PERMANENT MAGNET MACHINE AND METHOD WITH RELUCTANCE POLES FOR HIGH STRENGTH UNDIFFUSED BRUSHLESS OPERATION
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).
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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.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCHThis 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.
TECHNICAL FIELDThe 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 ARTThere 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.
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.
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.
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.
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.
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.
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 INVENTIONThis 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.
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.
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.
The invention provide increased power and torque without increasing the size of the machine.
The invention is applicable to both AC and DC machines, and to both motors and generators.
The invention is provides a compact electric machine structure for application to electric or hybrid vehicles.
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
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.
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.
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.
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.
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.
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.
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.
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.
Referring to
Referring to
Referring to
The cross section of this flux path is seen in the sectional views of the rotor at the axial locations shown in
As seen in
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.
The invention is applicable to both AC synchronous and DC brushless machines and to both motors and generators.
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.
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.
Type: Application
Filed: Dec 21, 2004
Publication Date: Nov 24, 2005
Applicant:
Inventor: John Hsu (Oak Ridge, TN)
Application Number: 11/019,075