Novel Switched Flux Permanent Magnet Machine Topologies

1 Introduction

Switched flux(SF),or flux switching,machines were initially developed as one of high frequency inductor generators[1].Single-phase SF permanent magnet(SFPM)machines[1]and Laws relays(limited angle actuators)[2]were developed in 1950s.In order to reduce the cost,the permanent magnet can be replaced by DC coil excitations[2,3],thus,a higher peak torque obtained,albeit with reduced efficiency and torque density.Later,this lead to the pioneering work by Prof.Pollock on both single-phase SF motors and drives for many low cost applications[4-13].Conse-quently,switched flux machines received world-wide awareness.In parallel,French researchers carried out extensive research activities on 3-phase SFPM machines[14-24]in terms of torque/power capability and efficiency,as well as hybrid excitations.At the University of Sheffield,we have been systematically developing single-phase,three-phase,and multi-phase rotary and linear SFPM machines having all stator poles wound and alternate stator poles wound,with multi-teeth,hybrid excited,and magnetless(less magnet or no magnet)for various applications,ranging from automotive to aerospace,and have been investigating their electromagnetic and control performance[25-76].Extensive researches on SFPM machines have also been carried out in China[77-113],most notably,at Southeast University[69-98]and Zhejiang University[99-112].Currently,SFPM machines are under intensive research all over the world[113-133],including UK,France,China,Norway,Netherlands,Denmark,USA,Japan,South Korea and Australia,etc.

In this paper,various novel SFPM machine topologies and their design and performance features will be overviewed,with particular emphasis on SF machines with reduced magnet usage or without magnet,as well as with variable flux capability.In addition,their relationships with doubly-salient PM(DSPM)machines and flux reversal PM(FRPM)machines will be described.

2 Features of SFPM machines

As shown in Fig.1,the main flux linked with coils is reversed or switched when the rotor rotates one stator pole pitch,i.e.from Fig.1a to Fig.1b[1].

Fig.1 Single-phase SFPM machine or laws relay

Alternative winding dispositions[31]are possible,viz.short-pitched,full-pitched,and toroidal,as illustrated in Fig.2 for a single-phase SFPM machine(also possible for a 3-phase SFPM machine).Fig.2a shows the layout of short-pitched concentrated winding wound on each stator pole.From Fig.2a,it can be seen that in the slots adjacent to permanent magnets,the two adjacent coil sides carry current of opposite polarity.Hence,it is possible to replace two short-pitched coils by one full-pitched coil,as shown in Fig.2b.Thus PM flux focusing effect may be utilized,while the copper loss can be significantly reduced.Alternatively,toroidally wound coils may be employed,Fig.2c,which may be beneficial when the machine axial length is very short.

Fig.2 Single-phase SFPM machines having different winding dispositions

As mentioned in the introduction SF machines usually have PM excitation,as shown in Figs.1 and 2,as well as DC coil excitation[4-12,60,114-116]in order to reduce the cost,as will be further described in the later sections.It is also possible to have any phase number[37]and stator/rotor pole number combinations,but optimal stator/rotor pole number combination exists[40-42].

As shown in Fig.3,for typical SFPM machines,the salient pole rotor is identical to that of a switched reluctance machine,which is simple and robust,without any magnet or coil.Both magnets(or DC excitation coils)and armature coils are housed on stator.The salient pole stator core consists of modular “U”-shaped laminated segments between which are placed circumferentially magnetized PM alternatively with opposite polarity.The stator winding is comprised of concentrated coils,each coil being wound on a pole formed by two adjacent laminated segments and a magnet.Flux focusing may be utilised and low-cost ferrite magnets employed.Since the windings and the magnets are effectively magnetically in parallel,rather than in series as in conventional PM machines,the electric loading and specific torque capability of a SFPM machine can be very high,at least comparable to those of surface-mounted and interior magnet machines.In addition,a high per-unit winding inductance can be readily achieved.Thus,such machines are eminently suitable for constant power operation over a wide speed range,i.e.they can have a high flux-weakening capability.

