WO2004054083A2

WO2004054083A2 - High-efficiency alternator system

Info

Publication number: WO2004054083A2
Application number: PCT/IL2003/001037
Authority: WO
Inventors: Victor Migdal
Original assignee: Victor Migdal
Priority date: 2002-12-09
Filing date: 2003-12-08
Publication date: 2004-06-24
Also published as: WO2004054083A3; IL153347A0; AU2003286399A1; AU2003286399A8

Abstract

Apparatus (10) for providing electrical power to a vehicle that includes a motor and a battery (14) having a predetermined charging voltage. The apparatus includes an alternator (11), which includes a rotor (22), coupled to be rotated by the motor so as to generate a rotating magnetic field, and a stator (21), including one or more stator coils that are inductively coupled to generate, responsively to the rotating magnetic field, an AC voltage that is at least 50% greater than the charging voltage of the battery. A transformer (12, 23) is coupled to receive and step down the AC voltage to an output voltage that is approximately equal to the charging voltage.

Description

HTGH-EFFICIENCY ALTERNATOR SYSTEM

FIELD OF THE INVENTION

The present invention relates generally to electrical power generation, and specifically to alternators that are coupled to motors in order to generate electrical power. BACKGROUND OF THE INVENTION

Alternators are widely used for supplying electrical power to vehicles, including (but not limited to) automobiles, motorcycles, tracks, boats, airplanes and locomotives. A typical vehicle alternator comprises a rotor and a three-phase stator. The rotor is coupled to the rotating shaft of the vehicle motor and is driven by an excitation current to producing a rotating magnetic field. The magnetic field causes currents to flow in the windings of the stator. The output currents from the stator are rectified in order to generate a DC current for charging the battery of the vehicle and powering the electrical systems in the vehicle. Alternators of this sort are used to generate electrical power based on the rotational motion not only of vehicular motors, but also of motors of other types and other sources of rotational motion.

Most alternators comprise a regulator circuit, which has the job of maintaining the alternator output voltage (and thus the vehicle system voltage) at a constant level irrespective of engine speed. The voltage output 1 evel o f a lternators k nown i n t he a rt must be close to the battery voltage, in order to avoid damage to the battery and other components of the electrical system of the vehicle. The regulator controls the level of the excitation current supplied to the rotor, and thus limits the rotor's magnetic field and hence the voltage generated by the stators. vehicular electrical systems with 12 V batteries, the regulator must typically limit the alternator output to around 14 V, while for systems with 24 V batteries, the regulator limit is around 28 V, i.e., within about 20% of the nominal battery voltage.

SUMMARY OF THE INVENTION hi alternators known in the art, power losses increase rapidly with increasing alternator speed. Factors contributing to this loss include "copper losses," due to resistance of the copper wiring used to wind the stator, and "iron losses," due to hysteresis and eddy currents produced by the alternating magnetic fields. These losses translate into increased heat generated by the alternator as its speed increases. Since the heat must be maintained below the level at which it can damage the electrical parts of the alternator (and other elements in the vehicle engine), these loss mechanisms effectively limit the maximum output current - and hence the maximum output power - that can be supplied by an alternator of a given size and weight.

Embodiments of the present invention provide methods for increasing the output power of a given alternator design, without increasing its size and weight. These embodiments operate by increasing the stator voltage of the alternator, to a level well above the nominal operating voltage of the circuit that the alternator is to drive, i the case of a vehicle battery, this means that the stator voltage of the alternator is substantially higher than the battery voltage, typically at least 50% higher than the battery voltage. A transformer is coupled to the stator output in order to step the voltage down to the nominal operating voltage that the alternator is to drive. A regulator senses either the high- voltage output of the stator, or the low- voltage output of the transformer, or both, and controls the excitation current of the rotor accordingly.

Because the power generated by the alternator is equal to the product of the stator voltage by the current, the present invention enables the alternator to generate much higher output power for a given rotor speed and stator current. As a result of the reduction in current- and speed-related losses, the alternator of the present invention is substantially more efficient that alternators known in the art. Thus, the principles of the present invention may be applied to produce smaller, lighter alternators for use in vehicular applications, as well as to increase the power that can be drawn from existing alternators. Although the embodiments described herein are directed particularly at vehicular applications, it will be appreciated that the principles of the present invention may be applied in alternators that are driven by substantially any source of rotational motion that is known in the art.

