WO2001073922A2

WO2001073922A2 - Improved inductor-type ac power generator

Info

Publication number: WO2001073922A2
Application number: PCT/IL2001/000283
Authority: WO
Inventors: Alexander Radovsky
Original assignee: S.H.R. Limited Bvi
Priority date: 2000-03-27
Filing date: 2001-03-27
Publication date: 2001-10-04
Also published as: WO2001073922A3; AU4450601A; JP2001275321A

Abstract

An inductor-alternator adapted to generate high-frequency AC power, the alternator being provided with a stator having a circular array of major teeth, each major tooth being slotted to define several minor teeth. Surrounding each major tooth in the array is an armature coil. The array is divided into sets each having a like number of major teeth. Surrounding each set of major teeth is an excitation winding to which an excitation voltage is applied to establish lines of magnetic flux passing through the minor teeth. Associated with the stator is a rotor having a circular series of small rotor teeth that project to face the minor teeth on the stator. When the rotor is driven, the rotor teeth then travel past the minor teeth on the stator to intercept the lines of magnetic flux, thereby inducing in the armature winding a high-frequency voltage

Description

IMPROVED INDUCTOR-TYPE AC POWER GENERATOR

FIELD OF INVENTION

This invention relates generally to AC power generators, and in particular to an inductor-alternator adapted to generate AC power having a high frequency.

BACKGROUND OF THE INVENTION Modern power stations make use of synchronous AC generators whose output is stepped up by a transformer and conveyed as a high AC voltage over transmission lines to a remote receiving station without significant IR losses. At the receiving station, the high-voltage AC is stepped down by a transformer to a locally useful level. The transmission of high voltage AC power requires a four- wire transmission line (three phases and zero). When the transmission line extends over long distances, this gives rise to significant reactance and problems due to the capacitance between adjacent wires in the line and between wires and the ground as well as line inductivity. An AC power generator in accordance with the invention is of the inductor- alternator type. A conventional inductor-alternator is a synchronous generator in which an excitation DC voltage is applied to field windings. The AC windings are distributed throughout stator slots whereby a voltage is induced therein in accordance with Faraday's law by changes in magnetic flux caused by changes in position of the rotor teeth.

In a conventional synchronous AC electric generator, the rotor winding is connected to a DC current source via rings and brushes. The reason that rings and brushes are necessary in the conventional synchronous machine is to provide electrical power from a stationary DC current source to a moving rotor winding. As the rotor is rotated, the changing magnetic field created by the DC current rotates along with the rotor, inducing an electromotive force (EMF) in the stator winding.

This known design suffers from several drawbacks. First, the rings and the brushes eventually wear out and must be replaced. Second, parts of the stator winding of the generator called "winding ends", protrude beyond the armature. These winding ends do not participate in the energy transformation process and unless the windings are made of superconductors, the winding ends contribute to resistance losses. In addition, the associated magnetic fields create eddy currents in electrical conductors outside of the armatures. These eddy currents are an additional drain on the power output of a generator.

To obviate the need for coupling electrical power out of the rotor of an inductor-alternator, the alternator disclosed in U.S. Patent 4,401,939 to Korbell provides a rotor that rotates within a stator. The rotor and stator have poles formed thereon at spaced positions on their facing surfaces, the rotor poles moving past the stator poles when the rotor is turned. Field coils to which an excitation voltage is applied are wound about the field poles of the stator to create an electromagnetic field about each pole. Armature coils are wound about a portion of the same field poles of the stator, a voltage being induced in the armature coils as the rotor poles travel past the stator poles. Hence, in this prior inductor-alternator, no sliding contacts are required to deliver the AC output of the alternator.

