Power Density Comparison for Three Phase Non-Slotted Double-Sided AFPM Motors

Transcription

1 Australian Journal of Basic and Applied Sciences, 4(1): , 010 ISSN Power Density Comparison for hree Phase Non-Slotted Double-Sided AFPM Motors S. Asghar Gholamian and A. Yousefi Electrical Engineering Department of Babol University of echnology, Babol, Iran Abstract: here are two topologies for non-slotted double-sided axial flux PM (AFPM) motors. he stator of the non-slotted AFPM machine is realized by non-slotted tape wound core with AC polyphase air gap windings and the rotor structure is formed by axially magnetized fan-shaped surface mounted Neodymium Iron Boron (NdFeB) permanent magnets. Selecting an AFPM motors with high power density is an important parameter in applications. So, comparison of power density between different topologies of double-sided AFPM motors seems to be necessary. In this paper, the sizing equations of axial flux non-slotted one-stator-two-rotor (ORUS) and two-stator-one-rotor (AFIR) type PM motors is presented and comparison of the ORUS and AFIR topologies in terms of power density is illustrated. Finally a high power non-slotted double-sided AFPM motor is introduced in the paper. Key words: PM motors, AFPM, power density and Double-sided. INRODUCION Double-sided axial flux PM motors (AFPM) are the most promising and widely used types. AFPMs (commonly called disc machines) are synchronous machines. In conventional machines, the air gap flux density has normally radial direction; in AFPMs, the air gap flux density presents mainly axial direction. In general, AFPMs exhibit an axial length much smaller than the length of a conventional motor of the same rating (Huang, S., 1998). here are two topologies for non-slotted double-sided AFPM motors. hese topologies are axial flux nonslotted one-stator-two-rotor (ORUS) and two-stator-one-rotor (AFIR) type PM motors. wo AFPM motors and their acronyms are selected ORUS-NS (Axial flux non-slotted external rotor internal stator PM stator) and AFIR-NS (Axial flux non-slotted internal rotor external stator PM motor) for detailed analysis. he stator of the non-slotted AFPM motors are realized by non-slotted tape wound core with AC polyphase air gap windings that are back-to-back wrapped around the stator core. he rotor structure is formed by axially magnetized surface mounted Neodymium Iron Boron (NdFeB) permanent magnets and shaft. Detailed views of the stator and rotor structures of the ORUS-NS and AFIR-NS motor are given in Fig.1. he portions between the windings are assumed to be filled with epoxy resin as in all non-slotted structures in order to increase the robustness of the structure and provide better conductor heat transfer. Moreover, the radial portions of the air gap windings are used for the torque production (Gholamian, S.A., 008; Gholamian, S.A., et al, 008). Flux directions of both AFIR and ORUS non-slotted topologies at the average diameter in D are also shown in Fig.a and b. Selecting a double-sided AFPM motors with high power density is an important parameter, especially in electrical vehicle applications. So, comparison of power density between different topologies of double-sided AFPM motors seems to be necessary. Increasing the air gap length, maximum power density will change in AFPM motors. hese changes are not the same in different topologies. Maximum power density of ORUS-NS is higher than AFIR-NS in large air gap length. In Section II, the generalized sizing approach for ORUS-NS and AFIR-NS types PM motors is briefly discussed. hen, some results of comparisons of the ORUS-NS and AFIR-NS topologies in terms of power density are illustrated in Section III. he conclusions are given in Section IV. Corresponding Author: S. Asghar Gholamian, Electrical Engineering Department of Babol University of echnology, Babol, Iran gholamian@nit.ac.ir 5947

2 Fig. 1: Axial flux non-slotted (a) one-stator-two-rotor. ORUS-NS type (Gholamian, S.A., et al, 008) (b) two-stator-one-rotor AFIR-NS type (Gholamian, S.A., 008) Fig. : One pole pair of the (a) ORUS-NS (Gholamian, S.A., et al, 008) (b) AFIR-NS (Gholamian, S.A., 008) II. Sizing Equations of AFPM Motors: In general, if stator leakage inductance and resistance are neglected, the output power for any electrical machine can be expressed as m P et ().() it dtmke I out p pk pk 0 (1) e(t) and E pk are phase air gap EMF and its peak value, i(t) and I pk are phase current and the peak phase current, is machine efficiency, m is number of phases of the machine and is period of one cycle of the EMF(Huang, S., 1999; Gholamian, S.A., 008). he quantity K p is termed the electrical power waveform factor and defined as 1 et ( ) it ( ) 1 K dt f (). t f () t dt p e i E 0 pk Ipk 0 () f e (t)=e(t)/ E pk and f i (t)=i(t)/ I pk are the expressions for the normalized EMF and current waveforms. In order to indicate the effect of the current waveform, a definition for current waveform factor, K i, is also useful, 5948

