Interior permanent magnet motor

Abstract

An interior permanent magnet motor comprising a rotor having a set of permanent magnets placed within the interior of the rotor and a stator which surrounds the rotor and has a set of stator teeth defining slots between adjacent teeth. The stator also includes a plurality of stator windings that extend around the teeth and within the slots, the rotor and stator defining an airgap there between, in which an outer peripheral surface of the rotor that faces the stator across the airgap is provided with a respective eccentric bulge in the region of each rotor magnet. The bulge has a part-elliptical shape, the centre of the circumference of the bulge lying on or close to an axis that passes through the axis of rotation of the rotor and through a point on or close to the circumferential centre of the associated magnet, and in which the two ends of the part-elliptical bulge respectively connect to a region of the periphery that interconnects to an adjacent bulge, in which the dimensions of the bulge are selected so as to optimise the motor in terms of minimising variations in cogging torque that arise due to geometric manufacturing errors.

Claims

1. An interior permanent magnet motor comprising: a rotor having a set of permanent ma ,nets placed within the interior of the rotor and a stator which surrounds the rotor and has a set of stator teeth defining slots between adjacent teeth, the stator also including a plurality of stator windings that extend around the teeth and within the slots, the rotor and stator defining an airgap there between, in which an outer peripheral surface of the rotor that faces the stator across the airgap is provided with a respective eccentric bulge in a region of each rotor magnet, the bulge having a part-elliptical shape, the centre of a circumference of the bulge lying on or close to an axis that passes through an axis of rotation of the rotor and through a point on or close to a circumferential centre of the associated magnet, and in which two ends of the bulge respectively connect to a region of a periphery that interconnects to an adjacent bulge, in which dimensions of the bulge are selected so as to optimise the motor in terms of minimising variations in cogging torque that arise due to geometric manufacturing errors, and in which the bulge has a ratio of long axis to short axis of between 5:1 and 8:1.

2. The interior permanent magnet motor according to claim 1 in which the bulge follows a path corresponding to substantially a half of an ellipse.

3. The interior permanent magnet motor according to claim 1 in which a length of the bulge along a major axis thereof is equal to a circumferential length of the magnet, or in a range between 0.9 times and 1.1 times the length of the magnet.

4. The interior permanent magnet motor according to claim 1 in which the part elliptical shape has a circumferential width of between 14 mm and 15 mm and a radial height of between 2 mm and 2.5 mm and in which the magnets have a length of between 12 mm and 13 mm.

5. The interior permanent magnet motor according to claim 1 in which the rotor has 8 permanent magnets and the stator has 12 teeth.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a plan view of an embodiment of an interior permanent magnet motor that falls within the scope of the present invention;

(2) FIG. 2 is a plan view of the stator showing the stator teeth and slots;

(3) FIG. 3 is a plan view of the rotor showing the location of the rotor magnets inside the rotor;

(4) FIG. 4 is an enlarged plan view of a part of the rotor showing the proportions of a semi-elliptical bulge associated with each rotor magnet;

(5) FIG. 5 is plot of the BH characteristics of the steel in an exemplary motor, and

(6) FIG. 6 shows the open circuit flux line distribution at the rotor position shown, and without current in the windings.

DETAILED DESCRIPTION OF THE INVENTION

(7) As shown in FIG. 1, an interior permanent magnet (PM) motor 100 comprises a rotor 102 and a stator 104, the rotor 102 having a smaller radius than the stator 104 and the two sharing a common axis of rotation. The rotor 102 defines an outer peripheral face 106 that faces away from the axis of rotation towards a similar, inwardly radially extending face 108 of the stator 104. An air gap 110 is defined between these two faces of the rotor 102 and the stator 104. The stator 104 is fixed in position and the rotor 102 is supported by bearings (not shown) so that it can rotate around the axis when in use.

(8) The stator 104 comprises a steel support or back iron 112 and is shaped to define a set of 12 inwardly projecting teeth 114, with slots 116 being defined between adjacent teeth 114. A set of windings of copper wire (not shown) are wound through the slots 116 and around the teeth 114 in a defined pattern. The layout of the teeth 114 can be seen clearly in FIG. 2. Note that the ends of the teeth 114 that face the rotor 102 are curved so that all points of the end face are at the same radius from the axis of rotation of the motor rotor 102.

