PERMANENT MAGNET EXCITED TRANSVERSE FLUX LINEAR MOTOR WITH INNER MOVER FOR ROPELESS ELEVATOR

PERMANENT MAGNET EXCITED TRANSVERSE FLUX LINEAR MOTOR WITH INNER MOVER FOR ROPELESS ELEVATOR DRAGOŞ OVIDIU KISCK 1, DO HYUN KANG 2, JUNG HWAN CHANG 2, JI WON KIM 2, DRAGOŞ ANGHEL 1 Key words: Linear motor, Synchronous machines, Transverse flux motor, Ropeless elevator, Permanent magnets. A transverse flux, PM-excited linear motor (TFM-LM) with inner mover for ropeless elevator was designed and built. It s output power density is higher and the weight is lower than for conventional permanent magnet excited linear synchronous motors (PM- LSMs). To get the maximum thrust force under the given volume, the thrust force density with respect to the ratio of the pole width, height of the PM and the length of pole pitch is optimized by 2-D finite element method (FEM). To verify the design optimization, the computed forces are compared with the experimentally measured forces detected on the experimental setup of the TFM-LM. The calculated and measured performances of TFM-LM with inner mover reveal a great potential of system improvements by reducing mass of the linear motor. For examples, when this motor is applied to a ropeless elevator, it is possible to increase the power density by more than 400[%] than the conventional PM-LSM. The results of this study recommend TFM-LM with inner mover for the ropeless elevators and for gearless direct linear driving systems. 1. INTRODUCTION Ropeless elevators witultiple cars in one shaft are considered to be the most promising answer to ultra-tall building s problems, including their hoist ways, halls machine room, and rope length limitations [1]. The successful implementation of ropeless elevators without counterweight depends on two main factors: the ratio of the thrust to mover weight and the motor size. The conventional permanent magnet excited linear synchronous motor (PM-LSM) was built for ropeless elevator application. PMs as mover it is placed between two sets of long armature windings [2]. But the ratio of the thrust to mover weight by the conventional 1 University Politehnica of Bucharest, Faculty of Electrical Engineering, 313 Spl. Independenţei 060042, Bucharest, Romania, dragos@dsp-control.pub.ro. 2 Korea Electrotechnology Research Institute, Mechatronics Research Group, Changwon, Kyungnam 641-120, Korea, dhkang@keri.re.kr. Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 55, 3, p. 268 277, Bucarest, 2010

2 Permanent magnet excited transverse flux linear motor for elevator 269 permanent magnet excited linear synchronous motor (PM-LSM) has only 83.3 N/kg which is not enough for ropeless elevators. A novel electrical machine based on the new concept of transverse flux configuration leads, simultaneously, to a considerable increase in the power density and efficiency. The topological investigations regarding the magnetic circuit geometry and winding form of the transverse flux machine have brought up a variety of constructing arrangements with different features and for several types of practical applications [3]. Here, the proposed TFM-LM with inner mover leads to a considerable increase in power density for the moving part. When the proposed TFM-LM with inner mover is used to a ropeless elevator, it is possible to increase power density more than 400% comparing with the conventional PM-LSM. The result of this study can be utilized by development of both ropeless elevators and gearless linear moving systems with high output. 2. CONFIGURATION AND PRINCIPLE OF THE TFM-LM The symbols used in this paper for TFM-LM are presented in Table 1. Table 1 SYMBOLS Symbol Remarks B 0 magnetic flux density in air gap by PM B r remanent magnetic flux density of the PM b p pole width b sp width of winding window F x thrust force per motor length F xd thrust force density h i pole length height of the PM h sp height of the winding window w number of turns for primary winding k winding factor W co magnetic coenergy W m magnetic energy δ length of the air gap µ m PM relative permeability µ 0 air permeability Θ a magnetomotive force by primary winding Θ m magnetomotive force by PM pole pitch τ p Figure 1 presents a general view of the TFM-LM with inner mover; the variant with long primary configuration to reduce the mass of the linear motor,

