Adoption of Linear Motor Propulsion System for Subway

(Osaka Subway Line No.7; Nagahori Tsurumi-ryokuchi Line)


TERAOKA, Shintaro
Rolling Stock Department
Rolling Stock Division
Construction and Engineering Headquarters
Osaka Municipal Transportation Bureau

Abstract
 
 On 20th March 1990, for the first time in Japan, we adopted a new propulsion system for Osaka City's 7th subway line named "Nagahori Tsurumi-ryokuchi Line".
 This new propulsion system uses linear induction motor(LIM) controlled by Variable Voltage Variable Frequency(VVVF) inverter and does not rely on ordinary adhesion that is to say friction between the wheels and rails.
 To achieve this new system, it has taken more than 10 years. While making a research for reducing the cost of the subway system, as a first step, Osaka City developed and put into practical use of the 3-phase squirrel-cage rotary induction motor driven car controlled by VVVF inverter, which has been called the railway system's dream. Moreover, after various kinds of experiments and repetitive running tests using the prototype of LIM driven car, this new system was finally completed.
 In 1990, this new system transported approximately 8 million visitors and played a great role as main access to the Expo'90(the International Garden and Greenery Exposition) with almost no troubles, and now this new system has met all the demands very satisfactorily.
 Along with its specifications and field data, the process of technological development and outline of this LIM driven car are introduced.
 
1 Introduction
1.1 Outline of Osaka Municipal Subway Network
 Osaka is a city covering an area of about 220.66kuwith a population of about 2.6 millions. (Daytime population is more than 3.8 millions.)
 Traffic of the city is maintained by municipally operating such transportation systems as "Subway", "Bus" and "New-Tram", with more than 3 million passengers a day being transported.
 The history of Osaka City's public transportation began on Sep. 12, 1903, when the city inaugurated the use of streetcars. The first subway in Osaka City was operated between Umeda and Shinsaibashi(3.1km) on May 20, 1933. The subway now has 7 lines which in total cover 115.6km, and mutual through service with private railways is operated in the Midosuji, Chuo, and Sakaisuji Lines.
 The number of subway cars amounts to about 1,200 with serving about 2.7 million passengers daily.
 
1.2 Outline of Osaka Subway Line No.7
Fig.1. Route Map of Osaka Municipal Subway Network
 
 As a first step, in order to tremendously enhance the convenience of transportation in the eastern district of Osaka City, the line No.7 was constructed 5.2km in length between Kyobashi and Tsurumi-ryokuchi station near by the site for the Expo'90. Kyobashi station is a junction of Keihan Main Line and JR Osaka Loop Line.
 In order to complete the municipal transportation network, extension from Shinsaibashi to Kyobashi began operating December 11, 1996, moreover extension from Tsurumi-ryokuchi to Kadomaminami and from Taisho to Shinsaibashi began operating August 29, 1997. Now this line No.7 is 15km in length with the minimum curve radius of 102m and the steepest grade of 50‰.
 Prior to completion of these extensions, in order to provide more efficient service while ensuring the safety, single-operator transportation began operating April 30, 1996, equipped with Automatic Train Operation(ATO)and a monitoring system to handle the look-out duties which used to be in charge of the conductor.
 
2 Background of Introducing LIM
 Osaka City has set up "Investigative Committee for Miniaturization of Subway" in September, 1979, aiming at reduction in subway construction cost.
 As for the car, minute examination was made, that is to say, what extent the miniaturization is practicable without making much sacrifice of the comfort to ride in passenger room, especially to what extent the floor height can be lowered considering bogie truck and new control system including equipment to be mounted under floor.
 The results of this investigation are shown in Fig.2. The tunnel cross-section area was reduced by about 40%, and the length, width and height of the subway cars were reduced by about 3m, 0.3m and 0.7m respectively. As a result of these reductions, estimated total construction cost was reduced by about 20%.
 
