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Description  |
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INTRODUCTION
This invention relates to inductor alternator rotors, and more particularly
to a high-speed homopolar inductor alternator rotor having poles comprised
of shaped laminations.
In high-speed flywheel energy storage systems, such as are useful in
generating mobile electrical power for vehicular transportation,
limitations on volume and weight of the drive system necessitate use of
small, lightweight alternators with high power output capabilities. This,
in turn, requires that the alternator operate at high speed. Conventional
wound rotor alternators, however, are limited in operating speed since, at
high angular velocities, typically above about 26,000 rpm, mechanical
stresses in the rotor windings become excessive. Solid rotor machines,
therefore, are virtual necessities at high angular velocities since only a
solid rotor can withstand the mechanical stresses incurred at the higher
operating speeds. Typical of such solid rotor machine is the homopolar
inductor alternator. In such machines, the rotor carries no windings and
is basically comprised only of magnetic material, thus facilitating rotor
operating speeds in excess of 90,000 rpm. The excitation field for
establishing the rotor magnetic poles is situated on the stator between
two longitudinally-separated stacks of stator laminations. A multi-phase
AC stator winding is wound through corresponding slots in the two stator
stacks.
In homopolar alternators of this type, voltage is induced in the stator AC
windings due to variation of the air-gap length (i.e., variation in
air-gap permeance) as the rotor poles rotate relative to the stator
windings. The permeance variations cause flux to vary from a very high
value when permeance is large, to a very low value when permeance is
small. Since the DC excitation from the field coil winding induces north
and south poles along the axis of the rotor such that opposite
complementary poles are at longitudinally-separated regions of the rotor,
respectively, it is customary to offset the opposite complementary rotor
poles by 180 electrical degrees from each other in order to induce voltage
of the proper polarity in both individual portions of the stator windings.
Otherwise, the north and south rotor poles would move past a stator coil
side having one half in one portion of the stator windings and the other
half in the other portion of the stator windings at the same time, so that
the voltages induced in the coil side halves, being of opposite
polarities, would cancel each other. Hence it is necessary to offset
mechanically the two rotor halves by an appropriate amount in order to
achieve the desired 180.degree. electrical offset. For an eight pole
stator design, for example, the 180.degree. electrical offset requires a
360.degree./8 or 45.degree. mechanical offset of the two rotor halves.
With the rotor poles mechanically offset by 45.degree. in an eight pole
machine, the center portion of the rotor located between the two offset
rotor pole sections must carry the entire flux from one rotor half to the
other. Being that the center portion of the rotor is necessarily of
smaller outer diameter than the salient rotor poles themselves, it
experiences a high flux density.
The typical high-speed inductor alternator rotor is fabricated of a single
piece of magnetic steel. Consequently, eddy currents are induced in the
pole faces due to the nature of the flux entering the faces. The eddy
currents flowing in each pole face cause electrical loss and attendant
heating of the pole face. Both of these conditions are detrimental to
normal alternator operation. Moreover, because the entire rotor is
magnetic, very large amounts of flux lines do not thread the air gaps, and
thereby fail to contribute to alternator output power, while saturating
the rotor iron.
A common technique for reducing losses due to eddy current flow is to
employ a laminated structure so as to interrupt continuity of the eddy
current paths. In an inductor alternator rotor, this approach heretofore
has not been very beneficial due to the three-dimensional nature of the
flux paths therein which makes it impossible to use planar laminations
that are everywhere parallel to the flux paths. The present invention
overcomes this problem.
Accordingly, one object of the invention is to provide an inductor
alternator rotor employing laminations which are everywhere generally
parallel to flux paths induced in the rotor.
Another object is to provide a high-speed inductor alternator rotor
exhibiting relatively low electrical losses.
Another object is to provide a high-speed, laminated pole, inductor
alternator rotor which exerts essentially no relative torque and
longitudinally-outward forces on nonmagnetic end clamps holding the
magnetic material of the rotor in place.
