Cryogenic converter for superconducting coil control

A two-stage conversion system is described for regulating the DC current in a superconducting coil, for example, the field winding of a superconducting synchronous machine. An ambient temperature converter provides a low, controlled current to a cryogenic converter, co-located with the superconductor, which steps up the current by over a hundred times to supply the superconducting coil. An array of parallel-connected metal oxide field effect transistors (MOSFETs) minimise losses in the cryogenic converter and a low-frequency, self-oscillating control technique is used for the DC link which minimises the capacitor requirements and ensures orderly switching conditions. Closed-loop control of the superconductor current is achieved through the ambient temperature converter, with current sensing power MOSFETs used at low temperature for current measurement. Practical results including measurements made on a superconducting machine are used to illustrate the system performance.