Fig.2. Comparison of Shield Tunnel Cross-section
 In the above-mentioned process, two propulsion systems were proposed, one is utilizing rotary induction motor and another is employing LIM. Osaka City took the initiative in preparing an experimental car for testing VVVF inverter control system utilizing rotary induction motor through 1981 and 1982. As for the LIM driving system, the Japan Railway Engineers' Association has prepared a test car under a project named "Study of Linear Motor Driven Electric Vehicle for Subway with Small Cross Section" from 1981 to 1983. Further, the Ministry of Transport of Japan and the Japan Subway Association have provided an experimental train consists of two electric cars in a project called "Feasibility Study of Small-sized Subway Vehicle Driven by Linear Motor" extended from 1985 to 1987. This train was subjected to various tests on a newly constructed 1.85km full-scale test track(Fig.3.) in Osaka Nanko(South Port).
 
Fig.3. Nanko Test Track
 Moreover Osaka City has attempted to manufacture its own experimental car to meet greater traffic demand and dissimilar track conditions of the Osaka Subway Line. And various minute tests were performed for about one and a half years.
 Through these tests, accelerating and braking performance was confirmed, the method of mounting LIM and the steering system were determined, and thus, overall safety and reliability of the car were ascertained.
 And then decision as to whether the introduction of LIM driven car to the line No.7 is appropriate or not was considered deliberately in "the Investigative Committee on the Rolling Stock for the Osaka Subway Route 7" composed of ten members of experience or academic standing. Accordingly, Osaka City chose the LIM driving system for subway line No.7.
 
3 Outline of cars; Series 70
Photo.1. LIM Driven Car series 70
 
 The small dimensions of car with keeping the comfort of passenger room were realized by lowering the floor height, not lowering the ceiling height.
 The above facts are accomplished by adoption of the flat LIM as well as endeavoring exhaustively to miniaturize and make thin the equipment and devices as much as possible. Moreover by adoption of the optical fibers, total length of cables used for controlling can be kept less than 1/5 as compared with the conventional cars.
 Some of main equipment are following.
 
3.1 Linear Induction Motor(LIM)
Fig.4. Development of Rotary Induction Motor into LIM
 The LIM is a flat plate-like shape formed by partially cutting a conventional AC rotary induction motor and moves linearly, as shown in the Fig.4.
 The primary side called the "LIM" consists of iron cores and coppery coils through which electric current flows, just like a rotary induction motor. The output is 100kW and natural cooling system is employed.
 
Fig.5. Exterior of LIM
 The secondary side called "reaction plate(RP)," achieves the same function as the rotor of a rotary induction motor. The reaction plate is a combination of an iron plate having a good permeability of magnetic field and a good conductive material such as aluminum. For steep slopes, aluminum may be substituted for copper. Reaction plates have three types according to the way of construction; flat, cap and end-bar.
 The primary side is mounted on the each bogie truck frame of the cars, and the reaction plate is attached to sleepers along the rail. The gap between the LIM and reaction plate can be easily adjusted to & 12mm by the turnbuckles located on the suspension units of LIM, considering the change during the travelling motion.
 
Fig.6. Reaction Plate(flat type)
 Then, the principle of movement is as follows. When AC current flows through the primary side, it creates a moving magnetic field along the longitudinal direction. And this primary side's moving magnetic field generates an eddy current and another magnetic field in the reaction plate on the ground. Then these two kinds of magnetic fields push and pull against each other, and the linear movement can be produced.
 
 
3.2 Control Equipment(VVVF Inverter)
 The car is equipped with VVVF inverter which controls the accelerating, braking force and speed of the LIM by converting DC 1,500V of overhead supply into AC 3-phase with modulating the voltage and frequency.
 The control system consists of the inverter unit, line breaker box, filter reactor, charging and current reduction resistor, and over-voltage suppression resistor. The inverter unit is a single-unit box about 3m long, 2m wide and only 500mm height to reduce the floor height, consist of the logic and cooling unit of main circuit element, brake-command receiving unit, filter condenser, and command converter which is a part of the train remote operation network system, etc.
 The main circuit element of the inverter employs Gate Turn Off(GTO) thyristor of 4500V-2500A capacity, controls four 100kW linear induction motors.
 
3.3 Air Brake Equipment
 This brake system is an electric-pneumatic blended brake, which lightens the load for adhesive air brake by making the best use of the non-adhesive propulsion of the LIM. In other words, necessary braking force and electric regenerative braking force are calculated by computer, and air brake makes up for only when the shortage of electric regenerative brake is detected.
 
