Compact and Low Cost Magnetic Bearing with Saturated Coil for Gas Turbine Generators

Transcription

1 Compact and Low Cost Magnetic Bearing with Saturated Coi for Gas Turbine Generators Van Xuan Thai, Bohwan Choi, Suyong Choi, Seungjin Yoo, Chuntaek Rim Dept. of Nucear and Quantum Engineering KAIST Daejeon, Korea Abstract A nove magnetic bearing system for gas turbine generators of next generation nucear power pant is proposed. This system newy adopts saturated cois for generating main bias force and sma size inear auxiiary cois for stabiizing the suspension. The saturated cois operate in the region of magnetic saturation in order to maximize the magnetic fux density and reduce the bias current. This resuts in compact size of core, significant reduction of copper oss; hence, a ow cost and high efficiency magnetic bearing is achieved. The proposed system is under deveopment for the goa of 5 ton rotor mass and 3MWe project. I. INTRODUCTION Over the past 4 years, magnetic bearings have been widey researched and appied to many appications such as fywhees, turbines, and high-speed machines. In genera, magnetic bearings can be cassified into 3 types: passive magnetic bearing (PMB), active magnetic bearing (AMB), and hybrid magnetic bearing (HMB) [1]. Recenty, the HMB is widey studied because it combines the advantages of the two former types, which are ow power oss and stabe suspension contro. However, the HMB is inferior to AMB for high oad capacity and high temperature appications because of the imited bias fux generated by permanent magnets which cannot be controed. Furthermore, the HMB is quite compex compared to AMB; so it often costs higher. Therefore, AMB is sti ikey to be used for high power and high temperature appications []. However, the AMB sti has some probems. First, the size of cois used in the AMB is quite buky in order to generate arge force, i.e. high magnetic fux arge. Second, the power oss of AMB system is often high because of reativey high bias current. There are severa authors tried to sove the probems of power oss and the size of the magnetic bearing. Some authors tried to find out the optimum number of poes to minimize the stator outside diameter of the magnetic bearing [3]-[5] but they are just the optimization method, not the one which can reduce the size much. Then, if the high oad capacity is needed, the poe shoes area shoud sti be arge eading to arge size magnetic bearing. On the other hands, some papers combined the radia and axia magnetic bearing to get smaer size [6]-[8]. However, they are quite compex and not ikey to be appied in the rea cases. To reduce the power oss, recenty, some authors try to improved the structure of the hybrid magnetic bearing to reduce the reuctance of the bias fux [1], [9]-[11], optimize the number of magnetic poes to reduce contro oss [3],[1], or using contro strategy to decrease the oss [13]. Previousy, to reduce hysteresis oss, a famous method is to use homopoar magnetic bearing which are now researched and improved [], [14], [15]. Nevertheess, a of the papers are just the optimization of using magnetic bearing, not the way to reduce the bias current which mainy causes the power oss of magnetic bearing. To sove the probem of compactness and power oss, a nove magnetic bearing system, which incudes saturated main cois for generating bias fux and inear auxiiary cois for generating contro fux, is proposed. The bias fux is appropriatey controed to sustain the weight of the suspension whereas the contro fux is used to stabiize the axis. The possibe configurations and the effectiveness in the size reduction of core and the reduction of copper oss are anayzed in this paper. The proposed AMB system is under deveopment for the goa of 5 t rotor mass in order to appy to 3MWe gas turbine generators of next generation nucear power pant simiar with [16]. II. DESIGN OF THE PROPOSED MAGNETIC BEARING A. The Operating Principe of the Proposed AMB The proposed AMB adopts the combination of saturated main coi(s) (red ine) and inear auxiiary coi(s) (bue ine) to achieve the wanted magnetic fux, as shown in Fig. 1. For exampe, the saturated coi may be made of ferromagnetic materia [17] and the inear coi may be made of nonoriented eectrica stee [18]-[19]. shown in Fig. 1, the saturated coi provides with major bias fux B ave which is a few times arger than the maximum fux ΔB A / generated by /13/$ IEEE 486

