# Microstructure, Magnetic Properties, and Application of FINEMET-Type Alloys with Co Addition
## 3 Results and Discussion
Fe\({}_{58}\)Co\({}_{25}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) alloy shows the lowest core losses and the best thermal stability of core losses. The stability of magnetic properties at high temperatures is required in devices operating at an H-class temperature. Figure 4: Hysteresis loops for Fe\({}_{83-x}\)Co\({}_{x}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) (x = 6 (a), 25 (b), and 58 (c)) alloys annealed at 750 K for 10 min. Figure 7 shows a schematic of a DC/DC converter in the interleaved topology used in the power supply systems of the rail vehicle. Three single-phase chokes operate in the output filter of the converter. The converter operates in a sealed enclosure on the roof of the rail vehicle. For this reason, reducing the weight and loss of inductive components are important design criteria. Choke designs were made for three different core materials. A well-known Fe\({}_{33}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) alloy was taken as the base material. Then, comparative designs were made for alloys with different cobalt contents. The design input assumptions are shown in Table 2, and they characterize the electrical forcing at the choke operation point and their operating conditions. Figure 5: Core losses of Fe\({}_{83-x}\)Co\({}_{x}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) (x = 0 (a), 6 (b), 25 (c), and 58 (d)) alloys annealed at 750 K for 10 min measured for the 1 kHz frequency. Figure 6: Core losses (P) as a function of temperature for Fe\({}_{83-x}\)Co\({}_{x}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) (x = 0 (a), 6 (b), 25 (c), and 58 (d)) alloys annealed at 750 K for 10 min. The chokes shown in Figure 7 are the output filter of the DC/DC converter. The output current of the converter flowing through the chokes contains mid-frequency current harmonics in the 8-40 kHz range, in addition to the main DC component. The dominant one is the 8 kHz component, which has the highest amplitude and causes the highest losses in the choke core. Amplitudes and frequencies of the current harmonics have a decisive influence on the value of choke losses. The designed chokes are based on a UI-type core in block design with additional air gaps (Figure 8). Air gaps introduced in the columns of the core make it possible to achieve adequate inductance and linearity of the inductance characteristics during overloads throughout the design and subsequent calibration of the choke. Efforts have been made to minimize the width of the introduced slots due to the intense losses associated with flux dissipation around the slots [28]. A CoreECOTM bonded multigap choke core was designed [29]. The core would have eight slots in each column. The slots would be filled with glass-epoxy spacers. The stiffness of the core structure would be ensured by flat aluminum column bars placed on the face of the core column. The column flat bars, slotted spacers, and core blocks would be joined with high-temperature-resistant epoxy adhesive. The fill factor of the nanocrystalline iron core is assumed to be 85%. Basic criteria for the selection of the magnetic material of the choke core are the lowest possible loss and a relatively high saturation induction (Table 3). Inductive components in power electronics are designed in temperature classes in which the choke insulation materials used can operate permanently. These are usually class F (T\({}_{\rm max}\) = 155 \({}^{\circ}\)C) and class H (T\({}_{\rm max}\) = 180 \({}^{\circ}\)C). The temperatures listed are the long-term permissible temperatures during normal operation of the choke under rated conditions. During overloads, higher