3.4 Bogie Truck
Fig.7. Exterior of Bogie Truck
 The truck powered by LIM can reduce noise because there are no gear unit, and can be equipped with a simple steering function called self-steering system. This steering system is performed by not using steering link and the like at all, but only softening the supporting rigidity in longitudinal direction of axle-box supports. So when passing on curves, the axle can be naturally placed along the rail, and this placement realizes smoother curving and comfortable riding.
 The air brake unit uses a disc brake system equipped with rubber cylinders and one disc per axle.
 
 
4 Field Data
4.1 Influence of the gap change
(1)Dynamic change of the gap in the operation line
 According to the measurement through actual running with a gap sensor attached on the end part of LIM, the range of gap changing is constant of about ±3mm, even if we adjust the dimension of the gap to install the LIM between 10mm and 13mm.
(2)Relations with acceleration/deceleration
 The acceleration (α:planning value is 2.5km/h/s)and deceleration (β:planning value is 3.5km/h/s)in the case of the gap adjustment 10mm was respectively 7-10% and 6-9% higher than the case of 13mm.
 The relations between the gap adjustment and the suction and traction force were measured with the gauge attached on the link, but we could not find any relations.
(3)Relations with the electric power consumption
 At the Nanko test truck we measured the electric power consumption of typical pattern (level truck of 1km in length, 90 seconds' running withαandβ:2.5km/h/s), and the electric power consumption in the case of 10mm gap was 4% less than that in the case of 13mm gap.
 But in the operation line, there was a opposite result that the electric power consumption of 10mm gap was more than that of 13mm gap. For examples the each electric power consumption of three different operating trains in the case of 10mm gap was 59.27, 59.79 and 55.05Wh/t/km, and that in the case of 13mm gap was 59.05, 58.20 and 55.57Wh/t/km respectively.
 The reason of this result was the condition of measurement was different. For exampleαandβwas not adjusted according to the adjustment of the gap, that is to say, when the gap size lessens, not onlyαandβ, but also the input current increase. And then the ratio of increasing the input electric power exceeds that of becoming the travel time short. So, on the whole, the electric power consumption in the operation line increases.
(4)Strength of LIM and Stability of bogie truck
 We measured following subjects,
  • The acceleration of the vibration and intensity of the stress generated in the each part of LIM.
  • The each part's stress of the bogie truck.
  • The ratio of a wheel load coming off and deviation from the center of gravity.
     And the numerical value of the result indicated the superior durability and fully secured safety, even if the gap is changed.
     
    4.2 Self-steering function of bogie truck
     The evaluation of riding comfort about up-down and back-forth shaking was "excellently good", nevertheless that about right-left shaking was "usual" at first, when the spring constant of axle-box support toward back-forth was "soft" of 280kgf/mm/box.
     It takes for a while to make a good relation between brand-new wheel and rail, so we exchanged some of axle-box supports for "hard" type of 470kgf/mm/box. The follow-up survey and comparative examination were done to improve the riding comfort. And the evaluation of "hard" type was "excellently good".
     Although the evaluation of riding comfort about the "soft" type became "excellently good" after a while, in case of newly changing the rails and wheels there is a possibility the evaluation goes down to "usual".
     Generally we expected the "hard" type was good for the "rectilinear" travelling and the "soft" type was good for the "steering" and in fact this expectation was true. For example the angle of steering at the sharp curve of the radius 119m was 0.248degree/axle in the case of the "soft" type and it was 0.150degree/axle in the case of the "hard" type.
     But the evaluation of riding comfort indicates the grease-up to the rail improves the traveling quality of the "hard" type at the sharp curve. And even if the "soft" type is adopted, the grease-up to the rail is indispensable to prevent the abrasion of the rail and the wheel flange.
     So we can recommend the "hard" type and adopted it for the newly-manufactured cars. Though it is under the follow-up survey, thus far there are no problems.
     
    4.3 Conditions of the rail corrugation
     When the quantity of the grease-up to the rail was shortage, wave-like pattern of wearing appeared on the rail surface and left-right swinging of the trains became large. On the other hand, there were no wave-like patterns of wearing appeared when the rail was greased-up appropriately, and furthermore even the old patterns of wearing disappeared.
     Therefore, the appropriate grease-up on the rail surface is inevitably necessary and really effective to prevent the rail corrugation.
     