3 I 1 pk it ( ) Ki dt Irms I 0 pk 0.5 (3) I rms is the rms value of the phase current. he peak value of the phase air gap EMF for AFPM in (1) is given by: Epk Ke Nph Bg. f.(1 ) Do (4) p K e is the EMF factor which incorporates the winding distribution factor K w and the per unit portion of the total air gap area spanned by the salient poles of the machine (if any), N ph is the number of turn per phase, Bg is the flux density in the air gap, f is the converter frequency, p is the machine pole pairs, is the diameter ratio for AFPM defined as D i /D o, D o is the diameter of the machine outer surface, D i is the diameter of the machine inner surface. he peak phase current in (1) is given by: 1 Do Ipk A Ki. mn 1 ph (5) m 1 is number of phases of each stator and A is the electrical loading. Combining (1) through (5), the general purpose sizing equations take the following form for AFPM. m f 1 P K K K AB (1 )( ) D 3 out e p i g o m1 p (6) he machine power density for the total volume can be defined as Pout Pden (7) DtotLtot 4 D tot is the total machine outer diameter including the stack outer diameter and the protrusion of the end winding from the iron stack in the radial direction, L tot is the total length of the machine including the stack length and the protrusion of the end winding from the iron stack in the axial direction (Huang, S., 1999; Gholamian, S.A., 008). A. Sizing Equations for the ORUS-NS: he generalized sizing equation approach can easily be applied to axial flux permanent magnet ORUS type motor (Gholamian, S.A., et al, 008). he outer surface diameter D o can be written as m f 1 Do Pout Ke KpKi ABg (1 )( ) m1 p 1/3 (8) he machine total outer diameter D tot for the ORUS-S motor is given by D D W tot o cu (9) 5949

4 W cu is the protrusion of the end winding from the iron stack in the radial direction. For the back-to-back wrapped winding, protrusions exist toward the axis of the machine as well as towards the outsides and can be calculated as W cu ADg Di Di Kcu J s (10) D g is the average diameter of the machine, J s is the current density and K cu is the copper fill factor. he axial length of the machine L e is given by L L L g e s r (11) L s is axial length of the stator, L r is axial length of the rotor and g is the air gap length. he axial length of the stator L s is L L W s cs cu (1) he axial length of the stator core L cs can be written as Bg p Do(1 ) Lcs (13) 4 pbcs B cs is the flux density in the stator core and p is the ratio of average air gap flux density to peak air gap flux density. Since there is no rotor core in rotor PM topologies, the axial length of rotor Lr is L r L PM Also, the axial length of the rotor core L cr is (14) L cr Bu Do(1 ) 8 pb cr (15) B cr is the flux density in the rotor disc core, and B u is the attainable flux density on the surface of the PM. he PM length L PM can be calculated as r Bg LPM ( gwcu ) K f Br Bg Kd r is the recoil relative permeability of the magnet, B r is the residual flux density of the PM material, K d is the leakage flux factor, K c is the Carter factor, K f =B gpk /B g is the peak value corrected factor of air gap flux density in radial direction of the AFPM motor. hese factors can be obtained using FEM analysis (Gholamian, S.A., et al, 008). (16) 5950

5 B. Sizing Equations for the AFIR-NS: he concept of Double-sided Axial Flux two-stator-one-rotor (AFIR) type PM motors was presented in (Huang, S., 1999; Gholamian, S.A., 008). he outer surface diameter D o is obtained from (6). D P m K K K AB 1 o out e p i g (1 )( ) m1 p f 1/3 (17) he machine total outer diameter D tot for the AFIR type machines is given as D D W tot o cu (18) W cu is the protrusion of the end winding from the iron stack in the radial direction and can be calculated as W cu D AD g i Di Kcu J s (19) he axial length of the machine L e is L L L g e r s (0) L s is axial length of the stator, L r is axial length of the rotor and g is the air gap length. he axial length of a stator L s is L L W s cs cu L cs is the axial length of the stator core. he axial length of the stator core L cs can be written as Bg p Do(1 ) Lcs 8pB cr (1) () Since there is no rotor core in rotor PM topologies, the axial length of rotor Lr is L r L PM (3) he PM length L PM can be calculated as r Bg LPM ( gwcu ) K f Br Bg Kd (4) III. Comparison of ORUS-NS and AFIR-NS: Comparison of two different Double-sided axial flux non-slotted PM motors in terms of power density is accomplished for 10HP output power, 6 poles and 60Hz drives. In this comparison, other constant parameters of motors are tabulated in table

