High speed motors

The research has been carried in order to design two PM machines:

(1) a two-pole, 30 krpm, 1kW motor for a hand tool;
(2) a four-pole, 12 krpm, with flux-weakening capability generator for on-board energy generation on trucks.


For the first application, the required speed was 30 krpm and the motor power about 1 kW. Different topologies were investigated, including slotted and slotless stator, laminated and sinterized stator, Ferrite and NdFeB magnets. The geometry of each topology has been optimized taking into account thermal, mechanical and magnetic limitations of the materials. After this research, the most promising solutions have prototyped: a slotted machine and a slotless machine, both of them with a NdFeB magnet rotor. Flux density in various parts of the machine has been measured and compared with the analytically computed one, obtaining a good agreement. The slotted stator-laminated solution has been chosen and industrialized by the industrial customer.

For the second application, a flux-weakening capability was required. The maximum speed was fixed to 12 krpm. The maximum-to-base speed range is about 3.5. A further constraint is fixed by the maximum back EMF at the maximum speed, in order to prevent excessive voltage. The external dimensions were also fixed, so that an optimization of the rotor structure was necessary to satisfy the requirements. The machine rotor was optimized so as to achieve a high torque density, but with a limited torque ripple, to avoid vibration and acoustic noise. A first prototype of the electrical generator was manufactured and tested. The results are quite good, even if a better quality lamination should be adopted in order to reduce the iron losses at the highest speeds.

The activity on this topic moved then to the study of a 60-100 krpm PM generator to be applied in a microturbine system for electric energy generation. This was a research project financed by an University fund, involving also other research groups of the Department on different issues.

  • N. Bianchi, S. Bolognani and Fabio Luise, Analysis and Design of a PM Brushless Motor for High Speed Operations, in IEEE Trans. on Energy Conversion, Vol. 20, No. 3, September 2005.
  • N. Bianchi, S. Bolognani and F. Luise, Potentials and Limits of High Speed PM Motors, IEEE Trans. on Industry Applications, vol.IA-40, No.6, Nov.-Dec. 2004.
  • N. Bianchi, S. Bolognani, F. Luise, High Speed Drive Using a Slotless PM Motor, Proc. of 35th IEEE Power Electronics Specialists Conference, PESC'05, Aachen, Germany, 20-25 June 2004.
  • N. Bianchi and S. Bolognani, A 30-krpm 1-kW PM motor for hand-tool applications, in Proc. of 11-th International Power Electronics and Motion Control Conference, EPE-PEMC 2004, Riga, Latvia, 2-4 Sept. 2004.
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