connection. To simplify prototype fabrication, the machine employs stator steel laminations and some hydrostatic parts with standard sizes of existing products, and the corresponding parameters are shown in Table I.
Then this section concentrates on making tradeoffs of some significant parameters including permanent magnet sizes as well as oilgap length, piston reference diameter, and swash plate angle, as summarized in Table II. The parameter variation ranges are based on the following empirical considerations. First, the oilgap length is not less than 1.0 mm for ease of manufacturing. Second, the permanent magnet thickness is approximately linear to the oilgap length when the flux density in the oilgap is kept constant. Third, the optimized ratio of radial permanent magnet’s arc to pole pitch is around 0.55. Fourth, the piston reference diameter is restricted by other structural sizes such as the stator inner diameter and the piston diameter. Fifth, a swash plate angle less than 15° is adopted to improve the loading condition of the compound rotor.The oilgap length is taken into account at first. Since the compound rotor is immersed in hydraulic oil, when it rotates there will be additional frictional loss acting on its surface, of which the expression is given as [24]