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Description
There is growing interest to use superconductors for actuator applications. High-field superconducting electromagnets enable a much higher performance. DC-excited stator coils in such systems are subjected to time-varying magnetic fields of up to several hundred mT, leading to significant AC loss. Inherently stable no-insulation coils make motion systems more reliable, but come at the cost of even higher AC loss. Here we focuss on the AV loss in such NI ReBCO coils, determining them both numerically and experimentally.
We developed a specific test arrangement and measured the AC loss in 200 mm long and 20 mm wide NI ReBCO racetrack coils made of single 4 mm wide tape conductor at 4.2 and 77 K. The combined effect of a parallel AC magnetic field with an amplitude of up to 100 mT and a DC transport current up to 600 A was investigated. The loss was measured simultaneously with magnetic, electric and calorimetric methods. The results largely confirm the numerical model predictions, which were obtained a-priori from only the coil dimensions, its critical current and the effective time constant τ associated with radial current redistribution. For this research, the coils were solder-impregnated resulting in a relatively low turn-to-turn surface resistance of 1 μΩ∙cm^2 and a τ-value of approximately 65 s, corresponding to a characteristic frequency f_c of 2.4 mHz.
The AC loss of the coils is determined in the frequency range of 0.5 mHz to 1.25 Hz. For f<f_c, coupling currents dissipate homogeneously throughout the winding pack. Whereas for f>f_c, the effect of current concentration due to the skin-effect (P_ac∝√f) that is predicted by the model is also clearly visible in the data. A DC transport current in the coils further increases the AC loss as the induced currents cause a displacement of the transport current in the superconductor. Experiments at 4.2 K and at 77 K confirmed that the dominant loss contribution being the coupling loss can be readily predicted from the value of the time constant τ, which can be straightforwardly measured with a fast current discharge test.
Despite the inherent thermal stability of NI ReBCO coils, the significantly higher AC loss compared to insulated coils makes this technology a less viable option for high-dynamic applications such as the magnetic field coils in a linear motor system. However, an application-specific optimization of the turn-to-turn resistance, seeking a smart compromise between coil operational robustness and acceptable thermal budget, can make such coils a feasible option for high-dynamic motion systems.
