## 4 Test Results  
### AC Losses Tests of an Assembled Phase  
The solitary coil was tested on a sinusoidal current (according to the method from p. 2.1) and at modulated currents (according to the method of p. 2.2). The frequency of the main harmonic was 50, 100, and 200 Hz. The results are shown in Figure 6a,b. On these charts (and beyond) losses are presented in watts, as this makes it easier to understand what the increase in the load on a potential cryogenic system is from experiment to experiment. Figure 5: (**a**) Schematic representation of an annular winding with an inside-mounted rotor; (**b**) locking device for the rotor. 1—assembled machine with HTS windings, 2—fixed plank with 15 holes, and 3—arrow” device, fixed on shaft of the rotor. One can see that loss values for BLDC and BLAC modes are significantly higher even for low currents, and remain several times higher at maximum current amplitudes. For more clarity, on the Figure 7a,b, two periods of current are shown: BLDC and BLAC. Due to the concept of obtaining currents of this form, which is associated with chopping relatively high voltage using transistors [30; 31], a large amount of electromagnetic interference occurs, which significantly complicates the computer processing of experimental data. For a pure sine, 3 to 5 signal periods are sufficient to accurately determine the losses at a given current amplitude. However, for the currents shown in Figure 7a,b, the situation changes significantly. Figure 8 shows the dependence of the losses calculated on the period for three current levels. Moreover, the representation of the dependence of losses on the amplitude value of the current, in contrast to the pure sine, is difficult and ambiguous. In this regard, all the graphs in this paper, where the current form BLDC and BLAC appears, use the effective (RMS) current value. Figure 6: (a) Solitary coil losses at pure sine; (b) solitary coil losses modulated waveforms (BLAC, BLDC). Figure 7: (a) One period of BLDC current; (b) one period of BLAC current. On the graphs, there is a certain average value, with noise superimposed on it. If the calculation of losses was carried out for a small number of periods, this will lead to a large error. The high-frequency component (Figure 7) makes a significant contribution to the loss value since its amplitude is significant (about 10 A) and invariant to the amplitude of the main harmonic. The current in armature produces a magnetic field, which opposes the main field of machine and thus, will distort the main field of machine [32]. The frequency of the current ripple is not constant and floats between approximately 7 and 12 kHz, due to the transistor switching control algorithm. If we consider the form of the BLAC current (Figure 7b), the situation is similar to the case described in [33], where losses on a sinusoidal current were modeled with the addition of higher harmonics up to the 20 th. In our case, on the one hand, the ripple current has a frequency of at least 140 th harmonic (for 50 Hz fundamental), and, on the other hand, is not sinusoidal, but rather sawtooth. Thus, this signal also has its own higher-order harmonics. In comparison, for a constant frequency modulation mode, the current ripple can be reduced by adding a filter in the converter circuit, while the filter itself will have high weight and size characteristics. In our case, due to the floating frequency, the creation of such a filter is difficult; therefore, considering the above, we place the HTS coils in the most difficult operating conditions in terms of modulation. ### AC Losses Tests of an Assembled Phase  
The next step was to test the assembled winding. Now the single coils are combined into a system, while they are located on a common charged magnetic core made of 2412 steel. Figure 9a shows the direction of the current in the phase coils. The measurement scheme for sine current is similar to Figure 1, where _(L)_ now represents the assembled phase rather than a separate coil.