Current limiting means for a generator Number:7,436,098 from the United States Patent and Trademark Office (PTO) owispatent

Title: Current limiting means for a generator

Abstract: The invention provides means of limiting maximum current conducted through windings of an electric machine having a rotor and a stator. By encouraging an appropriate leakage flux around a winding, a leak impedance can be achieved which may be used, according to the invention, to limit the maximum current in the winding as a matter of machine design.

Patent Number: 7,436,098 Issued on 10/14/2008 to Dooley

---

Inventors:	Dooley; Kevin Allan (Mississauga, CA)
Assignee:	Pratt & Whitney Canada Corp. (Longueuil, Quebec, CA)
Appl. No.:	11/832,121
Filed:	August 1, 2007

---

Related U.S. Patent Documents

---

	Application Number	Filing Date	Patent Number	Issue Date
	11103610	Apr., 2005	7309939
	10393252	Oct., 2006	7119467

---

Current U.S. Class:	310/201 ; 310/204; 310/254
Current International Class:	H02K 3/04 (20060101)
Field of Search:	310/201,204,242,254

---

References Cited [Referenced By]

---

U.S. Patent Documents

911713	February 1909	Frankenfield
1756672	April 1930	Barr
3707638	December 1972	Nailen
3753068	August 1973	Walker, Jr.
3812441	May 1974	Sakamoto et al.
3961211	June 1976	Vergues
4004202	January 1977	Davis
4025840	May 1977	Brissey et al.
4032807	June 1977	Richter
4039910	August 1977	Chirgwin
4190794	February 1980	Mikulic
4237395	December 1980	Loudermilk
4250128	February 1981	Meckling
4311932	January 1982	Olson
4346335	August 1982	McInnis
4392072	July 1983	Rosenberry
4401906	August 1983	Isobe et al.
4445061	April 1984	Jackson, Jr.
4492902	January 1985	Ficken et al.
4503377	March 1985	Kitabayashi et al.
4511831	April 1985	McInnis
4547713	October 1985	Langley et al.
4562399	December 1985	Fisher
4605874	August 1986	Whiteley
4617726	October 1986	Denk
4625135	November 1986	Kasabian
4638201	January 1987	Feigel
4709180	November 1987	Denk
4763034	August 1988	Gamble
4799578	January 1989	Matsushita
4852245	August 1989	Denk
4887020	December 1989	Graham
4896756	January 1990	Matsushita
4897570	January 1990	Ishikawa et al.
4924125	May 1990	Clark
4994700	February 1991	Bansal et al.
5030877	July 1991	Denk
5124607	June 1992	Rieber et al.
5184040	February 1993	Lim
5214839	June 1993	Rieber et al.
5235231	August 1993	Hisey
5304883	April 1994	Denk
5397948	March 1995	Zoerner et al.
5519275	May 1996	Scott et al.
5555722	September 1996	Mehr-Ayin et al.
5583387	December 1996	Takeuchi et al.
5585682	December 1996	Konicek et al.
5714823	February 1998	Shervington et al.
5729072	March 1998	Hirano et al.
5742106	April 1998	Muraji
5770901	June 1998	Niimi et al.
5793137	August 1998	Smith
5793178	August 1998	Biais
5798596	August 1998	Lordo
5822150	October 1998	Kelsic
5825597	October 1998	Young
5834874	November 1998	Krueger et al.
5838080	November 1998	Coiderchon et al.
5903115	May 1999	Taylor
5912522	June 1999	Rivera
5917248	June 1999	Seguchi et al.
5925999	July 1999	Lakerdas et al.
5936325	August 1999	Permuy
5942829	August 1999	Huynh
5952757	September 1999	Boyd, Jr.
5953491	September 1999	Sears et al.
5955809	September 1999	Shah
5962938	October 1999	Bobay et al.
6097124	August 2000	Rao et al.
6100620	August 2000	Radovsky
6114784	September 2000	Nakano
6124660	September 2000	Umeda et al.
6177747	January 2001	Maeda et al.
6239532	May 2001	Hollenbeck et al.
6242840	June 2001	Denk et al.
6249956	June 2001	Maeda et al.
6255756	July 2001	Richter
6265801	July 2001	Hashiba et al.
6271613	August 2001	Akemakou et al.
6286199	September 2001	Bobay et al.
6291918	September 2001	Umeda et al.
6313560	November 2001	Dooley
6323625	November 2001	Bhargava
6342746	January 2002	Flynn
6369687	April 2002	Akita et al.
6373162	April 2002	Liang et al.
6414410	July 2002	Nakamura et al.
6429615	August 2002	Schmider et al.
6437529	August 2002	Brown
6459186	October 2002	Umeda et al.
6504261	January 2003	Fogarty et al.
6525504	February 2003	Nygren et al.
6541887	April 2003	Kawamura
6965183	November 2005	Dooley
7081697	July 2006	Neet
7309939	December 2007	Dooley
2001/0030483	October 2001	Masumoto et al.
2002/0074871	June 2002	Kikuchi et al.
2002/0074889	June 2002	Kikuchi et al.
2002/0084705	July 2002	Kawamura
2002/0084715	July 2002	Kakuta et al.
2002/0093252	July 2002	Kang et al.
2002/0093269	July 2002	Harter et al.
2002/0096960	July 2002	Tong et al.
2002/0149281	October 2002	Saint-Michel et al.