There is therefore provided, in accordance with an embodiment of the present invention, apparatus for providing electrical power to a vehicle that includes a motor and a battery having a predetermined charging voltage, the apparatus including: an alternator, which includes: a rotor, coupled to be rotated by the motor so as to generate a rotating magnetic field; and a stator, including one or more stator coils that are inductively coupled to generate, responsively to the rotating magnetic field, an AC voltage that is at least 50% greater than the charging voltage of the battery; and a transformer, coupled to receive and step down the AC voltage to an output voltage that is approximately equal to the charging voltage. hi some embodiments, the AC voltage generated by the stator coils is at least four times the charging voltage of the battery. Optionally, the AC voltage generated by the stator coils may be at least ten times the charging voltage of the battery. In one embodiment, the charging voltage of the battery is approximately 14 VDC, and wherein the AC voltage generated by the stator coils is at least 78 VAC.

In disclosed embodiments, the transformer includes primary and secondary coils, wherein at least one of the primary coils is coupled to receive the AC voltage from the stator, and the output voltage is provided on at least one of the secondary coils, and the apparatus i ncludes a r ectifier, c oupled t o t he s econdary coil so as to r ectify t he o utput voltage in order to charge the battery. Typically, the apparatus also includes a regulator, which is coupled to receive feedback from one or more of the secondary coils of the transformer, and to control an excitation current supplied to the rotor responsively to the feedback.

In one embodiment, the secondary coils include an auxiliary coil, which is adapted to generate an auxiliary voltage greater than the charging voltage, and the regulator is coupled to generate the excitation current using the auxiliary voltage. The regulator may be adapted to sense a condition of the rotor, and to apply the auxiliary voltage to the rotor responsively to the condition so as to increase a strength of the rotating magnetic field. Additionally or alternatively, the regulator is further coupled to sense a level of the AC voltage generated by the stator, and to control the excitation current responsively to the level. Typically, the regulator is adapted to sense an occurrence of a fault responsively to the level of the AC voltage, and to cut off the excitation current responsively to the fault. Further additionally or alternatively, the secondary coils are arranged to provide at least first and second output voltages, wherein the first output voltage is approximately equal to the charging voltage, and the second output voltage is substantially different from the charging voltage.

In a disclosed embodiment, the apparatus includes a rectifier, which is coupled to rectify the AC voltage generated by the stator coils so as to generate a DC voltage that is at least 50% greater than the c harging v oltage o f t he b attery, a nd a D C/DC c onverter, which includes the transformer and is adapted to generate the output voltage as a DC voltage. Typically, the apparatus includes a regulator, which is coupled to receive feedback from the DC/DC converter, and to control an excitation current supplied to the rotor responsively to the feedback. There is also provided, in accordance with an embodiment of the present invention, electrical apparatus, including: an alternator, which includes: a rotor, which is adapted to be rotated by a source of rotational motion so as to generate a rotating magnetic field; and a stator, including one or more stator coils that are inductively coupled to generate, responsively to the rotating magnetic field, an AC voltage at a first voltage level; a transformer, coupled to receive and step down the AC voltage to generate an output voltage at a second voltage level, such that the first voltage level is at least 50% greater than the second voltage level; and a regulator, which is coupled to receive the output voltage at the second voltage level and, using the output voltage, to generate a control input to the alternator.

There is additionally provided, in accordance with an embodiment of the present invention, a method for providing electrical power to a vehicle that includes a motor and a battery having a predetermined charging voltage, the method including: coupling a rotor of an alternator to be rotated by the motor so as to generate a rotating magnetic field; inductively coupling one or more stator coils of the alternator to generate, responsively to the rotating magnetic field, an AC voltage that is at least 50% greater than the charging voltage of the battery; and stepping down the AC voltage to an output voltage that is approximately equal to the charging voltage.