Also, disclosing a brushless synchronous generator is the Radovsky U.S. Patent 5,952,759. And of prior art interest is the inductor-alternator disclosed in U.S. Patent 3,714,480 to Voldemazovich et al. in which the stator is provided with slots accommodating both a polyphase excitation winding and an armature winding. Inductor- alternators are known in which the excitation winding is disposed in relatively big slots in the stator whereas the armature winding is placed in multiple small slots. Thus, U.S. Patent 3,564,312 to Bournea discloses a medium frequency inductor-alternator in which the excitation winding is contained in a plurality of large stator slots while the armature winding is placed in small slots in the stator. The drawback to this arrangement is that were a heavily insulated high-voltage winding crowded into these small slots, the resultant lack of space would preclude adequate cooling of this winding.

While an AC inductor alternator, because its frequency is high, is inherently capable of producing a voltage having a very high magnitude, the capacity of the generator to do so may be inhibited by the inability of the insulated AC windings to sustain this high voltage without breakdown.

In my pending U.S. patent application SN 09/535,542, there is disclosed an inductor- alternator adapted to generate AC power having a high frequency. The stator of this generator is provided with a circular array of large teeth separated from each other by relatively large spaces, each of these large teeth having a head slotted to define a set of small teeth. An armature winding and an excitation winding disposed in these spaces surround each large tooth of the stator. The armature winding is connectable to an external load and the excitation winding is connectable to an excitation voltage source to establish about each of the large teeth lines of magnetic flux.

A rotor associated with the stator is provided with a circular series of small projecting teeth which face the small teeth of the stator, whereby when the rotor is driven, its small teeth cyclically scan the small teeth of the stator to modulate the lines of magnetic flux and induce in the armature winding a high-frequency AC voltage which is applied to the load.

Because in this inductor-alternator, each of the stator large teeth must be surrounded both by an armature coil and excitation coil which are received in the spaces between adjacent teeth of the stator, this requirement makes it difficult to manufacture the generator. It also raises the manufacturing cost.

Moreover when both armature and excitation windings surround each large tooth of the stator and occupy the spaces between adjacent teeth, the windings are then displaced from the rotor to a degree which reduces the magnetic coupling between the stator and the rotor with a resultant increase in flux leakage and a loss in efficiency.

SUMMARY OF THE INVENTION

In view of the foregoing, the main object of this invention is to provide an efficient, reliable and low cost inductor-alternator adapted to generate high- voltage AC power having a relatively high frequency and a voltage of any desired magnitude, even as high as 1000 kilovolts.

A significant advantage of an alternator in accordance with the invention is that, though much smaller in size, it is capable of producing the same amount of electrical energy as the largest AC synchronous generator. And the more compact alternator is far less costly to manufacture and to install.

More particularly, an object of this invention is to provide a brushless inductor-alternator in which the excitation and armature windings are associated with the stator, the rotor being free of windings, thereby obviating the need for sliding contacts to supply power to the rotor.

Also an object of this invention is to provide a power transmission system whose power station is provided with an inductor-alternator which generates high-voltage AC power of high frequency, which power is rectified to yield a high-voltage DC that is conveyed over a two-wire transmission line to a remote receiving station. At the receiving station the incoming high DC voltage is converted into standard low-frequency AC power. Or the DC high voltage power 5 produced at the power station, may be inverted to an AC high voltage having a standard low frequency (50 or 60 Hz) and conveyed over existing three-phase transmission lines.

Yet another object of the invention is to provide an inductor-alternator in which the insulated high-voltage windings are concentrated rather than being _l o distributed and therefore can be cooled without difficulty.

A further object of the invention is to provide an inductor- alternator whose AC windings are protectively enveloped in insulating receptacles, making it possible for the alternator to generate very high voltages without breakdown.

Briefly stated, these objects are attained in an inductor-alternator adapted

₁5 to generate high-frequency AC power, the alternator including a stator provided with a circular array of relatively large major teeth, each major tooth being notched to define several minor teeth. The array is divided into sets each containing a like number of major teeth. Surrounding each major tooth in the array is an armature winding and surrounding each set of major teeth is an 0 excitation winding to which an excitation voltage is applied to establish a magnetic field whose lines of flux pass through the minor teeth.

