WO2013026088A1

WO2013026088A1 - Interior permanent magnet machine

Info

Publication number: WO2013026088A1
Application number: PCT/AU2012/000973
Authority: WO
Inventors: Muhammed Fazlur RAHMAN; Rukmi DUTTA; Lester Guang-Er CHONG
Original assignee: Newsouth Innovations Pty Limited
Priority date: 2011-08-19
Filing date: 2012-08-17
Publication date: 2013-02-28

Abstract

An electric machine having an N-slot stator with a concentrated non-overlapping winding and a P-pole interior permanent magnet rotor with pairs of magnets each formed in a V-shape addressing a rotor pole and the magnets within each pair separated by an iron bridge, such that the number of slots and poles (N,P) are selected from the group consisting of ((18,14), (12,10) and (18,16)) and integer multiples thereof, and an air gap between the stator and rotor having an air gap length satisfying the relation: (air gap length) > (0.006 X (stator outer diameter)). Such a machine can deliver a CPSR > 4.2:1.

Description

INTERIOR PERMANENT MAGNET MACHINE

TECHNICAL FIELD

The present invention relates to electrical machines and, in particular, to an interior permanent magnet machine that exhibits a wide constant power speed range.

BACKGROUND

Electric machines have existed for over a century and many types of such machines have been developed for a wide variety of applications. Numerous standard design goals apply within the field to achieve desired performance. These include minimising the air gap between rotor and stator relative to the machine size so as to increase overall efficiency, and maximising the saliency ratio to maximise torque.

The development of rare earth magnets has seen their significant use in commercial applications since the 1990's, notably for so-called green energy technologies as permanent magnet (PM) machines, often used for small wind generators, and as both motors and generators for electric/hybrid vehicles, to name but a few applications. In machines designed for such traction applications, a wide field-weakening constant power speed range (CPSR), high torque/power density and high efficiency, are desirable characteristics. Structural simplicity thereby affording reduced manufacturing cost is also desirable. PM machines include surface permanent magnet (SPM) machines and interior permanent magnet (IPM) machines.

Distributed windings (DW) have been a preferred choice for most present-day IPM machines due to an ability to produce sinusoidal electromotive force (EMF) waveforms.

However, considerable space is required to hold the end-turns of DW machines, making their use problematic in applications where space is a major constraint. A machine using a concentrated non-overlapping winding (CW) is an alternative in such applications.

Concentrated windings are characterised by a non-sinusoidal EMF and a low winding factor. Nevertheless, it has been shown that sinusoidal back EMF waveforms with high winding factors are achievable in a CW-PM machine through appropriate selection of the slot and pole combination.

SUMMARY

It is an object of the present disclosure to substantially overcome, or at least ameliorate, one or more deficiencies of existing arrangements of CW machines.

In accordance with one aspect of the present disclosure, there is provided an electric machine comprising a fractional slot interior permanent magnet configuration having a concentrated winding formed on a stator thereof and pairs of magnets each formed in a V- shape and addressing each rotor pole, and an air gap length satisfying the relation: ((air gap length) / (stator outer diameter)) > 0.006.

Preferably each pair of magnets forming a rotor pole is separated by a corresponding iron bridge formed in the rotor, and adjacent pairs of magnets are separated by iron inter-pole link sections.

According to another aspect of the present disclosure there is provided an electric machine haying an N-slot stator with a concentrated non-overlapping winding and a P-pole interior permanent magnet rotor with pairs of magnets each formed in a V-shape addressing a rotor pole and the magnets within each pair separated by an iron bridge, such that the number of slots and poles (N,P) are selected from the group consisting of ((18,14), (12,10) and (18,16)), and an air gap between the stator and rotor having an air gap length satisfying the relation: (air gap length) > 0.006 x (stator outer diameter).

According to another aspect of the present disclosure there is provided an electric machine configured for field weakening applications, said machine comprising a fractional slot interior permanent magnet configuration having a concentrated winding formed on a stator thereof and pairs of magnets each formed in a V-shape creating a rotor pole and the magnets within each pair being separated by an iron bridge, a relatively large air gap between rotor and stator, and a field weakening constant power speed range (CPSR) of at least 6.2:1. Preferably the relatively large air gap is defined by an air gap length, being a radial distance between an outer diameter of the rotor and an inner diameter of the stator, satisfying the relation: (air gap length) > 0.006 x (stator outer diameter).

