Keywords: train application, high speed trains, electrical drive, permanent magnet machines, synchronous machines, permanent magnet-electrical excitation.

The benefits of hub motors are always central to the discussion concerning the choice of the most appropriate drive concept for new high speed trains. This is especially true for low-floor vehicles or double deck vehicles where it is likely that the space, which is now taken by the chassis, will be used as a walkthrough area. The distribution of the drive power between several axles represents another advantage. Finally, the electric high-speed railway traffic is an environmentally friendly alternative to the national air traffic. The German Aerospace Center, especially with its Institute of Vehicle Concepts, is working on coordinating the special research disciplines which are involved in the development of a high-speed double-floor train concept.

A double-floor train concept has two power cars and ten centre wagons. In this concept there is not much space in the axial direction for installing an electric motor near to the wheel. This means that the electric motor has to be placed within the wheel. The motor should have a disk-shaped structure. This is a technically unfavourable condition compared to the electric drive motors used today. It results in the design of ring-shaped machines with high pole pairs because of the thin iron cores in the radial direction and normally in large winding ends.

In the example, given in this paper, one wheel has to deliver a power of 250 kW, which is based on a torque of a little more than 1 kNm. The starting torque is greater by a factor of 4 (4400 Nm).

The machine is a combined permanent magnet electrically excited synchronous machine. Through the use of electrically field winding connected in parallel to the magnetic potential of the permanent magnets, the rotor iron core of each pole no longer carries the whole excitation flux of one pole. Ideally, they need to carry only half of the flux, so they are only half as thick. This is an advantage for such a ring-shaped machine with its small iron yokes. The magnetic flux of the magnets adds to the flux of the field winding and is thereby generated where it is needed, near to the air gap. In the electrically unexcited case, the flux of the permanent magnet closes briefly through the rotor yoke and is therefore magnetically invalid to the outside. This can result in a very large field weakening area without additional current heat losses, a significant advantage over pure permanent magnet excited machines.

In this paper the essential calculations are described and the advantages and disadvantages of the concept are discussed. The result is a concept proposal for an electrically and permanent magnet excited synchronous machine which may be used as a wheel hub motor for a high-speed train application.

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