GB2205693A

GB2205693A - Alternating current generator for automotive vehicles

Info

Publication number: GB2205693A
Application number: GB08813442A
Authority: GB
Inventors: Kiyoshi Tsuchiya; Yoshikazu Hashimoto
Original assignee: Mitsuba Electric Manufacturing Co Ltd
Current assignee: Mitsuba Corp
Priority date: 1987-06-08
Filing date: 1988-06-07
Publication date: 1988-12-14
Anticipated expiration: 2008-06-07
Also published as: GB2205693B; GB8813442D0; DE3819341A1; CA1303106C; FR2616278A1

Description

f-& 4v U j (,. 0., I 1 - z ALTERNATING CURRENT GENERATOR FOR AUTOMOTIVE

VEHICLES The present invention relates generally to an alternating current generator f or use in automotive vehicles such as cars, buses, trucks, motorcycles and the like, and in particular to improvements in the design of rotor cores f or such a generator.

Generally, the generator of the aforedescribed type includes a pair of rotor cores rotatably accommodated in and supported on a housing, a rotor winding wound around the rotor cores, a stator core disposed in and fixedly secured to the housing so as to encompass the rotor cores with a gap formed therebetween, and stator windings wound around the stator core. Each rotor core comprises a body portion and a plurality of claw-like pole-pieces formed integrally therewith so as to extend axially thereof. The rotor cores are coupled together and disposed with the pole-pieces of one of the rotor cores arranged alternately with those of the other rotor core.

In the prior art generator, however, each rotor core is so formed that when viewed radially of the stator core, its pole-pieces entirely overlap the stator core with their distal ends located on a common plane on which the corresponding end of the stator core lies, and hence the whole length of each pole-piece is inevitably great. As a result, the weight of the rotor cores is increased unduly.

Besides, inasmuch as the pole-pieces are subjected to a great centrifugal force when the rotor cores are rotated at a high speed, they are liable to be distorted outwardly. To this end, the gap between the stator core and the pole-pieces has to be increased sufficiently to prevent the pole-pieces from contacting the stator core, so that the manufacture of the generators that develop a higher output power is obstructed.

The prior art design of the rotor cores also has another drawback that the distal end portions of the pole-pieces are sutjected to a vibration since they alternately repulse and attract the stator core due to the known alternating arrangement of teeth and slots of the stator core, thereby increasing undesirable noise.

It is therefore an object of the present invention to provide an alternating current generator which can positively prevent the distortion of pole-pieces of rotor cores as well as the production of vibration, and which is lightweight and can achieve a higher output power.

According to the present invention, there is provided an alternating current generator for. automotive vehicles, comprising a housing; a stator core disposed in and fixedly secured to the housing, the stator core being of a generally cylindrical shape having a prescribed length and having front and rear ends; stator coil means disposed around the stator core; a core shaft having an axis of rotation therethrough and rotatably supported on the housing; a pair of front and rear rotor cores fixedly mounted on the core shaft for rotation therewith, each of the rotor cores having a body portion and a plurality of claw-like pole-pieces formed thereon in circumferentially equally spaced relation and extending along the core shaft,'the pair of rotor cores being coupled together such that the pole-pieces of the front rotor core are arranged alternately with the pole-pieces of the rear rotor core, each of the rotor cores having an overlap portion of a prescribed length overlapping the stator core as viewed radia,lly of the stator core; and rotor winding means disposed on the rotor cores; characterized in that each pole piece of the front rotor core is dimensioned to have a distal end spaced forwardly of the stator core from the rear end while each pole-piece of the rear front rotor core is dimensioned to have a distal end spaced rearwardly of the stator core from the front end, in such a manner that an overlap ratio of the length of the overlap portion of each rotor core to the length of the stator core is from about 0.88 up to smaller than 1.0.

Reference is now made to the accompanying drawings, wherein:- Fig. 1 is a partially cutaway side elevation of an alternating current generator in accordance with the present invention; Fig. 2 is a perspective view of a rotor core used in the generator of Fig. 1; Fig. 3 is a schematic side elevation of one of pole- pieces of the rotor core of Fig. 2; Fig. 4 is a diagrammatical illustration showing the arrangement of the pole-piece of the rotor core with respect to a stator core; Fig. 5 is a diagrammatical illustration showing the distortion of the pole-piece of the rotor core; Fig. 6 is a graphical presentation showing the relationship between the overlap ratio and the amount of distortion; Fig. 7 is a graphical presentation showing the relationship between the overlap ratio and the output current; and Fig. 8 is a graphical presentation showing the relationship between the tapering ratio and the output current.