Desirably the electric machine has a rated power in excess of 5 kW, and exhibits an efficiency in excess of 92% with a CPSR in excess of 8: 1 , or more preferably has a rated power in excess of 30 kW, and exhibits an efficiency in excess of 95% with a CPSR in excess of 8: l .

According to another aspect of the present disclosure there is provided an electric machine having an 18-slot stator with a concentrated non-overlapping winding and a 14-pole interior permanent magnet rotor with pairs of magnets each formed in a V-shape creating a corresponding rotor pole, the magnets within each pair being separated by an iron bridge, and an air gap between the stator and rotor having an air gap length satisfying the relation (air gap length) > 0.006 x (stator outer diameter), such that the machine has a field weakening constant power speed range (CPSR) of at least 4.2:1, and preferably 6.2:1.

According to another aspect of the present disclosure there is provided a concentrated winding fractional slot interior permanent magnet motor having V-shaped magnets characterised by an air gap length satisfying the relation: (air gap length) > 0.006 x (stator outer diameter).

According to another aspect of the present disclosure there is provided a concentrated winding fractional slot interior permanent magnet motor having V-shaped magnets characterised by an air gap length in the range of 120% - 320% of air gap lengths of alternative comparable sized motors. Desirably the air gap length is about 240% of air gap lengths of alternative comparable sized motors. Preferably the magnet V-angle is between 30 and 160 degrees, and most preferably between 30 and 90 degrees.

Desirably the machine or motor includes inter-pole link sections separating adjacent magnetic poles of the rotor have a thickness satisfying the relation: ((inter-pole link section thickness) / (rotor outer diameter)) < 0.015 .

Preferably the machine or motor includes an iron bridge separating an apex of the V- shaped magnets in each rotor pole section, the iron bridge having a width satisfying the relation: ((iron bridge width) / (rotor outer diameter)) < 0.015 .

Other aspects are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

At least one embodiment of the present invention will now be described with reference to the following drawings, in which:

Fig. 1 is an illustration of an exploded view of a prototype motor according to the present disclosure;

Fig. 2 is a plot of the back EMF waveform of the motor of Fig. 1 ;

Fig. 3 is a plot of the cogging torque for the motor of Fig. 1 ;

Fig. 4 is a plot of the d and q-axis inductances establishing the saliency ratio of the motor of Fig. 1 ;

Fig. 5 is a plot illustrating verification of measured efficiency of the motor of Fig. 1 throughout the field weakening range;

Fig. 6 A is a schematic representation of a prior art segmented IPM machine with DW (SEG-IP )

Fig. 6B is a schematic representation of a prior art singe piece pole IPM (IPM-I) machine with DW; Fig. 6C is a schematic representation of the CW-IPM machine with V-shaped magnets according to the motor of Fig. 1;

Fig. 7 A is a plot of measured normalized output torque for the machines of Figs. 6A, 6B and 6C;

Fig. 7B is a plot of the measured field-weakening performance comparison of the machines of Figs. 6A, 6B and 6C;

Fig. 7C is a plot of the measured normalized cogging torque over one cycle for the machines of Figs. 6A, 6B and 6C;

Figs. 8A and 8B are plots illustrating modelled optimisations of efficiency of motors according to the present disclosure;

Fig. 9 is a plot of the measured and modelled input power characteristics of the motor of Fig. 1 ;

Fig. 10 is a plot of the power and torque versus speed characteristic of the motor of

Fig. 1 ;

Fig. 11 is a plot of the modelled air gap length versus CPSR and input power;

Fig. 12 is a plot of the modelled air gap length versus efficiency and core loss;

Fig. 13 is a plot of the modelled input and output power and efficiency for a 5 kW implementation;

Fig. 14 is a plot of the modelled input and output power and efficiency for a 30 kW implementation;

Figs. 15A and 15B illustrate rotor magnetic fields for differing V-angles;

