Electromagnetic Performance Analysis of Novel Flux-Regulatable Permanent Magnet Machines for Wide Constant-Power Speed Range Operation

Transcription

1 Article Electromagnetic Performance Analysis Novel Flux-Regulatable Permanent Magnet Machines for Wide Constant-Power Speed Range Operation Yunchong Wang, Shuangxia Niu * Weinong Fu Received: 27 August 215; Accepted: 3 November 215; Published: 1 December 215 Academic Editor: Timothy Gordon Department Electrical Engineering, Hong Kong Polytechnic University, Hong Kong, China; wangycee@gmail.com (Y.W.); weinong.fu@polyu.edu.hk (W.F.) * Correspondence: eesxniu@polyu.edu.hk; Tel.: (ext. 6183); Fax: Abstract: Two novel structures permanent magnet (PM) machine, namely a hybrid flux modulation machine (HEFMM) a variable flux memory machine (), which have excellent field-weakening capability, are presented in this paper. HEFMM incorporates advantages parallel hybrid structure flux modulation structure, so as to increase torque density as well as increase constant-power speed range. Inspired by HEFMM, aiming to furr improve efficiency machine, with aluminum-nickel-cobalt (AlNiCo) PMs in inner stator which can be magnetized by current pulse direct current (DC) windings is developed. With double-stator structure, flux modulation effect in both machines can be employed to realize hybrid regulate air-gap flux density readily. operation principle is illustrated static steady performances machines are analyzed compared with time stepping finite element analysis, which validates effectiveness proposed designs. Keywords: field weakening; finite element method (FEM); memory machine; wide speed range; wind power 1. Introduction Permanent magnet (PM) machines with high efficiency high torque density have found extensive applications in various fields. Wide speed range constant-power operation is usually required in a variety industrial applications such as in electric vehicles wind power generation systems. Flux weakening-control PM machines has been studied extensively by researchers [1,2]. field-weakening performance PM machines is usually limited by magnet flux-linkage saliency ratio. For machines without saliency effect, due to uncontrollable PM flux, it is difficult to adjust flux in air-gap realize field-weakening control. To improve field-weakening capability PM machines, hybrid concept in which PM wound field are combined toger is investigated. Li Lipo [3] proposed a new type hybrid excited doubly salient PM machine capable field weakening, which incorporates both PMs wound field windings in stator. Owing to field winding, it has one hundred percent field weakening capability. field windings are able to provide field-weakening characteristics even for machines without saliency. However, shortcoming this design is its relatively high cogging torque. Several or hybrid synchronous machines have been proposed [4 8]. Based on former study, generally, hybrid machines is divided into two different types: series hybrid machines parallel hybrid machines. To demonstrate difference two topologies, basic equivalent magnetic circuit model is given in Figure 1. F PM F coil are magnetic motive force (MMF) Energies 215, 8, ; doi:1.339/en

