3846 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013

Comparative Investigation on Integrated System of Permanent MagnetSynchronous Generator and Power Converter Based on Machine

Topology for Small-Scale Wind Power ApplicationYu-Seop Park , Seok-Myeong Jang , Min-Mo Koo , Jang-Young Choi , and So-Young Sung

Department of Electrical Engineering, Chungnam National University, Daejeon 305-764, KoreaOcean System Research and Development Division, Department of Electrical Engineering, Korea Ocean Research and

Development Institute, Chungnam National University, Daejeon 305-764, Korea

This paper presents the comparative analysis and test results of permanent magnet (PM) synchronous generators based on machinetopology for small-scale wind power application. For reasonable comparison, various design considerations, such as flux direction, rotorposition, slot existence and PMmaterials, are dealt with under specific machine size limitation. Among the analysis models, five machinesare manufactured for experimental verification, and the generators are integrated with AC-DC-DC converter to convert originally gen-erated AC power to DC power. With our constructed experimental set, the electromagnetic characteristics based on measured current,and their performance are comparatively investigated under constant output voltage control condition.

Index Terms—Machine topology, permanent magnet (PM) wind power generator, power converter.

I. INTRODUCTION

A MONG various categories of wind power generators [1],permanent magnet synchronous generator (PMSG) type

is one of the attractive types due to its powerful performancewith higher energy density and efficiency. Since it was intro-duced, the PM has been actively applied to various electricalmachines, and it is also considered as a highly required materialin PMSG for wind power generation system. The type of PMmachines can be various according to the machine topology,and they were thoroughly investigated in previous studies. Inparticular, [2] sufficiently compared the performance of sevenmachine types while [3] included radial flux PMSG (RFPMSG)and axial flux PMSG (AFPMSG) in their research showingsimilar performance in lower power range while AFPMSGpresented superior performance to RFPMSG in higher powerrange. In addition, [4] applied rare-earth-free material to theirmachine to overcome the lack of rare-earth materials in in-dustry, and it can be applied to PMSG as well. On the otherhand, when PMSG is applied to its system, the generator isintegrated with a power converter, and the generated AC powerfrom PMSG is firstly converted to DC power. In this proce-dure, the phase current in PM generators contains harmoniccomponents, and their influence has been presented in [5]–[8]while they suggested conducting shield, segmented PM andrectifier structures as solutions. Therefore, in this paper, whilevarious generator types are dealt with according to machinetopology, their power loss calculation is also performed basedon the measured current to consider harmonics caused by theAC-DC-DC converter.

Manuscript received November 05, 2012; revised January 08, 2013 and Feb-ruary 06, 2013; accepted February 19, 2013. Date of current version July 15,2013. Corresponding author: S.-M. Jang (e-mail: smjang@cnu.ac.kr).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2013.2248700

TABLE IPM WIND POWER GENERATOR TOPOLOGY

II. COMPARATIVE INVESTIGATION ON PM GENERATORSACCORDING TO MACHINE TOPOLOGY

A. PM Generator Topology

In Table I, the PM generator topology dealt with in this paperis presented. This paper considers output power (0.3, 0.5, and1.0 kW), rotor position (inner rotor, outer rotor), stator structure(slotless, slotted), PM flux direction (radial, axial) and PM ma-terial (NdFeB, Ferrite). In addition, the bolded and underlinedmodels (type A, type C, type E, type F, and type H) are manufac-tured for experimental verification. In particular, type C, type Eand type F were dealt with to predict their electromagnetic char-acteristics based on analytical approach in our previous studies[9]–[12].

B. Analysis Models and Design Specifications

In this paper, for the reasonable comparison, the machineouter diameter and stack length are fixed, and the thickness ofeach material is identical. More specific dimensions are offeredin Table II. At first, for the comparison in terms of flux direction,the RFPMSG and AFPMSGwith slotless structure are designedas shown in Fig. 1. As dealt with later, manufactured Type A andType C show almost identical equivalent circuit parameters andperformance, and Type B has opposite rotor position comparedto Type A.As presented in Table II, the machine outer diameter of Type

C (AFPMSG) is 314 (mm) including end coil winding. Within

0018-9464/$31.00 © 2013 IEEE

PARK et al.: COMPARATIVE INVESTIGATION ON INTEGRATED SYSTEM OF PERMANENT MAGNET SYNCHRONOUS GENERATOR AND POWER CONVERTER 3847

Fig. 1. Analysis models without stator slot structure: (a) Type A (distributedwinding), (b) Type B (distributed winding), (c) Type C (distributed winding).