Fig. 16 is plot of modelled power for a number of V-angles; and

Fig. 17 illustrates a section of rotor laminate and the V-angle for the motor of Fig. 1. DET AILED DESCRIPTION INCLUDING BEST MODE Fig. 1 shows an electric machine 100 comprising an interior permanent magnet (IPM) rotor 102 and a stator 104 configured with a concentrated non-overlapping winding (CW) 108. The stator 104 is configured within a casing 105. The motor 100 is configured in a fractional slot arrangement having an unequal number of iron core slots 106 formed in the stator 104 relative to the number of magnetic poles 110 formed in the rotor 102. In the example illustrated, the motor 100 includes 18 slots 106 and 14 poles 110, the distinction between the slots and poles being more clearly seen in the schematic representation of Fig. 6C. Associated with the slots 106 is a concentrated winding (CW) 108, which may be implemented using double-layer stator windings. Concentrated windings are usually wound using either a horizontal slot fill method or a vertical slot fill method. The machine 100 used the horizontal method to simplify the winding process, although the vertical method may produce equivalent electromagnetic performance but with less copper and shorter end winding. Whilst Fig. 1 shows an exploded representation, in use the rotor 102 is positioned coaxially within the stator 104 such that an air gap 112 (seen in Fig. 6C) is formed there between, details of which will be discussed later in this specification. The length of the air gap 112 is the radial distance between the outer diameter of the rotor 102 and the inner diameter of the stator 104.

The prototype machine 100 has been constructed for a power output of 800 watts as a test machine that is reliably scalable for machines into the 30 kW - 110 kW range, particularly suited for use in traction applications such as electric motor vehicles. The present inventors do not foresee any impediment to scalability beyond this range, for example well into the megawatt range. The scalability of electrical machine design is well known in the art and can be reliably modelled using finite element (FE) analysis so as to transform prototype structural details and specifications into full scale working apparatus structural details and specifications. One feature of the scalability of electrical machine design is that relatively small motors tend to have a relatively low efficiency whereas correspondingly larger scale motors tend to have improved efficiency. For the motor of Fig. 1, an efficiency of approximately 82% at full load has been achieved. When the machine is scaled up keeping the dimensional relation the same as the prototype machine constructed, the efficiency of the machine was found to increase with power and size, reaching greater than 95 percent at 30 kW. The inventors expect the efficiency to improve further for larger machines of similar dimensional relationship.

Table 1 below indicates the key specifications of the prototype machine 100:

TABLE 1

KEY SPECIFICATIONS OF THE 800W CW-IPM Machine

Stator outer diameter 130 mm

Rotor outer diameter 80 mm

Air gap length 1.2 mm

Stack length (stator) 80 mm

Stack length (rotor) 79 mm

Number of poles 14 poles

Number of slots 18 slots / double-layer

Rated voltage 320 Vrms line-line

Rated current 2.55 Arms/phase

Magnet remnant flux density 1.04 T

Core material Non-oriented FeSi

Saturation mag. of laminations 1.68 T @5000 A/m

Predicted core loss at 50 Hz/1.5 T 2.60 W/Kg

Slot-fill factor 41%

Stator resistance at 25°C Ιθ Ω/ph Fig. 2 shows the measured and modelled back EMF waveforms for the 800 W motor 100 operating at 428 RPM. The waveforms demonstrate that the 18-slot, 14-pole double layer CW configuration produced a near-perfect sinusoidal back EMF waveform, contrasting accepted views regarding such configurations. Both the modelled line to line and line to neutral back EMF values were found to be within 1.9% error of the measured RMS quantities. When compared with an equivalent DW FE model, the winding factors achieved were considered high, at 0.902 for both measured and modelled values.

The motor 100 has a fractional-slot distribution in which the number of slots per pole per phase (S_pp) is not a whole number. Fractional-slot distribution significantly reduces periodicity between slots and poles, therefore leading to a significant decrease in the magnitude of cogging torque. The decrease can be quantified by the lowest common multiple (LCM) of the number of slots and poles. A higher LCM would yield a lower magnitude cogging torque. Compared to other commonly used fractional-slot winding layouts (i.e. 12- slot, 10 poles, or, 12-slot and 14 poles), the 18-slot 14 pole layout of the motor 100 results in a much higher LCM. However, a significant disadvantage to fractional-slot distribution is that such an arrangement creates high frequency fluctuations in the cogging torque.

For the motor illustrated in Fig. 1, peak points of cogging torque were measured and, using FE analysis, a plot of predicted cogging torque was found as illustrated in Fig. 3. From Fig. 3, the motor 100 produced a relatively low cogging torque having a value of 1.54% (peak to peak) which compares favourably against DW-IPM machines.

