ELECTRIC MACHINE

Abstract

The present disclosure relates to an electric machine (1) having a rotor (5) and a stator (4). The rotor (5) has a plurality of rotor poles (12). The stator (4) is part-annular and has a plurality of stator teeth (9) each having a winding. The stator (4) includes a plurality of sets (S1-9) of said stator teeth (9), each set (S1-9) comprising more than one stator tooth (9). The sets (S1-9) are magnetically de-coupled from each other. The windings (11) on the stator teeth (9) in each set (S1-9) are in different phases (A, B, C). The present disclosure also relates to a vehicle (2) having an electric machine (1) of the type described herein.

Claims

1. An electric machine for providing tractive force to propel a vehicle, the electric machine comprising: a rotor having a plurality of rotor poles, the rotor poles each formed by one or more permanent magnets; and a part-annular stator having a plurality of stator teeth, each tooth having a separate winding; the stator comprising a plurality of sets of said stator teeth, each set comprising more than one stator tooth and the sets being magnetically de-coupled from each other; wherein the windings on the stator teeth in each set are in different phases (A, B, C).

2. The electric machine as claimed in claim 1, wherein the electric machine is a three-phase electric machine and each set consists of a pair of said stator teeth.

3. The electric machine as claimed in claim 2, wherein the stator comprises eighteen stator teeth and a winding topology for said stator teeth is as follows:
AB+B+CCA+A+BBC+C+AAB+B+CCA.

4. The machine as claimed in claim 1, wherein the stator comprises a plurality of flux barriers to magnetically de-couple the sets from each other.

5. The electric machine as claimed in claim 4, wherein the flux barriers comprise one or more hollow cavity formed in the stator.

6. The electric machine as claimed in claim 5, wherein the flux barriers comprise two hollow cavities formed in the stator between each set of stator teeth.

7. The electric machine as claimed in claim 4, wherein the stator comprises a part-annular section from which the stator teeth extend radially inwardly, the flux barriers being formed in said part-annular section.

8. The electric machine as claimed in claim 7, wherein the flux barriers are formed in the part-annular section between said stator teeth.

9. The electric machine as claimed in claim 1, wherein the stator comprises a plurality of sub-sections which are magnetically de-coupled from each other, at least one of said sets being formed in each sub-section.

10. The electric machine as claimed in claim 9, wherein said sub-sections each comprise one or more flux barrier.

11. An electric machine for providing tractive force to propel a vehicle, the electric machine comprising: a rotor having a plurality of rotor poles, the rotor poles each formed by one or more permanent magnets; and a stator having a plurality of stator teeth, each tooth having a winding; the stator comprising a plurality of sets of said stator teeth, each set comprising more than one stator tooth; wherein the stator comprises a plurality of flux barriers to magnetically de-couple the sets from each other.

12. The electric machine as claimed in claim 11, wherein the flux barriers comprise one or more hollow cavity formed in the stator.

13. The electric machine as claimed in claim 12, wherein the flux barriers comprise two hollow cavities formed in the stator between each set of stator teeth.

14. The electric machine as claimed in claim 11, wherein the stator comprises an outer section from which the stator teeth extend radially inwardly, the flux barriers being formed in said outer section.

15. The electric machine as claimed in claim 14, wherein said outer section is annular or part-annular.

16. The electric machine as claimed in claim 11, wherein the windings on the stator teeth in each of said sets is in a different phase (A, B, C).

17. The electric machine as claimed in claim 11, wherein the electric machine is a three-phase electric machine.

18. A vehicle comprising the electric machine as claimed in claim 1.

19-20. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which:

[0031] FIG. 1 shows a schematic representation of a vehicle incorporating an electric machine in accordance with an embodiment of the present invention;

[0032] FIG. 2 shows a sectional view of the electric machine shown in FIG. 1;

[0033] FIG. 3 shows a first coil topology of the windings on the stator of the electric machine shown in FIG. 2;

[0034] FIG. 4 shows a sectional view of a variant of the electric machine shown in FIG. 2;

[0035] FIG. 5 shows a known second coil topology of the windings on the stator of an electric machine;

[0036] FIGS. 6A and 6B show the voltage characteristics of the electric machine incorporating the first and second coil topologies;

[0037] FIGS. 7A and 7B show the torque characteristics of the electric machine incorporating the first and second coil topologies; and

[0038] FIG. 8 shows a sectional view of a further variant of the electric machine shown in FIG. 2.

DETAILED DESCRIPTION

[0039] An electric machine 1 in accordance with embodiments of the present invention will now be described by way of example. As illustrated in FIG. 1, the electric machine 1 has particular application in a vehicle 2 to generate a traction force. The electric machine 1 can operate in the traction mode to provide the sole tractive force for propelling the vehicle, for example an electric vehicle (EV); or in conjunction with the internal combustion engine, for example a hybrid electric vehicle (HEV).