Fig.3 Single-and three-phase SFPM machines

The phase flux-linkage waveform of a SFPM machine is bipolar and the back-emf waveform can be designed to be essentially sinusoidal although concentrated stator windings are employed,Fig.4.When the rotor pole aligns with one side stator tooth,flux linkage reaches negative peak,whilst when it aligns with another side stator tooth,flux linkage becomes positive peak.When either the rotor pole or the rotor slot aligns with the stator magnet,the flux linkage is zero.The torque production relies on the double saliency of both stator and rotor.However,the torque results predominantly from the PM excitation torque and the reluctance torque is usually negligible.The advantages and disadvantages of conventional SFPM machines are summarized as follows:

Fig.4 Open-circuit field distributions and typical flux-linkage waveform of single coil in 12/10 stator/rotor pole SFPM machine

(1)Advantages:Simple and robust rotor;Easier to manage magnet temperature rise; Flux focusing/low cost ferrite magnets may be used; Sinusoidal back-emf waveform–suitable for brushless AC operation.

(2)Disadvantages:Reduced copper area; Low over-load capability due to heavy saturation; Complicated stator; Leakage outside stator; High magnet volume.

The specific design issues,such as stator and rotor pole number combinations,winding configura-tions,optimal split ratio,optimal rotor pole width,optimal stator pole width and magnet thickness,stator back-iron thickness etc,are summarized in Ref.[27].By way of example,the optimal rotor pole number is either close to the stator pole number or a multiple thereof,while the optimal ratio of rotor pole width to rotor pole-pitch is approximately 1/3[40-42].

3 Machines having permanent magnets on stator

3.1 Machines having PMs on stator

When the PMs are located on the stator,the PMs can be mounted in the stator back-iron(DSPM machines[134,135]),placed on the inner surface of the stator teeth(FRPM machines[136,137]),or sandwiched in the stator teeth(SFPM machines),Fig.5,Tab.1.As pointed out in Refs.[26,27],they have the same operating principle-switched flux,the identical rotor structure-salient pole rotor without magnet or coil,the similar stator topology-all having PMs on the salient pole stator which also carries non-overlapping windings.Irrespective of their location,however,the torque results predominantly from the PM excitation torque,i.e.the reluctance torque is negligible,although the torque production mechanism relies on the rotor saliency.Therefore,their common features may be summarized as follows:

Fig.5 Alternative machines having PMs on stator

(1)PM on stator.

(2)Salient pole stator with non-overlapping,concen-trated stator winding.

(3)Salient pole rotor without winding and magnet.

(4)Reluctance action.

(5)Dominant PM excitation torque.

(6)Negligible reluctance torque.

Tab.1 Comparison of alternative PM machines having magnets on stator

3.2 DSPM machines

A major disadvantage of DSPM machines is that due to unipolar flux-linkage,the torque density is relatively low.This is significantly improved in SFPM machines in which flux-focusing may be readily incorporated and the phase flux-linkage waveform is bipolar,the torque capability is significantly higher than that of a DSPM machine[73].Meanwhile,the back-emf waveform of a SFPM machine is essentially sinusoidal,while that of a DSPM machine is essentially trapezoidal.Thus,SFPM machine is more appropriate for brushless AC operation,while DSPM machine more suitable for brushless DC operation.

3.3 FRPM and SFPM machines

In both SFPM and FRPM machines each coil spans a pair of alternate magnetic poles.Therefore,the PM flux linkage is bipolar.However,in SFPM machines each coil encircles two laminated modules and a single magnet,and the fluxes produced by the magnet and the coil are in parallel.In contrast,in FRPM machines a pair of magnets is mounted on the surface of each stator tooth and the fluxes produced by the magnet and the coil are in series.Consequently,the FRPM machines exhibit lower winding inductances and the magnets are more vulnerable to potential irreversible demagnetization,may have a higher induced eddy current loss,and may experience a significant radial magnetic force,as they are directly exposed to the reluctance variation of the salient rotor poles.Due to flux focusing effect,SFPM machines also exhibit significantly higher torque density than FRPM machines.