As mentioned previously, the saliency ratio of electrical machines is desired to be high. However, the motor 100 as a consequence of the fractional-slot distribution, results in a decrease in the saliency ratio. Fig. 4 illustrates comparative saliency ratios for both measured and modelled versions of the motor 100, which are seen to provide saliency ratios in the range 1.05 - 1.14, thus approximately 1.1. Such a low value of saliency ratio would suggest poor torque and CPSR performances.

Surprisingly however, the present inventors found that by increasing the air gap length compared to that of correspondingly sized prior art machines, that a substantial increase in machine performances could be achieved.

Large air gaps are known in the art of electrical machines to provide for a higher overload capacity, better cooling and reduction of noise, and a reduction in unbalanced magnetic pull. However, the disadvantage of the large air gap length is that it results in a high value of magnetising current, leading to lower efficiency. Electrical machines used particularly for electric vehicle applications and also for wind generator applications, desire high efficiency thereby suggesting relatively small air gaps. Particularly, for induction motors it is desirable to make the air gap as small as possible so as to increase the power factor.

In the prototype motor 100 of Fig. 1, an air gap length of 1.2 millimetres was used, this being significantly larger by comparison to traditional IPM machines of comparable power output which normally have an air gap length of approximately 0.5 millimetres.

Fig. 5 illustrates measured and modelled efficiency and power output of the machine 100, which importantly is seen to display a near linear constant power speed ratio (CPSR) greater than 6.2:1 relative to a nominal motor speed of 573 RPM. Efficiency is also seen to be substantially constant at 82% over the power speed range. CPSR is taken as being the point at which field-weakening begins (the base speed), to the speed where power falls below the power at base speed, after which constant supply voltage and current cannot be maintained.

Figs. 6A - 6C and 7A- 7C provide a comparative analysis between the motor 100 and two other interior permanent magnetic machines of comparable designed power output. Fig. 6A shows a segmented IPM machine with a distributed winding (designated herein as "SEG- IPM") and which has 24 slots and 4 poles. Fig. 6B illustrates a single piece per pole IPM machine with a distributed winding (designated herein as "IPM-I"), again with 24 slots and 4 poles. Fig. 6C schematically shows the motor 100 with 18 slots and 14 poles (designated herein as "CW-IPM").

Fig. 7A illustrates a comparison of the normalised output torque between the three machines of Figs. 6A - 6C. It is seen that whilst the SEG-IPM machine of Fig. 6A and the IPM-I machine of Fig. 6B have the highest normalised torque value, the latter curve drops rapidly. The CW-IPM machine 100, whilst having a lower normalise torque value, has a much wider torque range.

Fig. 7B illustrates the field weakening performance of the machines of Fig. 6A, 6B, and 6C. It is seen that the SEG-IPM and IPM-I machines afford much lower power output than the CW-IPM machine 100, and further that the CW-IPM machine 100 has a much wider

CPSR. i Fig. 7C illustrates a normalised cogging torque period plot for the machines of Figs. 6A, 6B, and 6C indicating, that whilst the cogging torque of the machine 100 occurs at a higher frequency, the magnitude of that cogging torque is significantly smaller, at

0.77% peak, compared to the other two machines at 4.05% peak and 12.1% peak respectively.

A low cogging torque is particularly advantageous in wind generator applications.

The comparison of the three machines presented in Figs. 7A to 7C indicates that the CW-IPM motor 100 achieves at significant improvements in performance. Further, where a desired efficiency of greater than 70% is chosen, the CPSR of the motor 100 increases to be in excess of 7.2 to 1.

The verified measured efficiency throughout the field weakening range illustrated in Fig. 5 indicated that the majority of losses in the prototype motor 100 arose from copper losses. Efficiency may therefore be optimised by increasing the slot size to create space for a larger conductor size, as well as changing the manner in which the windings are formed (e.g. horizontal or vertical). Accordingly, an increase in the slot size was modelled to create space for larger conductor sizes as well as changing the manner in which windings may be performed. This resulted in two further CW-IPM modelled configurations, the first (designated IPM-R) having the same stator outer diameter and stack length as the motor 100, but with the rotor was made smaller to make space for a larger stator width. The second modelled version (designated IPM-S) included half the stack length, but increased stator outer diameter. The machine volume of both arrangements was kept the same as the motor 100 for an unbiased comparison. Specifications of these two designs are indicated in Table 2 below. Both arrangements were modelled with a vertical slot fill winding.

