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

Laércio Simas Mattos1,2, Roberto André Henrique de Oliveira2, Antônio Carlos Ferreira2, Richard Magdalena Stephan2 1 Federal Center of Technological Education, CEFET – MG, Leopoldina, MG, Brazil 2 Federal University of Rio de Janeiro – UFRJ, Rio de Janeiro, RJ, Brazil e-mails: [email protected], [email protected], [email protected], [email protected]

Abstract – This paper describes the constructive form of a linear induction motor applied to the traction of II. THE MAGLEV-COBRA VEHICLE MagLev-Cobra levitation vehicle and the required control to a secure and economic driving. Moreover, The great ability of doing small radius curves and the issues about linear induction motor intrinsic features, flexibility to transition between pronounced uphill and such as traction force and current, are presented. The downhill tracks characterize the innovative MagLev-Cobra linear induction motor (Patent No PI 1103525-0) was proposal. This is achieved because the vehicle is composed tested in various operating conditions. The results of several short length levitating modules, which makes it contribute to its improvement and to delineate its more articulated than conventional counterparts. The applicability, not only in MagLev Cobra, as in other "MagLev-Cobra" vehicle "snakes" without contact over vehicles types and industrial applications. magnetic rails, making it ideal for deployment in urban centers due to its ability to integrate the contours of roads, Keywords – LIM features, Linear Induction Motor rivers and dodge existent obstacles. (LIM), V/f driver. Initial studies of the MagLev-Cobra technology were conducted with the construction of a reduced scale 30 meters I. INTRODUCTION long oval path with a small train provided with High Temperature Superconductors (HTS) inside cryostats. When The search for passenger vehicles that add quality, agility a cryostat is filled with liquid nitrogen in the presence of the and comfort to public transportation is increasing worldwide. magnetic field generated by the permanent magnetic rail, a Electric traction vehicles have great environmental and “pinning effect” is created between the HTS and the economic advantages compared to propulsion by combustion magnetic rails. This enables stable levitation in the presence engine. Among them: lower noise generation, non-CO2 of the magnetic field [1]-[2]. emission, no soil contamination by leaking fuels and This levitation method deffers from the electromagnetics lubricants, lower explosion risk. The MagLev-Cobra vehicle and from the electrodynamics levitation methods, used in aims to attend the sustainable transportation requirements in Japan[3]-[4], Germany[5] and China[6]-[7]. urban areas, causing minimal environmental and With the success in tests conducted on the reduced scale architectonic impact during installation and operation. prototype, the next step was the construction of a real scale This paper focuses on the linear motor's behaviour. The prototype. undesirable performance of the motor during testing phase, The project aims a 200 meters test line in real scale in the whereupon current sags were observed (and consequently a campus of the Federal University of Rio de Janeiro [8]. loss of traction force) when the primary (movable part) passed under the junction of two secondaries (fixed part) A. Modular Characteristic prompted further study of the motor and driving constructive Fig. 1 shows the mock-up of a vehicle with two characteristics. intermediate modules and two modules used at the ends. Section II presents the MagLev-Cobra vehicle, with the Despite the different shapes, the electrical and mechanical peculiar characteristics that make it a suitable vehicle for configurations are identical for all modules. Intermediate urban outlines. modules can be added to increase the transportation capacity. The Linear Induction Motor is shown in section III. Traction tests for several motor gaps are presented in B. Energy supply section IV. In this section, the influence of the connection A linear induction motor of short primary and long between secondary modules is investigated. secondary gives the traction. This constructive characteristic The force that the linear induction motor exerts in the of the motor requires energy inside the moving vehicle. The vertical direction, assisting the vehicle levitation, is shown in system that brings energy to the MagLev-Cobra vehicle section V. Experimental results support the analyses. utilizes current collector brushes that run on a power bus The conclusions are present in section VI. 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.

978-1-4799-0272-9/13/$31.00 ©2013 IEEE 872 Primary resistance Secondary resistance reflected to the primary Primary leakage inductance Secondary leakage inductance reflected to the primary Magnetizing inductance Synchronous frequency Secondary frequency

/ (1)

(2)

/ / (3) Fig. 1. The MagLev-Cobra vehicle (Mock-up).

