FORCE AND CURRENT CHARACTERISTICS OF A LINEAR INDUCTION MOTOR USED FOR THE TRACTION OF A MAGLEV VEHICLE

Transcription

1 FORCE AND CURRENT CHARACTERISTICS OF A LINEAR INDUCTION MOTOR USED FOR THE TRACTION OF A MAGLEV VEHICLE Laércio Simas Mattos 1,2, Roberto André Henrique de Oliveira 2, Antônio Carlos Ferreira 2, Richard Magdalena Stephan 2 1 Federal Center of Technological Education, CEFET MG, Leopoldina, MG, Brazil 2 Federal University of Rio de Janeiro UFRJ, Rio de Janeiro, RJ, Brazil s: laercio@leopoldina.cefetmg.br, raholiveira@ufrj.br, ferreira@coep.ufrj.br, rms@ufrj.br Abstract This paper describes the constructive form of a linear induction motor applied to the traction of MagLev-Cobra levitation vehicle and the required control to a secure and economic driving. Moreover, issues about linear induction motor intrinsic features, such as traction force and current, are presented. Keywords Linear Induction Motor (LIM), LIM features, V/f driver, bidirectional power converter. I. INTRODUCTION The search for passenger vehicles that add quality, agility and comfort to public transportation is increasing worldwide. Electric traction vehicles have great environmental and economic advantages compared to propulsion by combustion engine. Among them: lower noise generation, non-co 2 emission, no soil contamination by leaking fuels and lubricants, lower explosion risk. The MagLev-Cobra vehicle aims to attend the sustainable transportation requirements in urban areas, causing minimal environmental and architectonic impact during installation and operation. This paper focuses on the linear motor's behaviour. The undesirable performance of the motor during testing phase, whereupon current sags were observed (and consequently a loss of traction force) when the primary (movable part) passed under the junction of two secondaries (fixed part) prompted further study of the motor and driving constructive characteristics. Section II presents the MagLev-Cobra vehicle, with the peculiar characteristics that make it a suitable vehicle for urban outlines. The Linear Induction Motor is shown in section III. Traction tests for several motor gaps are presented in section IV. In this section, the influence of the connection between secondary modules is investigated. The force that the linear induction motor exerts in the vertical direction, assisting the vehicle levitation, is shown in section V. Experimental results support the analyses. The conclusions are present in section VI. proposal. This is achieved because the vehicle is composed of several short length levitating modules, which makes it more articulated than conventional counterparts. The "MagLev-Cobra" vehicle "snakes" without contact over magnetic rails, making it ideal for deployment in urban centers due to its ability to integrate the contours of roads, rivers and dodge existent obstacles. Initial studies of the MagLev-Cobra technology were conducted with the construction of a reduced scale 30 meters long oval path with a small train provided with High Temperature Superconductors (HTS) inside cryostats. When a cryostat is filled with liquid nitrogen in the presence of the magnetic field generated by the permanent magnetic rail, a pinning effect is created between the HTS and the magnetic rails. This enables stable levitation in the presence of the magnetic field [1]. With the success in tests conducted on the reduced scale prototype, the next step was the construction of a real scale prototype. The project aims a 200 meters test line in real scale in the campus of the Federal University of Rio de Janeiro [2]. A. Modular Characteristic Fig. 1 shows the mock-up of a vehicle with two intermediate modules and two modules used at the ends. Despite the different shapes, the electrical and mechanical configurations are identical for all modules. Intermediate modules can be added to increase the transportation capacity. II. THE MAGLEV-COBRA VEHICLE The great ability of doing small radius curves and the flexibility to transition between pronounced uphill and downhill tracks characterize the innovative MagLev-Cobra Fig. 1. The MagLev-Cobra vehicle (Mock-up).