TABLE 2

KEY SPECIFICATIONS OF TWO OPTIMIZED CW-IPM MACHINE DESIGNS

IPM-R IPM-S

Stator outer diameter 130 mm 183.85 mm

Rotor outer diameter 36 mm 45.7 mm

Air gap length 1 mm 1.41 mm

Slot opening width 1.2 mm 1.2 mm

Stack length 80 mm 40 mm

Rated current 2.2 Anns/ph 2.2 A_rms/ph

Conductor size AWG 20 AWG 21

No. of turns per coil 163 turns 127 turns

Stator resistance 4.5 Ω/ph 5.5 Ω/ph

Mag. remanent flux 1.13 T 1.13 T Slot fill factor 45% 45%

Figs. 8A and 8B illustrate power curves for the IPM-R and IPM-S modelled configurations respectively, which provide comparable CPSR's and efficiencies thereby indicating the motor 100 is able to be readily adapted into a number of shapes and sizes for specific power outputs.

Fig. 9 is a plot of the input power for both modelled and measured for the motor 100. Fig. 10 plots output measurements of power 1003 and torque 1000 for the motor 100. It is seen that torque 1000 is maintained constant 100.1 over the speed range from zero up to the nominal operating speed of 573 RPM, after which the torque 1000 decays generally inversely 1002 with speed over the entire CPSR. Figs. 5 and 10 clearly illustrate that the higher value of the measured efficiency in excess of 80 percent can be achieved throughout the entire 6.2:1 field-weakening range of the motor 100.

In the motor 100, magnets are configured within the rotor 102 and are formed in a V- shape, the detail of which is seen in Fig. 17 for the specific dimensions (in millimetres) used in the motor 800. Fig. 17 in particular illustrates a section 1700 of lamination of the rotor 102 which includes a pair of inclined slots. 1702 and 1704 into which rare earth permanent magnets are positioned when the laminations are stacked together. The V-shape formed by the pair of slots 1702 and 1704 when occupied by the corresponding pair of magnets addresses and thereby forms a rotor pole within the V-shape, seen in Fig. 17 designated as P, of which there are 14 in the motor 100, generally designated N and S in the illustration of Fig. 1. The slots in this example, and thus the magnets, have a width of 2.30 mm. As seen, between the slots 1702 and 1704 at an apex of the V-shape, an iron bridge 1706 provides for magnetic coupling between the magnets (not illustrated in Fig. 17). The slots 1702 and 1704 are arranged at V-angle of about 60 degrees. Other preferred dimensions of this component of the prototype 100 are illustrated. The magnets are desirably sintered rare earth magnets, and chosen in preference to bonded magnets due to higher remanent flux densities and an ability to withstand higher operating temperatures.

The arrangement of Fig. 17 illustrates an iron bridge 1706 having a width of 0.58 millimetres, being smaller than 0.6 millimetres as used for a 40 millimetre rotor 102 in the prototype 100. This suggests to the present inventors an iron bridge width to rotor outer diameter ratio of less than 0.015. Further, Fig. 17 also shows an iron inter-pole link section 1708 also smaller than 0.6 millimetre (in this case 0.5 millimetres) implying a distance of the pole tips to the rotor outer diameter ratio also less than 0.015. That is:

((iron bridge width) / (rotor outer diameter)) <0.015 , and

((inter-pole link section thickness) / (rotor outer diameter)) < 0.015 .

Whilst the prototype 100 is constructed using a V-angle of about 60 degree, the empirical results obtained therefrom were then modelled using FE analysis for a range of V- angles to assess the changes in performance for those ranges. Fig. 15 A shows the fields for a V-angle of 120 degrees and Fig. 15B shows the relative fields for a V-angle of 60 degrees. Fig. 16 shows a plot of normalised power verses speed characteristics with variation of the V- angle of the permanent magnets. From these results, with a 14 pole rotor, the present inventors considered magnets with a V-angle of between 30 and 70 degrees will be most suitable for motors of this form of configuration. Nevertheless, the experimental results obtained from the motor 100 with the arrangements shown in Fig. 17 together with the results of Fig. 16 indicate that V-angles of between 30 degrees and 90 degrees will provide useful results affording a CPSR in excess of 6.0: 1. Magnet V-angles up to 160 degrees may be used depending on operational requirements.

