High-speed solid rotor technology has proven itself useful in nearly every high-speed application. High-speed solid rotors can be used in any application where the load can be connected directly to the electrical machine without a gearbox and the circumferential speed of the rotor is higher than 100 m/s.
Developed to address the existing gap between applications running below 200 kW and those running at multi-megawatt powers, solid rotors perform better than laminated rotors at higher speeds due to the mechanical and thermal limitations of laminated rotors.
Applications requiring high power density, high reliability and robustness, as well as those operating in harsh environments, find high-speed solid rotors especially helpful.
Characteristics of high-speed solid rotors
Solid rotors are robust, efficient and reliable. Their characteristics include:
- Direct drive system with no gearbox
- More compact size
- Fewer components, requiring less maintenance
- Speeds from 6,000 rpm to 25,000 rpm, depending on rotor diameter
- Powers from 300 kW to 2,000 kW for low-voltage systems
- Power levels above 2,000 kW for medium-voltage technology
In addition, their high effective bending stiffness achieves subcritical rotor dynamic behavior at high speeds, while their stiffness and balancing class are stable and not sensitive to thermal cycling. These characteristics make high-speed solid rotors extremely useful.
Current uses
The industries using high-speed solid rotors today include manufacturing, transportation and renewable energy.
The chemical and oil and gas industries use solid rotors in gas compressors and turbines. Solid rotors are also used in aircraft in auxiliary power units and in electric vehicles in propulsion subsystems. A partial list of other applications varies from hermetic natural gas compression to vacuum pumps, microturbines, machining tools, pneumatic actuators, sandblasting, fermentation, instrumentation and air polishing.
Need for higher speeds, higher power density
Traditionally, high-speed solid rotors have operated in speed ranges from 10,000 to 30,000 rpm. Today, there is an ever-increasing demand for higher speeds and higher power densities.
Spindle applications such as drilling, grinding and milling are looking for high-speed machines. Conventional spindles use belt drives, which limit their maximum speed. Speed and power requirements range widely in spindle applications, from 9,000 to 180,000 rpm and 1 to 24 kW.
Flywheel energy storage systems (FESSs) are becoming a viable alternative to traditional chemical battery systems and can benefit from high-speed solid rotor technology. FESSs are classified as either low speed (< 6,000 rpm) or high speed (10,000–100,000 rpm).
Turbomolecular pumps with rotational speeds up to 100,000 rpm, used to produce high vacuums in high-tech applications, are also benefiting from high-speed solid rotors. Some examples include film deposition, semiconductor manufacturing, high-energy physics, optical/glass industry, mass spectrometry and fusion technology.
All these applications need rotors that can meet their requirements. As new rotor designs are developed to handle these higher speeds and power densities, more uses will undoubtedly emerge.
Developments at The Switch
The Switch has been developing high-speed solid rotor technology for many years. The latest development work met the requirements of industrial-scale high-temperature heat pumps for higher speeds as part of the Business Finland-funded research project, “Novel Heat Pump MW-class Compressor Motor” (9119/31/2021).
The performance of solid rotors has improved with axially slitted rotors, rotors with copper bars, with the use of end rings and copper coating. Copper, and especially the copper cage, reduces rotor resistance, which in turn decreases slip and improves flux penetration. This results in a higher power factor for the system.
Currently, The Switch high-speed solid rotors in the multi-MW class reach a maximum continuous rotor circumferential speed of 200 m/s and an overspeed of even 240 m/s, while tolerating rotational velocity and temperature fluctuations. The circumferential speed of smaller machine sizes ranges from 130 to 178 m/s, depending on the copper material used in the end ring, but these sizes are currently available only for the S1 duty type and not for applications with rotational velocity and temperature fluctuations. These circumferential speeds result in rotational speeds between 20,000 and 25,000 rpm, depending on the machine size.
Spindle-, flywheel- and turbomolecular pump applications are currently outside the scope of The Switch high-speed machine portfolio due to their very high rotation speed requirements. However, we are always pushing the boundaries and investigating new ways to meet customer requirements for higher speeds and power densities.
It’s exciting to consider the future possibilities for high-speed solid rotors. We look forward to being part of progress in this field!
Feel free to get in touch to discuss your high-speed rotor needs or ideas.
R&D Project Manager
Jonna Tiainen
Jonna Tiainen currently works as R&D Project Manager at The Switch, Finland. She has a background in turbomachinery and over a decade of experience in Computational Fluid Dynamics, measurements, project management and teaching. Tiainen holds a D.Sc. (Tech.) degree in heat transfer and fluid dynamics from the LUT University, Finland.
References
Bilek, V. et al. (2025) A comprehensive overview of high-speed solid-rotor induction machines: Applications, classification, and multi-physics modeling. International Journal of Electrical Power & Energy Systems, Vol. 166, 2025, 110520, doi: 10.1016/j.ijepes.2025.110520.
Choudhury, T. et al. (2022) Design of Thick-Lamination Rotor Configuration for a High-Speed Induction Machine in Megawatt Class, 2022 International Conference on Electrical Machines (ICEM), Valencia, Spain, 2022, pp. 738-744, doi: 10.1109/ICEM51905.2022.9910880.
Gerada, D. et al. (2014) High-Speed Electrical Machines: Technologies, Trends, and Developments, IEEE Transactions on Industrial Electronics, vol. 61, no. 6, pp. 2946-2959, June 2014, doi: 10.1109/TIE.2013.2286777.
Ortiz, F. et al. (2024) Analysing the performance of a High-Speed Solid Rotor Induction Machine with Copper Coating. 2024 International Conference on Electrical Machines (ICEM), Torino, Italy, 2024, p. 5, doi: 10.1109/ICEM60801.2024.10700235.
Pyrhönen, J. et al. (2008) High-speed, 8 MW, solid-rotor induction motor for gas compression, 2008 18th International Conference on Electrical Machines, Vilamoura, Portugal, 2008, p. 6, doi: 10.1109/ICELMACH.2008.4799819.
Pyrhönen, J. et al. (2013) Design of Rotating Electrical Machines. John Wiley & Sons. p. 612. doi: 10.1002/9781118701591.
Yi, J. & Zhu, Z. (2025) Design and analysis of electromagnetic and mechanical structure of ultra-high-speed slotted solid rotor induction motor. Sci Rep 15, 2386 (2025). https://doi.org/10.1038/s41598-025-86405-0.