 |
Description  |
|
|
BACKGROUND
The present invention is directed toward alternating current generators.
More particularly, the present invention is concerned with high output
automotive alternators.
Automotive alternators are known utilizing both field coil and permanent
magnet fluxes coupled to a stator coil, hereafter referred to as hybrid
alternators. For example, U.S. Pat. Nos. 4,882,515 (hereafter '515) and
4,959,577 (hereafter '577), both to Radomski and both assigned to the
assignee of the present invention disclose two such hybrid alternators
based upon a Lundell rotor structure having a unitary field coil. Lundell
structures are notorious for undesirable flux leakage paths during power
generation, thereby resulting in less flux coupling through the stator
windings and reduced efficiency. The structure disclosed in '577 reduces
such leakage paths and increases stator flux, however it has magnetic flux
contribution limitations due to magnet size and retention limitations. The
structure disclosed in '515 can successfully accommodate more permanent
magnet material into its structure but still exhibits the undesirable flux
leakage paths characteristic of Lundell structures. However, hybrid
alternators of the variety disclosed in the two patents to Radomski
exhibit the desirable characteristic of low stator iron losses due to
substantial magnet flux shunting through the rotor structure.
Another variety of hybrid alternator is disclosed in U.S. Pat. No.
5,397,975 (hereafter '975) to Syverson and assigned to Ecoair Corporation.
As taught by the disclosure of '975, such a machine has a salient,
multi-pole, field coil controlled rotor portion with each pole supporting
respective windings of the field coil and a permanent magnet portion in
longitudinally spaced relationship to the field coil controlled rotor
portion. The permanent magnet portion maintains a permanent magnet flux
across an air gap between magnet poles and one portion of a divided stator
structure, thereby coupling significant magnet flux through the stator
structure. Such a spaced arrangement disadvantageously exhibits
significant iron losses due to the substantial magnet flux that is
continually coupled through the stator iron. Furthermore, the field coil
controlled rotor portion having multiple winding sets characteristically
exhibits greater power dissipation than unitary field coil winding
configuration such as practiced with Lundell variety rotor structures, and
requires each winding set to develop the magneto-motive force (mmf)
necessary to drive the flux across the air gap thus requiring greater
field currents. Additionally, the salient pole construction of Syverson
has some drawbacks relative to coil retention at rotor speeds typically
encountered in an automotive application and higher inertia due to the
multiple windings.
SUMMARY
Therefore, the present invention provides an alternator having a higher
power output for a given rotor inertia. This is accomplished in an
alternating current generator including a housing, a stator and output
winding, and a rotor having a field coil controlled Lundell portion and an
adjacently placed permanent magnet flux portion. The Lundell portion of
the machine has opposing claw-pole members with interleaved fingers. The
tips of the fingers of a first claw-pole member extend to the back of the
fingers of the second claw-pole member thereby aligning substantially in a
common plane therewith. The permanent magnet portion includes a permanent
magnet carder formed of magnetic material, adjacent the second claw-pole
member and having a plurality of permanent magnet poles at an outer
periphery thereof. Each of the permanent magnet poles is axially aligned
with one of the claw-pole fingers such that an axial space exists
therebetween. The permanent magnet poles alternate polarity, adjacent
permanent magnet poles having opposite magnetic polarity with adjacent
permanent magnet poles having opposite magnetic polarity.
A single field coil positioned between the pair of claw-pole members is
bi-directionally energizable to establish first and second magnetic
polarities of the claw-pole members such that one energization direction
establishes like polarity between each claw-pole finger and the respective
axially adjacent permanent magnet pole, and the other energization
direction establishes dissimilar polarity between each claw-pole finger
and the respective axially adjacent permanent magnet pole.
In accordance with one preferred aspect of the invention, all permanent
magnet poles comprise either a radially magnetized permanent magnet or a
radially magnetized portion of the magnetic material of the carrier.
In accordance with another preferred aspect of the invention, permanent
magnet poles alternate between radially magnetized permanent magnets of
like polarity and a portion of the permanent magnet carder at the outer
periphery thereof.