TABLE 3 Power and CPSR with variation of V-angle

V-angle Power at base speed CPSR

120° 600 W 4.2: 1

90° 725 W 6: 1

60° 1050 W 10:1

30° 1835 W 8:1

One aspect of scalability of the motor 100 is the need for optimizing materials, sizes and specifications to maintain the CPSR, preferably in excess of 6.2:1.

A fractional-slot CW configuration results in increased harmonic and sub-harmonic content in the MMF waveform. These additional harmonics affect rotor losses, torque ripple and the maximum field weakening range of the machine 100. The application of a double- layer stator windings as well as larger air gaps in the motor 100 assist in lowering the effects of these harmonics. Figs. 11 and 12 illustrate effects of varying the air gap length on the performance of the machine 100 as modelled and based upon previous modelled and measured parameters.

Fig. 11 shows the effects of the CPSR and input power at base speed (i.e. 573 RPM) when the air gap length is varied from 0.6 millimetres to 1.6 millimetres. As will be apparent from the dashed line in Fig. 11 , input power decreases as the air gap length is increased, whereas the CPSR increases generally exponentially. From Fig. 11 , there is a clear trade off between input power and the maximum field weakening speed of the machine 100. That trade-off will guide as to the upper usable air gap length.

Fig. 12 shows that, although having a wider air gap length is beneficial in decreasing core losses, the overall efficiency remains inversely proportional to the air gap length, as known in the art. This is due to the significant increase in input power as well as the fact that copper loss is more prominent compared to core losses with larger air gap lengths.

Significantly, in comparison to traditional IPM machines of similar size which have an air gap of about 0.5 millimetres, Figs. 1 1 and 12 clearly illustrate that for the 800 watt motor 100 and modelling derived therefrom, air gap lengths of between 0.6 millimetres and 1.6 millimetres can be useful in achieving increased CPSR.

From the modelling, and the experimental results obtained from the motor 100, the present inventors have determined the following relation ( ) as useful in specifying the air gap length for CW-IPM machines, particularly those having V-shaped magnets formed with an iron bridge:

K = ((air gap length) / (stator outer diameter)), > 0.006 , or

(air gap length) > 0.006 x (stator outer diameter).

In the example of Table 1, this relation ( ) equalled 0.0092, in contrast to a comparable traditional machine with a 0.5 mm air gap giving a value of K = 0.003. In the examples of Table 2, K = 0.0077. Particularly, the above relation is practically operative when a CPSR > 4.2: 1 is desired.

From Figs. 11 and 12 and the modelling results derived therefrom, the air gap length of motors according to the present disclosure may be increased over the range of 120% - 320% of traditional air gap lengths used in comparable prior art motors, the specific implementation of the motor 100 being increased by 240%.

According to the modelling and experimental results obtained from the 800 watt motor

100, modelling has been performed by the present inventors for comparable machines at 5 kW and 30 kW. Key design specifications of these machines are listed below in Table 4.

TABLE 4

KEY SPECIFICATIONS OF THE TWO MODELLED CW-IPM MACHINE CONFIGURATIONS

5 kW CW-IPM ^'30 kW CW-IPM

Stator outer diameter 0.26 m 0.39 m

Rotor outer diameter 0.160 m 0.240 m

Air gap length 3.0 mm 3.6 mm

Slot opening width 3.0 mm 4.8 mm

Stack length 0.080 m 0.160 m

Rated voltage 320 V_rms(i_i) 320 Vnns .i)

Conductor size 13.9 Ar.ns/ph 75.2 Arms/ph

No. of turns per coil 53 turns 16 turns

Mag. remanent flux 1.34 T 1.36 T

Core saturation mag. l-67T@500A/m 1.67T@500A/m

Slot fill factor 41% 41%

K =(Air gap /Stator outer) 0.0115 0.0092

Figs. 13 and 14 illustrate the input power, output power and efficiency verses speed characteristics of each of the 5 kW and 30 kW modelled configurations. As apparent from these modelled plots, an efficiency in excess of 92 percent is obtained for the 5 kW configuration and in excess of 95% is obtained for the 30 kW configuration, each with a CPSR in excess of 8 : 1. The air gaps of 3.0 mm and 3.6 mm are relatively larger than those traditionally found in comparable sized motors.

The construction of the motor 100 and the corresponding FE analysis thereof, and of other modelled motors of similar construction has shown, as expected, that a concentrated winding reduces the saliency ratio, whilst convention indicates that a wider CPSR is unachievable with a lower saliency ratio. Surprisingly, the experimental and modelled results obtained by the present inventors indicate that a wide CPSR is achievable, even with a low saliency ratio of approximately 1.1 (as shown by Figs. 4 and 5).