Rotatable within the stator is a rotor having a circular series of small rotor teeth which face the minor teeth on the stator and have the same tooth pitch. When the rotor is driven, the rotor teeth then travel past the minor teeth on the 5 stator to intercept the lines of magnetic flux, thereby inducing in the armature windings an AC voltage whose frequency depends on the number of rotor teeth and the rotor speed. BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention as well as other objects and further features thereof, reference is made to the attached drawing in which:

Fig. 1 schematically illustrates a first embodiment of an inductor- alternator in accordance with the invention;

Fig. 2 illustrates a second embodiment of the inductor-alternator;

Fig. 3 shows a system in accordance with the invention for transmitting power from a power station provided with a driven, high-voltage, high-frequency inductor-alternator to a remote receiving station; Fig. 4 schematically illustrates a third embodiment of the invention;

Fig. 5 is a perspective view of a half-section of a further embodiment of an inductor-alternator in accordance with the invention; and

Fig. 6 is a section taken through a transverse plane in the generator shown in Fig. 5.

DETAILED DESCRIPTION OF INVENTION

First Embodiment: Referring now to Fig. 1, shown therein is an inductor-alternator in accordance with the invention which includes a rotor 10 mounted on a shaft 11 driven by an engine or turbine M. Rotor 10 is disposed within an annular stator 12, and when driven rotates within the stator to cause the alternator to generate a high-frequency AC power whose voltage may have any desired magnitude. The rotor and stator can be fabricated of the same metals as those included in existing inductor-alternators. Hence the metals hereinafter identified, are only by way of example.

Cylindrical rotor 10, which may be made, for example, of laminated steel, has an outer periphery which is notched or slotted to define a circular series of equi-spaced small rotor teeth 13. Stator 12 which is annular in form and may be created by laminated sheets of ferromagnetic material, has an inner periphery that is slotted by large slots S to define a circular array of large teeth 14, (hereinafter referred to as major teeth) the number of these teeth depending on the number of phases and the number of phase zones, such as six major teeth. Each major tooth is notched to create a set of several equi-spaced small teeth 15 (hereinafter referred to as minor teeth).

In a preferred arrangement, the small equi-spaced teeth 13 on the outer periphery of rotor 10 match the minor teeth 15 on the head of each major tooth 14 on the stator and therefore have the same pitch, this being determined by the spacing between the successive small teeth. The relationship of the machine tooth pitch t_p to the teeth pitch t_t is expressed by the following equation:

t_p = (n+k + 1/3) t_t, where n is the number of minor teeth in a major tooth and k - 0, 1, 2, 3... for a three-phase generator.

Therefore when the minor teeth on a particular pair of major teeth are in line with the teeth on the rotor, then the minor teeth on the other motor teeth are displaced by 1/3 t_t relative to the rotor teeth.

Surrounding major tooth 14, and disposed within the large slots S between successive major teeth, is an armature or AC coil 16. When the rotor is driven, an AC EMF is induced in the armature windings which are connected to an external load 17 to supply power thereto. Surrounding each armature coil 16 and concentric therewith is an excitation winding 18 that is connected to an excitation voltage source 19.

In practice, one may protectively enclose the armature windings in an insulating receptacle formed of ceramic or other dielectric material. The receptacles are filled with a dielectric gas or fluid, making it possible to produce very high voltages without breakdown. And in practice, the DC excitation winding may be situated within the AC winding or these windings may be arranged in tandem relation.

When rotor 10 is driven by engine M and caused to rotate within stator 12, then each tooth 13 on the rotor, in the course of a full revolution of the rotor, scans the sets of minor teeth 15 on the heads of the stator major teeth. In doing so, the rotor teeth successively modulate the magnetic flux passing through the major teeth of the stator to induce an EMF in the armature windings associated therewith. The resultant AC voltage produced by the alternator exhibits a high frequency. The greater the number of rotor teeth and the greater the speed of the rotor, the higher the frequency of the generated AC power.

