U.S. patent application number 11/019075 was filed with the patent office on 2005-11-24 for permanent magnet machine and method with reluctance poles for high strength undiffused brushless operation.
This patent application is currently assigned to UT-Battelle LLC. Invention is credited to Hsu, John S..
Application Number | 20050258699 11/019075 |
Document ID | / |
Family ID | 35374522 |
Filed Date | 2005-11-24 |
United States Patent
Application |
20050258699 |
Kind Code |
A1 |
Hsu, John S. |
November 24, 2005 |
PERMANENT MAGNET MACHINE AND METHOD WITH RELUCTANCE POLES FOR HIGH
STRENGTH UNDIFFUSED BRUSHLESS OPERATION
Abstract
A method and apparatus in which a rotor (11) and a stator (17)
define a radial air gap (20) for receiving AC flux and at least
one, and preferably two, DC excitation assemblies (23, 24) are
positioned at opposite ends of the rotor (20) to define secondary
air gaps (21, 22). Portions of PM material (14a, 14b) are provided
as boundaries separating the rotor pole portions (12a, 12b) of
opposite polarity from other portions of the rotor (11) and from
each other to define PM poles (12a, 12b) for conveying the DC flux
to or from the primary air gap (20) and for inhibiting flux from
leaking from the pole portions prior to reaching the primary air
gap (20). The portions of PM material (14a, 14b) are spaced from
each other so as to include reluctance poles (15) of ferromagnetic
material between the PM poles (12a, 12b) to interact with the AC
flux in the primary-air gap (20).
Inventors: |
Hsu, John S.; (Oak Ridge,
TN) |
Correspondence
Address: |
QUARLES & BRADY
411 EAST WISCONSIN AVENUE
MILWAUKEE
WI
53202-4497
US
|
Assignee: |
UT-Battelle LLC
|
Family ID: |
35374522 |
Appl. No.: |
11/019075 |
Filed: |
December 21, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11019075 |
Dec 21, 2004 |
|
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|
10848450 |
May 18, 2004 |
|
|
|
60607105 |
Sep 3, 2004 |
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Current U.S.
Class: |
310/156.56 ;
310/181; 310/191 |
Current CPC
Class: |
H02K 21/046
20130101 |
Class at
Publication: |
310/156.56 ;
310/191; 310/181 |
International
Class: |
H02K 021/12; H02K
001/00; H02K 019/26 |
Goverment Interests
[0002] This invention was made with Government support under
Contract No. DE-AC05-00OR22725 awarded to UT-Battelle, LLC, by the
U.S. Department of Energy. The Government has certain rights in
this invention.
Claims
1. A brushless electric machine comprising: a cylindrical stator; a
rotor having an axis of rotation, the rotor being spaced from the
stator to define an annular primary air gap that receives an AC
flux from the stator, the rotor having longitudinal pole portions
running parallel to the axis of rotation and alternating in
polarity around a circumference of the rotor; at least a first
stationary excitation coil assembly for receiving direct current
from an external source and being positioned across a secondary air
gap from one end face of the rotor so as to induce a DC flux in the
rotor which increases a resulting flux in the primary air gap when
said direct current is of a first polarity and which reduces the
resulting flux in the primary air gap when said direct current is
of a second polarity opposite said first polarity; and wherein
portions of permanent magnet (PM) material are positioned to form
boundaries separating the rotor pole portions of opposite polarity
from an interior of the rotor and from each other to define PM
poles for conveying the DC flux to or from the primary air gap and
for inhibiting flux from leaking from said pole portions prior to
reaching the primary air gap when said direct current is of the
first polarity; and reluctance poles of ferromagnetic material
positioned between the PM poles to produce reluctance torque in the
rotor in response to AC flux in the primary air gap, wherein said
reluctance poles have a cross section that varies in an axial
direction relative to the rotor.
2. (canceled)
3. The brushless machine of claim 1, wherein the reluctance poles
extend radially with respect to a geometrical center of the rotor
to an outer circumference of the rotor.
4. The brushless machine of claim 1, wherein the rotor has a hub
that provides a portion of the return path for the DC flux to the
first stationary excitation coil assembly.
