# Assembly and preliminary tests of a 250 kW partially superconducting machine

Rémi Dorget, Walid Dirahoui, Jean Lévêque, Thierry Lubin, Kévin Berger,

Mathis Durand, Adrien Cipriani, Paul Gning, Christophe Viguier

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Rémi Dorget, Walid Dirahoui, Jean Lévêque, Thierry Lubin, Kévin Berger, et al. Assembly and preliminary tests of a 250 kW partially superconducting machine. ISS 2025 - 38th International Symposium on Superconductivity, Dec 2025, Nagasaki, Japan. pp. 1-5, id. AP6-03. hal-05449154

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# Assembly and preliminary tests of a 250 kW partially superconducting machine

* R. Dorget⁴, W. Dirahoui³, J. Lévêque³, T. Lubin³, K. Berger³, M. Durand³, A. Cipriani², P. Gning¹, C. Viguier¹

Safran Tech, Electrical & Electronic Systems Research group, Rue des Jeunes Bois, Châteaufort, 78114

Magny-Les-Hameaux, France¹

Absolut System, 3 rue Raymond Chanas, 38320 Eybens, France²

GREEN, Université de Lorraine, F-54000 Nancy, France³

Airbus UpNext SAS, Toulouse, France⁴

## Introduction

This paper introduces the superconducting machine demonstrator developed within the FROST project (Flux-barrier Rotating Superconducting Topology), launched in 2020. The prototype is a 250 kW partially superconducting device that integrates three main subsystems:

* Rotor composed of HTS bulk superconductors (Figure 2).

* A stationary HTS coil, energized with direct current, to generate a strong magnetic field (Figure 3).

* A three-phase copper armature, wound with Litz wire and positioned symmetrically on either side of the rotor (Figure 4).

The inductor's static section, built around the superconducting coil, establishes an axial magnetizing field. Inside the coil bore, the rotating part is formed by superconducting bulks. These elements act as magnetic flux shields, producing a non-uniform field distribution: the flux density remains relatively low in the vicinity of a pellet but reaches higher values in other regions, ranging from near zero up to $B_{maximum}$.

This spatial modulation of flux creates a homopolar component that contributes to the overall machine excitation. The armature relies on conventional copper three-phase windings, designed without iron teeth in order to reduce losses and improve performance. The topology is inherently brushless: only the superconducting bulks rotate, while excitation remains controllable through the DC current applied to the stationary coil.

Such a concept has been explored for several decades, often through small-scale prototypes built mainly in radial-flux configurations [1]. Since the launch of the FROST initiative in 2020, efforts have shifted toward the development of a larger 250 kW demonstrator. Activities have included advanced
modelling [2], detailed design, and optimization of the machine layout. The long-term aim of the project is to scale up from the initial 40 kW prototype to a robust 250 kW superconducting machine, paving the way toward practical applications of this novel topology.

In order to reach this objective, several design improvements were implemented. The BSCCO tape originally used in the static coil has been replaced with RE-Ba-Cu-O tape, offering higher performance and improved operational margins. The geometry of the superconducting screen was also revised: instead of a circular profile, a trapezoidal shape was adopted to align more closely with the contours of the armature windings. In parallel, a liquid-cooling system was integrated into the armature to enhance thermal management and maintain stable operating conditions under higher loads.

Furthermore, the experimental program includes the preliminary magnetization of the superconducting bulks by means of the static coil. This strategy couples flux modulation with bulk magnetization, effectively suppressing the homopolar component of the magnetic field. As a result, the flux density can alternate between +$B_{maximum}$ and -$B_{maximum}$, thereby enabling the machine to potentially deliver twice the power output compared to its initial configuration.

## Experimental Investigations on the Cooling, Performance, and Equivalent Modeling of a Superconducting Coil at 30 K

The first stage of the experimental campaign is devoted to the characterization of a superconducting coil under cryogenic conditions at 30 K. This intermediate temperature is particularly attractive for high-temperature superconductors (HTS), offering enhanced critical current while avoiding the complexity of liquid helium. The experiments aim to assess cooling behavior, electromagnetic performance, and equivalent modelling as preparatory steps toward system-level applications.

