Study on Optimum Design of Linear Motor for Urban Transit

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Arch type test facility with the cylindrical rotor of single-sided linear induction motor (SLIM, 3-phase, 4-poles, 100kVA) was designed and constructed as the propulsion system for a ne urban transit. The test SLIM is driven by a pulse-width-modulate inverter (PWM Inverter, 3-phase, 150kVA, 1-50Hz, 40-440V). From the comparison between the measured results and calculated results, it was found as follows:1. As the SLIM which is installed for the purpose of the above operates with the low speed 40-50km/h and requires the large mechanical clearance over 10mm for the safety drive, the thrust decreases as the slip decreases under the constant voltage operation in any supply frequency.2. In the speed control with a constant ratio of the supply voltage and the frequency, the larger voltage is required to accelerate the motor speed constantly in the low frequency, compared with that in the conventional rotating induction motor, because of the large voltage drop by the large primary impedance.3. The flux density in the air gap is weakened at the entry end and considerably strengthened at the exit end in the small-slip region even in the low-speed SLIM, because of the end effect. As the large vertical force (attractive force) produces at the exit end of the motor, which makes the effective weight of the vehicle increase in the practical case, the method of reducing the vertical force is required.4. A study has been made on the design of the SLIMs for the urban transit with the maximum speed of 70km/h. The outline of SLIM design is given as follows: The primary core should be designed long, narrow and thin. The motor length of approximately 2.5m would be appropriate for about 40km/h SLIMs. The pole pitch is one of the most important dimensions also. The suitable value will be about 300mm. The flux density should be selected in relatively low value from 0.4T to 0.5T in order to decrease the vertical force as well as the magnetizing current.