Foreign Patent Documents

	0 022 379		Jan., 1981		EP
	368930		Apr., 1994		EP
	754365		Feb., 1998		EP
	750806		Aug., 1998		EP
	0 881 744		Dec., 1998		EP
	0 932 246		Jul., 1999		EP
	1555855		Dec., 1968		FR
	2618616		Jul., 1987		FR
	5-316677		Nov., 1993		JP
	6-22478		Jan., 1994		JP
	6-105488		Apr., 1994		JP
	07-123621		May., 1995		JP
	11-89197		Mar., 1999		JP
	11-164500		Jun., 1999		JP
	2000-166152		Jun., 2000		JP
	2001-25197		Jan., 2001		JP
	2001-86668		Mar., 2001		JP
	2001-186728		Jul., 2001		JP
	2001-186738		Jul., 2001		JP
	2002-51490		Feb., 2002		JP
	2002-153029		May., 2002		JP
	2002-325382		Nov., 2002		JP
	2002-354731		Dec., 2002		JP
	2002-369479		Dec., 2002		JP
	2003-88007		Mar., 2003		JP
	2003-164089		Jun., 2003		JP
	2003-180044		Jun., 2003		JP
	2003-199267		Jul., 2003		JP
	2003-204638		Jul., 2003		JP
	2003-264996		Sep., 2003		JP
	99/09638		Feb., 1999		WO
	99/66624		Dec., 1999		WO
	02/09260		Jan., 2002		WO
	03/003546		Jan., 2003		WO
	03/028202		Apr., 2003		WO

Other References

General Electric Company, "150Kva Samarium Cobalt VSCF Starter/Generator Electrical System, Final Technical Report", 1979. cited by other . M. Cronin, "The All-Electric Airplane as Energy Efficient Transport", SAE Journal, 1980. cited by other . Richter, E. et al., "Jet Engine Integrated Generator"Amcn Inst. Aeronautics & Astronautics, 1981. cited by other . B. Dishner et al., "A Novel Electromechanical Approach to Constant Frequency Power Generation", IEEE Journal, 1989. cited by other . M. Cronin, "The All-Electric Airplane Revisited", SAE Technical Series, 1989. cited by other . SAE Technical Paper Series 892252, Application Consideratins for Integral Gas Turbine Electric Starter/Generator revisited, 1989. cited by other . The Applicability of Electrically Driven Accessories for Turboshaft Engines, 1993. cited by other . R. Nims, "Development of an Oilless, Gearless, and Bleedable Under Armor Power Unit" ASME paper, 1995. cited by other . Richter et al., "Preliminary Design of an Internal Starter/Generator for Application in the F110-129 Engine", SAE Aerospace Atlantic Conference, 1995. cited by other . R. Nims, "Armor-plated auxiliary power", Mechanical Engineering, 1997. cited by other . PCT International Search Report, PCT/CA2004/000408, Mailed Aug. 30, 2004. cited by other.

Primary Examiner: Schuberg; Darren
Assistant Examiner: Mohandesi; Iraj A
Attorney, Agent or Firm: Ogilvy Renault LLP

---

Parent Case Text

---

CROSS-RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 11/103,610, filed Apr. 12, 2005, now U.S. Pat. No. 7,309,939 which is a continuation of U.S. patent application Ser. No. 10/393,252, filed Mar. 21, 2003, now U.S. Pat. No. 7,119,467, issued Oct. 10, 2006, all of which is hereby incorporated by reference.