[0040] With reference to FIG. 2, the electric machine 1 is a permanent magnet synchronous motor (PMSM). The electric machine 1 is a three-phase machine in the present embodiment. The electric machine 1 comprises a stator 4 and a rotor 5. The rotor 5 is configured to rotate about a longitudinal axis X of the electric machine 1 (extending perpendicular to the plane of the page on FIG. 2). The electric machine 1 is described herein with reference to a transverse cross-section disposed perpendicular to said longitudinal axis X.

[0041] The electric machine 1 is installed within a component housing 6, such as a transmission housing, of the vehicle 2. The component housing 6 defines a chamber 7 for the electric machine 1. A protuberance 8 projects inwardly into the chamber 7. The remainder of the chamber 7 has a circular profile in transverse cross-section. The protuberance 8 can, for example, be formed by one or more assembly or machine, such as a power transfer unit or a starter motor, which may be disposed within the chamber 7 or adjacent to the component housing 6.

[0042] The stator 4 has a part-annular profile in transverse cross-section. Specifically, the stator 4 consists of a major annular sector in transverse cross-section. The corresponding minor annular sector is an annular gap formed in the stator 4 to accommodate the protuberance 8. Thus, in transverse cross-section, the stator 4 is generally C-shaped and comprises first and second ends 4-1, 4-2 which are separated from each other. As described herein, the angular extent of the major and minor annular sectors is defined to accommodate the protuberance 8. The stator 4 is formed from a plurality of laminations arranged in face-to-face contact with each other to form a stacked core. The laminations can, for example, be made of electrical steel.

[0043] The stator 4 comprises a plurality of stator teeth 9 projecting radially inwardly from a radially outer part-annular segment 10. The stator 4 in the present embodiment extends over 300 and comprises eighteen (18) stator teeth 9. The stator teeth 9 are labelled 9-1 through 9-18 in a counter-clockwise direction starting from the first end 4-1 of the stator 4. The stator teeth 9 have an angular spacing of 16.67. Unlike fully swept electric machines (i.e. electric machines having a stator extending over 360), the angle between the stator teeth 9 is not an integer multiple of 360. The stator 4 can be formed from one or more segments provided the resulting flux paths are self-contained within each segment. The number of stator teeth 9 in each segment may, for example, be eighteen (18), nine (9), six (6) or two (2). It will be appreciated, therefore, that the stator 4 may have a modular design. The dimensions of each segment can be modified to reduce torque ripple and voltage harmonics.

[0044] The rotor 5 has a substantially circular transverse cross-section and is arranged coaxially with the stator 4. The rotor 5 comprises eighteen (18) permanent magnets which form rotor poles 12 having uniform angular spacing around the rotor 5. Thus, the rotor pole pitch is 20 in the present embodiment. The rotor poles 12 are each formed by one or more permanent magnets. The rotor poles 12 can, for example, be made of rare-earth materials to provide a high density of magnetic flux.

[0045] The stator teeth 9 carry windings 11 connected to the 3-phase supply. In the present embodiment the windings 11 are concentrated windings comprising separate coils wound on each stator tooth 9. The phase shift between the windings 11 is 150 electrical degrees. If the windings 11 of two adjacent slots are in the same phase, then these windings 11 are connected with opposite polarity, resulting in a phase shift of 30 electrical degrees. At least in certain embodiments this can enable a higher torque density and/or a lower current density. With reference to FIG. 3, the first coil topology [T1] of the windings 11 is as follows:

AB+B+CCA+A+BBC+C+AAB+B+CCA[T1]

[0046] It will be understood that the first coil topology [T1] defines the phase of the current supplied to the stator teeth 9-1 to 9-18 in a counter-clockwise direction starting from the first end 4-1 of the stator 4. With reference to FIG. 1, the windings 11 are electrically coupled to a high voltage (HV) battery 14 via a first inverter 15. A control unit 16 comprising a first electronic controller (not shown) is provided for sequencing the supply of current to the windings 11. The interaction of the current in the windings 11 and the magnetic flux generated by the permanent magnets in the rotor poles 12 cause the rotor 5 to rotate. The supply sequence is the same as a conventional three (3) phase PMSM having a circular stator. Thus, the electric machine 1 operates in a conventional manner.

[0047] With reference to FIG. 2, a circumferential shield element 17 extends between the ends of the stator 4 to reduce the transmittal of magnetic flux from the rotor poles 12 into the component housing 6. However, the first and second ends 4-1, 4-2 of the stator 4 are magnetically isolated and this can result in voltage imbalances within the electric machine 1.

[0048] At least in certain embodiments the configuration of the stator 4 helps to reduce the volume of the stator teeth 9 and to lower the back-EMF harmonic content (compared to an equivalent 3/2 topology). The stator 4 may also lower the motor frequency by up to 10% (compared to a known fractional slot topology); and reduce the voltage imbalance in back-EMF of the windings 11 belonging to the same phase (compared to an equivalent 3/2 topology).