There is further provided, in accordance with an embodiment of the present invention, a method for generating electrical power using an alternator having a rotor and one or more stator coils, the method including: coupling the rotor to be rotated by a source of rotational motion so as to generate a rotating magnetic field; inductively coupling the one or more stator coils to generate, responsively to the rotating magnetic field, an AC voltage at a first voltage level; stepping down the AC voltage to generate an output voltage at a second voltage level, such that the first voltage level is at least 50% greater than the second voltage level; and generating a control input to the alternator using the output voltage. The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram that schematically illustrates an alternator system, in accordance with an embodiment of the present invention;

Fig. 2 is a schematic electrical diagram showing an alternator system based on a regulator with a low-voltage input, in accordance with an embodiment of the present invention;

Fig. 3 is a schematic electrical diagram showing an alternator system based on a regulator with multi-voltage inputs, in accordance with another embodiment of the present invention; Fig. 4 is a schematic electrical diagram showing details of a transformer used in the system of Fig. 3, in accordance with an embodiment of the present invention;

Figs. 5A and 5B are schematic electrical diagrams showing details of a regulator circuit used in the embodiment of Fig. 3, in accordance with an embodiment of the present invention; and Figs. 6 and 7 are block diagrams that schematically illustrate alternator systems, in accordance with alternative embodiments of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 is a block diagram that schematically illustrates alternator-based apparatus 10 for generating electrical power, in accordance with an embodiment of the present invention. Apparatus 10 comprises an alternator 11, which is coupled to the rotating shaft of a motor (not shown). The motor may belong to substantially any sort of vehicle, including motors used to drive automobiles, motorcycles, trucks, boats, airplanes and locomotives. Alternatively, apparatus 10 may be coupled to other sorts of non- vehicular motors or to other sources of rotational motion, without limitation.

Unlike vehicular alternators known in the art, which generate an output voltage within a small tolerance range of the battery charging voltage, alternator 11 generates an output voltage that is at least 50% higher than the nominal charging voltage of the battery. As noted above, the high- voltage capability of the alternator means that it operates more efficiently than alternators known in the art, by reducing the current that must flow in the stator coils in order to produce a given level of output power. Optionally, for even greater efficiency, the output voltage of alternator 11 may be at least four times the charging voltage of the battery, or even in excess often times the charging voltage.

The high output voltage of alternator 11 may be achieved by rewinding the stator of an existing alternator with a greater number of turns of wire than in the original alternator configuration. As a result, for any given rotor speed and magnetic field strength, the stator will generate a higher voltage with a lower current than would have been generated in the original configuration. Since the current flowing in the wire in alternator 11 will be lower than in the conventional alternator, a narrower gauge of wire may be used, so that the existing alternator housing and core can accommodate the increased number of turns on the stator. Because the stator current is reduced, alternator 11 generates less heat relative to the output power that it generates, and may thus be run at a higher output power level than the conventional alternator that it replaces.

Alternatively, alternator 11 may be designed from the start to operate at the high output voltage. In this case, it will be possible to reduce the weight of copper wire and iron core material used to produce the alternator, by comparison with alternators known in the art that are designed to operate at low voltage with the same output power. As another point of comparison, alternator 11 will be capable of generating a given output power at a lower rate of rotation than alternators known in the art, and will consequently have reduced iron losses and enhanced efficiency.

In the example shown in Fig. 1 , each o f the three stator windings generates an output voltage of 78 VAC, while a battery 14 that is charged by the alternator has a nominal voltage of 12 VDC. This means that to generate a given output power, alternator 11 need produce only about 13% (10 VAC/78 VAC) as much current as a conventional alternator. Generally speaking, alternator systems in accordance with embodiments of the present invention may be used in conjunction with batteries of substantially any voltage known in the art, including 6, 12, 24 and 48 V, as well as other voltages, above, below and between these values, as required. The alternators used in such systems may generate anywhere from 12 to 400 volts per phase of the stator.

The high- voltage output of alternator 11 is coupled to the primary of a transformer 12, which steps the output voltage down to about 10 VAC, to match the nominal battery voltage. A rectifier 13, typically a full- wave diode bridge, rectifies the three-phase AC output on the secondary of transformer 12 so as to generate a DC voltage of 14 VDC in the present example. The voltage is suitable for charging battery 14 and running the other electrical systems of the vehicle.

A regulator 16 senses and controls the excitation current supplied to the rotor of alternator 11. Regulator 16 may comprise an existing, off-shelf product, as described below with reference to Fig. 2. Alternatively, regulator 16 may comprise a novel, high- voltage regulator, which designed to enhance the efficiency of apparatus 10 still further. For this purpose, transformer 12 provides an auxiliary output, which is rectified by an auxiliary rectifier 15 (typically a full-wave or half-wave diode bridge) to provide an auxiliary voltage input to regulator 16. The auxiliary voltage is typically in the range of about 30 to 95 VDC, although voltages above and below this range may also be used. Details of this sort of novel regulator and its use are described below with reference to Figs. 3 and 5A/B.

