Abstract:

In Superconducting Maglev, ground coil is one of the most important equipment which determine the structure and characteristics of the Maglev system. At Yamanashi Test Line we adopted a null-flux levitation and double-layered propulsion coil system.As a result of study, it was decided to adopt a more efficient null-flux levitation system for Yamanashi Test Line. In this system, levitation coils are set over the propulsion coils on the side wall. To reduce the harmful vibration of superconducting magnets caused by magnetic field’s change from propulsion coils, a double-layered propulsion coil system is adopted for Yamanashi Test Line. The following describes the features of this new structure.

Key Words : Maglev, Linear Motor, Ground Coils

1.Introduction

2.Structure of the electro-magnetic guideway on Yamanashi Test Line

2.1 Null-flux levitation system

Null-flux levitation and guidance system was invented by RTRI in 1988. In this system, “8”-figured coils are installed on the side walls, and the corresponding coils of both side walls are connected to each other by a null-flux cable to generate a guidance force (Fig.1). Compared with the repulsive levitation, the characteristics are as follows,

1) Larger levitation force

As all of the horizontal elements of the “8”-figured coil function effectively, it gives larger levitation force than the repulsive levitation coil.

2) Lower magnetic drag force

It is almost zero during the wheel running, on the contrary there is a drag peak at low speed range in repulsive levitation system. And while levitating, the drag force is about one third as large as that of repulsive type.

3) Harder magnetic suspension

It means the vehicle is less sensitive to load weight, and means that more regularity of the guideway is required to gain the same ride quality.

4) More suitable for high voltage system

Levitation coil can function as guidance one, so there are no longer required high voltage null-flux cables between the propulsion coils of both side walls.

5) More difficult levitation coil design

As for levitation coils and fixing equipment, it is important to examine the strength, because the direction of the levitation force is different for them from the repulsive system, and the difference is estimated to be harder for them. Comparing the above-mentioned merits and demerits, we have decided to choose the null-flux levitation system. We have appreciated especially the following two points, lower magnetic drag force and larger levitation force.

2.2 Double-layered propulsion coil system

At Miyazaki test track, propulsion coils are single-layered and are installed every120electrical angle on the both side walls. This type is simple, but it is evident that a large harmonic magnetic field produced by these coils induces a considerable temperature rise inside the superconducting magnets. It does not matter for short run time operation like Miyazaki test vehicle, but for long revenue service line, the operation will be difficult if the heat generated inside the superconducting magnets is larger than the capacity of on-board refrigerators which must keep the superconducting coils at a very low temperature.

So, we have decided to improve the magnetic fields produced by propulsion coils, by using bigger coils and a double-layered installation. One coil covers 180electricalangle, and coils are installed at 120 pitch along the side wall. Fig.2 is an image of the composite magnetic field produced by those double-layered coils. The harmful change of magnetic field is reduced remarkably. As the distance from the superconducting magnet is different between the front coil and the back one, it is necessary to adjust the turn number of coils between them.

3.Preliminary tests at Miyazaki Test Track

In order to check this new electro-magnetic guideway structure, we have remodeled Miyazaki Test Track. Repulsive levitation coils of about 2.0 km have been changed to null-flux type. There has not been observed any large difference between null-flux levitation and repulsive levitation concerning the vehicle’s levitating condition. On the contrary, we can observe a clear difference of drag force between the two systems .

As for propulsion coils, about 1 km of single-layered coils has been replaced by double-layered ones. Accordingly, at this section, triple-layered coils are installed along the both side walls, which are null-flux levitation coil and double-layered propulsion coils. We’ve confirmed the effect of propulsion coil’s new structure on the superconducting magnets’ vibration reduction .

4.Coils for Yamanashi Test Line

4.1 Levitation coil

As mentioned above, we have adopted a null-flux levitation system on Yamanashi Test Line, so coil circuit is “8”-figured, and two “8” circuits are molded into one levitation coil. This coil works not only as levitation coil, but as guidance one.

The cross-sectional area of the aluminum coil conductor is so determined as to producesufficient levitation and guidance forces. Considering the eddy current loss, the conductor is divided into 48,that is, parallel winding 24 turns. Each coil has four terminals for null-flux cables connection.