2 Energies 215, 8, Energies 215, 8, page page (MMF) PM coils, respectively. RPM Rgap are reluctance PM PM coils, respectively. R PM R gap are reluctance PM materials air gap, materials air gap, respectively. In Figure 1a, Riron is reluctance iron core, respectively. respectively. In Figure 1a, R iron is reluctance iron core, respectively. In Figure 1b, R iron1, R gap In Figure 1b, Riron1, Rgap Riron are reluctances PM materials, air gap iron core, R iron are reluctances PM materials, air gap iron core, respectively. structure respectively. structure series hybrid machines is simple. In se types series hybrid machines is simple. In se types machines, PM field machines, PM field control windings are in series. However, re are some control windings are in series. However, re are some disadvantages for se machines: disadvantages for se machines: (1) Due to low permeability PMs, magnetic circuit coil has high (1) Due to low permeability PMs, magnetic circuit coil has a high magnetic reluctance; refore, large ampere-turn windings are needed to realize magnetic reluctance; refore, large ampere turn windings are needed to realize field weakening. field weakening. (2) coil is for field weakening only; field-strengning effect is poor because (2) coil is for field weakening only; field strengning effect is poor because saturation iron core. saturation iron core. (3) (3) series series topology topology may may lead lead to to demagnetization demagnetization PMs PMs especially especially for for ferrite ferrite materials materials with with low low coercive coercive force. force. (a) (b) Figure Figure (a) (a) Series Series hybrid hybrid machines; machines; (b) (b) parallel parallel hybrid hybrid machines. machines. In parallel hybrid machines, PM is not in magnetic circuit In parallel hybrid machines, PM is not in magnetic circuit coils. coils are not only for field weakening but also for field strengning. coils. coils are not only for field weakening but also for field strengning. current is much smaller because magnetic reluctance is lower. current is much smaller because magnetic reluctance is lower. major problem hybrid machine is that field winding decreases major problem hybrid machine is that field winding decreases efficiency, worsens heating problem increases size machine. To solve heating efficiency, worsens heating problem increases size machine. To solve heating problems hybrid machines system level optimization [9 11] should be considered. problems hybrid machines system level optimization [9 11] should be considered. core loss [12 14] steels cooper loss windings should be matched to improve core loss [12 14] steels cooper loss windings should be matched to improve efficiency. Memory machine is an excellent alternative to prevent se problems [14,15]. efficiency. Memory machine is an excellent alternative to prevent se problems [14,15]. concept memory is derived from fact that aluminum nickel cobalt (AlNiCo) materials can concept memory is derived from fact that aluminum-nickel-cobalt (AlNiCo) materials can be easily magnetized by a temporary direct current (DC) current pulse it was firstly proposed by be easily magnetized by a temporary direct current (DC) current pulse it was firstly proposed Ostovic [16]. In memory machines, with different DC current pulses, field can be readily by Ostovic [16]. In memory machines, with different DC current pulses, field can be readily adjusted just as hybrid machines. Current pulse is injected into DC winding during adjusted just as hybrid machines. Current pulse is injected into DC winding operation machine with high current but very short time. refore loss during operation machine with high current but very short time. refore loss magnetization DC winding is very small can almost be ignored. magnetization DC winding is very small can almost be ignored. In this paper, two novel structures are proposed. A hybrid flux modulation machine In this paper, two novel structures are proposed. A hybrid flux modulation machine (HEFMM) with DC field windings AC armature windings is labeled as Machine A. (HEFMM) with DC field windings AC armature windings is labeled as Machine A. HEFMM HEFMM has a bidirectional flux modulation feature. With field modulation effect, HEFMM can has a bidirectional flux modulation feature. With field modulation effect, HEFMM can realize realize separation magnetic circuits PM coils. refore, it belongs to separation magnetic circuits PM coils. refore, it belongs to parallel parallel hybrid machines. It has significant merits parallel hybrid hybrid machines. It has significant merits parallel hybrid machines. machines. flux modulation effect is applied in magnetic gears first [17 22]. In HEFMM, re flux modulation effect is applied in magnetic gears first [17 22]. In HEFMM, re are two are two field modulation groups. rotor PMs, inner stator tooth AC power winding field modulation groups. rotor PMs, inner stator tooth AC power winding constitutes a constitutes a field modulation group. or field modulation group is DC field control field modulation group. or field-modulation group is DC field control winding, rotor winding, rotor modulation steels AC power winding. For this design, PMs are major source air gap flux. At low speeds, DC current enhances air gap flux for high