Since both the magnets and armature coils are housed in the stator in PM machines having PMs in the stator,the stator copper area is significantly reduced,particularly in SFPM machine in which high magnet number and volume are employed.But as will be described in next section,new topologies are being developed and it is possible to reduce the number and volume of PMs,while maintaining its torque capability.

4 Reduction of magnet usage on SFPM machines

In the conventional SFPM machines,the slot area in stator is significantly reduced due to both PMs and armature coils being on the stator,while the number of magnet segments is high and the amount of magnets used is usually increased,which increases the cost since the rare earth magnet is expensive and normally＞50% of total material cost.The large volume of magnets may also increase the eddy current loss in the magnets.Hence,for the SFPM machines,one of the key issues is to reduce the magnet usage.Although the magnet usage in the conventional SFPM machines can be directly reduced,e.g.by reducing the magnet thickness,the torque density is usually reduced as well.As will be shown in this section,the magnet usage in SFPM machines may be reduced by employing multi-tooth,E-core or C-core stators,respectively,Fig.6.

Fig.6 Alternative SFPM machine topologies employing much less magnets

4.1 Multi-tooth SFPM machines

A multi-tooth SFPM machine[32,33]produces higher torque when the electric loading is low,Fig.7,it requires less magnets,coils and laminated modules,thus the material and manufacturing costs can be significantly reduced.However,it effectively results in more stator teeth and,hence requires more rotor teeth.In Refs.[32,33],it is shown that the multi-tooth SFPM machine reduces the magnet usage by a half relative to the conventional SFPM machines.However,it maintains almost the same slot area as that of the conventional machines due to the multi-tooth structure which limits the slot area when the split ratio is fixed.

4.2 E-core and C-cored SFPM machines

In order to reduce the magnet usage and cost,and increase the slot area,the concept of E-core SFPM machines was proposed and systematically investi-gated in Ref.[43].It employs only a half volume of magnets and exhibits larger torque density relative to the conventional SFPM machines,Fig.6 and Fig.7.In Ref.[45],it is shown that the middle tooth in the E-core stator may be removed without sacrificing the torque density,Fig.6 and Fig.7,although the power factor is found to be reduced.

It is worth noting that in all of these SFPM machines having multi-tooth,E-core and C-core stators,the magnet usage is significantly reduced(halved in above mentioned examples),while the torque density is also increased.As shown in Fig.7,only when the electric loading(current)is very high,their torque will be less than that in the conventional SFPM machine due to higher armature reaction and magnetic saturation.

Fig.7 Comparison of torque against current characteristics in alternative SFPM machines employing much less magnets

5 Magnetless Switched Flux Machines

Although PM machines exhibit high torque density and high efficiency,the rare earth magnets,e.g.NdFeB,are expensive and have limited resources.Further,the working environmental temperature may also limit their application due to potential irreversible demagnetisation.One of the possible solutions is to replace the magnet excitation by DC coil excitation.

In the conventional DC coil excited synchronous machine the field winding is on the rotor,and,consequently,slip-rings are required to supply the field current.In PM machines having magnets on the stator,e.g.DSPM and SFPM machines,Section III,the excitation of magnetic field may be realized by a DC coil on the stator,as previous done in Laws relay[2]and single-phase SF machines[3-12],in which the 1-phase DC coil excited SF machine and its controller were proposed and extensively analyzed for automotive and power tool applications due to the extremely low cost of the machine and drive system.Since the field winding is excited by unipolar current,it can be directly connected in parallel or in series with the power converter which feeds the bipolar current into the armature winding.The 1-phase FS machine was shown to exhibit a higher output power density than the equivalent universal and induction machines,and comparable efficiency to that of the induction machine.Of course,as in any type of 1-phase machines,the 1-phase FS machine also has problems of low starting torque,large torque ripple,and fixed rotating direction.

In Ref.[138,139],a DC coil excited DS machine was proposed.But their torque capability is limited due to the inherent relatively low torque capability of DS machines,as mentioned earlier.