Transformer 12, rectifier 13 and regulator 16 (as well as rectifier 15 if used) may be contained together in the same housing as alternator 11. Alternatively, one or more of these elements may be housed in a separate box. The elements of apparatus 10 are typically provided with or coupled to appropriate cooling means, as are known in the art. Apparatus 10 may be grounded to the chassis of the vehicle, so that the DC output of the apparatus is provided relative to the chassis ground, wherein the output may be either positive, negative or both positive and negative relative to the chassis. Alternatively, the output of apparatus 10 may be at a floating level relative to the chassis, or the apparatus may provide one or more grounded outputs and one or more floating outputs.

Fig. 2 is a schematic circuit diagram showing alternator apparatus 20, in accordance with an embodiment of the present invention. In this embodiment, alternator 11 comprises a three-phase stator 21, with coils labeled u, v and w. The coils are connected in a star configuration, as is known in the art, with a neutral lead taken from the center of the star. Alternatively, the stator coils may be connected in a delta configuration, or in any other suitable configuration that is known in the art. Although the embodiments described herein are based on three-phase stators, the principles of the present invention may similarly be applied to alternators with larger or smaller numbers of stators, stator coils and/or phases. Alternator 11 further comprises a rotor 22, having an excitation winding, labeled

G, which is monitored and driven by a regulator 24. The excitation current is generated by regulator 24 between terminals D+ and DF. The excitation current is limited by the battery charging voltage. The regulator is labeled as an "original regulator," since it operates at or near the battery voltage (for example, 12 VDC) appearing between terminals D+ and D- of the regulator. Regulator 24 may therefore comprise an existing, off-shelf regulator.

As noted above, stator 21 is able to generate a high output voltage, typically at least 50% higher than the battery charging voltage. The high- voltage output of stator 21 is coupled to the primary of a transformer 23, which generates a low- voltage AC output to rectifier 13, as described above. Exciter diodes 25 also rectify the output of transformer 23 to generate feedback to regulator 24, for controlling the excitation current applied to rotor 22. An ignition switch 26, in series with a charging lamp 27, is used to provide the initial excitation current from battery 14 to the rotor, when starting the motor that drives alternator 11. Fig. 3 is a schematic circuit diagram showing alternator apparatus 29, in accordance with another embodiment of the present invention. In this embodiment, a novel regulator 30 is used to control the excitation current to rotor 22. T he regulator receives a low-voltage DC input from exciter diodes 25, and in addition receives at least one higher- voltage input. In the present example, a transformer 31 generates an auxiliary output, which is rectified by an auxiliary rectifier 33 (such as a full- wave or half- wave diode bridge) to provide an auxiliary voltage input to regulator 30. The auxiliary voltage is typically in the range of about 30-95 VDC, as noted above. In addition, a high-voltage rectifier 34, which may also comprise a diode bridge, rectifies the output of the coils of stator 21 to generate a high- voltage input to regulator 30. The regulator and associated circuits have appropriate isolation to prevent short-circuits at the high voltage. Regulator 30 uses the auxiliary voltage input that it receives from rectifier 33 in order to increase the excitation current to rotor 22 under certain conditions. In particular, regulator 30 increases the voltage (and hence the current) a cross the excitation coil of rotor 22 in order to compensate for increased resistance of the excitation coil, which may occur as the temperature of alternator 11 increases. If the excitation voltage supplied to the rotor were held constant as the rotor resistance increased (as is the case in existing regulators, which do not have an auxiliary voltage to draw on), the excitation current and hence the magnetic field of the rotor would be reduced, leading to reduced alternator output power. By increasing the excitation voltage, regulator 30 enables rotor 22 to continue to exert an optimal magnetic field on stator 21 even under high-temperature operating conditions. This improvement in regulator 30 is capable of increasing the output power of apparatus 29 by approximately 14-15% relative to the output power of apparatus 20 (Fig. 2), in which regulator 24 is limited in output voltage to the level of the battery charging voltage. Details of regulator 30 are described below with reference to Figs. 5 A and 5B. Fig. 4 is a schematic circuit diagram showing details of transformer 31, in accordance with an embodiment of the present invention. The u, v and w coils of stator 21 are coupled as inputs to a primary 41 of transformer 31. The primary is wound on a core 43 together with secondary windings 42, which are wound and coupled to provide the desired low voltage output (for example, 10 VAC per phase) to rectifier 13. Auxiliary secondary w indings 44 a re a lso w ound on core 43 in o rder to p rovide t he d esired AC auxiliary voltage level, generally an intermediate level, to rectifier 33. Core 43 typically comprises ferromagnetic material, and is optimized to operate in an AC frequency range centered at about 400 Hz. The coils of the transformer may themselves be wound in a star or delta configuration, or in any other suitable configuration known in the art. Although the primary and secondary coils of transformer 31 are shown in the figure as being isolated from one another, substantially any type of transformer may be used for the purposes of the present invention, including autotransformers.

