Frequency dependence of magnetic ac loss in a five strand YBCO Roebel cable

We have measured the frequency dependent magnetic ac loss in a five strand Roebel cable with 2 mm wide non-insulated strands and a transposition length of 90 mm. The cable is made from 12 mm wide SuperPower YBCO (yttrium barium copper oxide) coated conductor tape with a 20 μm thick surrounding copper stabilizer layer. The ac loss of the Roebel cable is measured in a sinusoidally varying external magnetic field in perpendicular orientation, with amplitudes up to 150 mT, in the frequency range of 30-200 Hz, and at a temperature of 77 K. No measurable loss is observed in the parallel orientation. In the perpendicular orientation, the cable loss is predominantly hysteretic in nature. The loss either increases or decreases with frequency, in different field ranges. To resolve the origin of the frequency dependence, we measure the loss in a single strand and in standard tape with similar copper stabilization. The cable and sum of strand losses converge in the high field limit, indicating that strands are not electrically coupled. We attribute the observed frequency dependent increase in loss to a sum of the eddy current loss in the copper stabilizer and the intrinsic frequency dependence of the hysteresis loss. Experimental results and finite element method calculated values for the frequency dependent contributions to the magnetic ac loss agree well with each other.