The permanent magnet carder in one preferred configuration abuts the second
claw-pole member, and in another preferred configuration is magnetically
isolated from the shaft and spaced from the second claw-pole member.
In accordance with another preferred aspect of the invention, the permanent
magnet carrier has a main hub portion that is in spaced adjacency with the
second claw-pole member, and a rim section at the outer periphery thereof
that supports the permanent magnets in extension toward the second
claw-pole member to thereby provide gaps between the second claw-pole
member and the permanent magnets that are less than the gap between the
main hub portion and the second claw-pole member.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view taken along lines 1--1 of FIG. 4 through an
exemplary alternator having one preferred structure and providing flux
detail for a field coil energized in a first direction;
FIG. 2 is a partial sectional view through an alternator embodying the
present invention and providing flux detail for a substantially
unenergized field coil;
FIG. 3 is a sectional view through an alternator embodying the present
invention taken and providing flux detail for a field coil energized in a
second direction;
FIG. 4 is an isometric view of a first rotor embodying various features of
the present invention;
FIG. 5 is an isometric view of a second rotor embodying various features of
the present invention;
FIG. 6 is a sectional view taken along lines 1--1 of FIG. 4 through an
exemplary alternator having another preferred structure and providing flux
detail for a field coil energized in a first direction;
FIG. 7 is a schematic view of an exemplary field coil bi-directional
energization circuit.
DETAILED DESCRIPTION
Referring first to FIG. 4, a first rotor embodying certain features of the
present invention is generally designated by the numeral 16. Rotor shaft 3
1 has affixed thereto a pair of claw-pole members 11 and 13. Claw-pole
member 11 is referred to herein as the first claw-pole member and
claw-pole member 13 is referred to herein as the second claw-pole member.
Each claw-pole member is characterized by a respective hub portion 11A and
13A and a respective plurality of circumferentially spaced claw-pole
fingers 11B and 13B. Each claw-pole finger is joined to a respective hub,
extends axially in one direction therefrom, and terminates at a distal
end. The pair of claw-pole members 11 and 13 are aligned such that the
respective claw-pole fingers of each interleave with those of the other.
The respective geometries of the first and second claw-pole members are
such that the distal ends of the claw-pole fingers 11B of the first
claw-pole member 11 terminate substantially in a plane normal to the rotor
shaft axis common with the proximal ends of the claw-pole fingers 13B of
the second claw-pole member 13 for reasons more apparent at a later point.
Each claw pole member is formed of magnetic material such as steel.
Located between the claw-pole members 11 and 13, and radially inward of
the claw-pole fingers, is a field coil 37. Also affixed to the shaft 31 is
a pair of conventional slip rings 51 for coupling field coil 37 to a
bi-directional current source (not shown) to thereby establish the desired
polarity of the claw-pole members. This portion of the rotor may generally
be referred to herein as the field coil flux portion.
In the present exemplary embodiments, the second claw-pole member 13 has an
overall axial dimension slightly over one-half that of the first claw-pole
member 11, the geometries cooperating to provide for the distal ends of
the claw-pole fingers 11B extending substantially to the back surface of
the second claw-pole member 13. The tips of the claw-pole fingers 11B,
therefore, are substantially in the desired planar alignment with proximal
ends of the claw-pole fingers 13B. This geometry may be seen more clearly
in the sectional view of FIG. 1.
In accord with a first preferred embodiment of the present invention,
affixed to the shaft 3 1 is a permanent magnet carrier 18 (hereafter PM
carrier) formed of a magnetic material such as steel. The rotor structure
illustrated in FIG. 4 shows a first preferred PM carrier 18 wherein a
plurality of permanent magnets 19A and 19B are magnetized radially and
secured to the outer periphery of the PM carrier 18 such as by a suitable
structural adhesive. Additionally, if high speed operation is envisioned,
conventional filament banding may be employed for further retention
integrity. Any of a variety of permanent magnet material may be used for
permanent magnets 19A and 19B such as neodymium-iron, samarium-cobalt, or
ferrite. Each of the permanent magnets 19B is axially aligned with one of
the plurality of claw-pole fingers 11B, and each of the permanent magnets
19A is axially aligned with one of the plurality of claw-pole fingers 13B.