Transformer 31 may also have additional windings and/or terminals, and regulator 30 may comprise an additional switch (not shown), enabling the output voltages supplied by apparatus 30 to be varied among different, selectable values. Both DC and AC voltages may be supplied and switched in this manner.

Fig. 5 A is a schematic circuit diagram showing details of regulator 30, in accordance with an embodiment of the present invention. One function of regulator 30 is to stabilize the high- voltage output of alternator 11, independent of any load on the output and o f t he s peed o f r otation o f t he r otor, by e ontrolling t he c urrent i nput t o excitation winding 22. A s noted above, regulator 30 also receives a low- voltage input (typically between about 12 and 80 VDC) from excitation diodes 25 and an auxiliary voltage input (typically between about 30 and 95 VDC) from rectifier 33.

The high- voltage input to regulator 30 is sampled by an opto-isolator 62, in order to maintain electrical isolation between the high- voltage circuits and battery 14, as well as between the high- voltage circuits and other parts of the vehicle. Resistors 61 divide the input voltage, such that when the voltage on a resistor 63 exceeds +2.5 V, a transistor 65 begins to conduct, and a current flows through a resistor 66 and through opto-isolator 62.

A controller 64 receives the output of opto-isolator 62 and outputs a pulse-width modulated (PWM) control signal to a gate 67. Controller 64 typically outputs about 800 pulses per second, with a pulse width varying between 0 and 97% duty cycle. When gate 67 is switched on by the PWM pulse, an input current flows from rectifier 33 or battery 14 via one of diodes 68 through the gate. The auxiliary output of rectifier 33 is isolated from battery 14 by diodes 68. When gate 67 is closed, rotor current can flow through a diode 69 and is therefore greatly increased. A comparator 71 is used to maintain the output of alternator 11 at its proper level when battery 14 is charging. Any drop in rotor excitation current at high temperature is thus avoided. Fig. 5B is a schematic circuit diagram illustrating further aspects of regulator 30, in accordance with an embodiment of the present invention. As this figure illustrates, the regulator may be coupled to directly sample the current in the coils of stator 21 and to control t he P WM o utput o f c ontroller 64 to cut offa lternator 1 1 w hen a n o verload or short circuit is detected in the high voltage circuit. For this purpose, regulator 30 includes or is coupled to at least one toroidal transformer 70, which is coupled in turn to at least one of the coils of alternator 11 and generates a voltage proportional to the output current. When the voltage rises above a predetermined level, indicative of a possible short-circuit or overload in the alternator output, a reset signal is provided to the input of controller 64, which then interrupts the PWM output to gate 67. Once the overload or short-circuit is relieved, the controller resumes normal operation after a predetermined shut-off period, typically about 3 sec. Alternatively, an optional switch 74 may be used to provide the reset.

Regulator 30 also samples the rotor current using one or two resistors 60 coupled to the source lead of gate 67 (Fig. 5 A), which convert the current to a voltage in the range 0-0.5 V. When the voltage input to regulator 30 from resistor(s) 60 exceeds a predetermined threshold, controller 64 receives an input which causes it to reduce the PWM duty cycle accordingly. A potentiometer 72 may optionally be used to control the threshold 1 evel. A current limiter.52 may also be used to set the maximum permitted output current of the alternator.