As will be evident from the equation below, the higher the frequency, the greater is the magnitude of the output voltage V from the alternator.

V = π V2 -/- N - S - B_m> wherein: V is the output voltage of the alternator,

/ is the frequency of the output voltage,

N is the number of turns in the armature winding,

S is the area of the magnetic core section,

B_m is the amplitude of the alternating induction. In induction synchronous generators, the amplitude of the alternating component of induction is about 30 to 45 percent of the maximal excitation magnitude.

With an increase in frequency of the generator, the resultant gain in the output voltage is considerable. For example, a typical existing alternator operates at a frequency of 50 Hz, whereas the frequency of an alternator in accordance with the invention may, for example, be 1600 Hz. Even though its induction is three times less than in the existing alternator, its gain is still more than ten fold greater. This makes it possible to substantially reduce the number of turns in the armature winding and hence the quantity of coil wire required, as well as the area of the steel core.

In this way, one can produce at a much lower cost a compact inductor generator whose power output matches that of a much larger alternator. And because the armature and excitation windings are in the form of coils or solenoids which fit over the major teeth of the stator rather than being threaded thereover, the alternator can easily be fabricated and assembled. It is to be noted that in the embodiment of the alternator shown in Fig. 1, the stator has an array of six major teeth 14 to create the six phase zones of a three-phase machine. However, the invention is not limited to this number.

It is important to note that in an alternator in accordance with the invention, the AC winding is accommodated in large stator slots, making it possible to construct the alternator with an AC winding capable of carrying very heavy currents and to produce exceptionally high voltages. This is not possible with conventional alternators having small slots, which receive the AC winding.

The stator has k-m major teeth, k being equal to 1, 2 or 3 which defines the number of phase zones, m being the number of phases. Each major tooth 14 is divided into several minor teeth 15, the pitch of which is equal to that of the teeth 13 of rotor 10. The number

of minor teeth on stator 12 and the number Z of small teeth on rotor are such as to ensure the following relationships:

Z₂ = a(aι + 1/m) + 2K where: "a" is the number of major teeth "ai" is the number of minor teeth in a major tooth

K = 0, 1, 2, 3 m = number of phases Because there is space available in the large stator slots which accommodate the excitation and armature coils 16 and 18, in practice the armature coils can be protectively encapsulated in an insulating capsule such as a ceramic capsule filled with a dielectric fluid. This makes it possible to design the AC generator to operate at an exceptionally high voltage.

Second Embodiment: In this embodiment which is illustrated in Fig. 2, the relationship of rotor 20 to stator 21 is reversed, but the resultant inductor-alternator otherwise works in the same way as the alternator shown in Fig. 1. In this embodiment, the array of major teeth 22 of the annular stator 21 project from the outer periphery of the stator. The heads of major teeth 22 of this stator are each notched to create a set of minor teeth 23.

The minor teeth 23 in stator 21 face the circular series of small projecting teeth 24 formed on the inner periphery of rotor 20. Rotor 20 is concentric with stator 21 and therefore rotates outside of the stator. The armature and excitation coils (not shown in Fig. 3), surround the major teeth of the stator in the same manner as in Fig. 1, these coils being accommodated in the large slots between successive major teeth 22.

One advantage of the arrangement in which the outer diameter of rotor 20 is greater than that of stator 21 is that the more massive large diameter rotor driven by a diesel engine is capable of functioning as an inertial flywheel to stabilize speed of rotation of the engine and in doing so stabilizing the output voltage of the alternator.

The System: The conventional practice in which AC power from a generating station is conveyed to a receiving station over a high voltage AC transmission line has several drawbacks, one of which is the need for a step-up transformer at the power station. Another disadvantage of the conventional AC power transmission system is the need for multiple wires to carry multi-phase AC power. Moreover, AC power transmission gives rise to reactive network problems as well as system stability problems.