Briefly, in accordance with a preferred embodiment of the invention, a
rotor for a high-speed inductor alternator having two sets of salient
poles, respectively, at longitudinally-separated regions of the rotor,
respectively, the poles of one set being offset circumferentially by a
predetermined angle from the poles of the other set, comprises a plurality
of nested laminations. Each of the laminations extends continuously from
the face of one pole in one of the regions to the face of an opposite pole
in the other of the regions. Nonmagnetic restraining means constrain the
nested laminations within a fixed radial distance from the rotor
longitudinal axis. An end clamp is situated at each longitudinal end,
respectively, of the nested laminations, each of the end clamps being
configured to abut substantially the entire longitudinally-outer surface
at each end, respectively, of the radially-innermost one of the
laminations.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are set forth with
particularity in the appended claims. The invention itself, however, both
as to organization and method of operation, together with further objects
and advantages thereof, may best be understood by reference to the
following description taken in conjunction with the accompanying drawings
in which:
FIG. 1 illustrates a homopolar inductor alternator rotor fabricated of
solid magnetic material as known in the prior art;
FIGS. 2A and 2B are a fragmentary end view and side view, respectively, of
the rotor configuration shown in FIG. 1, illustrating magnetic flux paths
extending, through a thin segment of magnetic material, from a pole face
in one region of the rotor to a pole face in the other region of the
rotor;
FIG. 3 is a side view of a laminated rotor of overall configuration such as
shown in FIG. 1, but wherein planar laminations are employed in making up
the poles;
FIG. 4 is an end view of a laminated rotor of overall configuration such as
shown in FIG. 1, but having curved laminations extending between a pole
face in one region of the rotor and a pole face in the other region of the
rotor as evident in FIGS. 5 and 6;
FIG. 5 is a partial sectional view of the laminated rotor of FIG. 4, taken
along line 5--5 thereof;
FIG. 6 is a partial sectional view of the laminated rotor of FIG. 4, taken
along line 6--6 thereof, showing nonmagnetic end clamps holding the
laminations in place;
FIG. 7 is a partial sectional view of a laminated rotor of overall
configuration such as shown in FIG. 1, but wherein shaped laminations
extend between each of the pole faces in one region of the rotor and each
of the pole faces in the other region of the rotor, with a nonmagnetic
restraining ring encircling the laminations between the two rotor regions;
and
FIG. 8 is a plan view of a typical unmachined lamination, opened flat, of
the type employed in the structure shown in FIG. 7.
DESCRIPTION OF TYPICAL EMBODIMENTS
In FIG. 1, a conventional rotor 10 for an eight-pole homopolar inductor
alternator is illustrated. This rotor, comprised of solid magnetic steel,
is employable with a conventional wound stator (not shown) of the type
described by E. Richter in the Conference Proceedings of the 1971
Intersociety Energy Conversion Engineering Conference, Boston, Mass.,
pages 132-139, Aug. 3-5, 1971. The rotor is formed with two general
regions 11 and 12, each region including half the total number of rotor
poles. The salient poles in region 11 are circumferentially offset from
the salient poles in region 12 by an angle .theta. which is defined as
360.degree./n, n being the total number of poles on the rotor. For an
eight-pole rotor, as shown in FIG. 1, angle .theta. equals 45.degree..
The only magnetic material required for proper rotor operation is that
which provides flux paths between corresponding pole faces, such as faces
13 and 14 of the rotor shown in FIG. 1. One thin segment of this material
is illustrated in FIG. 2A as it would appear if the rotor, shown in FIG.
1, is viewed longitudinally from the left end. Any more magnetic material
other than similar segments typified by that shown in FIG. 2A is
extraneous to the magnetic circuit for the flux path furnished by the
material of FIG. 2A, and can only increase the leakage flux. FIG. 2B
illustrates rotor magnetic flux paths when the rotor is viewed from the
side.
During operation of the homopolar alternator containing rotor 10, magnetic
flux which enters a pole face in one region exits the opposite
complementary pole face in the other region. Specifically, magnetic flux
entering pole face 13 may exit at pole face 14. Minor variations in the
flux passing through pole faces 13 and 14 due to relative motion of the
stator, produce eddy current flow in the faces, causing electrical loss
and attendant heating of the pole faces. Moreover, since the entire rotor
is magnetic, there exist large amounts of magnetic flux that do not pass
through the air gaps located between the stator and the pole faces on the
rotor. This flux tends to saturate the rotor iron without contributing to
the alternator output. Efficiency of the machine is thereby reduced.