270 Dragos Ovidiu Kisck et al. 3 especially in moving part. The passive back irons in stator are skewed by the pole pitch in order to generate the thrust force in one direction. 2τ p b p h sp h i δ b sp Fig. 1 Basic configuration of long primary TFM with inner mover. The principles of force generation for the TFM-LM with long primary configuration are shown in Fig. 2. Fig. 2 Principles of force generation. The passive back irons in stator are cut and developed to show the principles of force generation. When the primary winding turns on, the magnetic polarities between N, S in mover and N 1, N 2, N 3, S 1, S 2, S 3 in stator generate the total traction force F T in one direction. Here, Θ a is magnetomotive force by primary winding. The TFM-LM uses the PMs as excitation. The magnetic flux density in the air gap

4 Permanent magnet excited transverse flux linear motor for elevator 271 can be amplified, because the profile of the PM is bigger than the stator pole width in the air gap. The advantage of TFM-LM with inner mover is the system improvements by reducing mass of the mover. As with the switched reluctance motor drives, the TFM-LM has the traction force ripple, which has limited its use in some applications. This force ripple could be minimized by controlling the current shape adequately [3, 4]. Using the one-dimensional model of TFM-LM with inner mover and the magnetic equivalent circuit (where the saturation and stray magnetic flux component are not considered), the thrust force produced in the TFM-LM is explained by the magnetic coenergy conversion [5]. The magnetic energy, W m, and the magnetic coenergy, W co, stored in magnetic fields are as follows, [6]: W W m co = = V V B 0 H 0 H( B)dBdV, (1) B( H )dhdv. (2) In terms of coenergy, the expression for the thrust force, F x, is: Wco Fx ( x) =. (3) x The value of the thrust force density F xd for the TFM-LM is [5]: Θ F xd = kb0. (4) 2τ The thrust force density, F xd, is proportional to the magnetomotive force, Θ a, and the magnetic flux density in the air gap, B 0 and inversely proportional to the pole pitch τ p. The thrust force density, F xd, is corrected by factor, k from the analytically calculated formula, because the iron saturation and the stray flux have a significant influence on the thrust force of TFM-LM. a p 3. DESIGN OF TFM-LM FOR ROPELESS ELEVATOR COMPARATIVE WITH A LINEAR PM-LSM A. BASIC DESIGN OF THE TFM-LM For comparing with a 3kW PM-LSM used also for ropeless elevator by the Underground Development Utilization Research Center of the Engineering

272 Dragos Ovidiu Kisck et al. 5 Advancement Association of Japan (ENNA), the specification of PM-LSM is shown in Table 2 [2]. Table 2 Specification of the PM-LSM Parameters Values Remarks Thrust force [N] 3,000 speed 1 m/s; acceleration 1 m/s 2 Power [kw] 3 - Total mass [kg] (primary + moving part) 469 related to 1 m length Moving mass[kg] 270 max. load 140 kg; car mass 130 kg Primary (one side) Moving part: PM(NdFeB) Thrust force/mass length [mm] 3,055 - width [mm] 150 300 mm (double side) height [mm] 90 180 mm (double side) mass [kg] 36 - No. of poles 16 - length [mm] 1,000 - height [mm] 150 - width [mm] 30 - total [N/kg] 6.4 1 m PM-LSM moving part [N/kg] 83.3 - Airgap length [mm] 5 - Efficiency 0.62 - Power factor 0.91 - Trust force density [kn/m 2 ] 10 - Design of the TFM-LM for ropeless elevator requires the basic dimensions of the structure. An approximate sizing of the TFM-LM with inner mover is obtainable using the analytical procedure. Presuming an usual ratio of τ p /δ 10 and Θ a /δ 1,000At/mm, the initial data required to derive the basic design are: Θ a = 10 ka, τ p = 50 mm, B 0 = 1 T, δ = 5 mm, k = 0.6. Using these initial inputs data and (4), the value for the thrust force density for the TFM-LM, F xd is 60kN/m 2, which is approximately six times higher than the force density for the PM-LSM (10 kn/m 2 ) in Table 2. This result shows that TFM- LM has a great potential to increase propulsion force per unit mass or unit volume. In order to improve the performance of TFM-LM, specifically to increase the proportion of thrust force to TFM-LM weight, it is necessary to optimize the geometrical parameters of the basic design by using the 2D-FEA program. The 2D- FEA analysis considers the leakage fields as well as the saturation effects and allows a more accurate approximation at any current and mover position [7]. B. POLE WIDTH b p AND PM HEIGHT OPTIMIZATION The proportions bp τ p and hm / τ p are theoretically assumed to be between 0 and 1. Optimization occurs when parameters are varied within the following realistic boundaries: 0.5 b / τ 0.9; 0.15 h / τ 0.5. p p m p