In the system shown in Fig. 3, an inductor-alternator 25 in accordance with the invention is driven by a turbine or an engine 26. The high-frequency, high-voltage AC power supplied by alternator 25 is applied through a rectifier 26 to a two wire DC transmission line 28 which conveys the rectified high voltage to a receiving station 29. At this station, the incoming high DC voltage is fed to a DC to AC inverter whose AC output is fed to load 17. The use of a two-wire transmission line 28 to transmit a high voltage DC without reactance problems clearly has advantages over a four- wire line transmission needed to transmit a three-phase AC voltage. Moreover, in such a system there is no stability problem.

A heavy load current passing through the armature winding of the inductor-alternator 25 shown in Fig. 3 tends to limit the magnetic flux in the core produced by excitation winding 16 and to demagnetize the core. As a consequence, under heavy load condition, the output voltage of the inductor-reactor will be reduced.

To overcome this drawback, a portion or all of the output of rectifier 27 coupled to alternator 25 is fed to an auxiliary or secondary excitation winding 30 which acts to stabilize the magnetic flux of the core. (Fig. 1 does not show the winding 30 included in the machine).

The inventive concept underlying the inductor-alternator disclosed in this generator resides in an arrangement in which the stator of the generator has a circular array of the major teeth each surrounded by both an excitation and an armature winding, the head of each major tooth being slotted to define a set of minor teeth.

The same inventive concept can be applied to the construction of a generator having a relatively low rotational speed producing a high-frequency current by increasing the number of rotor teeth. It is also applicable to the construction of a very low-speed generator producing a standard 50 or 60 Hz frequency output. Also the inventive concept may be embodied in the construction of low-speed synchronous motors and low-speed synchronous generators including generators driven by wind or by water power.

Third Embodiment: In the first embodiment of the invention illustrated in Fig. 1, each major tooth 14 of the stator of the inductor-alternator is surrounded by an armature or AC coil 16. Concentric with each armature winding on the stator teeth is a DC excitation winding 18 connected to a DC excitation source 19. Surrounding the excitation coil in the modified arrangement disclosed in Fig. 3 is a secondary excitation coil 30 to which a portion of a rectified DC voltage is applied to stabilize the magnetic flux of the coil.

This arrangement is somewhat difficult to manufacture and also adds to manufacturing costs, for it is necessary to mount on each major tooth of the stator in slots S spacing adjacent poles both an armature winding and an excitation winding as well as a secondary excitation winding. And since all three windings are crowded into spaces S between adjacent major teeth, they are displaced from the rotor of the generator.

To overcome this drawback and also to make it easier and less expensive to manufacture a high-frequency inductor-alternator, in the embodiment of the generator illustrated in Fig. 4 it is only armature windings 16 which surround the major teeth 14 of the stator.

In this embodiment, the annular stator 12 is provided with a circular array of six major teeth 14 for a three-phase machine, the head of each tooth being notched to create several minor teeth 15. Surrounding each one of the six major teeth 14 is an armature coil 16 which is received in spaces S between adjacent major teeth. When rotor 10 is driven by engine M, an AC EMF is then induced in the armature coils to supply power to the external load connected to the armature windings.

However in the embodiment shown in Fig. 4 unlike that shown in the Fig. 1, there is no excitation winding concentric with each armature winding surrounding each stator major tooth. In the Fig. 4 embodiment, the circular array of stator major teeth 14 is divided into sets, each set including a like number of major teeth. In the case of a machine having two magnetic poles, one set is composed of three major teeth 14 in the left half section of the annular stator 12, while the other set is composed of the three major teeth 14 included in the right half-section of the stator. Each set of major teeth constitutes an arcuate segment of the stator. In practice, the number of magnetic poles may be 2, 4, 6,...