5. A brushless electric machine comprising: a cylindrical stator; a
rotor having an axis of rotation, the rotor being spaced from the
stator to define an annular primary air gap that receives an AC
flux from the stator, the rotor having longitudinal pole portions
running parallel to the axis of rotation and alternating in
polarity around a circumference of the rotor; at least a first
stationary excitation coil assembly for receiving direct current
from an external source and being positioned across a secondary air
gap from one end face of the rotor so as to induce a DC flux in the
rotor which increases a resulting flux in the primary air gap when
said direct current is of a first polarity and which reduces the
resulting flux in the primary air gap when said direct current is
of a second polarity opposite said first polarity; and wherein
portions of permanent magnet (PM) material are positioned to form
boundaries separating the rotor pole portions of opposite polarity
from an interior of the rotor and from each other to define PM
poles for conveying the DC flux to or from the primary air gap and
for inhibiting flux from leaking from said pole portions prior to
reaching the primary air gap when said direct current is of the
first polarity; wherein at least one pole portion in each pair of
rotor pole portions is provided by ferromagnetic pole material and
extends longitudinally from the secondary air gap towards a middle
of the rotor; and wherein the pole material has a relative greater
cross section at the secondary air gap and tapers to a relatively
narrower cross section proximate the middle of the rotor to conduct
flux that turns ninety degrees from the secondary air gap to reach
the primary air gap.
6. (canceled)
7. A brushless electric machine comprising: a cylindrical stator; a
rotor having an axis of rotation, the rotor being spaced from the
stator to define an annular primary air gap that receives an AC
flux from the stator, the rotor having longitudinal pole portions
running parallel to the axis of rotation and alternating in
polarity around a circumference of the rotor; at least a first
stationary excitation coil assembly for receiving direct current
from an external source and being positioned across a secondary air
gap from one end face of the rotor so as to induce a DC flux in the
rotor which increases a resulting flux in the primary air gap when
said direct current is of a first polarity and which reduces the
resulting flux in the primary air gap when said direct current is
of a second polarity opposite said first polarity; and wherein
portions of permanent magnet (PM) PM material are positioned to
form boundaries separating the rotor pole portions of opposite
polarity from an interior of the rotor and from each other to
define PM poles for conveying the DC flux to or from the primary
air gap and for inhibiting flux from leaking from said pole
portions prior to reaching the primary air gap when said direct
current is of the first polarity; further comprising a second
stationary excitation coil assembly for receiving direct current
from an external source and being positioned across a second
secondary air gap on an opposite end of the rotor from the
first-mentioned secondary air gap; and wherein at least one pole
portion in each pair of rotor pole portions is provided by
ferromagnetic pole material and extends longitudinally from the
secondary air gap towards a middle of the rotor; and wherein the
pole material in the at least one pole has a relative greater cross
section facing each of the secondary air gaps and tapers to a
relatively narrower cross section towards the middle of the rotor
to conduct flux from each end of the rotor that turns ninety
degrees from a respective one of the secondary air gaps to reach
the primary air gap.
8. The brushless machine of claim 6, wherein a return path for the
DC flux to the first and second stationary excitation coil
assemblies is provided by the rotor.
9. The brushless machine of claim 6, wherein a return path for the
DC flux to the first and second stationary excitation coil
assemblies is provided by the stator frame and stator core.
10. The brushless machine of claim 6, wherein a return path for the
DC flux to the first and second stationary excitation coil
assemblies is provided by the stator frame, stator core, and rotor
core.
11. The brushless machine of claim 1, wherein said rotor has a body
portion that is cylindrical except for longitudinally extending
grooves, wherein PM material is disposed in said grooves and
wherein elongated pole pieces are disposed in said grooves over the
PM material to form a cylindrical rotor with poles of alternating
polarity on a rotor circumference that are separated by PM
material.
12. The brushless machine of claim 1, wherein the machine is a
brushless AC synchronous machine.
13. The brushless machine of claim 1, wherein the machine is a
brushless DC machine.
14. The brushless machine of claim 1, wherein the machine is a
motor.
15. The brushless machine of claim 1, wherein the machine is a
generator.