Further, accordingly to accepted practice in the art, low cogging torque in a permanent magnet machine is only achievable with certain combinations of slots and poles. Contrary to those certain combinations of slots and poles, the 18 slot 14 pole combination used by the present inventors was found to provide the lowest cogging torque of a range of motor configurations. Other slot and pole combinations indicated by analysis to be similarly useful include 18 slots and 16 poles, and 12 slots and 10 poles.

The number of slots and poles may be varied by integer multiples to achieve machines of different sizes, and thus suited to different rotor speeds. For example, the preferred 18 slot 14 pole combination may be reproduced as a 54 slot 42 pole combination in a larger diameter machine. Such a machine may be particularly useful in wind generator applications where relatively lower rotational speeds are typical, compared for example to electric vehicle motor applications. A significant advantage of this feature is that PM CW-IPM generators according to the present disclosure may be used in relatively large wind generator

configurations, possibly in direct-drive configuration. Gearboxes are traditionally required with relatively inefficient double-fed induction generator wind turbine configurations, for example in the 1 MW to 6 MW range. By obviating a need for a gearbox, or reducing the gear ratio, significant operational savings for a wind turbine can be made. For example, the presence of a gearbox increases manufacture and maintenance cost and acts to amplify any cogging torque, thereby reducing efficiency, particularly at low speeds. The low cogging torque of the presently described arrangements and the likely absence of a gearbox thereby affords a significant advantage over traditional wind turbine installations.

The use of concentrated windings for permanent magnet AC machines is a relatively new art, having commenced only in about 2002. Further, prior research for field weakening applications focused on surface permanent magnet machines. The present inventors have found that an interior permanent magnet machine having magnets at particular V-angles in combination with a concentrated winding can afford a wide CPSR. Further, contrary to established conventions for electrical machines, that large air gaps produce low efficiency and reduce power density and CPSR, the motor 100 and corresponding FE analysis has shown that a larger air gap may be used to extend the CPSR relative to the comparable machines of other configurations.

Further, the prototype machine 100 indicates that by maximising the width of the magnets, in this case 2.3 millimetres as defined by the magnet angle, provides the ability to maintain an iron bridge between the magnets thereby reinforcing the steel sections of the rotor and providing for structural integrity thereof.

The prototype machine 100 afforded a 56% increase in power when compared with two other similar size IPM machines, whilst offering a significantly lower cogging torque, which in turn leads to increased stability and lower acoustic noise. The efficiency achieved was 82% over a CPSR of 6.2:1, which was comparable to DW machines of this size.

The use of concentrated winding provided for simplification of manufacture and also shorter winding lengths, thus providing additional space for increasing the effective length of the machine.

The larger air gap length, in addition to extending the CPSR, makes the machine 100 increasingly tolerant or sympathetic to dynamic load changes for traction applications, for example reducing gearbox requirements and the like.

Machines configured according to the present disclosure may be used in wheel hub applications for electric cars, thereby taking advantage of the direct drive low cogging torque and wide CPSR. In such implementations, it would be typical for the stator to be centrally fixed to a stub axle and be electromagnetically coupled to an annular rotor forming part of the rotating wheel of the motor vehicle. In such an implementation the permanent magnets of the rotor would be mounted to an interior of the external rotor thereby addressing the stator poles formed by concentrated windings radiating outwardly from the stator on the wheel hub. Such a machine remains a fractional slot interior permanent magnet motor.

INDUSTRIAL APPLICABILITY

The arrangements described are applicable to the development of electric motors and generators having a wide CPSR whilst maintaining high efficiency. Such may be industrially applied to traction applications including electric vehicles and wind generators.

The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope arid spirit of the invention, the embodiments being illustrative and not restrictive.

(Australia Only) In the context of this specification, the word "comprising" means "including principally but not necessarily solely" or "having" or "including", and not "consisting only of. Variations of the word "comprising", such as "comprise" and

"comprises" have correspondingly varied meanings.

Claims

CLAIMS:

1. An electric machine comprising a fractional slot interior permanent magnet configuration having a concentrated winding formed on a stator thereof and pairs of magnets each formed in a V-shape and addressing each rotor pole, and an air gap length satisfying the relation:

((air gap length) / (stator outer diameter)) > 0.006.