Prior attempts to alleviate the aforementioned problems due to eddy
currents arising when the rotor shown in FIG. 1 is employed, involved use
of planar laminations in the manner illustrated in FIG. 3. This structure
allows much of the rotor intermediate the regions containing the poles to
be fabricated of nonmagnetic material, so that leakage flux can be
reduced. Specifically, a solid ring 25 of magnetic material abuts, and is
situated between, first and second stacks 26 and 27, respectively, of
planar laminations of magnetic steel in order to form a low reluctance
path between radially-consecutive poles formed by the laminations and
separated axially. A nonmagnetic spacer 28 encircles ring 25 and helps
maintain lamination stacks 26 and 27 packed tightly together and oriented
normal to longitudinal axis 30 of rotor shaft 31. The stacks of
laminations are urged toward each other axially by nonmagnetic end clamps
32 and 33 in a manner well known in the art. This configuration, by
employing nonmagnetic spacer 28 to help keep the radially-outer portions
of lamination stacks 26 and 27 uniformly spaced apart from each other,
avoids the necessity of having to add iron around magnetic ring 25,
thereby holding the amount of leakage flux to a low value. However, the
main flux, in its path from rotor pole face 23 to rotor pole face 24, must
move longitudinally, as well as circumferentially, as it passes from
laminations to solid magnetic material 25 and back to laminations again.
In so doing, much of the flux in each of the rotor poles must cross from
lamination to lamination. Consequently, reluctance is thus added to the
air gap for each of the poles, so that this form of rotor construction is
not very practicable. Hence, while making much of the rotor non-magnetic
can result in greatly reducing the total amount of leakage flux, along
with the problems introduced thereby, the reluctance thereby added to the
air gaps makes the structure not very practicable.
If four stacks are made up of laminations configured according to the shape
of the magnetic material providing the flux path shown in FIG. 2A, and are
encircled by a nonmagnetic ring, the structure illustrated in FIGS. 4, 5
and 6 results, FIGS. 5 and 6 being based on section views taken along
lines 5--5 and 6--6, respectively, in FIG. 4. The four stacks 40, 41, 42
and 43 of laminations are assembled about a shaft 31 and encircled by a
nonmagnetic ring 44 which holds them circumferentially in place about the
shaft. The subassembly thus formed is assembled into a complete rotor, as
shown in FIG. 6.
In FIG. 6, laminated pole faces 47 and 48 are illustrated as being
fabricated of laminations of the type shown in FIG. 2A assembled in the
manner indicated in FIG. 4. The laminations are disposed about shaft 31
and are encircled by nonmagnetic ring 44, which retains them tightly about
shaft 31. The laminations are held in place, axially, by nonmagnetic end
clamps 45 and 46 in a manner well known in the art. The rotor of FIG. 6
thus employs a minimum amount of magnetic material. However, the shape of
the individual rotor laminations makes the rotor difficult to fabricate,
and the centrifugal force exerted upon them when rotating must be
withstood almost entirely by retaining ring 44. Moreover, the rotor
segments are skewed with respect to longitudinal axis 30 of shaft 31, so
that a relative torque is exerted between end clamps 45 and 46 as the
laminations of the rotor tend to reach a minimum energy position during
rotation. This minimum energy position tends to cause the lamination
portions at either section of the rotor to align themselves radially, and
also tends to cause end clamps 45 and 46 to move apart axially. The forces
thus exerted during high-speed operation may be sufficiently great to
cause a mechanical failure somewhere on the rotor, and hence this
configuration is not altogether satisfactory for a high-speed machine.
