CONVERTER

PROBLEM TO BE SOLVED: To obtain a prescribed resistance by AC half cycle or less by setting a combination of polarities that connect a first superconducting coil and a first thyristor switch and a combination of polarities that connect a second superconducting coil and a second thyristor switch the same. SOLUTION: A first superconducting coil 1 and a second superconducting coil 2 are on the secondary side and a coil 3 is on the primary side. The first superconducting coil 1 and the first thyristor switch 5 are connected in parallel, and the second superconducting coil 2 and the second thyristor switch 6 are connected in parallel. Then the combination of polarities that connect the first superconducting coil 1 and the first thyristor switch 5 and the combination of polarities that connect the second superconducting coil 2 and the thyristor switch 6 are permitted to be same. Thus, a prescribed resistance can be obtained by AC half cycle or less, and switching of a persistent current circuit using the superconducting coil can be performed at a high speed.