Referring to Figs. 1 to 4, there is illustrated one embodiment of an alternating current generator, generally designated at 10, in accordance with the present invention, which is similar in its basic structure to the prior art generator. Specifically, the generator 10 comprises a housing 12 or a pair of case brackets, a core shaft 14 having an axis of rotation therethrough and rotatably supported on the case brackets 12, a pair of Lundell type front and rear rotor cores 16 and 16 fixedly mounted on the core shaft 14 for rotation therewith, a bobbin 18 mounted on the rotor cores, a rotor coil or winding 20 wound around the bobbin 18, a generally cylindrical stator core 22 received in and fixedly secured to the housing 12, and stator coils or windings 24 wound therearound. Each rotor core 16 comprises a body portion 26 including a disc-shaped end plate 26a and a reduced-diameter cylindrical portion 26b formed on one surface of the end plate 26a so as to extend coaxially therewith, and a plurality of claw-like pole-pieces 28 formed integrally with the end plate 26a of the body portion 26 in circumferentially equally spaced relation and extending axially thereof. The front and rear rotor cores 16 and 16 are coupled together, with the end faces of the cylindrical portions 26b mated with each other, in such a manner that the pole-pieces 28 of the front rotor core are arranged alternately with those of the rear rotor core, and are disposed in such a manner that the pole-pieces 28 of the front and rear rotor cores are surrounded by and opposed to the stator core 20 with a uniform gap G formed therebetween.

The generator 10 further includes slip rings 30 disposed around the core shaft 14 and connected to the rotor winding 20, brushes 32 retained in a holder 34 and adapted to be held in resilient contact with the slip ring 30, a rectifier 36 for rectifying generated alternating current, cooling fans 38 and other known members.

As is the case with the prior art generator, the disposition of the rotor cores 16 and 16 is-such that all the pole-pieces 28 overlap the stator core 22 as viewed radially of the stator core 22. In the generator 10, however, each rotor core 16 is so formed that its pole-pieces 28 have a length smaller than the prior ones have. In other words, each pole-piece 28 of the front rotor core is dimensioned to have a distal end 28a spaced forwardly of the stator core 22 from its rear end. Similarly, each pole-piece of. the rear rotor core is dimensioned to have a distal end 28a spaced rearwardly of the stator core 22 from its front end.

More specifically, each pole-piece 28 of the rotor core 16 is dimensioned to overlap the stator core 22, as viewed radially of the stator core, in such a manner that an overlap ratio 0( given by d/L is selected to satisfy 0 < 0(( 1.0, where L is the axial length of the stator core 22; and d is the overlap length of each rotor core, i.e., the length of that portion of the rotor core disposed in exactly radially opposed relation to the stator core 22. The structural difference between the embodiment of the invention and the prior art generator will be diagrammatically illustrated in

Fig. 4, in which 28' denotes the pole-piece for the prior art generator. As clearly seen from Fig. 4, the rotor core 16 of the generator 10 is provided with an overlap length d2 which is smaller than the overlap length d, for the rotor core of the prior art generator, and hence the overlap ratio 0( becomes smaller than 1.0 in the generator 10 although it is equal to 1.0 in the prior art generator. As will be verified later on by way of an example, the overlap ratio 0( as described above should preferably range from about 0.95 to z about 0.88, more preferably from about 0.93 to about 0.88.

Further, in the generator 10, each pole-piece 28 of the rotor core 16 is tapered from its proximal end 28b to its distal end 28a not only when viewed circumferentially of the rotor core but also when viewed radially thereof, although the outer surface of the pole-piece 28 is defined by a surface of a cylinder coaxial with the core shaft 14 and is hence maintained parallel to the stator core 22 when viewed circumferentially of the rotor core. The pole-pieces in the prior art' generator are also tapered similarly. In the generator 10, however, each pole-piece 28 is tapered as viewed radially of the rotor core 16 in such a manner that a tapering ratio P given by W2 I W1 is preferably set to range from about 0.20 to about 0.45, where W, is the width or circumferential length of the outer surface of the pole-piece at its proximal end 28b; and W2 is the width at its distal end 28a, as illustrated in Figs. 2 and 3.

As described above, in the generator 10, the pole- pieces 28 of the rotor cores are dimensioned to be short as compared with those for the prior art generator.

Accordingly, the rotor cores 16 can be made lightweight sufficiently, and hence the overall weight of the generator can be reduced substantially. Besides, the rotor cores 16 are less susceptible to distortion during the high-speed rotation. Therefore, the gap G between the rotor cores 16 and the stator core 22 can be reduced sufficiently to enhance the efficiency of the generator 10.

In those cases where the pole-pieces 28 are dimensioned to be too short, magnetic flux which is to flow from the rotor core to the stator core would be decreased unduly, so that it may be impossible to ensure a sufficient output power. However, if the overlap ratio is set to be within the above preferable range, the overlap length of the rotor cores would be great enough to ensure that a sufficient amount of magnetic flux flows, thereby permitting the generator 10 to develop a sufficiently high output power.

This may be probably because the magnetic flux flowing from an end face of the distal end 28a of each pole-piece, as shown in Fig. 4, would compensate for lack of magnetic flux which should have been generated from that portion of the prior art rotor core removed therefrom.