---

Claims

---

What is claimed is:

1. A method of making a stator assembly for an electrical machine, the method characterized in that it includes the steps of providing a stator having a first face and a second face, the first face adapted to be positioned adjacent a rotor assembly such that an annular rotor air gap separates the rotor assembly from the stator assembly, the second face having a first set of slots defined therein and adapted to receive electrical windings therein; inserting at least one electrical winding into the first set of slots; providing a back iron adapted to be mounted to the second face; mounting the back iron and stator to one another to provide a stator assembly; and then providing a second set of slots in the first face of the cylindrical stator, the second set of slots communicating with at least some of the first set of slots, the second set of slots having a circumferential width Gg, a radial height Hg and an axial length Lg, defining an area of the second slots as a function of Hg and Lg, said stator assembly allowing for a leakage magnetic flux to flow across said second slot and around the at least one winding when current flows in said winding, this leakage magnetic circuit having a given leakage inductance, said leakage magnetic flux inducing a counter-voltage in said winding of polarity in opposition to the main voltage that causes the primary current, further characterized in that the second slots are sized by selecting given values of circumferential width Gg or area of the slot so as to have an associated leakage inductance such that the flux across said slot and around the winding induces a counter-voltage of a given magnitude so as to limit a maximum output current in the at least one winding to a pre-selected value, the pre-selected value selected to be below a current at which the machine is thermally damaged by heat generated in the at least one winding.

---

Description

---

TECHNICAL FIELD

The technical field relates to means of limiting the maximum current, including a short circuit current, in windings of an electric machine such as a generator or motor.

BACKGROUND OF THE ART

The need for preventing short circuit overloading of circuits in electric generators and motors is well recognized. Protective external circuits and equipment are often provided with fusible materials or electronic controls external to the windings of the electric machine, however internal short circuit conditions may occur within the windings of a motor/generator that would not be detected or controlled by external fusing or controls.

One approach to providing protection within the windings is disclosed in the inventor's U.S. Pat. No. 6,313,560. Due to the heat generated by high currents and high rotational speeds of machines used in "more electric" aircraft engines, the electric motor/generator and the insulated wiring conductors must be sufficiently protected from the unlikely event of internal faults during operation, to ensure redundant safety systems exist. Therefore, it is desirable to build in failsafe means for controlling maximum machine current.

DISCLOSURE OF THE INVENTION

Oftentimes, the stator of an electric machine has a plurality of slots, each slot having windings and having a slot gap adjacent to the rotor and in communication with an annular air gap separating the rotor from the stator. The improved machine and method use a leakage flux phenomenon (which is considered in transformer design but not considered in machine design) to limit the current in an electric machine. By controlling the leakage flux impedance, the maximum current in a winding can be controlled by this phenomenon since the leakage flux induces a secondary voltage across in the winding which has opposite polarity to the primary voltage, thereby permitting the current to be controllably reduced/limited. For example, to encourage and control leakage flux the designer may vary the gap width; axial length; radial height; gap surface area; gap surface topography; and gap magnetic flux permeability.

The prior art teaches that flux leakage is inefficient, and to be avoided, reduced or compensated for wherever possible, to increase machine efficiencies and reduce unwanted magnetic interactions, etc. See, for example, U.S. Pat. No. 5,955,809 and U.S. published patent application US2003/0042814. The improved machine and method, therefore, depart significantly from the prior art by encouraging a leakage flux, and do so for the novel purpose of controlling a machine maximum current, for example in the event of an internal short circuit.

In one aspect, there is provided an electric machine operable as an alternator, the machine comprising: a rotating magnetic rotor assembly; and a stationary stator assembly mounted adjacent the rotor assembly such that an annular rotor air gap separates the rotor assembly from the stator assembly, the stator assembly including a plurality of lengthwise extending slots on a cylindrical surface of the stator assembly and opposed to the annular rotor air gap, the stator assembly further including at least one electrical winding disposed in at least one of the lengthwise extending slots defined in the stator assembly, the at least one electrical winding being electrically connected to a machine output adapted to deliver generated output electricity from the machine, the slots having slot gaps on the stator surface thus defining teeth members, the teeth members being disposed between the at least one winding and the rotor, the slot gaps having a circumferential width Gg, a radial height Hg and an axial length Lg, defining an area of the gap as a function of Hg and Lg, wherein, in use, movement of the rotor assembly induces a first alternating voltage and current in the at least one winding, characterized in that said slot gap is sized by selecting given values of circumferential width Gg or area of the gap so as to have an associated leakage self-inductance such that, in use, a leakage magnetic flux flows across said slot gaps and around the at least one winding in order to induce in said winding a counter-voltage of a given magnitude in response to said induced alternating current in said winding, thereby limiting a maximum output current in the at least one winding to a pre-selected value.

