If the MG has permanent magnets, these can be held in compression without need of a containment ring and the magnetic airgap can be reduced. The external rotor design, Fig. 11.6-2(B), has been used particularly for composite rotors since it is convenient to place such a machine inside the hollow rotor. On the other hand, if such a machine is already in volume production, costs should be lower. It may be attractive to adapt an existing machine for use in a flywheel and it will operate in a similar way apart from its vacuum atmosphere which creates challenges for rotor cooling. The most common topology shown in Fig. 11.6-2(A) denoted “conventional” has an external active magnetic surface and is the topology of choice since the vast majority of electric machines are designed this way. Typical values for J are between 10 kN/m 2 and 20 kN/m 2 and maximum peripheral speeds can be up to 200 m/s.
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