Fig. 6 is a block diagram that schematically illustrates alternator apparatus 80, in accordance with an alternative embodiment of the present invention. In this case, the high- voltage output of alternator 11 is immediately rectified by a rectifier 81, which typically comprises a diode bridge, as described above. Rectifier 81 provides a high- voltage DC output to a DC/DC converter 82, which steps the DC level down to the appropriate voltage for charging battery 14. Alternator 11 may be configured, for example, to generate an AC output between about 25 and 400 V per phase, so that the output of rectifier 81 is between about 30 and 600 VDC. Converter 82 typically comprises a high-frequency oscillator and step-down transformer (not shown), as are known in the art. Apparatus 80 has the advantage of simplicity of implementation, using off-shelf components, although there is some loss of efficiency in this embodiment, relative to the embodiments described above, due to the use of converter 82. The DC/DC converter may comprise a switch (not shown) for selecting the desired output voltage, or for setting multiple output voltage levels.

Fig. 7 is a block diagram that schematically illustrates alternator apparatus 90, in accordance with yet another embodiment of the present invention. Apparatus 90 comprises high- voltage rectifier 81, followed by a DC/DC converter 91, as in the preceding embodiment. In the present embodiment, however, a novel regulator 92 is used to control the excitation current to the rotor of alternator 11, as in apparatus 29 (Fig. 3). Regulator 92 receives the high- voltage DC output from rectifier 81, and also receives an auxiliary, medium- voltage DC input for increasing the voltage supplied to the rotor under certain conditions, as described above. The auxiliary voltage may be supplied either from an additional output of converter 91 (labeled "OP 1" in the figure), or from a separate rectifier 93 coupled to alternator 11 ("OP 2"). Regulator 92 is thus able to drive apparatus 90 to generate greater output power than apparatus 80 under high-temperature conditions. Although the alternator systems in the embodiments shown above have only a single output, at the charging voltage of battery 14, these systems may be modified easily in order to p rovide a dditional D C a nd/or A C o utputs a t o ther, h igher v oltages. T hese voltages may be provided by use of a suitable transformer, as described above. Alternatively or additionally, the alternator may itself comprise additional stators and/or rotors for the purpose of generating these additional voltages.

It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Claims

1. Apparatus for providing electrical power to a vehicle that includes a motor and a battery having a predetermined charging voltage, the apparatus comprising: an alternator, which comprises: a rotor, coupled to be rotated by the motor so as to generate a rotating magnetic field; and a stator, comprising one or more stator coils that are inductively coupled to generate, responsively to the rotating magnetic field, an AC voltage that is at least 50% greater than the charging voltage of the battery; and a transformer, coupled to receive and step down the AC voltage to an output voltage that is approximately equal to the charging voltage.

2. The apparatus according to claim 1, wherein the AC voltage generated by the stator coils is at least four times the charging voltage of the battery.

3. The apparatus according to claim 2, wherein the AC voltage generated by the stator coils is at least ten times the charging voltage of the battery.

4. The apparatus according to claim 2, wherein the charging voltage of the battery is approximately 14 VDC, and wherein the AC voltage generated by the stator coils is at least 78 VAC.

5. The apparatus according to claim 1, wherein the transformer comprises primary and secondary coils, wherein at least one of the primary coils is coupled to receive the AC voltage from the stator, and the output voltage is provided on at least one of the secondary coils, and comprising a rectifier, coupled to the secondary coil so as to rectify the output voltage in order to charge the battery.

6. The apparatus according claim 5, and comprising a regulator, which is coupled to receive feedback from one or more of the secondary coils of the transformer, and to control an excitation current supplied to the rotor responsively to the feedback.

7. The apparatus according to claim 6, wherein the secondary coils comprise an auxiliary coil, which is adapted to generate an auxiliary voltage greater than the charging voltage, and wherein the regulator is coupled to generate the excitation current using the auxiliary voltage.

8. The apparatus according to claim 7, wherein the regulator is adapted to sense a condition of the rotor, and to apply the auxiliary voltage to the rotor responsively to the condition so as to increase a strength of the rotating magnetic field.

9. The apparatus according to c laim 6 , wherein the regulator is further coupled to sense a level of the AC voltage generated by the stator, and to control the excitation current responsively to the level.

10. The apparatus according to claim 9, wherein the regulator is adapted to sense an occurrence of a fault responsively to the level of the AC voltage, and to cut off the excitation current responsively to the fault.