2 the inear coi, but the controabe fux of the saturated coi ΔB M is a few times smaer than that of the inear coi ΔB A. ΔBM Bave Br 1.5 B (T) Saturated main coi curve ΔBM HM ΔBA.5 Linear auxiiary coi curve H (A/m) HA Figure 1. Proposed B-H curves for a saturated main coi (red) and a inear auxiiary coi (bue) [17]-[19]. The hysteresis oop of the saturated coi in the B-H curve is sma so far as the contro range of H is kept positive, and this hysteresis oss is negigibe because the proposed main coi sedom changes the operating point frequenty. The core use of the saturated coi is much ess than that of a conventiona inear coi for giving same magnetic fux eve. The proposed inear coi has simiar B-H curve with that of conventiona one, but it requires a few times ess contro current, which resuts in sma size and conduction oss. B. The Configurations of the Proposed AMB The proposed AMB system for horizonta axis is shown in Fig. (a), where a saturated main coi, four inear auxiiary cois, and a inear radia rotor constitute a radia bearing, and two inear circuar cois and a inear axia rotor constitute an axia bearing []. (a) horizonta axis AMB (b) vertica axis AMB Figure. Proposed AMB configuration. (The shaded area shows the saturated core) The saturated cois and inear cois are expained in the previous section. The distance d between different types of cois shoud be arge enough to avoid the unwanted interaction between them. Moreover, the thickness w M of saturated main core shoud be a few times arger than that of inear auxiiary core w A to withstand the arge weight of the suspension. Note that the rotor of this AMB for horizonta axis is never saturated, but the stator of the saturated main coi for radia bearing ony is saturated. The proposed AMB system for vertica axis is shown in Fig. (b), where a saturated circuar main coi, a saturated axia rotor, two inear circuar auxiiary cois, and a inear axia rotor constitute an axia bearing, and four inear cois and a inear axia rotor constitute a radia bearing []. In this case, the stator and rotor of the axia bearing are fuy saturated; hence, the thickness h t can be reduced significanty compared to a conventiona inear axia bearing. The reason why this rotor can be saturated is because of the symmetry of fux distribution over the rotor w. r. t. the axis, which resuts in no magnetic fux density change in time though it is saturated. Due to the further reduction of core use by the proposed magnetic saturation, the proposed vertica axis AMB is tentativey preferred to the proposed horizonta axis AMB; the preference is, of course, dependent on the detai design. C. Magnetic Force Generation The magnetic force F s generated by the proposed saturated coi, as shown in Fig. 1 (a), is proportiona to the 487

3 square of the magnetic fux density B s and the area of the core cross section A s [1]. Fs = cosθ for θ μ μ The magnetic force F of the conventiona inear coi with the magnetic fux density B and the area of the core cross section A is as foows: (1) B A F () μ Comparing (1) with () for giving the same magnetic force, the proposed saturated coi has smaer core area because of the higher magnetic fux density as foows: B A Fs = = = F μ μ B = A B s D. Core Saving Rate The reduced core area of the proposed saturated coi resuts in the core saving of the proposed AMB. The voume of the core use of the saturated coi V s can be roughy estimated for the simpified mode as shown in Fig.. (3) the auxiiary cois is ess than a third of the main coi, as shown in Fig. 1. The overa cost saving effect coud be anayzed after a specific and detai design of the cois. III. CONTROL OF THE PROPOSED MAGNETIC BEARING The contro of the proposed AMB invoves severa tricky issues because of the combination of the saturated main coi and the inear auxiiary cois. The non-inear coi with a hysteresis oop does not permit the use of conventiona inear contro; furthermore, its characteristics may change according to the temperature and aging. To determine the appropriate bias eve of the saturated coi together with the maximum operating range of the auxiiary coi is aso an important design issue, which may affect to the controer design as we. The detai controer design of the proposed AMB which wi be used for high temperature gas turbines shoud consider the foowing issues. A. High Temperature and Aging Issues The magnetic properties of the ferromagnetic materia vary for temperature in a non-inear way. In genera, the saturation magnetic fux density of ferromagnetic materia decreases when the temperature increases. Fig. 3 shows the variation of B-H curves of AFK 5 [], a ferromagnetic materia, depending on different temperature T and different exposure time t. Nevertheess this variation, the contro strategy shoud assure that the magnetic fux density must maintain the wanted eve B M. V A (4) s s s where s is the tota ength of the core in Figs. (a) or (b). In (4), it is assumed that the core has even cross section area and the corner effect can be ignored. Simiary, the voume of the conventiona inear coi V for the tota ength of its core can aso be roughy derived from as foows: V A (5) Therefore, the core saving rate α w. r. t. the conventiona coi can be determined using (3)-(5) as foows: V Vs Vs s B s α = 1 = 1 V V A (6) 1 The tota ength of the saturated core s tends to be much smaer than that of the conventiona core because the drasticay reduced coi current with a virtue of arge hysteresis, as shown in Fig. 1. suming that this ratio s / is.7 the saving rate for the B /B s =1.4/1.6 exampe becomes 46%. This rate may increase for the proposed vertica axis AMB because it has more reduced s / ratio as discussed in the previous section. This remarkabe saving rate may be mitigated when considering the inear auxiiary cois necessary for the proposed AMB; however, the tota amount of core saving shoud be sti significant because the size of Figure 3. Magnetic fux contro depending on temperature and aging. Vin 5V 1 khz L1 L R R Vout Mutipier (AD633N) Low Pass Fiter Figure 4. Proposed fast dispacement sensing circuit. Ampifier 488

4 B. Dispacement Sensing Issue To sense the dispacement of the rotorss of the proposed AMB in a few tens of µs, an anaog mutipier synchronized with the sinusoida source votage is adopted, as shown in Fig. 4, for processing the differentia votagee obtained from a pair of inductive proximity sensors L1 and L, as depicted in Fig. 1. The source frequency is seected as higgh as 1 khz in order to achieve high speed response of the ow pass fiter. The output votage is dc and proportiona to the dispacement d1 d of Fig. 1. IV. B. Experiment for the Proposed d Sensing Circuit and Dispacement Sensors The sensing circuit, contro circuit, and inductive proximity sensors were impementeed for the experiment for the prototype saturated magnetic beearing, as shown in Fig. 6 (a). The DC and AC output votage waveforms of the sensing circuit are measured, as sho own in Fig. 6 (b), and the DC output votage for the dispaccement of the inductive proximity sensors, d1 - d is measureed, as shown in Fig. 6 (c). SIMULATION AND EXPERIMENTA AL RESULTS A. Magnetic Fux Simuation for the Proposed Axia Bearing The saturated axia bearing of the propoosed vertica axis AMB, as shown in Fig. 5 (a), was simuateed by the Ansoft Maxwe 3D mode. The outer radius is 35cm m and the air gap is 3 mm, and it is assumed that the core hhas the saturated characteristics [9]. For minimum use of corre, the stator and rotor are tapered to give even magnettic fux density distribution over the whoe core. Fig. 5 (b) shows the simuated magnetic fux density (B) distribbution for 1 ka turn, where the yeow part shows saturattion and the fux density is eveny distributed over the corre except for the corner and edge yet. (a) experiment kit for the proximity sensor orms (b) votage wavefo (a) 3D mode of the tapered axia magneticc bearing Output votage [V] d1- d [mm] (c) DC output votage vs. dispacement d Figure 6. Experiments for the proposed sensing circuit and inductive proximity senso ors. (b) Ansoft Maxwe simuation resut showing partia magnetic saturation Figure 5. Simuation resuts for the tapered saturatedd axia bearing of the proposed vertica axis AMB 489