## Cooling and Thermal Conditions

Reaching 30 K requires an efficient cryogenic system combining a cryocooler, thermal links, and adequate insulation. Temperature sensors distributed along the coil ensure uniform cooling and prevent harmful gradients. Once steady-state operation is established, the correlation between thermal stability and electromagnetic response is evaluated, providing insight into the effectiveness of the cooling design and its impact on coil performance.

## Hall Effect Instrumentation

To capture magnetic dynamics, the superconducting pellets are equipped with Hall effect probes. These sensors, calibrated for cryogenic operation, measure local flux density with high precision. Their distributed arrangement enables mapping of flux penetration and redistribution, offering a detailed picture of the magnetic environment. The instrumentation is critical for linking applied excitations
with the coil's internal field distribution.

## Shielding Verification

The superconducting state is further examined through shielding experiments. By applying external magnetic fields and recording the Hall probe signals, the bulk's ability to screen magnetic flux is quantified. The results confirm efficient shielding, validating both superconductivity and structural homogeneity. It is crucial to take this step before subjecting the coil to increased electrical stress.

## No-Load and Short-Circuit Tests

The second experimental phase addresses operational performance. No-load tests determine intrinsic parameters such as inductance and flux distribution without external constraints. These experiments reveal the coil's current-carrying capacity, dynamic stability, and loss mechanisms. The Hall probes provide real-time information on flux redistribution during these regimes.

## Equivalent Circuit Model

Data collected from the above tests are analyzed to establish an equivalent circuit model of the superconducting machine. Parameters such as inductance, residual resistance linked to flux motion, and effective losses are extracted. This model provides a simplified yet accurate representation of the coil, supporting predictive simulations and enabling direct comparison with conventional copper-based systems.

## Conclusion

The results demonstrate that the system can be cooled and reliably operated at 30 K, exhibiting strong shielding and stable performance under both steady-state and transient tests. Hall effect instrumentation proves essential for magnetic diagnostics, while the equivalent model offers a valuable design tool. These findings confirm the potential of HTS coils for advanced electrical machines and pave the way for further integration studies.

**Acknowledgement:** This work is supported by "Direction Générale de l'Armement" and the company SAFRAN Tech and ANR FROST - ANR-19-ASMA-0001

## References

[1] E. H. Ailam, D. Netter, J. Leveque, B. Douine, P. J. Masson, and A. Rezzoug, "Design and Testing of a Superconducting Rotating Machine," IEEE Transactions on Applied Superconductivity, vol. 17, no. 1, Art. no. 1, Mar. 2007, doi: 10.1109/TASC.2006.887544.

[2] A. Colle, T. Lubin, S. Ayat, O. Gosselin, and J. Leveque, "Test of a Flux Modulation
Superconducting Machine for Aircraft," J. Phys.: Conf. Ser., vol. 1590, p. 012052, Jul. 2020, doi:
10.1088/1742-6596/1590/1/012052.

[3] R. Dorget, T. Lubin, S. Ayat, and J. Lévêque, “3-D Semi-Analytical Model of a Superconducting Axial Flux Modulation Machine,” IEEE Transactions on Magnetics, vol. 57, no. 11, Art. no. 11, Nov. 2021, doi: 10.1109/TMAG.2021.3108632. Example reference – replace or delete

Acknowledgment

The authors would like to thank the Direction Générale de l'Armement (DGA), the Agence de l'Innovation de Défense (AID), and the Agence Nationale de la Recherche (ANR).

Figures

<figure><img src="image_2.png" /><figcaption>Figure 1. Representation of the active components of a flux modulation machine in its axial flux form.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 1. Picture of the assembled rotor</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2. Picture of the HTS coil</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4. Picture of an assembled cooling carter of the armature</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5. Picture of the assembled machine</figcaption></figure>

**Keywords:** Axial Field Machine, High Temperature Superconductors, Superconducting Motor, Synchronous Machine