16. A method of controlling flux in a brushless electrical machine,
the method comprising: inducing an AC flux in a rotor from a stator
across a radial air gap by conducting a current in a primary
excitation winding on the stator; positioning a first secondary
excitation coil at one end of the rotor; conducting a direct
current through the first secondary excitation coil so as to
produce a DC flux in the rotor across at least one axial air gap
and to produce a resultant flux in radial air gap resulting from
the AC flux and the DC flux; providing portions of permanent magnet
(PM) material as boundaries separating the rotor pole portions of
opposite polarity from an interior of the rotor and from each other
to define PM poles, and conveying the DC flux between the primary
air gap and the axial air gap through the PM poles and for
inhibiting flux from leaking from said PM poles prior to reaching
the primary air gap when said direct current is of the first
polarity; and spacing the portions of PM material so as to include
reluctance poles of ferromagnetic material between the PM poles to
interact with the AC flux in the primary air gap, and providing
said reluctance poles with a cross section that varies in an axial
direction relative to the rotor.
17. The method of claim 16, wherein said second flux has a first
component that is controlled in the rotor by current in the first
secondary excitation coil and further comprising conducting a
direct current through a second secondary excitation coil at an
opposite end of the rotor from the first secondary excitation coil,
so as to induce a second component of said DC flux across a second
axial air gap.
18. The method of claim 16, wherein the machine is operated as a
brushless AC synchronous machine.
19. The method of claim 16, wherein the machine is operated as a
brushless DC machine.
20. The method of claim 16, wherein the machine is operated as a
motor.
21. The method of claim 16, wherein the machine is operated as a
generator.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a continuation-in-part of U.S. patent application
Ser. No. 10/848,450 filed May 18, 2004. The benefit of priority
based on U.S. Provisional Patent Application No. 60/607,105, filed
Sep. 3, 2004, is also claimed herein.
TECHNICAL FIELD
[0003] The field of the invention is brushless machines, including
both AC and DC machines, including both motors and generators, and
including induction machines, permanent magnet (PM) machines and
switched reluctance machines.
DESCRIPTION OF THE BACKGROUND ART
[0004] There are three major types of brushless electric machines
available for the electric vehicle (HV) and hybrid electric vehicle
(HEV) drive systems. These are the induction machine, the PM
machine, and the switched-reluctance machine.
[0005] Permanent magnet (PM) machines have been recognized for
having a high power density characteristic. A PM rotor does not
generate copper losses. One drawback of the PM motor for the
above-mentioned application is that the air gap flux produced by
the PM rotor is limited, and therefore, a sophisticated approach is
required for high speed, field weakening operation. Another
constraint is that inductance is low, which means that current
ripple must be controlled.
[0006] It is understood by those skilled in the art that a PM
electric machine has the property of high efficiency and high power
density, however, the air gap flux density of a PM machine is
limited by the PM material, which is normally about 0.8 Teslas and
below. A PM machine cannot operate at an air gap flux density as
high as that of a switched reluctance machine. When the PM motor
needs a weaker field with a reasonably good current waveform for
high-speed operation, a sophisticated power electronics inverter is
required.
[0007] When considering a radial gap configuration for undiffused,
high strength operation, several problems have to be overcome. It
is desirable to provide a compact design with a shape similar to a
conventional radial gap machine.
[0008] It would also be beneficial to further enhance the control
of the field above that which is available with known PM rotor
constructions. This would increase the motor torque. It is also an
objective to accomplish this while retaining the compactness of the
machine.
[0009] The enhanced field weakening can reduce the field strength
at high speed to lower the back emf produced in the winding.
Therefore, for a specified DC link voltage, the speed range of the
machine can be increased over that it otherwise would be. This will
meet the compactness objective and allow simplification of the
drive system requirements.
[0010] The present invention continues the ability to enhance and
weaken flux in the primary air gap, while improving the
construction of the rotor.
SUMMARY OF THE INVENTION
[0011] This invention provides a high strength PM machine and
method for brushless undiffused operation in which reluctance poles
are added to permanent magnets (PM's) in a machine rotor to allow
enhanced field control.
[0012] The invention is incorporated in a method and apparatus in
which a rotor and a stator define a radial air gap for receiving AC
flux and at least one and preferably two DC excitation assemblies
are positioned at opposite ends of the rotor to define secondary
air gaps. Portions of PM material are provided as boundaries
separating the rotor pole portions of opposite polarity from an
interior of the rotor and from each other to define PM poles for
conveying the DC flux to or from the primary air gap and for
inhibiting flux from leaking from said pole portions prior to
reaching the primary air gap. The portions of PM material are
spaced from each other so as to leave reluctance poles of
ferromagnetic material between the PM poles to interact with the AC
flux in the primary air gap.