    4.4 Countermeasure for resonating noise
    Graph 1. Old pulse mode switching pattern
     
     There were various kinds of noise which caused by running, but the noise of resonance just after the start was a little big, so we changed the pulse mode switching pattern in the condition of start and low speed.
     The pulse mode switching pattern in the past is indicated in the Graph 1.









    Graph 2. Old noise pattern of inside and outside
     
     At the turning point(=point A) from the 27 pulse mode to the 15 pulse mode the outside noise level reached 102dB, and at the turning point(=point B) from the 15 pulse mode to the 9 pulse mode the outside noise level reached 105dB.
     Obviously this is peculiar in comparison with the level change of the running noise and the aerodynamic noise which increases in proportion to the speed.









    Graph 3. Improved pulse mode switching pattern
     
     It was considered the resonance with the specific frequency decided by the subway structure's dimension, for example tunnel, caused this noise.
     In fact, the switching frequency at the point A and B are about 114Hz and 150Hz respectively, and supposing the speed of sound is 345m/s, the wavelength of the point A and B are calculated about 2.9m and 2.3m respectively.






    Graph 4. Improved noise pattern of inside and outside
     
     It is possible these numerical values correspond with some of the main dimensions of the structure. So improvement pattern of the Graph 3. was confirmed by the field test-run. In the Graph 4., there are no peak level like Graph 2., and we can judge this countermeasure of the changing the pulse mode switching pattern made satisfactory effect.
     In fact by judging from the listening, the unpleasant resonance was disappeared and the improvement was confirmed. But the calorific power of the snubber resistor around GTO of the main circuit increased, so finally, by enlarging the capacity of the resistor, we decided to adopt the pulse mode switching pattern of Graph 3.



    4.5 The special reaction plate for the steep slope
     In case of a trouble like driving 4 cars in the steep slope section of 50‰with only 1 control-unit, there is a possibility of shortage to accelerate the train for the start. Therefore, the special reaction plate was laid, and we tried to improve the performance in this section.
     Most of all reaction plate is composed by aluminum board of 5mm and the iron plate of 22mm like the Fig.6. But this special reaction plate is composed of copper board of 3mm and the iron plate of 32mm.
     Before actual operation, this special reaction plate was laid in the expanded area in the "Tsurumi car inspection shop", and it was confirmed that the LIM showed the expected power as the design. The condition of the test was that 1 control-unit drove 4 cars with 200% load of the usual capacity at a speed of 15km/h.
     

    Table 1. The result of operation with special RP


     

    the
    Material

    Normal
    Max-Power

    Extra
    Hi-Power


    Traction
    (kgf/LIM)
     

    Cu
     

    1,739
    (1,570)

    2,113
    (2,021)

    Al
     

    1,706
    (1,500)

    1,992
    (1,982)


    Suction
    (kgf/LIM)
     

    Cu
     

    1,880
    (2,150)

    2,065
    (2,600)

    Al
     

    1,810
    (2,080)

    1,890
    (2,480)


    Efficiency
    (%)
     

    Cu
     

    49.2
    (50.7)

    49.6
     

    Al
     

    49.2
    (50.2)

    48.4
     


    Power rate
    (%)
     

    Cu
     

    79.4
    (60.7)

    63.5
     

    Al
     

    77.2
    (58.3)

    61.4
     

    Input
    current
    (A/LIM)
     

    Cu
     

    96
    (91)


     

    Al
     

    92
    (86)


     


         ( ):the inside is a calculation
     
     We measured various kinds of characters, and as for the traction force, the numerical value of actual measurement was a little higher than the calculation.
     Generally we could judge the operating performance in the steep slope line was fully satisfied.
     
    4.6 The ratio of electric regeneration
     When the operation line was between Kyobashi and Tsurumi-ryokuchi(5 stations, 5.2km), the ratio of electric regeneration was low of about 11% and actual electric power consumption was big.
     This result of LIM was unfavorable in comparison with Rotary Induction Motor, for example, which ratio is 33.20% on average of Osaka subway.
     However, this ratio improved drastically to about 17% when the operation line was extended to Shinsaibashi(12 stations, 10.9km) in December, 1996. On the other hand, after the completion of the line between Taisho and Kadomaminami, the ratio of electric regeneration goes up very little to about 17.5% as of Sep., 1997.

