Optimal-Airgap Control of a Linear Induction Motor for Korean Railway Transit 1

There have been many researches to develop linear induction motors for railway systems for last three decades. However, most of them are regarding design of the primary or secondary reaction plate to increase the performance of the system. Control of the airgap length has been a tough topic because of lack of technology, expensive construction cost, and so on. In this research, the optimum airgap length of a linear induction motor used for Korean railway transit has been derived by using an analytical method and verified by simulations. Moreover, feasibility of the optimum airgap control which is incorporated with actuators has been done. Introduction As demands for cost-effective, less susceptible to weather, and reliable means of public transportation have increased, Korean government has taken into account the linear railway system since the late twenty century. The linear railway system is called the non-adhesion drive system, where as conventional railway system is called the adhesion drive system which gets the propulsion force from the friction between wheel and rail (wheel-on-rail type). The non-adhesion drive system has lots of advantages over the adhesion derive system as follows; (1)Excellent acceleration and deceleration, (2)Capability of climbing steep gradients, (3)Lower construction cost due to the small tunnel crosssection, (4)Enable flexible route planning(travel through sharp curves), (5)Less susceptibility to weather conditions, (6)Quiet and smooth running(No mechanical couplings), (7)Less maintenance cost. Consequently, the linear railway transit is already in use in all over the world and continuously being extended. Linear railway system gets the propulsion force from a linear motor different from a conventional rotary motor and it produce thrust force electromagnetically without any contact. Therefore, it does not use any mechanical coupling for the rectilinear movement. Figure 1 shows the concept of a linear motor derived from a rotary motor. A linear motor is a conventional rotary motor whose stator, rotor and windings have been cut open, flattened, and placed on the guideway. Among many kinds of linear motors, linear induction motor(LIM) is prefer for the urban railway transportation system because the construction cost is much cheaper than linear synchronous motor(LSM), even though the efficiency and performance is little bit poor. The operating principle of a LIM is identical to a rotary induction motor. Space-time variant magnetic fields are generated by the primary part across the airgap and induce the electromotive force (EMF) in the secondary part, a conducting sheet. This EMF generates the eddy currents, which interact with the airgap flux and so produce the thrust force known as Lorenz’s force. There are two types of LIM as follows; (1) Short primary type (SP): stator coils are on board and conducting sheets are on the guideway, (2) Long primary type (LP): stator coils are on the guideway and conducting sheets are on board. For the LP type, construction cost is much higher than SP type but it does not need any current collector for operation. In high speeds, the LP type is usually used because transfer of energy using a current collector is difficult. On the other hand, the SP type is very easy to lay conducting sheets on the guideway and thereby reducing construction costs. However, the SP type has low energy efficiency because of the drag force and leakage inductance caused from the end effect. In general, the SP type LIM is applied for the low-medium speed urban lightweight transit[1]. Figure 2 represents the configuration of a SP type linear induction motor. Even though the operating principle is exactly the same as a rotary motor, the linear motor has a finite length of the primary or secondary parts and it causes static and dynamic end-effect which is the discontinuous airgap flux phenomenon. This end-effect causes the deterioration of the system performance, especially in high-speed operation. Another problem is that construction tolerance restricts the minimum airgap in order to prevent a collision between the primary part and the secondary reaction plate. Moreover, as the airgap length is getting smaller, the attraction force between the primary and secondary parts is getting larger dramatically and the attraction force would be another friction against propulsion. Therefore, large airgap is inevitable. However, this large airgap is one of the biggest problems in the system efficiency. Figure 1: Concept of a linear motor from a rotary motor [2]. Figure 2: Configuration of a SP type linear induction motor. Optimum airgap The large airgap(around 9 ~ 15 mm) increases the reluctance and leakage flux of the magnetic circuit, and decreases the magnetizing inductance as shown in Fig. 3. Consequently, much more excitation current is required. The increased excitation current results in significant decrease of power factor and efficiency. Therefore, it is necessary to figure out the optimum airgap length and develop a control unit such as a rapid sensing unit, interface algorithm, and an airgap controller in order to control the airgap length, consequently accomplish the high-efficiency system and energy savings. Figure 3: Magnetizing inductance according to variation of the airgap length. Typical variation of the normal force and thrust force of a LIM can be derived as in (1) and (2), based on [3]. ) 18 . 2 ( ) 58 . 8 ( ) 334 . 1 ( 151 45 . 0 82 114 52 . 4 ) ( e g g g normal e e e g F − − − − + + = (1) ) 06 . 1 ( ) 163 . 0 ( ) 81 . 5 ( 143 66 . 9 6 . 14 8 . 109 15 . 9 ) ( e g g g thrust e e e g F − − − + + + = (2) Where g is an airgap length. The novel function H(g) is derived to figure out the optimum airgap length which maximizes the thrust force and minimizes the normal force. ) ( ) ( ) ( g F g F g H normal thrust = (3) Figure 4 shows each force and H(g) function according to variation of the airgap length. From the figure, the thrust force is getting smaller with the airgap increase and the H(g) is small below 5[mm] that is, the attraction force is relatively big to the thrust force. And above 11[mm], the thrust force is too small even though the H(g) is little bit increased. Therefore, the optimum airgap length is around 5[mm]. 0 500