[0013] In a further aspect of the invention, the flux path through
the reluctance poles can be tapered in the direction of the flux
paths through the rotor to reduce the size and weight of
ferromagnetic material in the rotor. This also allows for two DC
flux paths from opposite ends as well as for return paths for the
DC flux.
[0014] The invention provide increased power and torque without
increasing the size of the machine.
[0015] The invention is applicable to both AC and DC machines, and
to both motors and generators.
[0016] The invention is provides a compact electric machine
structure for application to electric or hybrid vehicles.
[0017] Other objects and advantages of the invention, besides those
discussed above, will be apparent to those of ordinary skill in the
art from the description of the preferred embodiments which
follows. In the description reference is made to the accompanying
drawings, which form a part hereof, and which illustrate examples
of the invention. Such examples, however are not exhaustive of the
various embodiments of the invention, and therefore reference is
made to the claims which follow the description for determining the
scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a longitudinal section view of a brushless PM
machine with reluctance poles;
[0019] FIGS. 2 and 3 are end views of the rotor incorporated in the
assembly in FIG. 1;
[0020] FIGS. 4a and 4b are diagrams illustrating how the portion of
the rotor carrying the flux through reluctance poles can be tapered
and reduced to the portion actually carrying the flux;
[0021] FIG. 5 is a longitudinal section view of a brushless PM
machine having a rotor with reluctance poles and a tapered flux
path according to the present invention;
[0022] FIG. 6 is an end view of the rotor seen in FIG. 5;
[0023] FIGS. 7-11 are transverse sectional views through the rotor
of FIG. 5 taken in the planes indicated by the dashed lines in FIG.
5; and
[0024] FIGS. 12 and 13 are longitudinal section and end views of a
brushless PM machine of the present invention having a tapered flux
portion and showing the flow of flux through the rotor and
adjoining air gaps.
[0025] FIG. 14 shows that the externally excited DC flux return
path can go through the stator instead of the rotor if the frame
(or portion of the frame) is made of magnetically conducting
material.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] The principle of a high strength, undiffused brushless
machine has been previously disclosed in the Hsu, U.S. Pat. No.
6,573,634, issued Jun. 3, 2003, Hsu, U.S. patent application Ser.
No. 10/688,586 filed Sep. 23, 2003, and Hsu U.S. patent application
Ser. No. 10/848,450 filed May 18, 2004, the disclosures of which
are hereby incorporated by reference.
[0027] For a conventional PM machine the air-gap flux density is
about 0.6 to 0.8 Teslas and cannot be weakened without the aid of
some sophisticated power electronics. Both the stationary
excitation coil and the PM material in the rotor maximize rotor
flux in the PM machine of the present invention. It can produce two
to three times the air gap flux density of a conventional PM
machine. Because the torque produced by an electric machine is
directly proportional to the air gap flux density, a higher torque,
more powerful machine is provided with only small additions to size
and weight.
[0028] FIG. 1 shows a longitudinal section view of a radial gap,
high strength undiffused machine 10 with eight PM poles 12a, 12b in
a rotor assembly 11. FIGS. 2 and 3 each show the eight PM poles 12
bounded by eight sets of permanent magnets 14. Reluctance poles are
provided by the portions of the rotor 15 positioned in between
these sets of permanent magnets 14. The reluctance poles 15 allow
the flux 16 produced by a stator 17 to go through these reluctance
poles 15 easier than the path going through the PM poles 12a,
12b.
[0029] The rotor assembly 11 is preferably made as described in the
disclosures cited above, namely, the rotor has a hub 11a and a
plurality of laminations 11b of ferromagnetic material are mounted
and stacked on the hub 11a and clamped by non-magnetic metal end
pieces 11c. The rotor laminations 11b and end pieces 11c have keyed
projections 11d for insertion in keyways in the rotor hub 11a. The
stacked laminations 11c reduce the occurrence of eddy currents
resulting from the flux which travels through in an axial direction
through the rotor assembly 11.
[0030] PM pole pieces 12a (N), 12b (S) are disposed in longitudinal
grooves and retain the PM magnetic material 14 in place in still
deeper grooves with the assistance of adhesives. The PM magnetic
material 14 can be pre-formed pieces or the injected type. Between
pieces of PM material 14, an epoxy material can be used to fill
gaps. PM pole faces (not shown) are separate pieces attached to the
ends of the rotor assembly 11 to hold the PM pole pieces 12a, 12b
and magnets 14 in position.