It is well-known that the magnetic potential source of permanent magnets can be physically modelled by surface currents.However,surface current sheets are not possible to realise in practice and can usually be achieved by using volume currents.Therefore,the DC coil excited SF machine can be directly obtained from the corresponding SFPM machine,Fig.8a,by replacing the magnets with the volume coil excited model,Fig.8b.It should be noted that there is a lamination steel core in the middle of DC coil excited model to reduce the reluctance for DC coil excitation and to link the stator “U”-shaped lamination steel segments as a complete stator.However,the field produced by DC field conductors at the outer surface of the stator goes through the outside of the stator instead of the rotor.Hence,the DC field conductors close to the outer surface of the stator can be removed to enlarge the slot area for the field conductors close to the air-gap which produces the main flux,Fig.8c,and the DC field conductors can also be connected in an alternative way as illustrated in Fig.8d to reduce the number of coils.When the influence of end-effect and end winding is ignored,the machines having two kinds of DC field conductor connections exhibit the same electro-magnetic performance.Similar to the SFPM machine,the conductors in one DC field winding slot and its adjacent two stator teeth compose one stator pole,as illustrated in Fig.8 for 12-stator pole machines.

The electromagnetic performance of 12/10 stator/ rotor pole DC coil excited SF and SFPM machines having the same outer diameter and active axial length are predicted by 2-D FE analyses and compared in Ref.[60],Fig.9.It shows that the DC excited SF machine produces similar torque to the ferrite magnet SF machine,but significantly lower than the NdFeB magnet SF machine due to significant magnetic saturation in the stator teeth.

Fig.8 12/10 stator/rotor pole SFPM and DC coil excited SF machines

Fig.9 Comparison of torque-current characteristics of 12/10 stator/rotor pole SFPM and DC coil excited SF machines

A DC coil excited SF machine with modular rotor and non-overlapping windings was recently developed and investigated in Refs.[114-116].A modular rotor SFPM machine(Fig.18d)has similar torque density as a conventional SFPM machine[142],but a DC coil excited SF machine with modular rotor and non-overlapping windings(Fig.11)may exhibit higher torque density than the corresponding SF machines shown in Fig.8.

Fig.10 Single-phase DC coil excited SF machines

Fig.11 DC coil excited SF machines having modular rotor

6 Variable flux SFPM machines

The PM flux is not adjustable,which may limit the low speed over load capability and high speed operation,and reduce overall efficiency.Hence,there is a need to adjust the PM flux.This is usually achieved in vector control by employing either position or negative d-axis current,however,the q-axis current then needs to be reduced accordingly.

6.1 Variable-flux PM machines

Variable-flux PM(VFPM)machines include some means of adjusting the level of PM flux and are of interest today as they allow flexibility in terms of optimizing efficiency across a machine operation cycle.Many examples of VFPM machines have been studied and documented[63,141,142]including hybrid-excited machines with field coils and machines with mechanical adjustment.Some features of variable flux PM machines are summarized as follows:

(1)Advantages:Easy to achieve constant power operation(flux weakening); Potentially enhanced low speed torque; Reduced risk of high open-circuit back-emf at high speed during flux weakening; Possible high efficiency operation over whole torque and speed region.

(2)Disadvantages:Complex structure; Torque density likely reduced; Limited flux enhancing capability due to magnetic saturation;Extra DC source required,or Extra mechanical means required for mechanically adjusted flux.

6.2 Mechanically adjusted variable flux PM machines

Fig.12 shows the mechanical mechanism utilized for providing flux adjustment in SFPM machines[64].A short circuit bar is located behind the stator back-iron which can be moved to provide a short circuit path for the magnet flux.In the normal position,Fig.12a,the magnet flux links the rotor as in the conventional SFPM machine,Figs.13a and 13b,but when the bar is moved to the closed weakening position,Fig.12b and Fig.13c,much of the magnet flux is short circuited.Thus,the magnet flux linkage and the back-emf can be reduced,as shown in Fig.14.However,as a passive means for adjusting the magnet flux,it has disadvantages in that it is impossible to enhance the flux and it requires an extra mechanical mechanism.

Fig.12 Mechanically adjusted SFPM machine concept

Fig.13 Mechanically adjusted variable flux SFPM machines

Fig.14 Performance of mechanically adjusted variable flux SFPM machines

6.3 Hybrid PM and DC coil excited PM machines

Hybrid excited PM machines[63,141,142]can be further grouped based on the location of coil excitation sources on the stator or rotor,and the magnetic circuit configuration of the permanent-magnet and field-coil flux paths,series or parallel.