Surrounding each segment of the stator and conforming thereto is a single excitation winding 18E which encircles all three poles in the set. In Fig. 4, only shown is excitation winding 18E in the major teeth set on the left half-section of the stator. But in practice, there is also an excitation winding on the right half-section. Excitation windings 18E are connected to DC excitation voltage source 19.

In practice the excitation winding may be wound on a plastic carrier molded to conform to the segment of the stator containing the set so that it is only necessary when assembling the generator to slip the excitation winding onto the set.

When rotor 10 which is slotted to define a circular series of teeth 13 is driven by engine M and caused to rotate within stator 12, then each rotor tooth in the course of a full revolution of the rotor, scans the series of minor teeth 15 on the heads of the major teeth of the stator. In doing so, the rotor teeth successively modulate the magnetic flux passing through the teeth of the stator to induce an EMF in the armature windings associated with the major teeth of the stator. The AC voltage produced in this manner by the alternator exhibits a high frequency. The greater the number of rotor teeth and the greater the speed of the rotor, the higher the frequency of the generated AC power.

Because excitation winding 18E is shaped to encircle the three poles (one pair of N-S poles, 3 phases, 6 phase zones) of the set and is conformed to fit neatly onto and to conform to an arcuate segment of the stator, when in place it is then in close proximity to the rotor. As a consequence, the magnetic coupling between the rotor and stator is strong and leakage flux is not high.

Fig. 4 shows a stator having a circular array of six major teeth 14 divided into two sets, each set including three major teeth. When in practice a stator has a greater number of poles, such as twelve poles, then a greater number of sets is required, such as four sets, each having three poles. Each of these sets is fitted with a single excitation winding.

An inductor-alternator of the type shown in Fig. 4 is relatively easy to assemble, for it is only necessary to slip onto each pole in the stator an armature solenoid, and then slip over each set of poles an excitation coil which conforms to a segment of the stator.

Fourth Embodiment: The single-piece annular stator of the generator shown in Fig. 4 in which the circular array of major teeth lie within the stator make it necessary to provide an excitation winding that can be inserted inside the stator and then placed over an arcuate segment thereof containing a set of major teeth so as to encircle these teeth. This dictates an arcuate form to the side portions of the excitation winding which must conform to the arcuate shape of the segment. It becomes necessary when assembling this generator to operate within the confines of the annular stator when mounting the armature and the excitation windings.

In the embodiment of an inductor-alternator shown in Figs. 5 and 6, the structure of stator makes it easier to assemble the generator, for it is composed of identical lower and upper half-sections which are joined together, only the lower half-section 12L being shown in Figs. 5 and 6. Each half-section is received in a semi-cylindrical casing C having outwardly-projecting flanges F extending from opposite sides thereof. The flanges have bolt holes H to receive bolts to join the upper and lower sections together. In practice, the half-sections may include other expedients to hold them together.

The configuration of the stator shown in Figs. 5 and 6 is the same as that shown in Fig. 1 in which the stator has a circular array of six major teeth 14 equi-spaced by notched spaces S. Surrounding each major tooth in the array is an armature winding 16 X, only one of which is shown in Fig. 6. Armature winding 16X is displaced toward the axis of the electric machine, thereby enhancing the magnetic coupling between the stator and the rotor and diminishing flux leakage.

The three major teeth 14 in the half-section segment of the stator shown in

Figs. 5 and 6 are encircled by an excitation winding 31A which may also include a secondary excitation winding. Excitation winding 31A has a generally rectangular, frame-like form which is dimensioned to nest in the stator half section just below flanges F and to surround cylindrical rotor 10 so that the sides of the winding are closely adjacent the sides of the rotor and the ends of the winding are adjacent the rotor ends.