6 able 1: Parameters of motors Number of phases 3 Slot fill factor 0.8 Pole arc ratio 0.75 Slot per Pole per Phase 1 flux density in ststor 1.5 flux density in rotor 1.5 Efficiency 90% PM Residual flux density 1.1 In AFPM motors, the air gap flux density, Bg and diameter ratio, and are the two important design parameters which have significant effect on the motor characteristics. herefore, in order to optimize the motor performance, the diameter ratio and the air gap flux density must be chosen carefully. Fig.3 shows the power density variation as a function of air gap flux density and the diameter ratio for the AFIR-NS and ORUS-NS motors. (A) (B) Fig. 3: Power density vs. air-gap flux density and diameter ratio for A=0000 (A/m), g=1 (mm), J s = (A/m ) 595

7 a) ORUS-NS b) AFIR-NS: As can be seen from Fig3b, the maximum power density occurs at Bg=0.31 and = Varying air gap length, maximum power density occurs in different Bg and. able shows maximum power density with corresponding Bg and. able : Maximum power density with corresponding Bg and ype g (mm) Bg () Maximum power density (W/cm 3 ) ORUS-NS AFIR-NS Fig.4 shows the maximum power density variation as a function of air gap length for the AFIR-NS and ORUS-NS motors for A=0000 (A/m), J s = (A/m ).. Fig. 4: Maximum power density AFIR-S and ORUS-NS vs. air-gap length In as special air gap length (this air gap length is called Gs) maximum power density of AFIR-NS and ORUS-NS motors will be the same. Considering Fig.4, it can be concluded that in large air gap length, nonslotted ORUS motor has high power density. he considerable point is that the value of Gs will vary when the electrical loading 'A' and current density 'Js' changes. Fig.5 shows the maximum power density variation as a function of air gap length in A=5000 (A/m) and J s = (A/m ) for the AFIR-NS and ORUS-NS motors. Fig.6 shows the maximum power density variation as a function of air gap length in A=0000 (A/m) and J s = (A/m ), for the AFIR-NS and ORUS-NS motors also. AFIR-NS and ORUS-NS vs. Air-gap Length: According to Fig.5 it can be concluded that point Gs is shifted to larger air gaps and this means that in smaller air gaps AFIR-NS motor has higher maximum power density. According to Fig.6 it can be concluded that point Gs is shifted to smaller air gaps and this means that in higher air gaps ORUS-NS motor has higher maximum power density. Other value of Gs for various A and Js are tabulated in table

8 Fig. 5: Maximum power density AFIR-NS and ORUS-NS vs. air-gap length Fig. 6: Maximum power density AFIR-NS and ORUS-NS vs. air-gap length able 3: Other value of Gs for Various A and Js A Js Gs (mm)

9 IV. Conclusions: Selecting an AFPM motors with higher power density is an important parameter in applications. he main goal of this paper has been introducing to double-sided Axial Flux non-slotted PM Motors with maximum power density. here are two topologies for NON-slotted double-sided AFPM motors. he maximum power density is changed by different value of the air gap, electrical loading and current density. ORUS-NS topology has high power density in high current density and low electrical loading. But, AFIR-NS topology has high power density in low current density and high electrical loading. List of symbols E pk peak value of air gap EMF I pk peak phase current m number of phases period of one cycle of the EMF I rms rms value of the phase current K e EMF factor K w winding distribution factor N ph number of turn per phase Bg flux density in the air gap f converter frequency p machine pole pairs D o diameter of the machine outer surface D i diameter of the machine inner surface D tot total machine outer diameter L tot total length of the machine W cu protrusion of the end winding L e, axial length of the stator L cs axial length of the stator core PM length L PM REFERENCES Aydin, M., S. Huang,.A. Lipo, 001. Optimum design and 3D finite element analysis of nonslotted and slotted internal rotor type axial flux PM disc Machines, Power Engineering Society Summer Meeting. IEEE 3: Aydin, M., Surong Huang,.A. Lipo, 001. Design and 3D electromagnetic field analysis of non-slotted and slotted ORUS type axial flux surface mounted permanent magnet disc machines, Electric Machines and Drives Conference. IEMDC 001. IEEE International., pp: Gholamian, S.A., M. Ardebil, K. Abbaszadeh and S. Mahmodi Charati, 008. Optimum Design of 1kW Axial Flux Permanent Magnet Slotted ORUS Motor, European Journal of Scientific Research, 1: Gholamian, S.A., M. Ardebili and K. Abbaszadeh, 008. Analytic and FEM Evaluation of Power Density for Various ypes of Double-Sided Axial Flux Slotted PM Motors, International Journal of Applied Engineering Research, 3: Huang, S., J. Luo, F. Leonardi and.a. Lipo, A General Approach to Sizing and Power Density Equations for Comparison of Electrical Machines, IEEE rans. On Energy Conversion, IA-34(1): Huang, S., J. Luo, F. Leonardi and.a. Lipo, A Comparison of Power Density for Axial Flux Machines Based on the General Purpose Sizing Equation, IEEE rans. on Energy Conversion, 14():