In another aspect, there is provided a method of making a stator assembly for an electrical machine, the method characterized in that it includes the steps of providing a stator having a first face and a second face, the first face adapted to be positioned adjacent a rotor assembly such that an annular rotor air gap separates the rotor assembly from the stator assembly, the second face having a first set of slots defined therein and adapted to receive electrical windings therein; inserting at least one electrical winding into the first set of slots; providing a back iron adapted to be mounted to the second face; mounting the back iron and stator to one another to provide a stator assembly; and then providing a second set of slots in the first face of the cylindrical stator, the second set of slots communicating with at least some of the first set of slots, the second set of slots having a circumferential width Gg, a radial height Hg and an axial length Lg, defining an area of the second slots as a function of Hg and Lg, said stator assembly allowing for a leakage magnetic flux to flow across said second slot and around the at least one winding when current flows in said winding, this leakage magnetic circuit having a given leakage inductance, said leakage magnetic flux inducing a counter-voltage in said winding of polarity in opposition to the main voltage that causes the primary current, further characterized in that the second slots are sized by selecting given values of circumferential width Gg or area of the slot so as to have an associated leakage inductance such that the flux across said slot and around the winding induces a counter-voltage of a given magnitude so as to limit a maximum output current in the at least one winding to a pre-selected value, the pre-selected value selected to be below a current at which the machine is thermally damaged by heat generated in the at least one winding.

Still other aspects will be apparent from the attached description and figures. The invention is applicable to at least an electric machine with an internal rotor and a surrounding external stator, where the stator slots extend axially on an internal surface of the stator; and an electric machine with an internal stator and a surrounding external rotor, where the stator slots extend axially on an external surface of the stator. Application to other machines is also taught.

DESCRIPTION OF THE DRAWINGS

In the figures:

FIG. 1 is an example of an axial sectional perspective view of a stator and rotor, as improved, with axially extending slots housing conductor windings, and showing the axially extending slot gaps.

FIG. 2 is an example of a detailed axial sectional view showing two stator slots with conductor windings and slot gap width dimension (G.sub.g), also showing the circulation of leakage flux with arrows.

FIG. 3 is a detail perspective view of a flux conducting surface of a slot gap of FIG. 2 indicating the axial length dimension (L.sub.g) and the radial height dimension (H.sub.g) of the slot gap.

FIG. 4 is a partial axial sectional view showing an example of an alternative stator construction using a cylindrical internal back iron and T-shaped stator members to define stator slots and slot gaps independent of the width of conductors.

FIG. 5 is an enlarged cross-section of a rotor and stator according to the prior art.

FIG. 6 is an enlarged cross-section similar to FIG. 5, however instead depicted is the improved device of FIG. 1.

FIG. 6a is an enlarged portion of FIG. 6;

FIG. 7 is an enlarged cross-section of an example of a rotor and stator according to a further alternate embodiment of the present invention.

FIG. 8 is an enlarged isometric view of an example of a stator according to yet further alternate embodiment of the present invention.

FIG. 9 is a graph representing an example of the relationship between maximum current, leakage inductance and slot gap width, as improved.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The improved apparatus and method are used for controlling and/or limiting a primary circuit current, including a short circuit current, through the windings of an electric machine such as a motor, generator or alternator, either permanent magnet or otherwise.

By using a physical behaviour of the electromagnetic circuit in a machine, known as the leakage flux, it is possible to limit maximum currents in the windings of the machine. Leakage flux is well-known and understood in transformer design, but heretofore the machine designer has typically only considered flux leakage in the pursuit of minimising such leakage to improve machine efficiency and prevent unwanted interactions (e.g. magnetic coupling) between permanent magnet rotors and other subsystems.

In the improved machine, however, leakage flux is encouraged as a means to limit current inside the machine--as a matter of design. By creating a leakage flux which counters the primary flux path, the maximum current in the machine may be limited. Leakage flux is encouraged through the choice and selection of certain parameters of the machine's configuration, as will be better described below.