The polarities of the permanent magnets alternate such that adjacent
magnets are of opposite polarity. Therefore, it can be appreciated that
claw-pole fingers 11B have axially aligned therewith permanent magnets 19B
having a first common polarity, and claw-pole fingers 13B have axially
aligned therewith permanent magnets 19A having a second common polarity.
The permanent magnets 19A and 19B are shown having a substantially
trapezoidal shape which provides substantially symmetrical abutting
surfaces at the respective PM carrier to claw-pole finger interface. The
present trapezoidally shaped permanent magnets 19A and 19B are illustrated
herein as an exemplary shape, it being understood that other shapes for
the permanent magnets will be apparent to the skilled artisan.
Referring additionally to FIG. 5 wherein like numerals correspond to like
features described with respect to FIG. 4, a second rotor embodying
certain features of the present invention is also generally designated by
the numeral 16. A second preferred PM carder 18' includes a plurality of
permanently magnetized areas 19A' and 19B' comprising permanently
magnetized portions of the PM carrier 18'. Such a PM carder 18' having
integral permanently magnetized areas 19A' and 19B' can be manufactured,
for example, by selectively patterning the magnetic areas using
conventional electromechanical fixtures. Permanent magnets 19A and 19B and
permanently magnetized areas 19A' and 19B' are hereafter referred to
interchangeably as permanent magnets.
In both FIGS. 4 and 5, permanent magnets, regardless of composition or
structure, define permanent magnet poles at the periphery of the carrier.
The terminology "permanent magnet poles" is not intended to as being
descriptive of the material comprising the poles, rather, of the nature or
characteristic of flux emanating therefrom as distinguished from the field
coil flux portion. The two permanent magnet carrier structures heretofore
described wherein all permanent magnets 19A and 19B comprise either
permanent magnet material or permanently magnetized areas of the steel
carder are given by way of non-exhaustive example. An alternate structure
later described will be seen to have permanent magnet poles comprising
permanent magnet material and permanent magnet poles comprising
non-magnetized steel at the outer periphery of the permanent magnet
carrier.
Turning now to FIG. 1, a sectional view of an alternator in accord with the
present invention is illustrated. The section shown is essentially through
two coincident planes represented by section line designated 1--1 in FIG.
4 in order to more completely illustrate certain details of the rotor
otherwise obscured with a planar sectional view owing to the symmetry of
the rotor. The alternator is generally designated with the numeral 10 in
the figure with the rotor structure again being generally designated by
the numeral 16. A pair of end frames 12A and 12B provide a housing,
support and mounting structure for the alternator. Typically the end
frames are comprised of a non-ferrous material such as aluminum and held
together by a plurality of through-bolts (not shown) in a well known
manner. Each end frame houses a respective bearing 43 and 45 for rotatably
supporting the rotor shaft 31. The end frames also cooperate to retain a
stator assembly comprising a conventional stacked lamination core 23 and
output winding 25 by sandwiching the core therebetween. Conventional slip
tings 51 are electrically coupled to field coil leads 37A and 37B and are
fixedly secured to the rotor shaft 31 in a conventional manner. In
slidable contact with the outer surfaces of the slip rings are
conventional brushes shown generally as part of brush assembly 33. At both
ends of the rotor 16 are cooling fans 39A and 39B for rotation with the
rotor to provide air circulation about and through portions of the rotor
and housing.
First and second claw-pole members 11 and 13 are shown as is the field coil
37 interposed therebetween. Conventionally, a core 14 comprising magnetic
material such as steel is also interposed between the two claw-pole
members and provides an outer mounting diameter for the field coil 37.