2. An electric machine according to claim 1 wherein each pair of magnets forming a rotor pole are separated by a corresponding iron bridge formed in the rotor, and adjacent pairs of magnets are separated by iron inter-pole link sections.

3. An electric machine having an N-slot stator with a concentrated non-overlapping winding and a P-pole interior permanent magnet rotor with pairs of magnets each formed in a V-shape addressing a rotor pole and the magnets within each pair separated by an iron bridge, such that the number of slots and poles (N,P) are selected from the group consisting of ((18,14), (12, 10) and (18,16)) and integer multiples thereof, and an air gap between the stator and rotor having an air gap length satisfying the relation:

(air gap length) > 0.006 x (stator outer diameter).

4. An electric machine configured for field weakening applications, said machine comprising a fractional slot interior permanent magnet configuration having a concentrated winding formed on a stator thereof and pairs of magnets each formed in a V-shape creating a rotor pole and the magnets within each pair being separated by an iron bridge, a relatively , large air gap between rotor and stator, and a field weakening constant power speed range (CPSR) of at least 6.2:1. "

5. An electric machine according to claim 4 wherein the relatively large air gap is

5 defined by an air gap length, being a radial distance between an outer diameter of the rotor and an inner diameter of the stator, satisfying the relation:

(air gap length) > 0.006 x (stator outer diameter).

6. An electric machine according to claim 4 or 5 wherein the machine has a rated power 10 in excess of 5 kW, and exhibits an efficiency in excess of 92% with a CPSR in excess of 8 : 1.

7. An electric machine according to claim 4 or 5 wherein the machine has a rated power in excess of 30 kW, and exhibits an efficiency in excess of 95% with a CPSR in excess of 8: 1.

15 8. An electric machine having an 18-slot stator with a concentrated non-overlapping winding and a 14-pole interior permanent magnet rotor with pairs of magnets each formed in a V-shape creating a corresponding rotor pole, the magnets within each pair being separated by an iron bridge, and an air gap between the stator and rotor having an air gap length satisfying the relation (air gap length) > (0.006 x (stator outer diameter)), such that the -20 machine has a field weakening constant power speed range (CPSR) of at least 4.2: 1

9. A concentrated winding fractional slot interior permanent magnet machine having V- shaped magnets characterised by:

an air gap length satisfying the relation:

25 (air gap length) = x (stator outer diameter), where K > 0.006; and a CPSR > 4.2:1.

10. A machine according to claim 9 wherein K > 0.009 and a CPSR > 6.2 : 1.

11. A machine according to claim 9 wherein K > 0.0092 and a CPSR > 8:1.

12. A concentrated winding fractional slot interior permanent magnet motor having V- shaped magnets characterised by an air gap length in the range of 120% - 320% of air gap lengths of alternative comparable sized motors.

13. The motor of claim 12 wherein the air gap length is about 240% of air gap lengths of alternative comparable sized motors.

14. An electric machine according to any one of the preceding claims wherein the magnet V-angle is between 30 and 160 degrees.

15. An electric machine according to claim 14 wherein the magnet V-angle is between 30 and 90 degrees.

16. An electric machine according to any one of the preceding claims wherein inter-pole link sections separating adjacent magnetic poles of the rotor have a thickness satisfying the relation:

((inter-pole link section thickness) / (rotor outer diameter)) < 0.015 .

17. An electric machine according to any one of the preceding claims having an iron bridge separating an apex of the V-shaped magnets in each rotor pole section, the iron bridge having a width satisfying the relation:

((iron bridge width) / (rotor outer diameter)) < 0.015 .

18. An electric generator having an N-slot stator with a concentrated non-overlapping winding and a P-pole interior permanent magnet rotor with pairs of magnets each formed in a V-shape addressing a rotor pole and the magnets within each pair separated by an iron bridge, such that the number of slots and poles (N,P) are selected from the group consisting of (( 18, 14), ( 12, 10) and (18, 16)) and integer multiples thereof, and an air gap between the stator and rotor having an air gap length satisfying the relation:

(air gap length) > 0.006 x (stator outer diameter).

19. A wind turbine comprising an electric generator according to claim 18.

20. An electric machine substantially as described herein with reference to any one of the embodiments as that embodiment is illustrated in the drawings excluding Figs. 6A and 6B.