In addition, in the preferred embodiment, the pole- piece 28 is tapering from its proximal end 28b to its distal end 28a as viewed radially of the rotor core, with the X5 of about 0.20 to about 0.45. As will be tapering ratio / verified later on by way of an example, this tapered construction ensures that the generator 10 develops a maximum output power. Furthermore, the pole-pieces 28 of the rotor cores 16 of the aforedescribed particular construction have been found to be less susceptible to vibration, and hence noise produced by the generator.10 can be reduced substantially.

The present invention will now be illustrated more detailedly by way of the following examples.

Example 1

There were prepared various generators which had different overlap ratios but had the same tapering ratio of 0.30. Then, to demonstrate the efficacy of the generators of the invention, the rotor cores were rotated at prescribed speeds. and the amount of distortion H of the pole-pieces of the rotor cores as well as the butput current A was measured.

Specifically, as regards the amount of distortion H of pole pieces, the rotor cores were rotated at a high speed, i.e., at 20,000 rpm. On the other hand, when measuring the output current A, they were rotated at a low speed (2,000 rpm) which corresponded.to the idling speed of the engine.

The results obtained are graphically depicted in Figs 6 and 7, with the first showing the relationship between the amount of distortion H and the overlap ratio C and Fig. 7 showing the relationship between the output current A and the overlap ratio 0(. It is observed from Fig. 6 that the amount of distortion H decreases in a linear fashion as the overlap ratio C decreases. Fig. 4 also depicts the amount of distortion diagrammatically,, wherein H2 and d2 respectively designate the amount of distortion and the overlap length for the rotor core with an overlap ratio of 1.0 while H, and d, respectively designate the amount of distortion and the overlap length for the rotor core with an overlap ratio of 0.88. On the other hand, from Fig. 7, it is observed that the output current A is maintained high until the overlap ratio decreases to about 0.88, but it gradually decreases as the overlap ratio CK further decreases below the value.

From these results, it can be concluded that even though the pole-pieces 28 of the rotor cores 16 should be dimensioned to be short, not only is the amount of distortion reduced, but the output current A is kept high, as long as the overlap ratio Ce is maintained within the range of no smaller than about 0.88. Further, in order to reduce the amount of distortion practically sufficiently, the overlap ratio 0( should be no greater than 0.95, more preferably no greater than 0.93.

Example 2

There were prepared various generators having different tapering ratios P by varying the width W2 Of the pole-piece at its distal end for a fixed width W, at its proximal end.

Thereafter, the rotor cores were rotated at a low speed, i.e., at 2,000 rpm, and the output current A was measured.

The results obtained are graphically depicted in Fig. 7, which shows the relationship between the tapering ratio and the output current A.

It can be observed from Fig. 7 that the generator with the tapering ratio P of about 0.35 develops the maximum output current, and the generators with the tapering ratio greater or smaller than such value exhibit a lower output current. Practically speaking, the output current A required for the low-speed rotation zone is considered to be greater than about 42 ampere. Therefore, the tapering ratioshould preferably range from about 0.20 to about 0.45 in order to ensure a high output power.

While the alternating current generator according to the present invention has been specifically shown and described herein, the invention itself is not limited to be restricted by the exact showing of the drawings or the description thereof. It is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

CLAIMS:-

An alternating current generator for automotive vehicles, comprising:

(a) a housing; (b) a stator core disposed in and fixedly secured to said housing, said stator core being of a generally cylindrical shape having a prescribed length and having front and rear ends; (c) stator coil means disposed around said stator core; (d) a core shaft having an axis of rotation therethrough and rotatably supported on said housing; (e) a pair of front and rear rotor cores fixedly mounted on said core shaft for rotation therewith, each of said rotor cores having a body portion and a plurality of claw-like pole-pieces formed thereon in circumferentially equally spaced relation and extending along said core shaft, said pair of rotor cores being coupled together such that the pole-pieces of the front rotor core are arranged alternately with the pole-pieces of the rear rotor core', each of said rotor cores having an overlap portion of a prescribed length overlapping said stator core as viewed. radially of said stator core; and (f) rotor winding means disposed on said rotor cores; characterized in that each pole-piece of said front rotor core is dimensioned to have a distal end spaced forwardly of said stator core from said rear end while each pole-piece of said rear front rotor core is dimensioned to have a distal end spaced rearwardly of said stator core from said front end, in such a manner that an overlap ratio of the length of said overlap portion of each rotor core to the length of said stator core is from about 0.88 up to smaller than 1.0.
2. An alternating current generator according to claim 1, in which each of said pole-pieces has an elongated outer surface defined by a part of a cylindrical plane disposed coaxially with said core shaft, said outer surface having a circumferential width gradually decreasing from a proximal end thereof to the distal end in such a manner that a tapering ratio defined by the ratio of the width at the proximal end to the width at the distal end ranges from about 0.20 to about 0.45.
3. An alternating current generator for automotive vehicles, constructed substantially as herein described with reference to and as illustrated in the-accompanying drawings.

Published 1988 at The Patent Office, State House, 6671 High Holborn, London WCIR 4TP. Further copies may be obtained from The Patent Office.

SaJes Branch, St Marry Cray, Orpington, Kent BR5 3F.D. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1187.