6 Permanent magnet excited transverse flux linear motor for elevator 273 70 60 50 F xd [kn/m 2 ] 40 30 20 10 / τ p =0.5 / τ p =0.4 / τ p =0.3 / τ p =0.2 0 0,5 0,6 0,7 0,8 0,9 1,0 b p / τ p Fig. 3 Thrust force density F xd for different magnet height and pole width b p. Fig. 3 shows the dependence of thrust force density F xd with relation to magnet height for different pole width b p. Optimum traction force density is achieved when 0.65 b τ 0. 75 and 0.35 h τ 0. 45. p p C. POLE PITCH τ p OPTIMIZATION From an analytical view, in equation (4), the thrust force density F xd grows as the pole pitch τ p decreases, in such a way that in theory if the pole pitch τ p becomes to zero, the thrust force density F x goes to infinity. In reality, the thrust force density F xd will be limited by iron saturation and the dimensions. Figure 4 reveals the optimum area of the pole pitch for practical use. The optimal pole pitch becomes τ p = 40 60 mm. m p 80 60 F xd [kn/m 2 ] 40 20 0 0 50 100 150 200 τ p [mm] Fig. 4 Thrust force density F xd for different pole pitch τ p. Fig. 5 Thrust force density F xd different magnetomotive force.

274 Dragos Ovidiu Kisck et al. 7 The thrust force density at different magnetomotive force are presented in the Fig. 5. For small magnetomotive force (15,000 At), the relations between magnetomotive force and the thrust force density by (4) is proven: Fxd ~ Θa. Table 3 presents the optimized dimensions for the TFM-LM dedicated for ropeless elevators. Using the optimized dimensions, the thrust force per moving part weight is 291N/kg and the thrust force density is 63kN/m 2 which is approximately six times higher than the ordinary thrust force density of the corresponding 3kW, PM-LSM. The primary mass can be also reduced by using TFM-LM instead of using PM-LSM. Table 3 Specification of the TFM-LM for Ropeless Elevator Parameters Values Remarks Thrust force [N] 3,000 speed 1 m/s; acceleration 1 m/s 2 Power [kw] 3 - Total mass [kg] (primary + moving part) 107.84 related to 1 m length Moving mass [kg] 270 max. load 140 kg; car mass 130 kg length [mm] 3,055 - width [mm] 215 - height [mm] 200 - Primary τ p [mm] 50 - b p [mm] 40 - Moving part: PM (NdFeB and iron) Trust force/mass h sp [mm] 40 - b sp [mm] 65 - mass [kg] 10.3 - length [mm] 317 - height [mm] 75 - width [mm] 55 - [mm] 20 - total [N/kg] 27 1 m TFM-LM Moving part [N/kg] 291 - Airgap length [mm] 5 - MMF [At] 10,000 - Trust force density [kn/m 2 ] 63-4. EXPERIMENTAL TEST OF A DOWN-SCALED MODEL A. THE CONSTRUCTION OF A DOWN-SCALED MODEL To verify the results of the calculation, stationary thrust forces are measured for a down-scaled model of the TFM-LM, whose dimensions are shown in Table 4.

8 Permanent magnet excited transverse flux linear motor for elevator 275 Table 4 Specification of the DOWN-SCALED TFM-LM Model Parameters Values Remarks Airgap (δ) 1 mm - Polar pitch (τ p ) 20 mm - PM(NdFeB) height ( ) 10 mm /τ p = 0.5 Pole width (b p ) 14 mm b p /τ p = 0.7 Pole length (h i ) 20 mm - Height of window (h sp ) 20 mm - Width of window (b sp ) 30 mm - Stator 70 65 800 mm 3 - Mover 28 20 150 mm 3 - Conductor 1 3 mm 2 - Turns number (w) 78 2 - Winding factor (k) 0.6 - The photos from Fig. 6 show the down-scaled mover and the stator. a) b) Fig. 6 The down-scaled model of the TFM for experimental test: a) mover; b) stator. B. EXPERIMENTAL TESTS & RESULTS Because the 3D-FE analysis considers the saturation effects as well as the leakage fields for 3-dimention at any current and mover position, the thrust forces are calculated by using 3D-FEM for different positions and current [8]. Figure 7 shows the calculated thrust forces using 3D-FEM analysis and the measured thrust forces for different positions. The directions of the magnetic flux between the PM and primary winding are opposite when x/τ p = 0 and are the same when x/τ p = 1.0. Figure 8 shows the thrust force density F xd for 2D-FEM analysis, 3D-FEM analysis and experimental data. Thus, it is observed that the thrust force densities are 52 kn/m 2, 46 kn/m 2 and 37 kn/m 2 for 2D-FEM analysis, 3D-FEM analysis and experimental data at 2,000 At (Θ a /δ 1,000 At/mm) respectively. The obtined results permit a comparision between the two considered 3kW linear-motor

