

The axial airgap versions of other types of machines, such

as the induction and switched reluctance machines, are also

possible. The structure of an axial-airgap induction machine

652 D. J. Perreault et al.

is similar to that of an axial-airgap permanent magnet machine

except that windings are used instead of permanent magnets.

25.7 Dual/High Voltage Automotive

Electrical Systems

The electrical system of a 1920s internal combustion engine

(ICE) automobile had only a few loads: a starter, an ignition

device, a horn, and some lamps [48]. The mean power con-

sumption of these loads was less than 100 W. An engine-driven

dc generator charged a 6 V lead-acid battery that provided

electrical power. The power was distributed via point-to-point

wiring, with most loads controlled directly by manually oper-

ated primary switches located within the reach of the driver.

Only the starter was switched indirectly by an electromechan-

ical relay. After the Second World War, the automotive elec-

trical system started to grow rapidly in complexity and power

consumption as additional features, including radios, multi-

speed windshield wipers, and power windows, were added. The

introduction of higher compression engines stretched the 6-V

system beyond its technological limits. The 8.5 to 1 compres-

sion ratio engines required 100–200% greater ignition voltages

than the 6.4 to 1 engines. As a result, the primary side cur-

rent of the ignition coil was doubled or tripled and the life of

the distributor contacts was reduced to an unacceptable level.

To overcome this problem, the battery voltage was increased

to 12 V in the mid-1950s [49, 50].

Over the past four decades, the electrical power require-

ments of automobiles have increased even more rapidly. From

a mere 400 W in 1955, the power rating of a luxury vehicle’s

generator has increased to over 1800 W [51, 52]. However,

the electrical system of a modern automobile is architecturally

identical to the 12-V point-to-point system of the 1950s. The

only changes that have taken place have been at the compo-

nent level, such as the replacement of the dc generator by

a three-phase alternator-rectiﬁer, the replacement of wound-

ﬁeld dc motors by permanent magnet ones, and an increased

use of relays. The rapid growth in the electrical system is

expected to continue due to environmental regulations, con-

sumer demand for increased functionality, safety, security and

comfort, and replacement of some mechanical actuators by

electrical counterparts. The average electrical power require-

ment of a modern luxury vehicle is about 800 W. With the

addition of such loads as electric power steering, engine-

cooling fan, water pump, and electromechanical engine valves,

the average power requirement could increase to 2.5 kW by

2005 [53]. The traditional solution of increasing the size of the

alternator and the battery is not practical due to space limita-

tions and fuel efﬁciency requirements. Furthermore, the peak

power requirements of some of the anticipated loads – heated

windshield (2.5 kW), heated catalyst (3 kW), electromechani-

cal engine valves (2.4 kW at 3000 rpm), and active suspension

(12 kW) – cannot be met economically using the present

architecture. These factors have motivated the development

of new dual/high voltage electrical architectures that incorpo-

rate a higher-voltage bus in addition to the standard 14 V bus

[39, 54–56]. A dual/high voltage approach allows an efﬁcient

supply of power to many loads which beneﬁt from operat-

ing at a higher voltage, while retaining the 14 V bus for loads

(such as lamps and electronics) which do not beneﬁt from a

higher voltage. High-voltage architectures that do not retain

the 14 V bus are also possible, but will require a substantial

investment in the design and production of new high-voltage

components. This section describes some of the characteristics

and preliminary speciﬁcations of the new dual/high voltage

electrical system architectures. It also discusses some of the

widely considered implementation approaches.

25.7.1 Trends Driving System Evolution

The conventional 12-V automotive electrical power system has

many defects, including a widely varying steady-state system

voltage and large transients, which force the electrical func-

tions to be over-designed. However, these limitations alone

have not been a strong enough driver for automotive compa-

nies to seriously evaluate advanced alternatives. Now a number

of new factors are changing this situation. The most important

of these are future load requirements that cannot be met by

the present 12-V architecture.

25.7.1.1 Future Load Requirements

Table 25.6 gives a list of electrical loads expected to be intro-

duced into automobiles in the next ten years [53]. Some of

these loads (electrohydraulic power steering, electric engine

fan, electric water pump, and electromechanical valves) will

replace existing mechanically or hydraulically driven loads.

The remaining are new loads introduced to either meet

government mandates or satisfy customer needs.

TABLE 25.6 Electrical loads expected to be introduced into automobiles

in the next decade [53]

Load Peak power (W) Average power (W)

Exhaust air pump 300 10

Electrohydraulic power

steering

1000 150

Electric engine fan 800 150

Heated catalytic converter 3000 90

Electric water pump 300 150

Heated windshield 2500 120

Electromechanical engine

valves (6 cylinders at

6000 rpm)

2400 800

Active suspension 12,000 360

Total 1830