FIG. 7 illustrates an assemblage of nested rotor laminations about rotor
axis 30, which is superior to the configuration illustrated in FIGS. 4, 5
and 6. This construction permits the four pole quadrants at either end of
the rotor to be joined to those at the opposite end of the rotor through
their mid regions, and yet each lamination provides a continuous, low
reluctance path between each pole face, respectively, at one end of the
rotor and each pole face of the opposite, complementary pole,
respectively, at the other end of the rotor. Since each lamination is
continuous circumferentially between the set of poles situated at each end
of the rotor (while the configuration shown in FIGS. 4, 5 and 6 employs
non-magnetic material between circumferentially-consecutive poles), there
is more magnetic material employed in the configuration shown in FIG. 7
than in that shown in FIGS. 4, 5 and 6. The extra magnetic material
employed in the configuration of FIG. 7 is situated radially inward from
the pole faces so that its contribution to leakage flux is essentially
negligible. The partially cutaway illustration in FIG. 7 shows merlon
regions 50 and 51 forming circumferentially-successive salient poles in
one section of the rotor, and merlon regions 52 and 53 comprising separate
salient poles in the opposite region of the rotor. A portion of
radially-innermost lamination 54 is visible in FIG. 7 and comprises a
unitary piece of magnetic material, such as magnetic steel. A nonmagnetic
band 55 encircles and constricts the laminations of the rotor in order to
hold them circumferentially in place against the rotor even when
experiencing high centrifugal force due to high-speed operation of the
alternator. Band 55 may comprise fiber-reinforced glass. In the
alternative, band 55 may be comprised of nonmagnetic stainless steel of
the 300 series, titanium wire, or other suitable nonmagnetic material. The
magnetic material radially-beneath restraining ring 55 aids in resisting
centrifugal forces and, additionally, provides support for each magnetic
pole on the rotor. This support is symmetric about longitudinal axis 30 of
the rotor.
FIG. 8 is a plan view of a single, planar lamination 60, conveniently of
rectangular shape, as it appears prior to being rolled into a cylinder for
use on the rotor of the instant invention. Lamination 60 may be assembled
into a stack along with other laminations of similar configuration, and
the stack rolled into a cylinder of coaxial laminations having its inner
diameter corresponding to the outer diameter of the rotor shaft about
which it is to be situated, and its outer diameter conforming to the inner
diameter of the retaining ring which is to encircle the laminations. It is
feasible to use only one lamination of sufficient length. The cylinder
thus formed is crenellated by removing segments 61, thereby forming
merlons 62 of the width of each pole piece. Crenels 61 and merlons 62 at
either end of the cylinder are circumferentially offset from the
respective crenels and merlons at the opposite end of the cylinder by an
angle of 360.degree./n, n being the total number of rotor poles. After the
rotor shaft is inserted into the cylinder, the entire cylinder is clamped
into a restraining ring and the rotor poles are formed by bending the
merlons radially outward into protuberances of axial thickness determined
by the total thickness of the laminations. This may be accomplished, for
example, by use of a shaping punch. The overall length of the cylinder is
thus determined by the radially-innermost lamination, which is the longest
lamination. After the rotor is assembled as illustrated in FIG. 7, each of
the radially-outer surfaces of the rotor poles is machined to provide a
cylindrical pole face normal to a respective radius of the rotor. This
manufacturing sequence is simpler than that for the rotor illustrated in
FIG. 3 and the rotor illustrated in FIGS. 4, 5 and 6.
As an additional advantage of the structure illustrated in FIG. 7, a
surrogate clamp may readily be employed in place of ring 55 during
manufacture of the rotor. After the rotor is assembled, the surrogate
clamp is replaced with a composite band wound under tension directly on
the pole laminations in order to gain the added strength of a composite
material in resisting the large centrifugal forces experienced by the
rotor when operating at high speeds.
The foregoing describes an inductor alternator rotor employing laminations
which are everywhere generally parallel to the rotor flux paths. The
high-speed inductor alternator rotor thus formed exhibits relatively low
electrical losses and essentially no relative torque nor
longitudinally-outward forces on nonmagnetic end clamps holding the
magnetic material of the rotor in place.
While only certain preferred features of the invention have been shown by
way of illustration, many modifications and changes will occur to those
skilled in the art. It is, therefore, to be understood that the appended
claims are intended to cover all such modifications and changes as fall
within the true spirit of the invention.
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