Placed above excitation winding 31A in the lower half section 12L of the stator is a like excitation winding 3 IB for the upper half-section of the stator (not shown). In order to avoid contact with the shaft 10R of rotor 10 which extends through the center of the rotor, the ends of the excitation windings 31A and 3 IB are bowed.

The close proximity of these excitation coils in the stator to rotor 10 acts to strengthen the magnetic coupling between the stator and rotor and to minimize flux leakage, thereby to provide a highly efficient generator. The arrangement shown in Figs. 5 and 6 is easy to assemble, for the major teeth in the half sections are exposed and it is a simple matter to install the armature and excitation windings thereon.

While there have been disclosed preferred embodiments of the invention, it is to be understood that many changes may be made therein without departing from the spirit of the invention. Thus while the alternator disclosed herein is adapted to operate as a power station generator, an alternator of essentially the same design may be constructed to function as an automobile alternator. And the alternator may simply be associated with a rectifier to provide a DC voltage supply for those applications which require such a supply.

Claims

CLAIMS:

1. An inductor-alternator adapted to generate AC power having a high frequency comprising:

A. a stator provided with a circular array of major teeth, each being slotted to define several smaller minor teeth; said array being divided into at least two sets each containing a like number of major teeth;

B. an armature winding surrounding each of said major teeth and connectable to an external load;

C. an excitation winding surrounding each set of major teeth and connectable to an excitation voltage source to establish lines of magnetic flux about said minor teeth; and

D. a rotor associated with the stator provided with a circular series of small rotor teeth facing the minor teeth of the stator, whereby when the rotor is driven, the rotor teeth cyclically scan the minor teeth of the stator to modulate the lines of magnetic flux and induce in the armature winding a high- frequency AC voltage which is applied to the load.

2. An alternator, as set forth in Claim 1, in which the stator is formed of ferromagnetic material.

3. An alternator, as set forth in Claim 2, in which the stator is formed by laminated metal sheets.

4. An alternator, as set forth in Claim 1, in which the rotor teeth have the same pitch as the minor teeth on the stator.

5. An alternator, as set forth in Claim 1, in which said armature winding is in the form of a coil that is mounted on each of said major teeth.

6. An alternator, as set forth in Claim 1, further including an auxiliary excitation winding mounted on said stator to which is fed a rectified load current.

7. An alternator, as set forth in Claim 1, in which the circular array of major teeth is formed on the inner periphery of an annular stator, and the circular series of rotor teeth is formed on the outer periphery of the rotor, the rotor being disposed within the annular stator.

8. An alternator, as set forth in Claim 1, in which said excitation winding is in close proximity to said rotor whereby the magnetic coupling between the stator and the rotor is strong.

9. An alternator, as set forth in Claim 1, in which said stator is formed by a single annular piece of core material from which is projected a circular array of six major teeth, each of said sets of major teeth having three major teeth to define a stator segment which is surrounded by said excitation winding.

10. An alternator as set forth in Claim 9, in which said segment has an arcuate shape, and said excitation winding has a corresponding shape.

11. An alternator as set forth in Claim 1, wherein said annular stator is constituted by half-sections which are joined together.

12. An alternator, as set forth in Claim 11, in which each section is provided with a pair of opposing flanges, the flanges of the sections being joined together to complete the stator.

13. An alternator, as set forth in Claim 12, in which the excitation winding for each half-section has a rectangular frame-like shape and is nested therein adjacent the ends thereof.

14. An alternator as set forth in Claim 13, in which the rotor has a cylindrical form and is supported by a rotor shaft, and said rectangular excitation winding nesting in said half section surrounds the rotor so that the sides of winding are close to the sides of the rotor and the ends of the winding are close to the ends of the rotor.

15. An alternator as set forth in Claim 14, in which the ends of the rectangular excitation winding are bowed to avoid contact with said motor shaft.

16. An alternator as set forth in Claim 1, in which the armature winding surrounding each major tooth is displaced toward the rotor to improve the magnetic coupling between the rotor and the stator.