This core 14 may be a separate piece as illustrated or may be integral
with one or both of the claw-pole members. PM carrier 18 is shown with
permanent magnets 19A and 19B at the outer periphery thereof. Preferably,
PM cartier 18 also includes a number of axial passages 15 therethrough
which advantageously reduce inertial mass and provide for air circulation
when preferably aligned intermediate the claw-pole fingers 13B. It is
apparent from the sectional view of FIG. 1 that PM carder 18 abuts the
second claw-pole member 13. Since, as previously described, the tips of
the claw-pole fingers 11B of the first claw-pole member 11 are
substantially in planar alignment with the back surface of the second claw
pole member 13, the permanent magnet 19B illustrated in the portion of the
sectional view above the rotor shaft in the figure is adjacent the
claw-pole finger 11B of the first claw-pole member 11. Likewise, the
permanent magnet 19A illustrated in the portion of the sectional view
below the rotor shaft in the figure is adjacent the claw-pole finger 13B
of the second claw-pole member 13. Further, it can be seen that the
polarities of the two respective permanent magnets 19A and 19B shown
sectionally in the figure have opposite polarities. It is here noted that
the placement of the magnets 19A axially with respect to the abutting
surfaces of the second claw-pole member 13 and the PM carrier 18
preferably provides for a small air gap therebetween to prevent certain
undesirable local flux leakage from the magnets adjacent the second claw
pole during power generation.
Further with respect to FIG. 1, it is assumed that the alternator is being
operated with a field excitation current to establish the indicated
polarity of the claw-pole members; that is to say claw-pole fingers 11B
have a north (N) magnetic polarity and claw-pole fingers 13B have a south
(S) magnetic polarity. Excitation current resulting in such claw-pole
polarity is referred to herein as forward excitation or forward field.
This establishes magnetically homopolar pairs of axially aligned pole
fingers and permanent magnets. Thus, flux at the respective outer surfaces
of any pair of axially aligned claw-pole fingers and permanent magnets is
in the same direction. This excitation scenario results in field generated
flux patterns F.sub.F and permanent magnet generated flux patterns F.sub.M
as generally illustrated. Choosing an arbitrary start point of the outer
surface (N pole) of the permanent magnet 19B, the permanent magnet flux is
seen to essentially leave the permanent magnet 19B, cross the air gap
between the rotor and stator, penetrate deeply into the stator core, cross
the air gap to an adjacent, opposite polarity permanent magnet, and close
the path through the magnetic material of the PM carrier 18 to the south
pole of the magnet. Similarly, the flux generated by the field coil 37 is
seen to leave the claw-pole finger 11B, cross the air gap between the
rotor and stator, proceed deeply into the stator core, cross the air gap
to an adjacent, opposite polarity claw-pole finger 13B, and close the path
through the magnetic material of the rotor structure including the hub
portion 13A, core 14 and hub portion 11A. All pairs of circumferentially
adjacent claw-pole fingers and pairs of circumferentially adjacent
permanent magnets have respective flux patterns generally following those
immediately previously described. Notwithstanding the degree of proximity
of the substantial magnetic material of the claw-pole finger 11B to the
permanent magnet 19B, very little flux exchange occurs therebetween since
the polarity and magnitude of the mmf of the claw-pole finger 11B opposes
the adjacent magnet mmf and flux emanating from the outer surface of the
permanent magnet 19B (N pole). The pairs of axially aligned claw-pole
fingers 11B and magnets 19B exhibit inherently more desirable flux
patterns as can be appreciated when examining the polar relationship
between the PM carrier 18 in the locality of the permanent magnet 19B (S)
and the claw-pole member 13, also (S). The polar opposition that these
homopolar localities exhibit serve to advantageously thwart flux leakage
between them by directing respective flux to lower reluctance and more
desirable paths (i.e. those established by respective circumferentially
adjacent poles through the output winding). The adjacent pairs of axially
aligned claw-pole fingers 13B and magnets 19A, however, exhibit inherently
less desirable flux patterns as can be appreciated by similar examination
of the polar relationship between the PM carder 18 in the locality of the
permanent magnet 19A (N) and the claw-pole member 13 (S). The polar
attraction that these abutting dipolar localities exhibit serve to aid
flux leakage F.sub.L into non-beneficial paths (i.e. shunting through PM
carder 18, claw-pole finger 13B and stator core 23).
FIGS. 2 and 3 illustrate additional magnetic circuit features with the aid
of the upper portion of the sectional view of FIG. 1, with new assumptions
of a substantially null field coil excitation current and a field coil
excitation current opposite to that of the forward excitation current,
respectively. The latter excitation current is referred to herein as
reverse excitation or reverse field.