In one embodiment, the given effect results through design control of the stator slots. While it is indeed well-known to provide winding slots in a stator, the configuration and design steps taken to provide such slots are both believed to be novel. In a second aspect, the stator slots are filled with a slot cap material. These and other specific embodiments will now be discussed.

Referring first to FIG. 5, a prior art stator 50 and permanent magnet rotor 52 are shown schematically and, for ease of depiction only, are both shown partially and as planar bodies. Rotor 52 has magnets 54 arranged in and alternating `North` and `South` pole arrangement, and is separated from stator 50 by a rotor air gap 56. Winding or windings 58 are provided in slots 60 between adjacent teeth 62 in stator 50. Each slot 60 has a winding gap 64 which permits windings 58 to be inserted into slot 60, typically by an automated winding machine, as is well known in the art. The winding gap 64 is typically at least several winding-widths (or diameters) wide, to permit the winding to be provided accurately and efficiently in each slot 60. In use, as rotor 52 passes over stator 50, a primary magnetic flux path 66 is set up, as magnetic flux travels from rotor 52, across rotor air gap 56, down a particular tooth 62, around slots 60 and along the core of stator 50 and then back up a further successive tooth 62 to rotor 52, across rotor air gap 56. As air has a magnetic permeability of almost zero, and because the winding gap 64 is quite large (relatively speaking), almost no magnetic flux passes along the stator face (i.e. adjacent rotor air gap 56), because such flux is impeded by the winding gap 64. Magnetic flux along such a "tooth-top" path is also intentionally impeded, by design, to ensure the primary flux path usefully passes around windings 58.

Referring now to FIGS. 1-4, FIG. 1 illustrates an example of an internal stator 1 for placement inside an external rotor 10. In the embodiment shown, the stator 1 has a plurality of stator slots 2 extending axially on an external cylindrical surface of the stator 1. Stator slots 2 have a slot gap 7 on a surface 8 to be disposed adjacent rotor 10. As will be described in greater detail below, however, unlike the prior art the slot gaps 7 are not provided for the purpose of inserting winding conductors 3 into stator and, in fact, slot gaps 7 are preferably as narrow, and more preferably narrower than, the width of an individual winding conductor 3. For example, a slot gap 7 accordingly to the present invention may be only 0.040'' in width (depending on a machine's design), which will clearly be understood by the skilled reader to be too narrow to permit windings to be inserted therein by prior art winding methods. This embodiment is thus clearly distinguished from prior art winding gaps, as will be discussed in greater detail below. The invention is also applicable to an electric machine with an internal rotor and a surrounding external stator and, in such a case, the stator slots 2 would of course extend axially on an internal surface of the stator (ie. adjacent the rotor), as will be apparent from this description. The invention applies to a motor and a generator/alternator.

FIGS. 1 and 4 illustrate the general structure of an improved stator 1, having a cylindrical back iron 4 upon which are mounted axially extending T-shape teeth members 5 to define rectangular axially extending stator slots 2 to house a plurality winding conductors 3. Stator 1 is thus a composite stator composed in this case of back iron 4 and stator 5. The stator 1 has a plurality of stator slots 2 where each slot 2 houses conductor windings 3. Each slot 2 has a slot gap 7 adjacent to the rotor 10 and in communication with an annular rotor air gap 20 (see FIG. 6) separating the rotor from the stator 2.

Referring now to FIG. 6, in use, as rotor 10 passes over stator 1, a primary magnetic flux path 20 is set up, as magnetic flux travels from rotor 10, across rotor air gap 22, down a particular tooth 5, around slots 2 and along back iron 4, and then back up a further successive tooth 5 back to rotor 10, across rotor air gap 22. However, unlike the prior art, a secondary or leakage flux 6 flow is encouraged around each group of winding conductors 3 (in this embodiment), by reason of the increased leakage inductance caused by the width of the winding gap 7 in relation to the magnitude of the current passing through winding 3. The secondary or leakage flux 6 is proportional to the current flowing through winding 3 and in a direction opposite to the primary flux 20. This phenomenon, as it is presently understood, will now be described in greater detail.