11. The apparatus according to claim 5, wherein the secondary coils are arranged to provide at least first and second output voltages, wherein the first output voltage is approximately equal to the charging voltage, and the second output voltage is substantially different from the charging voltage.

12. The apparatus according to claim 1, wherein the apparatus comprises: a rectifier, which is coupled to rectify the AC voltage generated by the stator coils so as to generate a DC voltage that is at least 50% greater than the charging voltage of the battery; and a DC/DC converter, which comprises the transformer and is adapted to generate the output voltage as a DC voltage.

13. The apparatus according to claim 12, and comprising a regulator, which is coupled to receive feedback from the DC/DC converter, and to control an excitation current supplied to the rotor responsively to the feedback.

14. Electrical apparatus, comprising: an alternator, which comprises: a rotor, which is adapted to be rotated by a source of rotational motion so as to generate a rotating magnetic field; and a stator, comprising one or more stator coils that are inductively coupled to generate, responsively to the rotating magnetic field, an AC voltage at a first voltage level; a transformer, coupled to receive and step down the AC voltage to generate an output voltage at a second voltage level, such that the first voltage level is at least 50% greater than the second voltage level; and a regulator, which is coupled to receive the output voltage at the second voltage level and, using the output voltage, to generate a control input to the alternator.

15. The apparatus according to claim 14, wherein the first voltage level is at least four times the second voltage level.

16. The apparatus according to claim 15, wherein the first voltage level is at least ten times the second voltage level.

17. The apparatus according to claim 15, wherein the first voltage level is approximately 14 VDC, and wherein the second voltage level is at least 78 VAC.

18. The apparatus according claim 14, wherein the regulator is coupled, responsively to the second voltage level, to control an excitation current supplied to the rotor using the output voltage.

19. The apparatus according to claim 14, wherein the transformer comprises primary and secondary coils, wherein at least one of the primary coils is coupled to receive the AC voltage from the stator, and the second voltage level is provided on at least one of the secondary coils, and comprising a rectifier, coupled to the secondary coil so as to rectify the output voltage.

20. The apparatus according to claim 19, wherein the secondary coils comprise an auxiliary coil, which is adapted to generate an auxiliary voltage at an auxiliary voltage level that is greater than the second voltage level, and wherein the regulator is adapted to generate the control input using the auxiliary voltage.

21. The apparatus according to claim 20, wherein the regulator is adapted to sense a condition of the rotor, and to apply the auxiliary voltage to the rotor responsively to the condition so as to increase a strength of the rotating magnetic field.

22. The apparatus according to claim 19, wherein the secondary coils are arranged to provide a 11 east first a nd s econd o utput v oltages, w herein t he s econd o utput v oltage i s provided at the second voltage level, and the first output voltage is substantially different from the second voltage level.

23. The apparatus according to claim 14, wherein the regulator is further coupled to sense the first voltage level, and to generate the control input responsively to the first voltage level.

24. The apparatus according to claim 23, wherein the regulator is adapted to sense an occurrence of a fault responsively to the first voltage level, and to cut off an excitation current to the rotor responsively to the fault.

25. The apparatus according to claim 14, wherein the apparatus comprises: a rectifier, which is coupled to rectify the AC voltage generated by the stator coils so as to generate a DC voltage at a DC voltage level that is at least 50% greater than the second voltage level; and a DC/DC converter, which comprises the transformer and is adapted to generate the output voltage as a DC voltage at the second voltage level.

26. The apparatus according to claim 25, wherein the regulator is coupled to receive feedback from the DC/DC converter, and to generate the control input responsively to the feedback.

27. A method for providing electrical power to a vehicle that includes a motor and a battery having a predetermined charging voltage, the method comprising: coupling a rotor of an alternator to be rotated by the motor so as to generate a rotating magnetic field; inductively coupling one or more stator coils of the alternator to generate, responsively to the rotating magnetic field, an AC voltage that is at least 50% greater than the charging voltage of the battery; and stepping down the AC voltage to an output voltage that is approximately equal to the charging voltage.

28. The method according to claim 27, wherein the AC voltage generated by the stator coils is at least four times the charging voltage of the battery.

29. The method according to claim 28, wherein the AC voltage generated by the stator coils is at least ten times the charging voltage of the battery.