Hybrid excited DSPM machines,Fig.15a,were investigated in Refs.[138-142].In addition to the inherent relatively low torque density as mentioned earlier,they have long end windings for the field windings which over-lap with the armature windings.

A novel hybrid excited SFPM machine employing non-overlapping field and armature windings was proposed in Refs.[21],Fig.15c.However,the machine outer diameter is significantly enlarged in order to accommodate the DC field winding,which signi-ficantly reduces the torque density.The magnets in the SFPM machine may be partially replaced by the DC excitation windings and consequently several hybrid excited topologies were developed[39,87,88,90],Fig.15b,but they have overlapping armature and field windings,albeit with significantly reduced torque capability.

Fig.15 Hybrid SFPM machines

A new hybrid excited SFPM machine was developed in Ref.[44]to eliminate the foregoing disadvantages.It employs non-overlapping field and armature windings and also exhibits a simple structure.Compared with the conventional SFPM machine,the magnet material in the proposed hybrid excited SFPM machine is also halved while the torque density is improved since it is based on the developed E-core SFPM machine[43].Fig.16 compares predicted and measured torque-DC excitation current characteristics.

Fig.16 Torque-DC excitation current characteristics of E-core hybrid excited SFPM machine

7 Other SFPM machine topologies

In addition to the forgoing mentioned SFPM machines topologies,numerous other machine topologies have been developed recently,to name a few:Linear and tubular SFPM machines[46,58,67,68,106,108,110,113,126],

Fig.17[143,144]; Fault-tolerant SFPM machines with isolated armature windings[37,38,43,44,82-86],Fig.6c and Fig.18b; Axial-field SFPM machines; Transverse-flux SFPM machines[76],Fig.18e;Modular rotor SFPM machines,Figs.18d; Multi-PM sandwiched SFPM machines which may be advantageous to employ low cost ferrite magnets,Fig.18c[100,130]; Multi-3-phase inverters fed SFPM machines[27,81].

Fig.17 Linear hybrid step motor(Sawyer motor)[143,144]

Fig.18 Other SFPM machines

It is worth noting that linear SFPM machines inherit all the advantages of rotary SFPM machines,they are particularly attractive for long-stroke applica-tions,since,as in hybrid step motors and Sawyer motors,Fig.17[143,144],both magnets and coils can be mounted on the short mover so that less permanent magnets or copper will be used than conventional long-stroke linear PM machines,which will significantly reduce the cost.Therefore,linear SFPM machines have re-attracted much attention recently in academic and industrial field[46,58,67,68,106,108,110,113,126].

8 Conclusions

Numerous novel and new SFPM machine topologies have been developed over the last decade.They are briefly reviewed in this paper with particular emphasis on SF machines with reduced magnet usage or without using magnet,as well as with variable flux capability.

Since the torque results predominantly from the PM excitation torque and the reluctance torque is usually negligible,control of a SFPM machine is similar to that of surface-mounted PM machines,but with large winding inductance[65,66].

Due to their novel features,e.g.PM on the stator,simple and robust rotor,flux focusing,and higher torque density compared with conventional DSPM and FRPM machines,as well as at least similar torque/power density compared with conventional surface-mounted and interior PM machines,SFPM machines have significant potential for applications,including domestic appliances,power tools,automo-tives,aerospace,and wind power generation etc.

Acknowledgements:This paper is based on the author’s keynote speech at International Conference on Electrical Machines and Systems(ICEMS2011),20～23 August,2011,Beijing.The contributions on this subject made by the PhD and ex-PhD students at the University of Sheffield,Dr Y.Pang,Dr J.T.Chen,Dr A.S.Thomas,Dr Y.Chen,Dr R.L.Owen,Dr X.Liu,Dr J.X.Shen,Dr W.Hua,Dr W.Min,Dr H.S.Liu,Mr Z.Azar,Mr M.Al-ani,are greatly appreciated.

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