As indicated in FIGS. 2 and 4, when the machine is operated, the primary current conducted through the windings 3 generates a magnetic leakage flux circuit 6 about the periphery of the stator slot 2 and passing across the slot gap 7 as best shown in FIG. 4. It will be understood by those skilled in the art that the high permeability of the materials used to construct the back iron 4 and the T-shaped member 5 such as silicon iron, have a tendency to confine the magnetic circuit, (although as indicated in FIG. 4, a certain amount of magnetic flux fringing 9 will occur in the peripheral edges of the slot gap 7). Materials preferred by the inventor for construction are: samarium cobalt permanent magnets, maraging steel (preferably 250 or 300) retention sleeve, aluminum yoke, copper primary and secondary windings, silicon iron, SM2 or other soft magnetic material for the stator teeth and laminated silicon steel for the back iron. The stator material is rigid. Slot 2 is sized sufficiently to house conductors 3. Preferably, slot gap 7 is sized to provide a suitable leakage flux 6, as will now be described.

Referring again to FIGS. 2 and 4, the leakage flux 6 circulation is indicated with arrows. FIGS. 2 and 3 show details of the slot gap 7 parameters in a simple embodiment having an axially extending slot gap 7 of width dimension G.sub.g having an axial length dimension of L.sub.g and a radial height dimension of H.sub.g. The gap surface area may be derived by multiplying L.sub.g.times.H.sub.g. However, it will be understood in light of this disclosure, that the gap geometry and surface topography may be varied considerably, for example as shown in FIG. 8, to include ridges, or other surface features to improve or otherwise affect the transmission of magnetic flux or vary the distribution of magnetic flux extending across the slot gap 7. Further, the magnetic permeability and therefore the gap flux density can be adjusted by the selection of materials and dimensions for the back iron 4 and T-shaped members 5. Alternately, a slot gap cap may be provided to cap slot gap 7 partially or entirely, as described further below.

Optionally, the stator material including the back iron 4 and or T-shaped member 5 can be selected such that at least a portion of the stator has a Curie temperature which is below maximum design operating below a temperature for the machine, such that (according to the teachings of the inventor's U.S. Pat. No. 6,313,560, the teachings of which are fully incorporated into this disclosure by reference) the magnetic flux circulation through the stator material will be impeded when the stator material acquires a temperature above the Curie temperature. Such design is preferably configured such that the secondary or leakage flux 6 is less affected by the Curie-point `effect` than the primary flux path. It is preferably that the inventor's Curie-point effect be maximized for primary flux flow and minimized for secondary or leakage flux flow to gain the most satisfactory benefit from the use of such feature with the present invention. In any event, the improved machine may be used in conjunction with one or more means to thereby assist in providing maximum current protection to an electric machine.

The multi-piece stator of FIGS. 1-4 and 6 may be provided in the following steps: providing by any suitable means, a stator ring 5 having slots 2, wherein slots 2 are on the side opposite rotor face 8; providing windings 3 into slots 2; mounting back iron 4 to the tooth-ring by any suitable method (e.g. by bonding); and then providing slot gaps 7 by any suitable method (e.g. cutting).

Referring to FIG. 6a, the path 6 that the leakage flux travels around winding 3 may be thought of as comprising several path components--in this case components A-D. In the discussion above and below, the skilled reader will understand in light of this disclosure that, although the inventor prefers to focus design attention in applying the present invention to the portion of the stator denoted as path D in FIG. 6a, one or more path components may be designed appropriately implement the present invention.

Referring to FIG. 7, an example of a second embodiment is shown. The reference numerals defined above will also be used to denote the analogous features in this embodiment. In FIG. 7, stator 1 is composed of a single piece (i.e. back iron 4 and teeth 5 are integral with one another) and winding 3 comprises a single conductor. Also in this embodiment, a winding gap 30 may be partially or completely capped or filled by one or more filler or slot cap members 32, optionally composed of a material having higher magnetic permeability than air, but less permeability than the stator material, and thus permitting a sufficient leakage flux 6 to be induced, in use to permit the current in windings 3 to be limited to a desired level, as was described with respect to the previous embodiment. It will be understood, however, that in this embodiment slot cap members 32 replace (preferably completely) winding gaps 7, in both space and function. The designer may select the slot cap and stator materials and dimensions according to the teachings of this disclosure to manage the leakage inductance, and thus limit the maximum current of the machine, in a manner as will now be described in more detail.

Referring now to FIGS. 1-4, the means by which the invention limits the primary current conducted through the windings 3 of the stator 2 will be described below.

