Twin-rotor Lundell Alternator

25.6.2.3 Twin-rotor Lundell Alternator

of more sophisticated power electronics. Power electronics The maximum power capability of the Lundell alternator is technology offers tremendous value in this application. For

limited in part by the limit on its length-to-diameter ratio example, replacing the conventional diode rectifier with a imposed by mechanical stresses on the stamped pole pieces. switched-mode rectifier provides an additional degree of This prevents the Lundell alternator from being arbitrarily design and control freedom, and allows substantially higher scaled up in size. The power capability of conventional designs levels of power and efficiency to be attained from a given is probably limited to 3 kW, which is likely to be unacceptable machine. One such design is shown in Fig. 25.14. It employs in the foreseeable future [39]. One way to retain the cost-

a simple switched-mode rectifier along with a special load- effectiveness of the claw-pole alternator while achieving higher matching control technique to achieve dramatic improvement

output power is to place two claw-pole rotors back-to-back on in alternator output power, efficiency, and transient perfor-

a common shaft inside a common stator [40]. This effectively mance [37]. The switched-mode rectifier provides improved increases the length of the claw-pole alternator without chang- control without the cost and complexity of a full active ing its diameter. This design allows higher power alternators converter bridge. By controlling the duty ratio of the switched- to be built while retaining most of the cost benefits of the mode rectifier based on available signals such as alternator claw-pole design.

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- Another approach for improving the output power and tor of two and average efficiency improvements on the order efficiency of the Lundell alternator is through the use of 20% are possible with this technology. Furthermore, the

25.6.2.4 Power Electronic Control

Field Current Regulator

field

b Battery

v sa v sb v sc

FIGURE 25.14 Lundell alternator with a switched-mode rectifier.

25 Automotive Applications of Power Electronics 657 switched-mode rectifier can be employed to achieve greatly The cylindrical rotor is made from steel laminations and the

improved load-dump transient control. field winding is placed in the rotor slots. The cylindrical rotor is similar to the armature of a dc machine except that the con- nection of the field winding to the external circuit is made

25.6.3 Alternative Machines and Power

through slip rings instead of a commutator. The cylindrical

Electronics

rotor structure leads to quiet operation and increased out- put power and efficiency. Unlike the claw-pole alternator, the

The demand for increased alternator power levels, efficiency, length of the machine can be increased to get higher output and performance also motivates the consideration of alter- power at a higher efficiency. The efficiency is higher since the native electrical machines, power electronics, and design effect of the end windings on the machine performance is less approaches. While no alternative machine has yet displaced in a machine with a long length. It is also possible to build the Lundell alternator in production vehicles, primarily due to the machine with a salient-pole rotor instead of a cylindrical cost considerations, some potential candidates are reviewed rotor. However, a machine with a salient-pole rotor is likely to in this section. These include machines that are mounted produce more noise than a machine with a cylindrical rotor. directly on the engine rather than driven from a belt. These

A machine with a cylindrical wound rotor has similar power direct-driven machines become important as power levels electronics and control options as a claw-pole machine. If

rise. This section also addresses the more general case of generation-only operation is required, a diode bridge and field the combined starter/alternators. While the use of a sin- current control is sufficient to regulate the output voltage. Bet- gle machine to do both starting and generation functions ter performance can be achieved by using a switched-mode is clearly possible, a separate (transient-rated) dc machine rectifier in conjunction with field control [37]. If motoring is presently used for starting. This is because the large mis- operation is desired (e.g. for starting), or even better perfor- match in starting and generating requirements has made the mance is desired, a full-bridge (active-switch) converter can combined starter/alternator approach unattractive. However,

be used, as shown in Fig. 25.15. Since this is a synchronous as alternator power ratings increase, the mismatch is reduced, machine, some form of rotor position sensing or estimation

and a single starter/alternator system becomes more practical. is typically necessary. The full-bridge converter allows maxi-

A combined system has the potential to eliminate the need for mum performance and flexibility but carries a significant cost

a separate flywheel, starter, solenoid switch, and pinion engag- penalty. ing drive. It also has the potential to allow regenerative braking and “light hybrid” operation, and to provide idle-stop capabil- ity (i.e. the ability to turn off the engine when the vehicle is

25.6.3.2 Induction Machine

stopped and seamlessly restart when the vehicle needs to move) The stator of a three-phase induction machine is similar to for reduced fuel consumption. A move to this more sophisti- that of a three-phase synchronous machine. The rotor is either cated approach relies upon advanced electrical machines and

a squirrel-cage or wound rotor. The machine with the squirrel- power electronics.

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

25.6.3.1 Synchronous Machine with a Cylindrical

rotor is cylindrical and is constructed from steel laminations.

It is also possible to use a solid rotor instead of a laminated The claw-pole rotor can be replaced with a cylindrical rotor rotor. However, a solid rotor leads to higher losses as com- to achieve better coupling between the stator and rotor. pared with a laminated rotor. The losses in a solid rotor can be

Wound Rotor

v sa

v sb v sc

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

658 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.