    Table 2. Data of electric power



    Ratio of
    electric
    regeneration
    (%)

    Electric power
    consumption
     


    Note
     

    (kWh/car/km) (Wh/t/km) 


    1993
     


    11.3
     


    1.41
     


    54.3
     



    Kyobashi〜
    Tsurumi-ryokuchi
    (5 stations, 5.2km)
     


    1995
     


    10.6
     


    1.46
     


    56.1
     

    Jan.,
    1997
     


    17.0
     


    1.63
     



     

    Shinsaibashi〜
    Tsurumi-ryokuchi
    (12 stations, 10.9km) 

    Sep.,
    1997
     


    17.5
     


    1.89
     



     

    Taisho〜
    Kadomaminami
    (17 stations, 15.0km) 

    Rotary
    Induction
    motor


    33.20
     


    1.41
     


    40.27
     


    As of 1993
     


     
     There are mainly two kinds of reasons about drastic change of the ratio of electric regeneration as follows.
     
    (1)Owing to the line's condition
    Fig.8. Comparison of the line's condition
     Between Kyobashi and Tsurumi-ryokuchi, 3 intervals out of 4, have an ideal condition to save energy, because the positions of stations are higher than those of the intervals and so there is little necessity to use brake. On the other hand, in the newly extended line between Shinsaibashi and Kyobashi, there is only 1 interval has an ideal condition out of 7. So the ratio of electric regeneration is improved due to the line extension.
     But in the extension line, the total quantity of electric power consumption increased because the increase of accelerating electric power was much more than that of electric regeneration. So in fact, we should construct the ideal line to save electric power consumption.
    (2)Owing to the operation's condition at the terminal
     The rate of the number of the terminal stations divided by that of all stations decreases drastically to 2/12 from 2/5 just after the line extension in December, 1996.
     At the terminal stations, there are crossover lines where reaction plates can not be placed and the speed of train is low, so the electric regenerating brake system can not be utilized effectively.
     
    5 Conclusion
     Although Osaka is the oldest city in Japan, the forward-looking and enterprising spirit of its people allowed Osaka to transform itself dramatically at each historical turning point, and to play a leading role in the development of Japan's city planning, industry, and culture with keeping Osaka's rich historical heritage.
     For examples in the world of railway, though the cars driven by rotary-type motor already have had more than 100 years' history, about 15 years ago Osaka City made efforts to use the 3-phase induction motor practically for the first time in Japan and then we felt no further improvements in performance and maintenance-free can be achieved. In fact, nowadays the most of newly built cars in Japan are adopting this technology.
     But the LIM driven car could solve the problems concerning about adhesion between wheel and rail, and brought many advantages. Following to Osaka, line No.12 of Tokyo subway is started the operation in December, 1991. And Kobe City, Yokohama City, Fukuoka City, and so on also have the plan of introducing the LIM driving system for subway.
     Although they say the LIM driven car is possible only for the adoption in a complete new line, introduction in existing lines is also possible. For example, if reaction plates could be laid on existing lines, both use of the rotary induction motor and LIM would become possible and could replace the conventional car gradually with the LIM driven car. Of course for this purpose, the advantages of LIM driving system must pay back the cost of laying reaction plates. This is our earnest hope along with development of the LIM drive system. Though we had only about 8 years' experiences, we will continuously cooperate with the diffusion and the development of the linear motor subway from now on.
     We would like to express our hearty gratitude to the persons and parties who guided and cooperated with us on the occasions of development, testing, introduction and putting into practical use of the LIM driven cars.
     Now we have a big dream of realizing to host the 2008 Olympic Games in Osaka. With all of your support and cooperation in our efforts, we hope to welcome you in the 2008 Olympic Games in Osaka through our transportation network.
     
     
    [References]
    (1)Prototype LIM Driven Railcar for Subways of Smaller Cross Section Tunnels.
     by Yoshihiko Toyama (1984)
     p.11, Japanese Railway Engineering Vol.24, No.3
    (2)Research and Development of Linear Motor Driven Metro System
     by Akira Matsumoto (1988)
     p.8, Japanese Railway Engineering Vol.28, No.2
    (3)Linear Motor Driven Car of Tsurumi-ryokuchi Line
     by Tadayoshi Yamanaka et al. (1990)
     p.13, Osaka and Its Technology No.19