(9) The structure of the rotor 102 is shown in FIG. 3 in more detail. The rotor 102 comprises a laminated cylindrical core (not shown), made up of layers of metal sheet, which has a set of eight permanent magnets 118 inset into slots 120 cut into the core 122, so that the magnets 118 are supported a small distance below the peripheral surface 106. The magnets 118 are spaced evenly around the rotor 102, and alternate between North and South poles facing radially away from the axis of the rotor 102. This is beneficial compared with mounting the magnets 118 on the peripheral surface 106 of the rotor 102, in particular provides radial containment of the magnets 118 without additional components.

(10) The peripheral surface 106 of the rotor 102 above each magnet 118 defines a part-elliptical bulge 124, in the example shown being exactly one half of an ellipse, with eight such bulges 124 in total spaced evenly around the rotor 102. The centre of the circumference of each bulge 124 lies on an axis that passes through the axis of rotation of the rotor 102 and through the circumferential centre of the associated magnet 118, and in which the two ends of the semi-elliptical bulge 124 respectively connect to a region of the periphery 106 that interconnects to an adjacent bulge 124.

(11) The air-gap function of the motor 100 shown in FIGS. 1 to 3 is defined as
x=a.Math.sin
y=b.Math.cos +rroutb(2)

(12) where is the angle of ellipse, x and y are the coordinate of ellipse, a and b are the width and height of ellipse, rrout indicates the maximum rotor radius. The ellipse centre depends on the minimum air-gap length as well as the height of ellipse, whose coordinate is (0,rroutb). FIG. 4 shows clearly the variables a and b used in the above equation.

(13) The applicant has determined that the selection of optimised part-elliptical shaped bulges 124 provide excellent flexibility compared with other rotor peripheral shapes in terms of insensitivity to geometric changes due to manufacturing errors in terms of the potential to be less sensitive to cogging torque.

(14) To test the performance of the semi-elliptical design, a motor was modelled in a simulation package having the properties set out in Table 1 below. In addition, the B-H characteristics of the lamination silicon steel is shown in FIG. 5. A range of different elliptical bulges were modelled. The applicant has realised that the cogging torque has positive correlation with the width of the part-elliptical bulge 124 but negative correlation with its height with the motor most insensitive to the manufacture error when a and b equals to 2.2 mm and 14.4 mm, respectively. In this example the minimum air gap was 0.6 mm.

(15) FIG. 6 shows the magnetic fields within the motor 100, and how the fields are concentrated in the air gap 110 by the bulges 124.

(16) TABLE-US-00001 TABLE I MAIN PARAMETERS OF EXEMPLARY IPM MOTOR Variable Value Unit Variable Value Unit Rotor Number of poles 8 Magnet Axial length 11.15 mm Axial Length 36 mm Magnet Thickness 3.025 mm (Radial) Shaft (Diameter) 6 mm Magnet Width 12.59 mm (Circumferential) Magnet De-centre 13.0 mm Remanent Flux 1.35 T Density Rotor Diameter 45.2 mm Recoil 1.05 Max Permeability Rotor Diameter Min 43.8 mm Web Width 1.5 mm Winding Number of Turns 24 (per Stator Segment) Web Length 1.65 mm Bridge (Width) 0.5 mm Magnet Cavity 3.1 mm Number of 4 Height (Radial) Parallel Branches (per Phase) Magnet Cavity 12.64 mm Wire diameter 1.2 mm Width (bare copper) (Circumferential) Stator Number of Slots 12 Others Rotor Pucks 5.4 Mech. Skewed Angle Deg. Axial Length 36 mm Number of Pucks 3 Stator Outer 85 mm Lamination Steel Diameter Material Inner Bore 46.4 mm Diameter Tooth Width 7 mm Tooth Tip Gap 3 mm Tooth Tip 0.5 mm Thickness Tooth Angle 110 mm

(17) In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained.