Laboratory testing of an active magnetic bearing supported permanent magnet 3.5 kw blower prototype

Transcription

1 Laboratory testing of an active magnetic bearing supported permanent magnet 3.5 kw blower prototype Rafal P. Jastrzebski, Alexander Smirnov, Janne Nerg, Ahti Jaatinen-Värri, Pekko Jaatinen, Tuomo Lindh, Olli Pyrhönen, Jussi Sopanen, Jari Backman LAPPEENRANTA UNIVERSITY OF TECHNOLOGY PO. Box 20, FI Lappeenranta, Finland Tel.: Fax: rafal.jastrzebski@lut.fi, alexander.smirnov@lut.fi, janne.nerg@lut.fi, ahti.jaatinenvarri@lut.fi, pekko.jaatinen@lut.fi, tuomo.lindh@lut.fi, olli.pyrhonen@lut.fi, jussi.sopanen@lut.fi, jari.backman@lut.fi Acknowledgements The authors would like to thank TEKES for their financial support ( SaLUT-FCB to Business). Keywords Fuel cell system, Control methods for electrical systems, Alternative energy, Magnetic bearings, Magnetic levitation, Prototype commissioning, MIMO control. Abstract A commissioning and laboratory tests of an active magnetic bearing supported PM 3kW blower are presented. The motor operation is evaluated in no-load and load tests. An identification of AMB-rotor system, testing of H centralized control and mass-flow of the blower for recirculation of anode gas in a SOFC are discussed. The key results include comparison between analytical and measured values of an output sensitivity function of different MIMO controllers, measured efficiency and output motor power, and blower measurements. Introduction The alternative energy systems, such as solid oxide fuel cells (SOFCs) are composed of many elements. Their overall efficiency depends on the efficiency of the supporting elements and on the efficiency of the energy conversion. In small SOFC the system net efficiency can increase about 5 % (when increasing the blowers efficiencies from % to 35 %) when customized and properly scaled blowers and not the off-the-shelf selected components are applied [1]. When applying an anode gas recirculation the overall fuel utilization can increase, for example according to [1], from 55% for a single-pass to about 93%. In addition to the efficiency, the anode gas recirculation in the SOFC has other advantages. The recirculation adjusts the temperature for reforming reactions and helps to avoid carbon deposition inside the reformer and the stack [2], and during the normal operation, the gas recirculation enables the omission of an external water supply [3] and [4], and lastly the part of the non-reacted fuel is fed back to the system [5]. Because of the difficult operational, often high-temperature, environments and other demanding requirements of the alternative energy systems, the blowers with contactless and oil-free rotor suspension are preferred over the ones with ball or journal bearings. Particularly SOFC applications place very high demands on high-speed blower (high efficiencies, high temperature, control of

2 position of the blower wheel, high reliability), which is a critical component in the system. Active magnetic bearing (AMB) rotor systems enable very high rotation speeds, high efficiency, contactless and lubrication-free operation, long and reliable operation without need for service, built-in diagnostics and active control of vibrations. They are suitable for demanding applications where precise motion control is required and where other types of bearings are not adequate. However, the AMBs require complex control electronics, accurate plant modeling, modal analysis of rotor dynamics, and challenging control system design and identification. The literature presents a very few comprehensive laboratory tests of the customized fully levitated electrical machines, which could be suitable for demanding blower applications [6-8]. Moreover these studies comprise only tests in limited environments, for example, without a blower wheel, and not in application specific environments. Therefore, the total blower efficiency is unknown. The industrial AMB systems require solving of complex and application specific technical issues as well as challenging control system design and identification. Consequently, academic AMB research focuses on narrow control problems investigated by using academic laboratory rig [9] and not application tailored industrial test-rig platforms. This study presents the results of the commissioning and laboratory tests of the fully custom designed AMB supported permanent magnet (PM) blower, which is suitable for SOFC applications, with two different electric rotor structures. Machine overview The studied AMB blower has a horizontal rotor (with austenitic stainless steel or titanium variants) supported on two eight-pole radial bearings, which control the 4 degrees of freedom (DOF). The thrust disc-type bearing on one end of the rotor controls the axial 5th DOF. The AMB system supports rotor of mass 2.5 kg. The major parameters of the system include: rotor length without the blower wheel m, nominal magnetic air-gap length 0.5 mm, bandwidth of the radial actuators 870 Hz, bias current 2.5 A, maximum current 6 A and nominal inductance of radial and axial bearing coils 3.5 mh and 0.5 mh, respectively. The control is tested with custom-built servo amplifiers in a half-bridge configuration. The custom amplifiers operate with a supply DC link voltage of V. During the tests, the amplifiers operate with a supply DC link voltage of 60 V, and a switching frequency of 40 khz. The sampling time is 100 µs. The AMB control is tested with a flexible custom control platform based on a dspace DS motion control solution and CMSS 65 eddy current probe system. This way different control architectures, for example, cascaded position-current control [10] or zerobias flux control [11] can be tested. The fully embedded single-chip control implementation can be achieved by using suitable Field-Programmable Gate Array (FPGA) [12]. The permanent magnet synchronous motor (PMSM) has three phases and two poles. The nominal values of the machine are: speed rpm, line-to-line voltage 400 V, and current 5.6 A. Because during the testing the maximum output frequency of the inverter was limited to 500 Hz, the speed and the supply line-to-line voltage were reduced to rpm and V, respectively. The motor is driven by an ABB machinery drive ACSM1 with a 16 khz switching frequency. Identification of AMB rotor system parameters The performance of the AMB control can be improved by identification of parameters. The identification is done in the frequency domain with a step-sine excitation. The excitation u is supplied through electromagnets of the bearings one pair at a time; and displacement y of the rotor is measured with the position sensors. The obtained measurements are combined and a multiple-input multipleoutput (MIMO) frequency response function is achieved with the following equation, (1)

3 where is the excitation frequency. The measured frequency response function is used to identify a set of parameters. To improve the quality of results an iterative routine is used. Individual frequency ranges are chosen so that they contain most significant information about the parameters in question. At first the most wide frequency ranges are fitted and later the narrow ones are investigated. The results obtained at a standstill for a titanium rotor are presented in Fig. 1. The same procedure is applied for the steel rotor and for both rotors rotating at rpm. Finally, all the AMB control parameters are identified [13]. For the radial suspension the results are summarized in Table I and Table II. Fig. 1: Iterative identification of the titanium rotor Table I: Parametric identification result for the Ti rotor Parameter Initial value Identified value Current stiffens A-end [A m -1 ] Position stiffens A-end [N m -1 ] Current stiffens B-end [A m -1 ] Position stiffens B-end [N m -1 ] Flexible mode frequency [Hz] Flexible mode damping 2000e e-6 Polar moment of inertia [kg m 2 ] 724e-6 864e-6 Table II: Parametric identification results for the steel rotor Parameter Initial value Identified value Current stiffens A-end [A m -1 ] Position stiffens A-end [N m -1 ] Current stiffens B-end [A m -1 ] Position stiffens B-end [N m -1 ] Flexible mode frequency [Hz] Flexible mode damping 2000e e-6 Polar moment of inertia [kg m 2 ] 849e e-6

4 The difference between the identified and initial values of the (polar) moment of inertia about the rotation axis of the steel rotor is due to an influence of the impeller at rpm pumping air in standard ambient conditions ( K, 100 kpa). Mushi et al. [14,15] discusses how the air flow in the housing can provide forces on the impeller, such as cross-coupled forces. These forces contribute to the polar moment of inertia. The thrust axial bearing has a solid iron stator. The ac currents result in eddy currents, possibly faster saturation and stray fluxes. Therefore the electrical parameters of the bearing are dependent on frequency. In Fig. 2 the measured inductance and resistance of axial bearing as a function of frequency are presented. Typically to account for these variations, the controllers with lower bandwidth and gain are used for the control of axial bearings. Fig. 2: Measured inductance and resistance of the axial bearing as a function of frequency Laboratory testing of AMB control To provide good performance and disturbance rejection at full operating range robust controllers are used. The controllers are synthesized for the system with the speed as an uncertain parameter and using the identification results for all other parameters. Three different synthesized H controllers are tested at different operational points with the prototype and the steel rotor. The output sensitivity functions are measured. The output sensitivity and their peak values provide a general index for evaluation of performance and stability of an AMB system [16]. The measured results are compared with theoretical values in Fig. 3. The solid lines connect the measured points and surfaces denote the theoretical data. The presented results indicate that there can be an additional foundation resonance close to 800 rad/s. This resonance is excited with the increased speed and is clearly seen in Figure 3b. In addition, in this frequency range, a subharmonic from the three-phase rectifier propagates as a noise to the dc link voltage, actuators and sensors [17]. The displacement of the structural resonance frequency for different controllers appears because of differences in loop shape between the controllers and because of the water-bed effect [18].

5 Fig. 3: Theoretical and measured values for the output sensitivity function of different controllers Laboratory testing of the motor No-load voltage of the designed PMSM was measured by rotating the impeller using compressed air. The no-load voltage was measured across the resistive load. The main peak correspond to the rotation frequency of 76 Hz, the second one is at 380 Hz which is approximately the fifth harmonic. The rootmean-square (RMS) value of the induced phase-no-load voltage was measured to be 23.8 V at the frequency of Hz. Because the no-load voltage is directly proportional to the frequency we can calculate the no-load voltage at nominal speed of rpm (750 Hz) by 750Hz E phase, rpm 23.8V 235.9V (2) Hz It must be noticed that in the measurements, the magnets were at the temperature of 20 C and the corresponding remanent flux density of the magnet material (Neorem 799t) was 1.11 T. Load measurements were performed using the measurement setup consisting of ABB ACSM1 frequency converter, Norma power analyzer, and a dynamometer acting as a load machine. Measured efficiency and output power of the stainless steel rotor structure at a speed of rpm are presented in Fig. 4. The reason why the measurements were performed only at rpm is that the maximum output frequency of the ACSM1 frequency converter was 500 Hz.

6 Efficiency (%) 90,0 % ,0 % 70,0 % ,0 % 50,0 % 40,0 % 30,0 % 20,0 % Efficiency (%) 10,0 % Output power (W) 500 0,0 % Current (A) 7 Fig. 4: Measured efficiency and output power as a function of stator phase current at rpm Output power (W) SOFC blower measurements A performance of the high-speed centrifugal blower, designed to recirculate anode gas in solid oxide fuel cells was studied experimentally. The initial performance tests were done under normal ambient laboratory conditions (Approximately 20 C and 100 kpa), and with air as the working fluid. The inlet mass flow was measured with a venture tube. The inlet and outlet pressures as well as the temperatures were measured for each operating point, along with the ambient pressure and temperature. A photo of the test setup is presented in Fig. 5. The blower operating map is presented in Fig. 6. The operating map was measured with four different throttle positions of the exhaust valve. Altogether 16 different operating points were recorded, and these points are marked to the operating map. The pressure rise of the blower is slightly lower than expected. However, the design rotational speed is almost twice the rotational speeds used in the tests. The measured and estimated output blower power is presented in Fig. 7. For the maximum measured blower output power at rpm the calculated blower efficiency is about 75% and the total net efficiency of the motor and blower is about 60%. The blower performance can be improved by adjusting the axial position of the rotor. Fig. 5: Test setup and the blower with pressure measurements

7 1, , , , ,07 1, , , , , ,02 0,04 0,06 0,08 0,1 0,12 Mass flow [kg/s] Fig. 6: The blower operating map at different rotational speeds Total-to-total pressure ratio 3,5 Blower gas power [kw] 3 2,5 2 1,5 1 0, ,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20 Mass flow [kg/s] Fig. 7: The measured blower gas power (red), the estimated gas power in nominal conditions (black) Conclusion The AMB control system identification results demonstrate the validity of applied models and design procedures. With identification the variation range of the parameters is reduced thus reducing the parametric uncertainty. The nominal parameters are tuned to the measured data. Therefore the application of the centralized robust high-performance model-based controllers is possible. The stainless steel and titanium variants of the rotor allow operation with up to and rpm, respectively. The PM motor operation has been tested in no-load and load tests with speeds up to rpm. The achieved motor efficiency at higher powers exceeds 80 % which is very good for small power machines. For the measured blower output power at rpm the best blower efficiency is about 75 %.

8 In general, the mechanical and control designs are scalable from 3 up to 40 kw and can be tailored to the particular demands of the application. The excellent experimental results indicate that project has significant potential for application in new technology products. As an example, the use of the presented customized AMB supported PM blower in the SOFC can significantly increase the net efficiency of the fuel cell and therefore increase a market attractiveness of that kind of alternative energy system. As a future outlook the modifications such as development of a new cooling system, application of zero-bias operation, PM biasing and optimization of bearing construction could further increase the blower efficiency while further reducing its footprint and cost. The elimination of eddy-current sensors, elimination of axial and potentially one of the radial bearings could drastically reduce the cost and broaden the potential application field. References [1] Powell, M., Meinhardt, K., Sprenkle, V., Chick, L., and McVay, G.: Demonstration of a highly efficient solid oxide fuel cell power system using adiabatic steam reforming and anode gas recirculation. Journal of Power Sources, Vol. 205, pp , 2012 [2] Halinen, M., Saarinen, J., Noponen, M., Vinke, I.C., Kiviaho, J.: Experimental Analysis on Performance and Durability of SOFC Demonstration Unit. Fuel Cells, Vol. 10, pp , 2010 [3] Halinen, M., Rautanen, M., Saarinen, J., Pennanen, J., Pohjoranta, A., Kiviaho, J., Pastula, M., Nuttall, B., Rankin, C., and Borglum, B.: Performance of a 10 kw SOFC Demonstration Unit, ECS Trans. Vol. 35, , 2011 [4] Peters, R., Riensche, E., Cremer, P.: Pre-reforming of natural gas in solid oxide fuel-cell systems. Journal of Power Sources, Vol. 86, pp , 2000 [5] Noponen, M., Halinen, M., Saarinen, J., Kiviaho, J.: Experimental Study of Anode Gas Recycling on Efficiency of SOFC. ECS Transactions, 5: , 2007 [6] Munteanu, G., Binder, a., Schneider, T., Funieru, B.: No-load tests of a 40 kw high-speed bearingless permanent magnet synchronous motor. Speedam 2010, 2(3), , 2010 [7] Munteanu, G., Binder, a., & Schneider, T.: Development and test of high-speed bearingless PM synchronous machines. e & i Elektrotechnik und Informationstechnik, Vol. 128, pp , 2011 [8] Munteanu, Gabriel, Binder, A., & Schneider, T.: Loss measurement of a 40 kw high-speed bearingless PM synchronous motor IEEE Energy Conversion Congress and Exposition, (6), , 2011 [9] Kejian, J., Changsheng, Z., & Ming, T.: A Uniform Control Method for Imbalance Compensation and Automation Balancing in Active Magnetic Bearing-Rotor Systems, Journal of Dynamic Systems, Measurements, and Control, 134, 13p., 2012 [10] Jastrzebski, R.P., Pöllänen, R., Centralized Optimal Position Control for Active Magnetic Bearings Comparison with Decentralized Control, Electrical Engineering, Vol. 91, No. 2, pp , 2009 [11] Jastrzebski, R.P., Smirnov, A., Pyrhönen, O.: Force Controllers for AMB Systems with Position and Current Feedback, Solid State Phenomena, vol. 198, Mechatronic Systems and Materials IV, pp , 2013 [12] Jaatinen P.: One Chip Solution for Low-Cost Active Magnetic Bearing System, Master s Thesis, LUT Energy, 2013 [13] Smirnov A.: AMB system for high-speed motors using automatic commissioning, Doctoral Dissertation, Acta Universitatis Lapeenrantaensis 508, LUT, Finland, 2012 [14] Mushi, S.E., Lin, Z., and Allaire, P.E., Design, Construction, and Modeling of a Flexible Rotor Active Magnetic Bearing Test Rig, IEEE/ASME Transactions on Mechatronics, Vol. 17, No. 6, , 2012 [15] Mushi, S.E., Lin, Z., Allaire, P.E., and Evans, S., Aerodynamic Cross-coupling in a Flexible Rotor: Control Design and Implementation, in Proc. of ISMB 11, 12 17, 2008

9 [16] Takahashi, N., Fujiwara, H., Matsushita, O., Ito, M., Fukushima, Y.,: An evaluation of stability indices using sensitivity functions for active magnetic bearing supported high-speed rotor, Transactions of the ASME, Journal of Vibration and Acoustics Vol. 129 No. 2, , 2007 [17] Jastrzebski, R.P., Hynynen, K.M., Smirnov A., Pyrhönen O.: Influence of the drive and dc link generated disturbances on an AMB control system, Electrical Review, No 1a, , 2012 [18] Skogestad, S., Postlethwaite, I.: Multivariable Feedback Control: Analysis and Design, John Wiley & Sons, 2005