276 Dragos Ovidiu Kisck et al. 9 variants: the PM-LSM and the TFM-LM with inner mover. In comparison to the PM-LSM, the TFM-LM with inner mover leads to the increase of thrust force/motor mass and thrust force/mover mass by 410 % and 250 % respectively. 1000 800 4000AT 3D-FEA 4000AT - Experiemnt Thrust force [N] 600 400 200 2000AT 3D-FEA 2000AT - Experiment 1000AT 3D-FEA 1000AT - Experiment 0-200 0 2 4 6 8 10 12 14 16 18 20 Displacement [mm] Fig. 7 Calculate vs. measured thrust force related to the mover position and magnetomotive force. Fig. 8 Thrust force density F xd for 2D-FEM analysis, 3D-FEM analysis and the measurement.

10 Permanent magnet excited transverse flux linear motor for elevator 277 Table 5 Final DESIGN Specification of the designed TFM-LM Parameters Values Remark Thrust force [N] 3,000 speed 1 m/s; acceleration 1 m/s 2 Power [kw] 3 - Total mass [kg] (primary + moving part) 112.04 related to 1 m length Moving mass [kg] 270 max. load 140 kg; car mass 130 kg Moving part: PM (NdFeB and iron) mass [kg] 14.5 - length [mm] 446 - height [mm] 75 - width [mm] 55 1 m TFM-LM [mm] 20 MMF At 10,000 10,000 Trust total [N/kg] 26.8 Trust force density kn/m 2 44.8 force/mass Moving part [N/kg] 206.9 Airgap lengtm 5 5. CONCLUSIONS The proposed TFM-LM with inner mover leads to a considerable increase in power density for the moving part. To get the maximum thrust force under the given volume, the thrust force density with respect to the ratio of the pole width, height of the PM and the length of pole pitch is optimized by 2-D finite element method (FEM). To verify the design optimization, the computed forces are compared with the experimentally measured forces detected on the experimental setup of the down-scaled TFM-LM model. The measured thrust force density is 29% less for 2D-FEM analysis and 20% less for 3D-FEM analysis. In comparison to the PM-LSM, the TFM-LM with inner mover leads to the increase of thrust force/motor mass and thrust force/mover mass by 410% and 250% respectively. The obtained ratio of the thrust to mover weight is 206.9 N/kg for the TFM-LM, whereas for PM-LSM this value is 83.3 N/kg. Received on February 3, 2009 REFERENCES 1. T. Duenser, R. Deplazes, M. Meier, A new elevator system and its implementation, MAGLEV 2002, Lausanne. 2. J. F. Gieras, Z. J. Piech, Linear Synchronous Motor, CRC Press LLC, 1999. 3. R. Krishnan, Switched reluctance motor drive, CRC Press, 2001. 4. L.A.P. Henriques, L.G.B. Rolim, W.I. Suemitsu, P.J. Branco, Torque ripple minimization in a SR drive by neuro-fuzzy compensation, IEE Transactions on magnetics, 36, 5, 2000, Sept. 5. D.H. Kang, Y. H.Chun, H. Weh, Analysis and optimal design of transverse flux linear motor with PM excitation for railway traction, IEE Proc. Electr. Power Appl., 150, 4, 2003, July. 6. S. A. Nasar, Electromagnetic energy conversion device and systems, Printice-Hall, Inc., 1970. 7. * * *, http://www.magsoft-flux.com/ 8. * * *, http://www.infolytica.com/