Novel dual-rotor counter-rotating PM machines with radial and axial configurations

This thesis systematically investigates radial-flux dual-rotor (DR) permanent magnet (PM) (DRPM) machines with (a) different slot/pole number combinations, (b) magnetic fluxes produced by two rotors in both series and parallel, and (c) two rotors operating at either the same or different speeds. All the investigations are carried out by analytical analyses and/or finite element analysis (FEA), together with experimental verification.
In the scenario that two rotors of DRPM machines operate at the same speed, the DRPM machines with series (S) and parallel (P) magnetic circuits (SDRPM and PDRPM machines) are compared. The analysis indicates that SDRPM machines generally exhibit higher torque. Moreover, the outer rotor of the DRPM machine contributes most to the total torque, which also shows the single rotor permanent magnet (SRPM) machine has a potentially higher torque than the DRPM machine. Therefore, the SDRPM machine is selected to compare with the SRPM. It reveals that the SDRPM machines have higher overload capabilities but lower torque density and less efficient PM utilisation than the SRPM machines.
Although with lower torque density and PM utilisation, DRPM machines inherently have two rotors compared to SRPM machines. Thus, DRPM machines can be employed in applications that require two rotors, thereby replacing two SRPM motors. The counter-rotating (CR) operation is selected for further analysis as a special case in a scenario with different rotational speed rotors since the CR operation systems are widely used in industrial applications, such as underwater vehicles, wind power turbines, household appliances, etc. Therefore, a yokeless counter-rotating dual-rotor PM (YCDRPM) machine, which features series magnetic circuits, is introduced and analysed. This YCDRPM machine has the synergies of a magnetic gear and a PM machine to achieve dual-rotor CR operation, significantly reducing the cost and saving the space compared to using two SRPM machines. However, the sandwiched stator of the YCDRPM machine results in a complex mechanical structure, and its radial flux configuration makes the torques of two rotors more challenging to adjust. Therefore, a novel counter-rotating single-stator dual-internal-rotor PM (CSDPM) machine is proposed. The CSDPM machine design includes a single shared winding set, one shared stator, and two robust rotors. Different from conventional radially arranged radial-flux magnetic geared machines, axially arranged radial-flux rotors of CSDPM machines allow for easier assembly and can modify the torque split of two rotors. The proposed CR operation system with the CSDPM machine shows higher torque than the conventional CR system using two SRPM machines.