TABLE IIDESIGN SPECIFICATIONS OF ANALYSIS MODELS

Fig. 2. Analysis models with stator slot structure: (a) Type F (distributedwinding), (b) Type G (distributed winding), (c) Type H (concentrated winding),and (d) Type I (concentrated winding).

the identical machine outer diameter limitation 314 (mm), othermachines were designed, and their stack length 101 (mm) wasalso fixed. Therefore, within the machine size limitation, thecomparative machine performance can be investigated. In ourprevious researches [9]–[12], the analytical method was devel-oped for those machines, and those results presented high re-liability in comparison with finite element method and exper-imental results. In particular, the solution for the bread shapePM in type F was presented for torque ripple reduction [11].With the method, as shown in Fig. 2, Type G, Type H, andType I were newly designed with the minimum torque ripple.From the electromagnetic characteristic analysis, the equivalentcircuit parameters can be derived, and the power curve versusvoltage can be predicted as well.

C. Generator Comparison According to Machine Topology

In Fig. 3, the predicted output power and voltage in DC loadcondition according to machine topology at 150 (rpm) of ro-tational speed are presented with experimental results. Due tohigher values of resistance, outer rotor type shows a bit lowerpower characteristics. In addition, in comparison with Type A,

Fig. 3. Predicted output power and voltage in DC load condition at 150 (rpm)and experimental verification: (a) Type A, B, and C; (b) Type D, E, F, G, H, andI.

TABLE IIICOMPARISON FOR THE AMOUNT OF GENERATOR MATERIALS

Type B and Type C, since the machine size is increased, thehigher power can be obtained from Type D, Type E, Type Hand Type I. However, when the Ferrite PM is applied to Type Fand Type G, they show similar performance with Type A, TypeB and Type C. Their power curves can be also compared withType H and I in that they have slotted stator core, but they usedifferent PM material. In [11], Type F was developed for thesubstitution of Type E. Although both machines can meet 0.5(kW) at 150 (rpm), the Type D and E (slotless stator and NdFeBPM) show much wider power curve than Type F and G (slottedstator and Ferrite PM) in identical machine outer diameter andstack length conditions.As can be confirmed in Table II, there are five groups ac-

cording to rotor positions. In our paper, for the reasonablecomparison, several design specifications were fixed in thosegroups. They are the number of poles, speed, output power, ma-chine outer diameter, stator core thickness, rotor core thickness,stack length, air-gap length, PM pole arc ratio, PM thicknessand PM residual flux density as presented in Table II. Sincethe PM thickness and its pole arc ratio are identical, more PMvolume could be used in outer rotor type machine. Besides,with the more PM volume, the outer rotor type machine re-quires less number of coil winding to meet identical outputpower compared to inner rotor type. The amount of generatormaterials in Table III was resulted from the size limitations.

3848 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013

Fig. 4. Manufactured generators and experimental results in no-load and ACload conditions at 150 rpm (ch1:100 V/1 V, ch2:1 A/0.1 V) : (a) TypeA; (b) TypeC; (c) Type E; (d) Type F; and (e) Type H.

D. No-Load and AC Load Test With Manufactured Models

Frist of all, Type C, Type E, and Type F were presented in[9]–[12] while Type A and Type H were newly manufacturedfor this paper. In Fig. 4, the manufactured models, PM shape,no-load and AC load test results are presented, and the AC loadtest were performed in their output power conditions presentedin Table II. Besides, the Type A and C show almost identicalperformance in both conditions in that their equivalent circuitparameters are very similar. When it comes to no-load voltage,the Type H shows highest value while Type E shows lowestvalue. In addition, that of the other models (Type A, Type C, andType F) is in the middle of them. However, this does not implythat Type F with higher value has better performance than TypeE with lower value. Due to higher copper loss of Type F, lowerefficiency of Type F is measured as will be presented later inFigs. 6 and 7.