30. The method according to claim 28, wherein the charging voltage of the battery is approximately 14 VDC, and wherein the AC voltage generated by the stator coils is at least 78 VAC.

31. The method according to claim 27, and comprising rectifying the output voltage from the transformer in order to charge the battery.

32. The method according claim 31, wherein coupling the rotor comprises controlling an excitation current supplied to the rotor responsively to feedback derived from at least one secondary coil of the transformer.

33. The method according to claim 32, wherein stepping down the AC voltage further comprises generating an auxiliary voltage greater than the charging voltage, and wherein controlling the excitation current comprises generating the excitation current using the auxiliary voltage.

34. The method according to claim 33, wherein generating the excitation current comprises sensing a condition of the rotor, and applying the auxiliary voltage to the rotor responsively to the condition so as to increase a strength of the rotating magnetic field.

35. The method according to claim 32, wherein controlling the excitation current comprises sensing a level of the AC voltage generated by the stator, and controlling the excitation current responsively to the level.

36. The method according to claim 35, wherein controlling the excitation current comprises detecting an occurrence of a fault responsively to the level of the AC voltage, and cutting off the excitation current responsively to the fault.

37. The method according to claim 27, wherein stepping down the AC voltage comprises coupling a transformer to step down the AC voltage, and providing at least first and second output voltages from secondary windings of the transformer, wherein the first output voltage is approximately equal to the charging voltage, and the second output voltage is substantially different from the charging voltage.

38. The method according to claim 27, wherein stepping down the AC voltage comprises rectifying the AC voltage generated by the stator coils so as to generate a DC voltage that is at least twice the charging voltage of the battery, and applying a DC/DC converter to generate the output voltage as a DC voltage.

39. The method according to claim 38, wherein coupling the rotor comprises controlling an excitation current supplied to the rotor responsively to feedback from the DC DC converter.

40. A method for generating electrical power using an alternator having a rotor and one or more stator coils, the method comprising: coupling the rotor to be rotated by a source of rotational motion so as to generate a rotating magnetic field; inductively coupling the one or more stator coils to generate, responsively to the rotating magnetic field, an AC voltage at a first voltage level; stepping down the AC voltage to generate an output voltage at a second voltage level, such that the first voltage level is at least 50% greater than the second voltage level; and generating a control input to the alternator using the output voltage.

41. The method according to claim 40, wherein the first voltage level is at least four times the second voltage level.

42. The method according to claim 41, wherein the first voltage level is at least ten times the second voltage level.

43. The method according to claim 41, wherein the first voltage level is approximately 14 VDC, and wherein the second voltage level is at least 78 VAC.

44. The method according claim 40, wherein generating the control input comprises controlling an excitation current supplied to the rotor using the output voltage.

45. The method according to claim 40, wherein stepping down the AC voltage comprises coupling at least one primary coil of a transformer to receive the AC voltage from the stator, and coupling a rectifier to at least one secondary coil of the transformer so as to rectify the output voltage.

46. The method according to claim 45, wherein stepping down the AC voltage further comprises generating an auxiliary voltage at an auxiliary voltage level that is greater than the second voltage level, and wherein generating the control input comprises generating the control input using the auxiliary voltage.

47. The method according to claim 46, wherein generating the control input comprises sensing a condition of the rotor, and applying the auxiliary voltage to the rotor responsively to the condition so as to increase a strength of the rotating magnetic field.

48. The method according to claim 40, wherein stepping down the AC voltage comprises coupling a transformer to step down the AC voltage, and providing at least first and second output voltages on respective secondary windings of the transformer, wherein the second output voltage is provided at the second voltage level, and the first output voltage is substantially different from the second voltage level.

49. The method according to claim 40, wherein generating the control input comprises sensing the first voltage level, and generating the control input responsively to the first voltage level.

50. The method according to claim 49, wherein generating the control input comprises sensing an occurrence of a fault responsively to the first voltage level, and cutting off an excitation current to the rotor responsively to the fault.

51. The method according to claim 40, wherein stepping down the AC voltage comprises rectifying the AC voltage generated by the stator coils so as to generate a DC voltage at a DC voltage level that is at least twice the second voltage level, and coupling a DC/DC converter to generate the output voltage as a DC voltage at the second voltage level.

52. The method according to claim 25, wherein generating the control input comprises generating the control input responsively to feedback from the DC/DC converter.