Levitation coil is not a high voltage equipment, but it is required to bear large forces of levitation and guidance. So, we should consider a larger mechanical strength than electrical insulation. Levitation coil is molded out of GFRP. We have adopted non-saturated polyester SMC (Sheet Molding Compound).(Fig.3)

4.2 Propulsion coil

Propulsion coil is required to endure a linear motor’s high voltage and the reaction force of propulsion at the same time. At Miyazaki Test Track, original propulsion coils are molded out of SMC, but on Yamanashi Test Line the propulsion coil’s voltage is higher, so they are molded out of epoxy resin which is widely used for the high voltage machines like switch gear or mold transformer. In high voltage machines, the terminal tends to be big, but in propulsion coils, there is not a space for a big terminal, and as there are a huge number of terminals along the guideway, cable-connecting should be quite simple work. Considering these points, we have developed a simple and compact one-touch-terminal (connector).

On Yamanashi Test Line, the northern line is fed with 22 kV, and southern line with 11 kV. So there are two types of propulsion coils according to the voltage level.

As for northern line coils, the turn number of winding is 8 for front coil and 10 for back coil. For southern line coils, they are 7 and 8 respectively.(Fig.4)

　

5.Coil installation

To reduce the coil’s level adjustment work, we have developed “beam” system. A beam is 12.6m long, and levitation coils and propulsion coils are set on one side of it. Instead of individual coils’ adjustment, beams are adjusted to arrange guideway. Also “panel” system is adopted in some section, the aim of which is similar to the beam.

So there are three types of side walls in Yamanashi Test Line, that are beam, panel and ordinary side wall.

5.1 Levitation coils installation

Levitation coils are installed on the both side walls at every 0.9 meters. One coils fixed on the wall with 8 bolts, which stand guide force and drag force.

For levitation force, which is very large, each coil has a projection on their back and it sits on the corresponding projection of the side wall, thus the coil is supported vertically. Every coil of both side walls is connected to each other by null-flux cable to produce a guide force.

5.2 Propulsion coils installation

To reduce the harmonic magnetic field, propulsion coils are set on the side wall in double. Front coils and back coils are installed at every 1.8 meters respectively.

At the end of beam (or panel), short “end coil” is installed to prevent a propulsion coil from spanning two beams.

We adopted the following 3 types of propulsion coil installation on Yamanashi Test Line. We will compare and assess these 3 types through the installation work and the running tests.

1) Fixed-with-bolts type

Each coil has flanges for bolt fixing. This type have been tested at Miyazaki Test Track.

2) Fixed-with-fastener type

Specially designed FRP fasteners ( we call it “partial spacer”.) pin down the coil, and the fastener is fixed with bolts on the wall.

3) Unit type

Double-layered propulsion coils are set in the FRP made vessel ( we call it “whole spacer”.), then a levitation coil is set over it to make a “unit”. The units are fixed with bolts on the wall.

　

6.Study for cost down

6.1 Single-layered propulsion coil

When we determined the basic design of Yamanashi Test Line, it was necessary to adopt the double-layered propulsion coil structure, but recently, superconducting magnets have been so improved that it has become possible to adopt single-layered propulsion coil structure. So, it has been decided to introduce it partially on Yamanashi Test Line. Single layered propulsion coil structure has many merits over thedouble-one such as,

1) coil length is shorter, consequently cheaper

2) it doesn’t need many kinds of coils ( front coil, back coil and end coil)

3) installation work is more simple, consequently cheaper

6.2 Reaction injection molding coil

To reduce the manufacturing cost of levitation coil, we are now developing RIM method. Dicyclo pentadiene, that is the major component in RIM, is injected as a low viscous liquid into the mold. It reacts and hardens in a short time, producing a suitable substance for levitation coil. As the molding process doesn’t require high pressure and high temperature, the molding equipment is inexpensive, and the insulation scheme of the wound coils can be simplified.

7.Conclusions

We have introduced a new electro-magnetic guideway structure and several ways of coil fixation for Yamanashi Test Line. The installation work of ground coils has already been finished. Through the running test and the bench tests, we will compare and assess the many types of coils. And besides, we will make efforts to reduce the coil cost. The development of the ground coil has been financially supported by the Ministry of Transport of Japan. Also, a part of this development was performed by Central Japan Railway Company.

References