A Theoretical and Experimental Study of the AC Losses of a High Field Superconductor and the Implications for Power Applications

摘要

A theory for the hysteresis loss of a high-field superconductor carrying ac is presented, based on the Riemersma critical state model (B x J = C). Voltage waveforms, energy loss per cycle, and limiting values of effective resistivity are derived. For sinusoidal current of frequency f in 10-mil wire the high (critical) current limit of resistivity is found to be 4.06 x 10 to the -14th power f (ohm-meter) compared to 0.93 x 10 to the -16th power f for a current peak of one-tenth the critical current; both values are independent of the high-field material used. An equivalent circuit consisting of a variable inductance is proposed for the hysteretic superconductor and three possible definitions of the inductance are discussed. In any case, the inductance is nonlinear, a transcendental function of the current, the function being different on the four segments of a hysteresis loop. The high-current limit of the absolute, internal inductance (the preferred definition) per unit length is mu sub zero/3 pi (henry/meter). Experiments were performed at 4.2K on bifilar windings of 10-mil, Nb25%Zr wire, both bare and copper plated. Currents up to the critical value were used at frequencies of 60, 400, and 800 Hz. The feasibility of applying superconductivity to two classes of ac power equipment is discussed: (1) conventional apparatus such as transformers and rotating machines, already highly efficient, and (2) low-voltage static inverters which are not so efficient. A novel circuit is presented for a superconducting inverter or oscillator. (Author)