Turning to FIG. 2, a substantially null field coil excitation current is
assumed. Proceeding with such an assumption, any flux crossing the air gap
between the rotor and stator originates with the permanent magnet 19B.
Because of the proximity of the permanent magnet 19B to both the stator
core 23 and the respective claw-pole finger 11B, a portion of the
permanent magnet flux is now shunted through this low reluctance path. In
essence, the stator core is utilized as a shunt path for a portion of the
permanent magnet flux. This permanent magnet flux path during null field
coil excitation advantageously reduces the emf generated in the output
winding through the diversion of a portion of the flux to the magnetic
material of the claw-pole members and advantageously reduces the iron
losses due to the same flux diversion. This, of course, results in less
drag on the vehicle engine and less iron loss induced heat generation. In
practice, what has been referred to herein as null field coil excitation
includes field excitation currents which result in mmf insufficient to
drive the resultant field coil flux across the rotor/stator gap. A general
representation of the flux pattern just described is shown by the
arrow-headed lines in FIG. 2.
Turning to FIG. 3, a reverse field coil excitation current is assumed and
results, in addition to the shunting just described, in even greater
permanent magnet flux diversion away from the output winding since now the
dipolar relationship between the claw-pole finger 11B (S) and the outer
surface of the permanent magnet 19B (N) provides for an even lower
effective reluctance path for the permanent magnet flux. Some of the
permanent magnet flux continues to couple to the output winding to induce
an emf therein. In order that the net emf of the output winding be
reduced, the reverse excitation is increased which both diverts more
permanent magnet flux away from the output winding and imparts a opposite
mmf to the output winding to oppose that of the permanent magnet. Thus,
when the emf in the output winding due to the remaining permanent magnet
flux still coupled thereto is equal and opposite to the emf induced
therein due to the reverse excitation field coil flux coupled thereto, the
alternator output is null. The advantages of the present construction in
this regard are that the reverse excitation requirements for null output
are reduced due to the natural and forced flux diversions and reduced iron
losses. All pairs of adjacent claw-pole fingers and pairs of adjacent
permanent magnets have respective flux patterns generally following those
immediately previously described. However, the pairs of axially aligned
claw-pole fingers 11B and magnets 19B exhibit inherently less leakage flux
outside of the desired shunt path as can be appreciated when examining the
polar relationship between the PM carrier 18 in the locality of the
permanent magnet 19B (S) and the claw-pole member 13 (N). The polar
attraction that these dipolar localities exhibit serve to advantageously
thwart flux leakage outside of the desired shunt path. The adjacent pairs
of axially aligned claw-pole fingers 13B and magnets 19A (see FIG. 1 ),
however, exhibit inherently less desirable flux panels as can be
appreciated by similar examination of the polar relationship between the
PM carrier 18 in the locality of the permanent magnet 19A (N) and the
claw-pole member 13 (N). The polar opposition that these homopolar
localities exhibit serve to aid flux leakage into other paths ousted of
the desired shunt path.
While the configuration shown in FIG. 1 provides high output power with
simple control, and uses flux diversion by virtue of the intended proximal
placement of the magnet carrier 18 to claw pole member 13 to minimize iron
losses, it may still be desirable to reduce the leakage flux F.sub.L. The
leakage flux F.sub.L can be further reduced by employing an additional
alternative configuration. Substitution of each magnet 19A that is in
axial alignment with claw-pole fingers 13B with steel poles provides for
such leakage reduction.. The configuration illustrated in FIG. 4 remains
essentially as shown, however, with alternate placement of permanent
magnet 19B and steel 19A poles. Those skilled in the art will recognize
that the magnet mmf can be altered by choice of magnet material or
redimensioning as desired. The leakage flux F.sub.L illustrated in FIG. 1
is greatly reduced since the local polarity influence of the formerly
placed permanent magnet 19A is eliminated by the deletion of the magnet;
the only local permanent magnet polarity influences remaining being
homopolar with respect to claw-pole member 13 and coming from permanent
magnets 19B.