3 Energies 215, 8, Energies modulation 215, 8, steels page page AC power winding. For this design, PMs are major source air-gap flux. At low speeds, DC current enhances air-gap flux for high torque density. When torque EMF density. (electromotive When force) EMF reaches (electromotive its maximal force) value, reaches DC its maximal current will value, decrease DC tocurrent weakenwill decrease magneticto field. weaken DCmagnetic windingfield. is able todc provide winding a reversing is able to field provide to realize a reversing one hundred field to percent realize one field-weakening. hundred percent field weakening. DC field-weakening DC winding field weakening AC power winding winding AC arepower distributed winding on two are distributed different stators. on two different DC windings stators. located DC in windings inner stator located doin not take inner space stator do not AC take power space winding AC dopower not obstruct winding housing do not obstruct AC power housing winding. AC power winding. Extending study HEFMM, a variable flux fluxmemory machine () is isinvestigated later later named as asmachine B. B. In In, DC DCfield fieldwindings HEFMM are arereplaced by by AlNiCo PMs DC DC magnetization magnetization windings. windings. AlNiCo AlNiCo PMs PMs are are magnetized magnetized by by current current pulse pulse DC windings DC windings housed housed on inner on stator inner stator regulate regulate field to realize field field to realize weakening. field weakening. inherits inherits merits HEFMM, merits which HEFMM, haswhich excellent has field excellent weakening field weakening capacity capacity wide speedwide range. speed Furrmore, range. Furrmore, improves efficiency improves power efficiency density dramatically. power density dramatically. principle HEFMM principle ishefmm illustrated by analytical is method. illustrated Time-stepping by analytical finitemethod. element Time stepping method (TS-FEM) finite [23] element is used method to simulate (TS FEM) [23] steady is used performance to simulate steady novel machines. performance FEM simulation novel machines. results are FEM reported simulation to validate results are reported accuracyto validate analytic accuracy model. analytic model. 2. Operation Principle Prototype Structure 2.1. Analysis Flux Modulation Effect basic idea flux modulation machine or Vernier machine is derived from flux modulation flux-modulation magnetic gear. magnetic gear is isshown in infigure 2 air gaps are exaggerated. To Toilluminate operation principle flux modulation effect, a portion a magnetic gear is shown in Figure 2. Figure 2. Portion a flux modulation magnetic gear. Figure 2. Portion a flux modulation magnetic gear. flux linkage excited by outer rotor PM within one pole range inner rotor is examined. flux N1 is linkage pole pair excited number by outer rotor outer PM rotor within PMs, one Ns is pole pieces range modulation inner rotor steels is examined. N2 is N 1 pole pair is pole-pair number number inner rotor outer PMs. rotor PMs, θ1 is N s outer is rotor pieces position modulation θ is steels angular N 2 is position pole-pair in number outer air gap. inner rotor ω1 is PMs. mechanical θ 1 is outer speed rotor position outer rotor. θ is fundamental angular position component in outer MMF air gap. outer ω rotor 1 is PMs mechanical FPM1 speed magnetic circuit outer permeance rotor. P are fundamental expressed component as: MMF outer rotor PMs F PM1 magnetic circuit permeance P are expressed as: 4 hm FPM1 θ,θ1 F1cos N1( θ θ1) B r cos N 1( θ θ1) (1) F πμ PM1 pθ, θ 1 q F 1 cos rn 1 pθ θ 1 qs 4 h m m B r cos rn π µ 1 pθ θ 1 qs (1) m P θ P P cos( N ) 1 sθ (2) P pθq P P 1 cospn s θq (2) P1 is permanence induced by flux modulation effect modulation steels. Based on permeance coefficients expressions slot effect [12], P P1 are given as: / P βc μ g g h μ (3) P a1 a2 m r μ β ga1 ga2 hm π.78 2c 3 sin(1.6 πc ) (4)

4 Energies 215, 8, P 1 is permanence induced by flux modulation effect modulation steels. Based on permeance coefficients expressions slot effect [12], P P 1 are given as: P p1 1.6βc q µ { pg a1 ` g a2 ` h m {µ r q (3) µ P 1 2β.78 g a1 ` g a2 ` h m π.78 2c 2 sinp1.6πc q (4) where g a1 g a2 are thickness air gaps, h m is thickness B r is residual flux density PM 1. c is space ratio modulation steels. β is nonlinear coefficient related to α g a1 + g a2 + h m. µ is vacuum permeability µ r is relative permeability PMs. According to Equations (1) (2), inner air gap flux density is shown as: h m B pθ, θ 1 q F PM1 pθ, θ 1 q P pθq 4 B r cosn π µ 1 pθ θ 1 q rp P 1 cospn s θqs m $, P cosn 1 pθ θ 1 q 4 h & m B r P1 /. π µ m 2 cos rpn 1 N s qθ N 1 θ 1 s % P1 2 cos rpn 1 ` N s qθ N 1 θ 1 s /- (5) flux linkage within one pole range inner rotor due to outer rotor PM is given as: π{n ş 2 φ pθ 1 q r g2 l m h m 4 r g2 l m B r π µ m P 1 " P B pθ, θ 1 qdθ N 1 sinn 1 pθ θ 1 q 2 pn 1 N s q sin rpn 1 N s qθ N 1 θ 1 s π P 1 2 pn 1 ` N s q sin rpn 1 ` N s qθ N 1 θ 1 s*ˇˇˇˇ N 2 (6) When N 1 = N 2, flux density due to flux modulation effect is negligible re is no gear effect between two rotors. To employ flux modulation effect, pole pair numbers should satisfy relationship N 1 N s = N 2. Equation (6) can be rewritten as: h m 4 φ pθ 1 q r g2 l m B r π µ m ˆ2P ` P1 sin pn N 1 N 1 θ 1 q (7) 2 For flux modulation machine, inner rotor PM poles are replaced by stator windings. stator flux linkage is same as expression Equation (7) due to flux modulation effect outer rotor PMs Dual Flux Modulation Effect Compared with conventional flux modulation machine, dual flux modulation machine with two sets PMs has much higher torque density. As shown in Figure 3b, for dual flux modulation machine, rotor PMs are surface-inserted into rotor yoke additional PMs are inserted into stationary modulation steels. inner rotor PMs middle stationary modulation steels consist first set flux modulation group. rotor salient poles PMs inserted into stationary modulation steels consists second modulation group. flux linkage stator winding is superposition se two fields can be expressed as: φ s φ PM1 ` φ PM2 pφ PM1 ` Φ PM2 q sin pn 1 θ 1 q (8) 13974