[0031] It is also possible add two end rings of a soft magnetic
material to the ends of the stack of laminations 11a before adding
the clamping pieces 11c. The end rings provide smoothing for flux
in a circumferential direction around an axis of rotation 19a. The
pole faces can also made of a soft magnetic material, such as
steel. They can be attached to the thin steel end rings by rivets,
screws, welds, or any feasible means. The thin steel rings hold the
pole pieces in place against centrifugal force. Alternatively, end
pole faces can be held by rivets.
[0032] The machine 10 has two DC excitation assemblies 23 and 24 at
opposite ends of the rotor assembly 11. The DC excitation
assemblies 23, 24 each include a stationary, ring-shaped excitation
core 23b, 24b and a multi-turn coil 23a, 24a for receiving direct
current from an external source. This DC current can be of a first
polarity or of a second opposite polarity. The cores 23b, 24b
encircle the rotor shaft 11 and are mounted to a machine housing
37. The cores can be made of iron, steel, another iron alloy or a
compressed powder ferromagnetic material. A stationary toroidal
excitation coil 23a, 24a fits in an annular recess in each
excitation core 23b, 24b.
[0033] The rotor assembly 11 rotates with a main drive shaft 19
around an axis of rotation 19a. The stator 17 is disposed around
the rotor 11 and has a laminated core 17a and windings 17b as seen
in a conventional AC machine. The rotor assembly 11 is separated
from the stator 17 by a radial air gap 20, which is also referred
to herein as the primary air gap. AC flux is produced in this air
gap 20 by the stator field. The rotor assembly 11 is separated from
the DC excitation assemblies 23 and 24 by air gaps 21 and 22,
respectively. These air gaps 21, 22 are oriented axially relative
to the axis 19a of the rotor 11. DC flux will be produced in these
air gaps 21, 22 by the DC excitation assemblies 21 and 22. Flux
collector rings 25 are disposed between the axial air gaps 21, 22
and the DC excitation assemblies 23 and 24 to smooth the DC flux
component and reduce the possible occurrence of eddy currents.
[0034] The drive shaft 19 is supported by bearings 31 and 32. The
cores 23b, 24b for the excitation assemblies form brackets for
these bearings 31, 32. The bearing brackets conduct DC magnetic
flux. If needed, the ceramic bearings or insulated bearings (i.e.,
an electrically insulating material is used to isolate the rotor
outer ring to the bearing housing) can be used. A short internal
shaft 30 is also coupled to the rotor 11. A shaft encoder 33 and a
pump 34 for lubricant for the motor 10 are situated inside a
passageway 35 through the core 24. A housing cover 36 closes the
passageway 33.
[0035] Referring to FIG. 2, the DC flux 16 produced by the
excitation assemblies 23, 24 is conducted into the rotor from one
set of the PM side poles 12a of N polarity, and then turns to flow
radially outward across the main air gap 20 into the stator core
17a, then loops and returns radially inward and is conducted
axially outward through adjacent poles 12b of S polarity at the
other end of the rotor 11 (FIG. 3). The DC flux 16 produced by the
excitation coils does not pass through the reluctance poles 15.
FIG. 1 illustrates a flux path 16 for only one of the pole pairs.
The other pole pairs would have flux paths of the same pattern. The
DC flux return path 16 shown in FIG. 1 is using the rotor 11 for
its return path. Normally, a return path is located in the rotor 11
is more compact than a return path through the aluminum motor
housing 37. This is because the diameter of the rotor 11 is smaller
than that of a stator frame for conducting the DC return flux.
However, it is possible to use the stator frame for its DC flux
return path. FIG. 14 shows that the externally excited DC flux
return path can go through the stator instead of the rotor if the
frame (or portion of the frame) is made of magnetically conducting
material.
[0036] Referring to FIGS. 2 and 3, the PM material 14 together with
the excitation current going through the excitation coils 23a and
24a produce the north (N) and south (S) poles on the exterior of
rotor 11 that faces the stator 17 and the radial air gap 20. This
rotor flux in the radial air gap 20 can be either enhanced or
weakened according to the polarity of the DC excitation in the
excitation assemblies 23, 24 that face the ends the rotor 11.
Subsequently, the radial air gap 20 receives the rotor flux from
the rotor 11, which interacts with the primary flux induced by the
stator windings 17b to produce a torque.