[0049] A variant of the stator 4 is shown in FIG. 4. Like reference numerals are used for like components. The configuration of the stator teeth 9 is unchanged from the configuration described with reference to FIG. 3. In particular, the stator 4 extends over 300 and comprises eighteen (19) stator teeth 9. The angular spacing of the stator teeth 9 is 16.67. Furthermore, the first coil topology [T1] is unchanged for the stator teeth 9.

[0050] As shown in FIG. 4, the stator 4 comprises a plurality of flux barriers 18 which sub-divide the stator 4 into a plurality of sets (S1-9) of said stator teeth 9. The flux barriers 18 control the flux density within the stator 4 by reducing the magnetic coupling between the stator teeth 9 in each set (S1-9). In certain embodiments, the flux barriers 18 may be decouple the stator teeth 9 in each set (S1-9). The reduced magnetic coupling of the stator teeth 9 may reduce voltage imbalances in the electric machine 1. The flux barriers 18 are arranged such that each set (s1-9) consists of two (2) stator teeth 9. The flux barriers 18 in the present embodiment are in the form of hollow cavities formed in the radially outer part-annular segment 10 of the stator 4. The hollow cavities are formed in each lamination of the stator 4 such that the flux barriers 18 extend parallel to the longitudinal axis X of the electric machine 1. Two of said flux barriers 18 are formed between the stator teeth 9 in each set (S1-9). The flux barriers 18 may promote magnetic saturation within the radially outer part-annular segment 10 of the laminations of the stator 4. The localised magnetic saturation may promote magnetic isolation of each set (S1-9) of said stator teeth 9.

[0051] The stator teeth 9 carry windings 11 connected to the 3-phase supply. The windings 11 are concentrated windings comprising separate coils wound on each stator tooth 9. The first coil topology [T1] of the windings 11 is the same as detailed above and illustrated in FIG. 3. It will be understood that the windings 11 on the stator teeth 9 in each set (S1-9) are in different phases. This arrangement is significant since it may reduce the back-EMF of the electric machine 1. In particular, the first coil topology [T1] may be effective in reducing the 5th and 7th voltage harmonics. These harmonics do not produce a useful torque component, but do increase the peak value of back-EMF. It will be appreciated that the windings 11 may be selectively coupled to an electrical load and function as an alternator.

[0052] With reference to FIG. 5, a second coil topology [T2] is as follows:

A+A+BBC+C+AAB+B+CCA+A+BBC+C[T2]

[0053] The second coil topology [T2] is a known coil topology. When the second coil topology [T2] is implemented on the electric machine 1, the stator teeth 9 in each set (S1-9) are in the same phase. A comparison was made of the operating characteristics of the electric machine 1 having the first coil topology [T1] and the second coil topology [T2]. The stator 4 of the electric machine 1 had the same number of laminations and the same number of turns per coil in each variation. The voltage characteristics for the first coil topology [T1] are shown in FIG. 6A; and the voltage characteristics for the second coil topology [T2] are shown in FIG. 6B. The first coil topology [T1] develops significantly lower line voltage than the second coil topology [T2]. In the present embodiment the line voltage in the first coil topology [T1] is approximately 51.3V; and the line voltage in the second coil topology [T2] is approximately 63V. The torque characteristics for the first coil topology [T1] are shown in FIG. 7A; and the torque characteristics for the second coil topology [T2] are shown in FIG. 7B. The first and second coil topologies [T1], [T2] develop approximately the same average torque, approximately 365 Nm in the present configuration, for the same configuration of the stator 4.

[0054] A variant of the stator 4 shown in FIG. 4 is illustrated in FIG. 8. Like reference numerals are again used for like components. In this arrangement, the stator 4 is divided into a plurality of sub-sections 19 each defining a set (S1-9) of said stator teeth 9. In the present embodiment, the sub-sections 19 each consist of two (2) stator teeth 9. The sub-sections 19 are separated from each other to magnetically de-couple the sets (S1-9) of said stator teeth 9. The stator teeth 9 in each sub-sections 19 are arranged in a generally U-shaped configuration with the stator teeth 9 projecting radially inwardly. It will be understood that the sub-sections 19 are supported in a frame or housing (not shown) within the electric machine 1. The stator teeth 9 carry windings 11 having the first coil topology [T1] detailed above and illustrated in FIG. 3.

[0055] It will be understood that the configurations of the sub-sections 19 may be modified. The sub-sections 19 may each consist of more than two stator teeth 9. In certain embodiments the sub-sections 19 may each consist of an even number of stator teeth 9. The stator teeth 9 may be disposed in pairs which are magnetically de-coupled from each other by one or more flux barriers 18. For example, the sub-sections 19 may each consist of four (4) stator teeth 9 arranged in two pairs which are magnetically de-coupled from each other by one or more flux barrier 18; or six (6) stator teeth 9 arranged in three pairs which are magnetically de-coupled from each other by one or more flux barrier 18. One or more hollow cavity may be formed in each sub-section 19 to form an intermediate flux barrier 18.

[0056] It will appreciated that further changes can be made to the electric machine 1 described herein without departing from the scope of the present invention.