5 Energies 215, 8, where Φ PM1 is flux linkage excited by inner rotor PMs Φ PM2 is flux linkage excited by middle rotor PMs. To achieve preferable field control capability, hybrid structure is shown in Figure 3c. Instead inner PMs, DC windings can excite field, which is modulated by middle rotor steels. scale magnetic field can be controlled by DC current conveniently. Energies 215, 8, page page (a) (b) (c) Figure 3. Structure flux modulation machine: (a) conventional flux modulation machine; Figure 3. Structure flux modulation machine: (a) conventional flux modulation machine; (b) dual (b) dual flux modulation machine; (c) hybrid flux modulation machine (HEFMM). flux modulation machine; (c) hybrid flux modulation machine (HEFMM) Variable Flux Memory Machine 2.3. Variable Flux Memory Machine hybrid dual flux modulation structure dramatically improves field control capability hybrid flux modulation dual flux machine. modulation However, structure power dramatically consumption improves on DC windings field control capability decreases flux machine modulation efficiency machine. increases However, rmal load power consumption machine. on DC windings decreases machine with efficiency memory materials increases AlNiCo owns rmal excellent load field machine. control capability same as hybrid dual flux modulation machine omits power losses in DC with memory materials AlNiCo owns excellent field control capability same as windings. For variable flux memory machine, field regulation is realized by AlNiCo PMs hybrid dual flux modulation machine omits power losses in DC instead DC field windings. refore, number pole pairs AlNiCo PMs is identical to pole pairs DC field winding hybrid dual flux modulation machine. Same as deduction dual flux modulation effect, effective flux in air gap can be written as: 5

6 Energies 215, 8, windings. For variable flux memory machine, field regulation is realized by AlNiCo PMs instead DC field windings. refore, number pole pairs AlNiCo PMs is identical to pole pairs DC field winding hybrid dual flux modulation machine. Same as deduction dual flux modulation effect, effective flux in air-gap can Energies 215, 8, page page be written as: φ s φ PM1 ` φ PM2 (9) s PM1 PM2 (9) magnetization DC winding is powered by current pulse at beginning operation, duty current pulse is very short, even though transient current is quite high, loss this winding is low size DC magnetization winding is much smaller than DC filed winding hybrid dual flux modulation machine. For field weakening operation, magnetization direction rotor PMs NdFeB is opposite with AlNiCo material which are housed on inner stator. maximum magnetic energy product NdFdB is is much higher than than AlNiCo coercive coerciveforce force AlNiCo AlNiCo is is low, low, refore refore demagnetization risk risk AlNiCo should be beconsidered for variable flux memory machine. To ensure that re is no demagnetization during operation, AlNiCo should be thick enough. working point point should should be be above above knee knee point point as as shown shown in Figure in Figure 4. B4. r Br H c are Hc are remanent remanent coercive coercive force force AlNiCo, AlNiCo, B k Bk H k are Hk are B B H value H value working working point K, point respectively. K, respectively. Figure 4. Nonlinearity involved parallelogram hysteresis model aluminum nickel cobalt (ALNiCo). Figure 4. Nonlinearity-involved parallelogram hysteresis model aluminum-nickel-cobalt (ALNiCo). By Ampere Law, magnetic equation can be written as: By Ampere Law, magnetic equation can be written as: Bl H h H l L L M PM L L (1) H μ M h PM H L l L B Ll L (1) µ where hpm is thickness AlNiCo HL is average value magnetic field intensity where magnetic h PM circuit is thickness outside AlNiCo, AlNiCo HlL L is average length value magnetic magnetic circuit. field intensity section above magnetic knee circuit point outside B H AlNiCo, curve AlNiCo l L is is length linear magnetic slope circuit. is μpm μ. section refore, above knee relationship point between B-H Bk curve Hk AlNiCo is: is linear slope is µ PM ˆ µ. refore, relationship between B k H k is: µ μ PM PMµ μ B r B M (11) H K Substitute Equation (9) into Equation (8), thickness AlNiCo PM is: μ PM Bl L L hpm (12) Br BK For AlNiCo materials, to ensure working point above knee point, BK should be larger than.9 Br. refore to avert demagnetization AlNiCo during operation, hpm should be: K

