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Power losses are an important phenomenon in type-2 superconductors. Precise evaluation of power losses in superconducting coils is crucial for designing novel machines such as superconducting motors or generators. Although ac losses are relatively easy to measure with electrical methods, it is more difficult to measure power losses in the dc mode, which is induced by varying external magnetic fields, such as in a real operating environment. In particular, the problematic one could be the direct-drive wind generator, where several elements introduce nonsynchronous disturbances to the magnetic fields. Modeling had been carried out to estimate the power losses in second-generation high-temperature superconducting coils under various external dc or ripple fields; however, experimental work in the area is less common due to the difficulty of conducting experiments and special equipment requirements. In this paper, power losses under various magnetic fields are experimentally measured by the calorimetric method.