E. Generator Current Characteristics Under Constant OutputVoltage Control by DC-DC Converter

The output power of the direct-drive PM wind power gen-erators is not constant with varying wind speed, so the con-stant output voltage control was applied with the duty ratio ofDC-DC converter as presented in [9]. The duty ratio was de-termined by . Here, , and are theduty ratio, charging capacitor voltage, and output voltage, re-spectively. When the PM wind power generator and DC-DCconverter are integrated, the phase current contains various har-monic components. The harmonic components deteriorate thegenerator performance, and we presented that the 5th harmoniccomponent due to phase diode rectifiers can be a main causeof rotor loss in [13]. However, the phase current is highly de-pendent on not only the diode rectifiers but also electrical ma-chine parameters and the duty ratio of the power converter whenthey are integrated. As presented in Fig. 5, we performed the155 constant voltage control under 300 (W) output power,and the measured current could be obtained.

Fig. 5. Phase current according to duty ratio of DC-DC converter with con-stant voltage control (ch1:100 V/1 V, ch2:1 A/0.1 V, DR : duty ratio of DC-DCconverter) : (a) Type A; (b) Type C; (c) Type E; (d) Type F; and (e) Type H.

F. Electromagnetic Losses in PM Wind Power Generators

In PM wind power generators, the electromagnetic losses areoccurred while they are being operated. The losses in PM ma-chines can be categorized by copper loss, core loss and rotorloss on PM surface. In low speed machines, the copper loss isdominant while the core loss and rotor loss are dramatically in-creased in high speed machines.The copper loss can be simply obtained by

. Here, is the resistance of 1-phase, and isthe rms value of phase current. Although AC resistance shouldbe considered in high speed machines, we considered DC resis-tance in our prediction and measurement in that our generatorspeed is relatively lower. On the other hand, the core loss canbe calculated by the revised Steinmetz equation presented in ourprevious study [10]. As dealt with in the study, the harmoniccomponents and magnetic field behavior due to the AC-DC-DCconverter are considered to calculate the core loss based on themeasured current in Fig. 5. In addition, the rotor loss can be alsocalculated based on the method presented in [13], and the har-monic components were considered by the measured phase cur-rent in the armature reaction field. In Fig. 6(a), the copper lossaccording to the duty ratio is compared. Since Type A and TypeC have much higher resistance, they show much higher copperloss values than the others according to the equation presentedin the figure. On the other hand, since the core loss and rotorloss are the function of frequency, their lower values are con-firmed in higher duty ratio (lower speed) in Fig. 6(b) and (c).In addition, when it comes to Type F with the Ferrite PM, sincethe conductivity of Ferrite PM is much lower than NdFeB PM,its rotor loss values are almost zero. The PMmaterial propertiesare presented in the figure.

III. DISCUSSION AND CONCLUSION

As shown in experimental comparison in Figs. 7 and 8, theirefficiency is decreased in lower speed (higher duty ratio) sincethe copper loss is dominant. In addition, from the efficiency and

PARK et al.: COMPARATIVE INVESTIGATION ON INTEGRATED SYSTEM OF PERMANENT MAGNET SYNCHRONOUS GENERATOR AND POWER CONVERTER 3849

Fig. 6. Power loss comparison based on measured current according to duty ratio of DC-DC converter with constant voltage control in 300 (W) : (a) copper loss;(b) core loss; and (c) rotor loss on PM surface.

Fig. 7. Efficiency of manufactured generators with DC-DC converter ac-cording to duty ratio under constant voltage control.

Fig. 8. Ratio of output power to speed of manufactured generators with DC-DCconverter according to duty ratio under constant voltage control.

power density (ratio of output power to speed), type H (slottedNdFeB) presented much better performance while it still hashighest core loss. Although it was not presented, we will applysoft magnetic composite presented in [14] for the core loss re-duction in our future study.In this paper, various PM wind power generators according

to the machine topology were comparatively investigated.We offered specific design specifications of those machinesincluding electrical parameters and mechanical size, and theirperformances were compared. In addition, for the power losscalculation, we applied the measured current to armature reac-tion field to consider the harmonic components and magneticbehavior caused by the AC-DC-DC converter.

ACKNOWLEDGMENT

This work has been supported by KESRI(20101020300520)and KETEP(20124030200090).

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