[0037] Referring to FIGS. 4a and 4b, the DC flux in an axial
direction turns to the radial direction (i.e. a 90-degree turn).
Assuming the depth (i.e. the distance going into the paper) of the
paths shown in FIGS. 4a and 4b is a constant, FIG. 4a shows that
the DC flux component 16e entering the bottom of the pole piece
material 12 makes the 90-degree turn first, followed by successive
flux components 16b-16d, until the component at the top 16a turns
upward last. This provides a tapered flux path 16 in which a
portion of the pole piece material 12 in the rotor 11 is not
utilized. FIG. 4b shows that a material-saving flux path can be
provided a tapered-shape of the pole piece material 12. As the
depth of the path changes, the contour of the tapered path is not a
straight line, in order to maintain a cross sectional area that is
inversely proportional to the distance down the path.
[0038] FIG. 5 shows a modification to the rotor 11. This provides a
pole piece 12a tapered in a direction parallel to axis 19a. The
tapered pole piece 12a means that the DC flux going into the first
side poles sees a gradually smaller cross sectional area. At the
middle section of the rotor 11, the cross-sectional area of the
pole piece 12a is nearly zero. The tapered flux path is separated
from other parts of the rotor by sets of PM material 14a seen in
FIG. 6. Second sets of PM material 14b are spaced from the first
sets of PM material 14a to define reluctance poles 15.
[0039] The cross section of this flux path is seen in the sectional
views of the rotor at the axial locations shown in FIGS. 7-11. As
seen in FIG. 7-11, the spacing between the sets of PM magnets 14a,
14b defines eight N-S PM poles 12a, 12b and eight reluctance poles
15, pairs of these poles 15 being connected through a narrow cross
sectional area 15a seen in FIG. 7. This cross sectional area 15b,
15c then becomes progressively wider in FIGS. 8 and 9. This cross
sectional area then becomes progressively narrower 15d, 15e in
FIGS. 10 and 11. This provides a flux path 18a, 18b shown in FIG.
13 for two of the reluctance poles 15.
[0040] FIG. 12 illustrates two parallel DC flux paths 16f, 16g for
the PM poles 12. Unlike the series DC flux path (see FIG. 1) that
has the flux going into the side poles at one end of the rotor 11
and coming out from the other end of the rotor 11, the parallel DC
flux paths 16f, 16g illustrated here have flux entering the rotor
from both sides through the secondary air gaps 21, 22. From there,
the flux turns ninety degrees to cross the primary air gap 20 and
then return across the primary air gap to the core assemblies 23,
24 across the secondary air gaps 21, 22 (the return path being
represented by the dashed line in FIG. 12).
[0041] FIG. 12 also illustrates two additional retaining pieces
each having a central ring-shaped portion 11f and four radially
extending flanges 11e for holding the rotor assembly 11
together.
[0042] As seen in FIG. 13, the invention provides a reluctance pole
flux path 18a, 18b between the reluctance poles 15 of the brushless
machine 10. In addition, FIGS. 12 and 13 show that the return path
for the DC flux 16f enters a south (S) polarity return pole 12b
situated between two of the second sets of PM magnets 14b, is
conducted into the laminations 11b, and then is conducted through
gaps in the PM material 14a, 14b to reach the cooperating N pole
12a. The north-south polarity of the pieces of magnetic material
14a around the N poles is such that the N-polarity material faces
the N poles and the S-polarity material faces away from the N
poles. The north-south polarity of the pieces of magnetic material
14b around the S poles is such that the S-polarity material faces
the S poles and the N-polarity material faces away from the S
poles. The DC flux paths 16f, 16g are generally of the same
configuration (symmetrical) and of equal strength in this
embodiment but could be asymmetrical and of unequal strength in
alternative embodiments.
[0043] By controlling energization of the core assemblies 23, 24,
field weakening can be used to reduce the DC field strength at high
speed to lower the back emf produced in the winding. Therefore,
under a given DC link voltage the speed range of the machine can be
increased. This again meets the compactness objective by
simplifying the drive system requirement.
[0044] The invention is applicable to both AC synchronous and DC
brushless machines and to both motors and generators.
[0045] This has been a description of the preferred embodiments of
the invention. The present invention is intended to encompass
additional embodiments including modifications to the details
described above which would nevertheless come within the scope of
the following claims.
* * * * *