7 Energies 215, 8, Substitute Equation (9) into Equation (8), thickness AlNiCo PM is: h PM µ PMB L l L B r B K (12) For AlNiCo materials, to ensure working point above knee point, B K should be larger than.9 ˆ B r. refore to avert demagnetization AlNiCo during operation, h PM should be: h PM ě 1µ PMB L l L B r (13) In, field is regulated by AlNiCo PMs, which is magnetized by DC current pulse. To realize various degree field weakening for different speed, remanent AlNiCo B r magnetization direction are determined by different magnetization or demagnetization DC current pulses applied in DC windings. As analyzed above, inner stator AlNiCo PMs enhance field when magnetization direction inner stator PMs AlNiCo rotor PMs NdFeB are identical. Orwise, y weaken field when magnetization directions are opposite Structures Two Prototypes For prototype HEFMM, Machine A, AC power winding with five pole pairs is housed on outer stator DC field control winding with 22 pole-pairs is accommodated on inner stator. Twenty-seven PMs magnetized in outward direction 27 modulation steels are arranged radially on rotor. In HEFMM machine, AC windings provide rotating magnetic field, which is coupled with lower speed PM rotor. HEFMM has two stators one rotor. For prototype, Machine B, outer stator middle rotor parameters are same as Machine A. DC field control windings in Machine A are replaced by magnetization DC windings AlNiCo poles. structure parameters Machine A Machine B are shown in Table 1. Table 1. Parameters hybrid flux modulation machine (HEFMM). Parameters Machine A Machine B Rated speed 3 rpm 3 rpm Outer stator diameter 2 mm 2 mm Inner rotor diameter 75 mm 75 mm Axial length 6 mm 6 mm Outer stator winding pole-pairs 5 5 Inner stator winding pole-pairs 27 - Inner stator AlNiCo pole-pairs - 27 Rotor pole-pairs (P r ) Figure 5a shows structure proposed HEFMM Figure 5b shows. major difference between se two structures is that each slot inner stator is inserted one piece AlNiCo PM. magnetization direction AlNiCo can be identical or opposite with magnetization direction rotor PMs. For Machine A, DC field control windings are fed by continuous DC current to control field. refore, cooper losses are much larger than Machine B heating problem Machine A is more serious than Machine B. performance PM depends on temperature greatly. However, two air-gap structure se machines can improve heat dissipation high temperature PM material is used for Machine A. For Machine B, inner windings are fed by pulse current ir cooper losses can be negligible