An alternative preferred embodiment of the present invention is now
described with reference to FIG. 6 wherein like numerals correspond to
like features previously described with reference to the various other
figures. The field coil controlled rotor portion of the machine is
identical to that shown and described with reference to various other
figures and hence the description thereof shall not be repeated here. In
contrast, however, the permanent magnet portion of the machine has bushing
21 fixedly coupled to the rotor shaft and is formed of a suitable
non-magnetic material. Fixedly coupled to bushing 21 is a substantially
annular hub 27 extending from bushing 21 and supporting PM carder 22. Hub
27 and PM carrier are both formed from a magnetic material such as steel.
PM carder 22, hub 27 and bushing 21 are designed such that magnetic
isolation is provided between the magnetic structures by the bushing 21
and the air space 26. By providing magnetic isolation between the PM
carder 22 and the second claw-pole member, undesirable leakage flux path
(F.sub.L in the embodiment shown in FIG. 1) is substantially reduced. Air
space 26 additionally provides a cavity for air circulation. Although not
shown in the illustration for the sake of clarity, it is further desirable
that air circulation passages be formed through the hub portion 27 of the
PM carder to reduce inertial mass and to provide for improved air
circulation.
The PM carrier 22 preferably tapers toward the second claw-pole member near
the outer perimeter forming a circumferentially continuous flange 24. This
flange 24 has an axial dimension substantially equivalent to that of the
permanent magnet 19A and 19B radially abutted thereto and is effective as
a pole piece or flux concentrator for the pole of the radially inward pole
of the permanent magnet. Flange 24 terminates axially to provide for an
air gap between the claw-pole fingers and the permanent magnets 19A and
19B which are substantially aligned with the terminal edge. This air gap
is intended to reduce the amount of permanent magnet flux leaking locally
to a respective axially adjacent claw-pole finger thereby increasing the
amount of permanent magnet flux coupled to the output winding during
forward field. However, since it remains desirable to locate the magnets
proximate to the claw-pole fingers to reduce the length of the flux
diversion shunt paths using the inner periphery of the stator and various
magnetic members of the rotor during null or reverse field, such a
geometry as illustrated allows for substantial optimization of the
desirable magnetic isolation of the body of the hub from the claw-pole
member 13 while allowing a more proximal, yet still spaced, axial
alignment of the permanent magnets with respective claw-pole fingers. An
air gap of substantially 1 mm to 2 mm between the magnet end and the
claw-pole fingers has provided acceptable performance in both these
competing regards. Thus, the embodiment shown in FIG. 6 provides better
utilization of magnet flux--though obtained at the expense of additional
magnetic isolation components--while retaining flux control and flux
diversion via the unitary field coil.
For the sake of completeness, FIG. 7 illustrates an exemplary circuit 100
for supplying bi-directional excitation current to the field coil of an
alternator in accord with the present invention. A battery 111, such as a
conventional 12 volt automotive battery, is shown as the primary
excitation current source for the field coil 37. The skilled artisan will
recognize of course that when the alternator is operating at less than
full output, the system voltage across the battery provides the necessary
excitation current. A first pair of power transistors 101 and 104 are
associated with a first excitation current direction when commanded on and
a second pair of power transistors 102 and 103 are associated with a
second excitation current direction when commanded on. Free wheeling
diodes 105-108 are shown coupled across respective power transistors
101-104 for managing well known inductive effects of the field coil. In
the exemplary circuit, the various transistors can be modulated to adjust
alternator output to compensate and regulate against load and speed
changes as is conventionally practiced using well known techniques. An
exemplary pulse width modulated controller is disclosed in U.S. Pat. No.
4,636,70 to Bowman et al. and is readily adaptable by one having ordinary
skill in the art to provide bi-directional field current control in
conjunction with the exemplary circuit 100 or other alternative circuits
which are known.
While the preceding description sets forth certain preferred embodiments of
the invention, it is to be understood that various alternatives will be
apparent to those having ordinary skill in the art. Therefore, the
embodiments contained herein are to be taken by way of example and not of
limitation, the invention to be limited only by the claims as appended
hereto.
* * * * *
|
|
 |
|
 |
Description |
|