5 C. Experiments for the prototype of saturated magnetic bearing using the proposed sensing circuit A prototype of the proposed magnetic bearing incuding a saturated coi, a inear auxiiary coi and a proposed proximity sensors and a motor to rotate the system is buit as shown in Fig. 7. The evitated objects incudes the sensor disk, the saturated disk, the inear disk, the rotor and the shaft as shown in Fig. 7 and have the tota weight of 1 kg. The air gap between the cores and the disks is designed as.7 mm. In the prototype, ferrite POT cores are used as the magnetic bearing cores. Their magnetization curves with different temperature are shown in Fig. 8. position of the rotor or in another word, to set up the operation position; and V gs is the gate-source votage of the MOSFET IRF 64B used in the experiment. Fig. 1 (a) and (b) shows the experimenta resuts with different rotationa speed of rpm, 15 rpm and rpm, respectivey. shown in Fig. 1 (a), the V out is stabiized around zero votage indicating the rotor is evitated around equiibrium position. When the rotationa speed is increased to 15 rpm and rpm, the V out is fuctuated around zero votage, refecting the fuctuation of severa tens micrometer around the equiibrium position of the rotor but the rotor is sti kept stabiized. The second experiment of the prototype was conducted to find out the maximum current I max in which the core is saturated. When the coi is saturated, its inductance is decreased sharpy as shown in Fig. 11. That is where the I max is find out. can be seen from the Fig. 11, can be seen from the Fig. 11, the experimenta I max for. 5 mm, 1 mm, and 1.5 mm air gap are around 1 A, 1.5 A, and 1.7 A, respectivey. The cacuated resuts for these different air gap are 1.6 A,.1 A, 3.18 A, respectivey. This difference is due to the fact that when the air gap is arger, the eakage inductance wi become more and more significant eading to inaccurate cacuation of inductance. Experimenta resuts shown in Fig. 11 give us important information for designing the controer to saturate the coi in the further works. V. CONCLUSION AND FURTHER WORKS The proposed AMB is found to be quite compact in core size and has significant reduction in copper oss. Therefore, a ow cost and high efficiency magnetic bearing can be achieved by using this method. These merits become dominant for the proposed vertica axis AMB because of the saturated rotor as we as the saturated stator of the axia bearing. Experiments with the prototype proved the abiity of the controer and the proposed sensors in evitating and stabiizing the rotor with different rotationa speed. Moreover, the maximum current with different air gap are aso figured out, which is a basis experiment for further works. Figure 7. Prototype of the proposed magnetic bearing. The first experiment for the prototype was conducted with the unsaturated core to verify the evitation of the 1 kg object as we as to verify the dynamic response of the proposed sensor and of the PID controer in stabiizing the evitated object. The entire circuit of the cosed oop PID contro for the first experiment is shown in Fig. 9 where V out is the output votage of the sensor circuit showing the position of the rotor; V ref is the reference votage to set up the equiibrium Figure 8. Magnetization curves of experimenta core. 49

6 Sensor 1 ko ko 1 ko AD633N 5V 1 khz 6 ko.1 µf Vout.33 µf 33 µf ko 1 ko -15V Vref Vgs -15V --15V IRF 64B L Magnetic bearing coi PID controer Figure 9. PID controer for the experiment of unsaturated core mm 1 mm 1.5 mm Inductance [mh] (a) rpm Current [A A] Figure 11. Inductance vs. current in n saturation experiment. REFERENCE ES [1] [] [3] (b) 15 rpm [4] [5] [6] [7] (c) rpm Figure 1. Experimenta resuts with different rootationa speed. [8] 491 F. Jiancheng, S. Jinji, L. Hu, T. Jiq qiang, A nove 3-DOF axia hybrid magnetic bearing, IEEE Transsactions on Magnetics, vo. 46, no. 1, pp , Dec. 1. G. Schweitzer, E. H. Masen, Magneticc bearings Theory, Design, and Appication to rotating machineery, Springer-Verag Berin Heideberg, 9. oto, Optima number of stator K. Matsuda, Y. Kanemitsu, S. Kijimo poes for compact radia active magnetic bearing, IEEE Transactions on Magnetics, vo. 43,, no. 8, pp , Aug. 7. oach to designing a magnetic K. Matsuda, S. Kijimoto, An appro bearing system for smaer rotating maachine, in IEEE Internationa Symposium of Industria Eectronics ISIE I 8, pp , June 8. nd, Design of compact active A.S. Reddy, B.K. Agarwa, S. Chan magnetic bearing with higher oad carrying capacity, in 1 Students Conference on Engineering g and Systems SCES, pp. 1-5, March 1. ound, J.W. Koar, Combined P. Imoberdorf, C. Zwyssig, S.D. Ro Radia-Axia Magnetic Bearing for a 1 kw, 5, rpm Permanent Magnet Machine, in nd IEEE E Appied Power Eectronics Conference, APEC 7, pp , Feb. 7. P. Imoberdorf, T. Nussbaumer, J.W. Koar, K Anaysis of a Combined Radia-Axia Magnetic Bearing for a High-speed drive system, in 5th IET Power Eectronics, Machines and Drives (PEMD 1), pp. 1-6, Apri 1. Q. Wu, W. Pan, Y. Hong, Y. Sun, H. Zhu, Principe and parameter design for an innovated radia-axia hybrid magnetic bearing, in Internationa Conference on Eectrrica Machines and Systems ICEMS 8, pp , Oct. 8.

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