8 magnetization direction rotor PMs. For Machine A, DC field control windings are fed by continuous DC current to control field. refore, cooper losses are much larger than Machine B heating problem Machine A is more serious than Machine B. performance PM depends on temperature greatly. However, two air gap structure se machines can Energies improve 215, 8, heat dissipation high temperature PM material is used for Machine A. For Machine B, inner windings are fed by pulse current ir cooper losses can be negligible. Energies 215, 8, page page (a) (b) Figure Figure 5. Structure 5. Structure proposed proposed machines: machines: (a) (a) HEFMM HEFMM (b) (b) variable variable flux flux memory memory machine machine (). (). 3. Performance Simulation Analysis 3. Performance Simulation Analysis 3.1. Performance Machine A 3.1. Performance Machine A 7 Using TS FEM, performances machines are analyzed. To verify mamatical model effectiveness Using TS-FEM, machine, performances magnetic field machines distribution are analyzed. HEFMM To verify refers mamatical as Machine A is studied. model Figure effectiveness 6 shows flux machine, line distribution magnetic field Machine distributiona with HEFMM a power refers current as Machine 2 A A is studied. Figure 6 shows flux line distribution Machine A with a power current 2 A a 75 A DC current in DC winding housed in inner stator. It is obvious that re is a a 75 A DC current in DC winding housed in inner stator. It is obvious that re five pole pair magnetic field in air gap. five pole pair magnetic field is modulation is a five pole-pair magnetic field in air-gap. five pole-pair magnetic field is modulation consequence consequence fundamental fundamental harmonic magnetic field. field. It isit also is also effective effective harmonic harmonic that that interacts interacts with with power power winding housed on outer stator. Figure 6. Figure Flux 6. lines Fluxdistribution lines for for Machine A with 2 A load load current current 75 A 75 A current. current. Figure 7 Figure shows 7 shows radial radial component flux flux density density its harmonic its harmonic spectra duespectra to rotor due to rotor PM PMin in outer air gap for for Machine A. Due A. Due to to flux modulation flux modulation effect, twenty-two effect, pole twenty two pairs pole PMs excite an obvious five pole-pairs space harmonic component. pairs PMs excite five pole pairs space harmonic component

9 Figure 6. Flux lines distribution for Machine A with 2 A load current 75 A current. Figure 7 shows radial component flux density its harmonic spectra due to Energiesrotor 215, PM 8, in outer air gap for Machine A. Due to flux modulation effect, twenty two pole pairs PMs excite an obvious five pole pairs space harmonic component. Figure 7. Radial component flux density due to rotor permanent magnets (PMs) its Figure 7. Radial component flux density due to rotor permanent magnets (PMs) its harmonic spectra for Machine A. harmonic spectra for Machine A. 8 Energies 215, 8, page page Figure 8 shows flux density distribution in outer air gap due to inner DC windings. Figure For 8 shows HEFMM flux Clearly, density distribution modulationin effect outer air rotor gap due modulation to inner steels DC inner windings. For HEFMM Clearly, modulation effect rotor modulation steels stator teeth result in a various kinds space magnetic harmonics in outer air gap. largest inner stator teeth result in a various kinds space magnetic harmonics in outer air gap. most important harmonic is five pole-pairs space harmonic as discussed in Section 2. largest most important harmonic is five pole pairs space harmonic as discussed in Section 2. Figure 8. Radial component flux density due to Stator DC current for HEFMM its Figure 8. Radial component flux density due to Stator DC current for HEFMM its harmonic spectra harmonic spectra Figures 9 1 show field weakening performance DC current HEFMM. output steady state maximum torque decreases along with DC current. With constant current 2 A in power winding, steady state maximum torque reduces from 46 Nm to 13 Nm when DC current decreases from 75 A to 75 A. amplitude back EMF also decreases along with DC current. no load EMF is reduced rapidly as DC current varies from 75 A to 75 A. decreasing steady state torque amplitude back EMF prove