2 B. Energy supply A linear induction motor of short primary and long secondary gives the traction. This constructive characteristic of the motor requires energy inside the moving vehicle. The system that brings energy to the MagLev-Cobra vehicle utilizes current collector brushes that run on a power bus with aluminum conductors installed over stainless steel plates. The power bus is fed with 550 Volts DC and currents up to 160 A. The maximum voltage drop allowed by the system is 2% and the maximum temperature supported is +55 C. III. THE LINEAR INDUCTION MOTOR (LIM) The LIM idealized to MagLev-Cobra has constructive characteristics that contribute to levitation. Due to the primary and secondary arrangement forming a C (Fig. 3), the attraction force between these parts contributes to the force required to levitate the vehicle. Each vehicle module (wagon) has a short primary. The secondary is distributed along the way. All modules work independently, but synchronized. The synchronism is achieved through sensors installed at the train and control signals given to the frequency inverter of each motor. The characteristics of the linear motor of MagLev-Cobra are shown on TABLE I. TABLE I LIM s plate data of MagLev-Cobra. Trifasic Linear Induction Motor Equipament Description Data MANUFACTURER Equacional MODEL EALP 1000 / 6 FORCE 900 N POWER 10 CV PRIMARY VOLTAGE 420 V Y PRIMARY CURRENT 53 A FREQUENCY 25 Hz NUMBER OF POLES 6 SPEED 7.8 m/s WORK REGIME 1 hour ISOLATION CLASS H PROTECTION IP 00 Duncan (1983) developed the equivalent circuit of LIM, as shown in Fig. 2 [3]. The dimensionless parameter Q indicates the end effects influence on the traditional model of a rotating induction motor. Time constant of the LIM Equivalent time of one turn in rotary motor Motor effective length Motor velocity Primary resistance Secondary resistance reflected to the primary Primary leakage inductance Secondary leakage inductance reflected to the primary Magnetizing inductance Synchronous frequency Secondary frequency Fig. 2. Equivalent Circuit of LIM. Fig. 3 shows the primary and secondary of the three-phase linear motor. The primary has 54 windings with 13 turns each and copper rectangular conductors of 1.3x9.2mm. It has 6 poles. With length of 1.27m, the short primary is compatible with the dimensions of MagLev-Cobra. The secondary has a laminated iron core which fits aluminum bars of 12.7mm x 12.7mm, attached to a short circuit bar of 12.7mm x 25.4mm at both ends. Fig. 3. Linear Induction Motor. IV. TRACTION FORCE The LIM develops a force in the longitudinal direction, responsible for the movement, and a force in the normal direction. The traction force is generated by the interaction between the induced current in the secondary with the travelling field in the air gap. The total electromagnetic power developed by the motor (P AG ) is given by the equivalent circuit and (4). The power effectively converted to mechanical form (P conv ) should consider the ohmic losses in the secondary. According to Chapman [7], the force F x is given by (5). (3) (4) (5) (1) (2) Where is mechanical frequency, defined in (6). (6)

3 The measurement system shown in Fig. 4 was used to obtain experimental data and validate the model. Fig. 4.a shows the arrangement with a load cell fixed in a rigid base through steel cables, the display also is shown in this figure. A 2000 kg load cell was used (Fig. 4.b) to measure forces. The anchoring of the motor primary with the cable is shown in Fig. 4.c. The tests were performed with the primary blocked for different air gaps and keeping the ratio V/f constant, as shown in TABLE II. The results for air gaps between 08 mm and 20 mm are shown in Fig. 5 (a and b). In rotary motors, the air gap varies between 0.2 and 0.3 mm. But, in general, linear motors need large air gaps to operate. The magnetic flux in the LIM is longitudinal, that is, the magnetic flux lines are parallel to the direction of the travelling magnetic field [6]. (a) (a) (b) Fig. 5. Force and current measurements, (a) Fxf and (b) Ixf (b) TABLE II V/f constant. V f V/f (c) Fig. 4. Traction force measurement, (a) arrangement, (b) load cell and (c) connection to the primary. The motor secondary consists of modular sections of 1.5m length, mechanically connected side by side along the line. During the tests, a malfunction when the primary passed under secondary junction was noted. The secondaries are united by painted metallic plates, screwed on their housing (Fig. 6). At these junction points, current sags were observed, which result in momentary force loss. Fig. 7 shows a current profile when primary passes through a connection (in blue).