III. THE LINEAR INDUCTION MOTOR (LIM)

The linear induction motor has intrinsically attractive forces between the primary and secondary. In conventional MagLev vehicles, this attraction operates in opposition to the levitation [9]-[12]. But the attractive force can also contribute to the levitation. For instance in [13], a "T" topology uses a synchronous frequency control to produce the levitation force. The LIM idealized to MagLev-Cobra has constructive Fig. 2. Equivalent Circuit of LIM. characteristics that contribute to levitation. Due to the Fig. 3 shows the primary and secondary of the three-phase primary and secondary arrangement forming a “C” (Fig. 3) linear motor. The primary has 54 windings with 13 turns [14], the attraction force between these parts contributes to each and copper rectangular conductors of 1.3x9.2mm. It has the force required to levitate the vehicle. Each vehicle 6 poles. With length of 1.27m, the short primary is module (wagon) has a short primary. The secondary is compatible with the dimensions of MagLev-Cobra. The distributed along the way. All modules work independently, secondary has a laminated iron core which fits aluminum but synchronized. The synchronism is achieved through bars of 12.7mm x 12.7mm, attached to a short circuit bar of sensors installed at the train and control signals given to the 12.7mm x 25.4mm at both ends. 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 Fig. 3. Linear Induction Motor. ISOLATION CLASS H PROTECTION IP 00 IV. TRACTION FORCE

Duncan (1983) developed the equivalent circuit of LIM, The LIM develops a force in the longitudinal direction as shown in Fig. 2 [15]. The dimensionless parameter Q , responsible for the movement, and a force in the indicates the end effects influence on the traditional model of normal direction. The traction force is generated by the a rotating induction motor. interaction between the induced current in the secondary with the travelling field in the air gap. The total electromagnetic

Time constant of the LIM power developed by the motor (PAG) is given by the Equivalent time of one turn in rotary motor equivalent circuit and (4). The power effectively converted to Motor effective length mechanical form (Pconv) should consider the ohmic losses in Motor velocity the secondary.

873 3 (4) According to Chapman [7], the force Fx is given by (5). 3 (5) Where is mechanical frequency, defined in (6).

(6)

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 (c) through steel cables, the display also is shown in this figure. Fig. 4. Traction force measurement, (a) arrangement, (b) load cell A 2000 kg load cell was used (Fig. 4.b) to measure forces. and (c) connection to the primary. 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 [16].

(a)

(a)

(b) Fig. 5. Force and current measurements, (a) Fxf and (b) Ixf

TABLE II

(b) V/f constant. V f V/f 085 05 17.0 165 10 16.5 245 15 16.3 320 20 16.0 390 25 15.7

874 The motor secondary consists of modular sections of 1.5m Fig. 10 shows the improvement. The force curve with length, mechanically connected side by side along the line. electric connection (blue) is between the curve without During the tests, a malfunction when the primary passed electric connection (green) and the ideal condition of under secondary junction was noted. The secondaries are continuous secondary (red), obtained when the primary is united by painted metallic plates, screwed on their housing exactly over a secondary module. (Fig. 6). At these junction points, current sags were observed, Fig. 11 shows that the corresponding currents produce the which result in momentary force loss. force. Fig. 7 shows a current profile when primary passes The results also show that the magnetic gap between the through a connection (in blue). secondary modules has an influence that could not be compensated for.

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”. V. ATTRACTION FORCE

The tests to check the influence of the electrical The normal force existing in the LIM is the result of the connection on the motor behavior meets the layouts shown in magnetic flux crossing the air gap and has two components. Fig. 9. The first component is the eletromagnetic attractive force

875 between the primary and the secondary iron core and the VI. CONCLUSION second component is the eletrodynamic repulsion force between the moving primary and secondary FMM. The The significant influence of the air gap in the linear motor repulsive force can be disregarded because the vehicle traction force was initially observed. operates at low speed [17]. The presented work clarified the importance of electrically For this test, longitudinal movement is blocked by a steel connecting the segmented parts of a long secondary linear cable anchored in the wall. The attraction force is measured induction motor. The magnetic gap between these segments by installing a load cell between the ground and the primary also influences the traction force. Therefore, the mechanical of the linear motor. The test adopted the airgap of 8mm. The construction must be accurate, since no other simple frequency varied between 1Hz to 7Hz and V / f ratio was compensation mean can be implemented to compensate for kept constant. The arrangement for the measurements is the magnetic gap. shown in Fig. 12. The attractive force reaches approximately The special motor construction is particularly interesting 3000N (Fig. 13). 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

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