10 Energies Figure 215, 8, Radial component flux density due to Stator DC current for HEFMM its harmonic spectra Figures show field-weakening field weakening performance DC DC current current HEFMM. HEFMM. output output steady steady state state maximum maximum torque torque decreases decreases along with along with DC current. DC current. With constant With constant current current 2 A in 2 A power in winding, power winding, steady steady state maximum state maximum torque torque reduces reduces from 46 from Nm46 tonm 13 Nm to 13 when Nm when DC current DC current decreases decreases from 75 from A to A to A. 75 A. amplitude amplitude back EMF back also EMF decreases also decreases along along with with DC current. DC current. no-load no load EMF EMF is reduced is reduced rapidly rapidly as as DC DC current current varies varies from from A to A to A. A. decreasing steady state state torque amplitude back EMF prove field weakening field-weakening effect DC DC winding. major major drawback drawback HEFMM HEFMM is that is that field weakening field-weakening operation operation is realized is realized by by DC DCcurrent which whichrequire a aquite high ampere turn ampere-turn to provide enough MMF. high ampere turn ampere-turn DC winding increases size machine decreases efficiency. refore is better option for high efficiency high power density applications I DC = 75A I DC = A I DC = -75A Torque [Nm] Energies 215, 8, page page Time [ms] Figure 9. Steady state torque with different field weakening currents. Figure 9. Steady state torque with different field-weakening currents. 9 No-load Back EMF [V] I DC =75A I DC =A I DC =-75A Time [ms] Figure 1. No load electromotive force (EMF) phase with different field weakening currents. Figure 1. No-load electromotive force (EMF) phase A with different field-weakening currents Performance Machine 3.2. Performance Machine B As performance analysis prototype HEFMM,, referred to as Machine B, As performance analysis prototype HEFMM,, referred to as Machine B, is analyzed with TS FEM since parameters outer stator middle rotor are identical for is analyzed with TS-FEM since parameters outer stator middle rotor are identical for both Machine Machine B. both Machine A Machine B. Figure 11 shows flux lines distribution with 2 load current. Br, remanent Figure 11 shows flux lines distribution with 2 A load current. B AlNiCo is 1.8 magnetization direction AlNiCo is identical to r, remanent NdFeB PMs AlNiCo is 1.8 T magnetization direction AlNiCo is identical to NdFeB PMs on middle rotor. function AlNiCo memory material is to substitute DC on middle rotor. function AlNiCo memory material is to substitute DC windings in HEFMM. Same as Machine A, five pole pair magnetic field in air gap is obvious windings in HEFMM. Same as Machine A, five pole-pair magnetic field in air-gap is obvious interacts with stator windings. interacts with stator windings. 1398

11 is analyzed with TS FEM since parameters outer stator middle rotor are identical for both Machine A Machine B. Figure 11 shows flux lines distribution with 2 A load current. Br, remanent AlNiCo is 1.8 T magnetization direction AlNiCo is identical to NdFeB PMs on middle rotor. function AlNiCo memory material is to substitute DC Energies 215, 8, windings in HEFMM. Same as Machine A, five pole pair magnetic field in air gap is obvious interacts with stator windings. Figure 11. flux lines distribution with 2 A load current. Figure 11. flux lines distribution with 2 A load current. Figure 12 shows radial component flux density its harmonic spectra due to Figure 12 shows 13 radial component fluxdensity its harmonic spectraspectra due to rotor PMs Figure shows radial component flux density its harmonic due to inner stator AlNiCo poles. rotor PMs Figure 13 shows radial component flux density its harmonic spectra due Figures 14AlNiCo 15 show field weakening performance. output steady state to inner stator poles. maximum torque decreases remanent performance AlNiCoPMs from 1.8 T to 1.8 T. state Figures show when field-weakening varies. output steady With constant current 2 A in power winding, steady state maximum torque reduces from maximum torque decreases when remanent AlNiCo PMs varies from 1.8 T to 1.8 T. 54 Nm to 5.2 Nm. amplitude no load back EMF decreases from 44 V to 4.5 V. With constant current 2 A in power winding, steady state maximum torque reduces from 54 can realize field weakening operation as simple as HEFMM. re is no continuous current in Nm to 5.2 Nm. amplitude no-load back EMF decreases from 44 V to 4.5 V. DC winding. DC winding is only for magnetizing AlNiCo PMs. can realize operation asvariation simple as HEFMM. is no continuous torque with speed forre HEFMM.current in Figure field-weakening 16 shows maximum DC winding. DC winding is only for magnetizing field is weakened by DC current or AlNiCo PMs when machines speedalnico is PMs.higher Figure 16 shows maximum torque variation with along speed for increasing HEFMM. than rated speed. As a result, torque decreases with speed. For, by maximum torque is 54 Nm below rated speed 3 rpm decreasesspeed to field is weakened DCsteady current or AlNiCo PMs when machines is 18than Nm as speed extended 1 rpm. For decreases HEVPM maximum steady torque is47 Nm higher rated speed. As atoresult, torque along with increasing speed. below rated speed 3 rpm decreases to 9isNm as below speed rated extended to 1 For, maximum steady torque 54 Nm speed 3 rpm. rpm decreases to 1 HEVPM maximum steady torque is 47 Nm 18 Nm as speed extended to 1 rpm. For below rated speed 3 rpm decreases to 9 Nm as speed extended to 1 rpm. Energies 215, 8, page page Figure 12. Radial component flux density due to rotor PMs its harmonic spectra for Figure 12. Radial flux density due to rotor PMs its harmonic spectra for Machinecomponent B. Machine B