4 Fig. 6. Mecanic coupling between secondary modules. Fig. 9. Primary and secondary position during test. Fig. 10. Propulsion force with cage continuous and discontinuous. Fig. 7. Current behavior in the transition between secondary parts (in blue). Trying to minimize this issue, an electrical connection between the secondary short circuit bars (presented separately in each module) was provided with aluminum bars, as shown in (Fig. 8). Fig. 11. Currents with cage continuous and discontinuous. Fig. 8. Electric connection between squirrel cages. The tests to check the influence of the electrical connection on the motor behavior meets the layouts shown in Fig. 9. Fig. 10 shows the improvement. The force curve with electric connection (blue) is between the curve without electric connection (green) and the ideal condition of continuous secondary (red), obtained when the primary is exactly over a secondary module. Fig. 11 shows that the corresponding current produce the force. The results also show that the magnetic gap between the secondary modules has an influence that could not be compensated for. V. ATTRACTION FORCE The normal force existing in the LIM is the result of the magnetic flux crossing the air gap and has two components. The first component is the eletromagnetic attractive force between the primary and the secondary iron core and the second component is the eletrodynamic repulsion force between the moving primary and secondary FMM. The repulsive force can be disregarded because the vehicle operates at low speed [7]. For this test, longitudinal movement is blocked by a steel cable anchored in the wall. The attraction force is measured by installing a load cell between the ground and the primary of the linear motor. The test adopted the airgap of 8mm. The frequency varied between 1Hz to 7Hz and V / f ratio was

5 kept constant. The attractive force reaches approximately 3000N (Fig. 12). Fig. 12 shows that the force remained constant between 3 and 6 Hz, because the force is proportional to the square of magnetic field (B), but the magnetic field is proportional to the ratio V/f (according to (7) [7]). When the frequency is low, between 1Hz and 3Hz, resistance R1 is significant. Above 6Hz the impedance becomes important. Hence the occurs fall of force in these regions. (7) Where, is air magnetic permeability and is area (constant). Fig. 12. Attraction Force; air gap = 8mm; M=450 kg. VI. CONCLUSION The significant influence of the air gap in the linear motor traction force was initially observed. The presented work clarified the importance of electrically connecting the segmented parts of a long secondary linear induction motor. The magnetic gap between these segments also influences the traction force. Therefore, the mechanical construction must be accurate, since no other simple compensation mean can be implemented to compensate for the magnetic gap. The special motor construction is particularly interesting for urban MagLev vehicles since the normal forces between primary and secondary contributes to the levitation. ACKNOWLEDGEMENT To BNDES, FAPERJ, CAPES (DINTER), CNPq and WEG for the financial support and to Dr. Ivan Chabu for the technical discussions. REFERENCES [1] F. C. Moon, Superconducting Levitation- Applications to Bearings and Magnetic Transportation, publisher Wiley- Interscience [2] R. M. Stephan, R. de Andrade Jr. and A. C. Ferreira, Superconducting Light Rail Vehicle: a transportation solution for highly populated cities, IEEE Vehicular Technology Magazine, December 2012, pp [3] J. Duncan, Linear induction motor-equivalent-circuit model, Electric Power Applications, IEE PROC, Vol. 130, PL B, No. 1, jan. 1983, pp [4] A. E. Fitzgerald, C. Jr. Kingsley, and D. U. Stephen, Máquinas Elétricas, Bookman, 6 th Edition, [5] S. J. Chapman, Electric Machinery Fundamentals, McGraw-Hill Series in Electrical Engineering, 6 th Edition, [6] E. R. Laithwaite, J. F. Eastham, H. R. Bolton and T. G. Fellows, Linear motors with transverse flux, PROC. IEE, Vol. 118, No. 12, December 1971, pp [7] G. G. Sotelo, R. M. Stephan, P. J. Branco and R. de Andrade Jr A didactic comparison of magnetic forces, International Journal of Electrical Engineering Education 48/2, March 2011, pp