Sumitomo Electric Industries Ltd unveiled a prototype vehicle equipped with a superconducting motor at a press conference June 12, 2008 (See related article). The motor is a claw-pole DC motor that uses a superconducting coil as a stator and copper wire as a rotor.
The company pointed out that the main purpose of the latest development is to make people aware that the use of superconducting coils is more closely related to our everyday lives, now that rail cars such as linear motor cars, superconducting cables and shipboard superconducting motors are being commercialized.
The prototype motor is not an AC motor and the superconducting coil is not used as a rotor because the company wanted to produce a test-ride vehicle as soon as possible. In view of safety, the superconducting coil used as a stator is only supplied with a current of up to 40A, although it is able to pass currents higher than 100A, according to the company.
A 240m long bismuth (Bi) wire rod, namely Sumitomo Electric's DI-BSCCO, is used for the superconducting coil. The wire rod is 4mm wide and 0.2mm thick.
The superconducting state is induced by cooling the stator with a 4-liter liquid nitrogen tank installed on the motor. With 4L of liquid nitrogen, the vehicle is reportedly able to run for approximately two hours while maintaining the superconducting state. The superconducting coil is monitored by a thermocouple or a current sensor so that power supply is cut off before the superconducting state is broken due to temperature rise.
The prototype vehicle is based on the Toyota Comfort. The engine was replaced with a motor and the motor was connected to a manual transmission system. The motor, measuring about 300mm in diameter and about 400mm in length, has an output of 31kW (at 3,000rpm).
It is powered by 12 units of 12V lead-acid batteries connected in series, which are used to increase the voltage to 144V. In the test drive, the manual transmission was set to the fifth gear while the vehicle was running because the motor has a sufficient torque even at a low motor speed.
As superconducting motors are on the way to being commercialized in vessels, Sumitomo Electric expects demand from the ground transportation industry as well. When the superconducting motor is used in electrically driven vehicles such as hybrid cars, plug-in hybrids and electric vehicles, the motor size and the voltage level can be reduced, according to the company.
The company hopes to incorporate the latest motor in industrial vehicles such as buses within the next 10 years. Meanwhile, the company predicts that it may take a longer time to equip passenger cars with the motor because they are less frequently used compared with industrial vehicles.
For industrial vehicles, it is conceivable to at first adopt a driving model where the vehicle carries liquid nitrogen in an amount sufficient to cool the motor for just one day, instead of installing a refrigerator, the company said.
The company believes that compact refrigerators will be small enough to fit in passenger cars as a result of future improvement, thereby enhancing the usability of the motor. Moreover, if the motor is combined with fuel cell vehicles that run on liquid hydrogen, the superconducting motor can be cooled at a lower temperature by using liquid hydrogen than by using liquid nitrogen, the company said. As a result, the amount of current that can be flown through the superconducting coil will be increased.
In fact, the existing coil will be able to pass a current of up to 1,000A by cooling it at 20K (-235°C) with liquid hydrogen, whereas the amount of current flow is limited to about 200A when cooled by liquid nitrogen, the company said. This reportedly contributes to the reduction of motor size and the enhancement of output.
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