12 Energies 215, 8, Figure Radial component flux density due to rotor PMs its harmonic spectra for Machine B. Figure 13. Radial component flux density due to Stator AlNiCo PMs for its Figure 13. harmonic Radial spectra. component flux density due to Stator AlNiCo PMs for its harmonic spectra. Energies 215, 8, page page 11 Energies 215, 8, page page No-load No-load Back Back EMF EMF [V] [V] Br =1.8T Br = T Br =1.8T =-1.8T Br = T Br =-1.8T Time [ms] Figure 14. No load EMF phase A with Time [ms] different remanent AlNiCo. Figure 14. No-load EMF phase with different remanent AlNiCo. Figure 14. No load EMF phase A with different remanent AlNiCo. 6 Torque Torque [Nm] [Nm] Br = 1.8T Br = T Br = -1.8T Br = T Br = -1.8T Time [ms] Figure 15. Steady state torque with Time different [ms] remanent AlNiCo. Figure 15. Steady state torque with different remanent AlNiCo. 6 Figure 15. Steady state torque with different remanent AlNiCo. 6 HEVPM 5 HEVPM 5 Torque Torque [Nm] [Nm]

13 Time [ms] Energies 215, 8, Figure 15. Steady state torque with different remanent AlNiCo. 6 5 HEVPM Torque [Nm] Speed [rpm] Figure 16. Maximum torque vs. speed waveform. Figure 16. Maximum torque vs. speed waveform. 4. Conclusions 4. Conclusions Two novel machine structures with excellent field weakening performance are presented. HEFMM Two novel machine with structures parallel magnetic with excellent circuit field-weakening topology have promising performance potential are presented. for wide speed HEFMM range applications. with parallel magnetic structure circuit is proposed topology based have on promising HEFMM, potential which for employ wide flux speed modulation range applications. effect to realize parallel structure hybrid is proposed operation. based on Without HEFMM, continuous which employ DC flux modulation current, effect to realize can realize parallel high hybrid efficiency high operation. power density. Without basic continuous principle DC both structures current, is elucidated can verified realize high by TS FEM. efficiency performances high power both density. machines basic are principle analyzed both result structures shows is elucidated excellent field weakening verified by performance TS-FEM. performances machines. both machines are analyzed result shows excellent field weakening performance machines. 12 Acknowledgments: This work was supported by research grants (Project 4-ZZBM, PolyU 5388/13E PolyU 15213/14E) Research Grants Council in Hong Kong Special Administrative Region, China. Author Contributions: Shuangxia Niu conceived idea research provided guidance supervision. Yunchong Wang implemented research, performed analysis wrote paper. All authors have contributed significantly to this work. Conflicts Interest: authors declare no conflict interest. References 1. Kou, B.; Li, C.; Cheng, S. Flux-Weakening Characteristic Analysis a New Permanent-Magnet Synchronous Motor Used for Electric Vehicles. IEEE Trans. Plasma Sci. 211, 39, Slootweg, J.G.; de Vries, E. Inside wind turbines-fixed vs. variable speed. Renew. Energy World 23, 6, Li, Y.; Lipo, T.A. A doubly salient permanent magnet motorcapable field weakening. In Proceedings 26th Annual IEEE Power Electronics Specialists Conference, PESC' 95 Record, Atlanta, GA, USA, June 1995; pp Fodorean, D.; Djerdir, A.; Viorel, I.A.; Miraoui, A. A double excited synchronous machine for direct drive application Design prototype tests. IEEE Trans. Energy Convers. 27, 22, [CrossRef] 5. Akemakou, A.D.; Phounsombat, S.K. Electrical Machine with Double Excitation, Especially a Motor Vehicle Alternator. U.S. Patent 6,147,429, 14 November Chan, C.C. Novel permanent magnet motor drives for electric vehicles. IEEE Trans. Ind. Electron. 1996, 43, [CrossRef] 7. Aydin, M.; Huang, S.; Lipo, T.A. A new axial flux surface mounted permanent magnet machine capable field control. In Proceedings 22 IEEE Industry Applications Conference, 37th IAS Annual Meeting, Pittsburgh, PA, USA, October 22; pp Tapia, J.A.; Leonardi, F.; Lipo, T.A. Consequent-pole permanentmagnet machine with extended field-weakening capability. IEEE Trans. Ind. Appl. 23, 39, [CrossRef] 13983

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