The present method involves controlling the slot gap parameters to provide a desired a leakage flux inductance and thus, leakage impedance. The leakage flux induces a secondary voltage in the windings 3 of polarity in opposition to the primary current, in accordance with "Lenz's law", which dictates that an electric current induced by a changing magnetic field will flow such that it will create its own magnetic field that opposes the magnetic field that created it. In the prior art, however, the winding's "own magnetic field" was not permitted to flow in any appreciable way by reason of stator geometry (i.e. the air gap was too wide). The inventor has recognized, however, that if an induced leakage flux is encouraged to encircle a winding, the effect of the opposing of magnetic fields on winding voltage can be used advantageously to limit the maximum possible current flowing through the winding, thus providing the machine designer with a tool to providing intrinsic short circuit protection.

In the prior art, the winding gap (with air therein between) was so large that very high magnetizing forces (B) would be required to force a magnetic flux to cross the gap. The magnetic permeability of air is, of course, very low, being essentially that of free space (.mu..sub.O). At prior art winding gap distances, insufficient magnetomotive forces (mmf) was provided by the magnetic circuit to force a magnetic flux to cross the winding gap. It is of course understood that, in prior art designs, as the machine size grows, so too can the associated magnetizing forces, however winding size also increases to handle the increased current load, and therefore so too does winding gap size. Thus, regardless of machine size, the magnetic permeability of the winding air gap remains a barrier to a magnetic flux path in the portion of stator (including the air gap) between the windings and the rotor.

It is generally known that the internal impedance of an electric generator/alternator governs the short circuit current of the machine. As is well understood, the internal impedance is related to the number of winding turns, magnetic flux parameters, among other things, and is commonly referred to as the "commutating inductance" or "commutating impedance" of the electric machine. Although a machine's commutating inductance typically determines the machine's short circuit current, it is well-understood that this property cannot be effectively used by the designer to control the maximum current inside the electrical generator/alternator, since increasing the commutating inductance also increases the voltage generated at a given speed. The short circuit current is calculated by voltage/commutating impedance, which both increase in proportion to speed (i.e. frequency). Therefore, increasing the commutating inductance also increases the voltage generated at a given speed and as such will not limit the short circuit current.

However, as improved, the total impedance of the electric machine can also be increased (to thereby limit maximum current) by an additional phenomenon, namely the "leakage inductance" of the machine. The leakage inductance has an associated leakage impedance which is independent of the machine's commutating inductance impedance. Therefore, leakage inductance can be used to adjust the maximum current with little effect on the unloaded output voltage of the machine. The leakage flux is a direct result of the current passing through the windings 3 and, according to Lenz's law, is opposite in polarity. Thus, leakage flux can be used to control the short circuit current through the windings 3. The leakage inductance is proportionally to leakage flux and thus varying the inductance permits the designer to achieve a flux sufficient to attain the necessary voltage to limit the current appropriately in the machine. The leakage inductance of a permanent magnet alternator (or any machine) can be accurately defined and controlled by defining an appropriate shape or configuration for the stator, and by selecting appropriate materials for construction of the stator. For example, the designer may ensure the secondary magnetic path has sufficient inductance by varying the gap 7 parameters between the winding slots 2. The leakage inductance (and hence the leakage impedance) can also be adjusted in design, according to the designer's wishes by, for example, varying the width G.sub.g of the slot gap 7 or area of the gap 7 (i.e.: H.sub.g.times.L.sub.g) (in the case of FIGS. 1-4), or by varying the permeability of the slot gap 7, i.e. by selecting a slot cap material and dimensions (in the case of FIG. 7). This, the inductance of the secondary magnetic circuit (i.e. the leakage path) can be selected in view of the expected current in the winding, in use, to encourage a sufficient leakage flux around the winding to induce the necessary counter-voltage to limit the maximum current in the winding to a value acceptable to the designer. Some iteration may be required in design.

It will be understood that complete elimination of the slot gap 7 is not preferred unless a slot cap material has a lower magnetic permeability and/or impedance than the stator teeth-back iron, so as not to detrimentally affect the primary magnetic circuit. It will also be understood that in this specification, including the claims, the terms "shape", "configuration" and "construction" are used to refer to stator design parameters such as dimensions, relative proportions, material selection (which may include the selection of "air" as a "material" in the case of the selection of an air gap), magnetic permeability, and so on, which affect the amount of magnetic flux which travels around the windings through the stator as the result of a current flow in cond