In order to study stability and a. c. losses in a. c. superconducting coils, the authors have developed a 50kVA coil and operated it at 50Hz. The coil was wound with an ultra-fine multifilamentary superconducting 42 strand cable. The conductor has 15367 NbTi filaments, 0.42µm in diameter with 1.1mm twist pitches. To prevent wire motions, the cable was embedded in grooves on coil formers and was wound under a large tension. The coil was quenched at 173A r.m.s., which was about 90% of the critical current, and central peak field was 1.54T. The measured a. c. losses were 2.2W/coil, when the coil was operated at 163A r.m.s., 1.45T. From a. c. loss measurements, it was shown that inter-strand coupling loss was very small, compared with inter-filament coupling loss.