ABSTRACT
Electrical machines can be divided into DC machines and AC machines, which are fed with DC current and AC current, respectively. Since brushes or slip rings are needed in DC machines, their relatively low reliability makes the DC machines less competitive. The AC machines can be further divided into induction machines and synchronous machines. Although induction machines are cheap and easy to manufacture, synchronous machines are more attractive due to their high efficiency and precise speed control. Currently, synchronous machines have a wide range of applications and have been extensively investigated by both academics and within the industry. In a synchronous machine, there are two rotary magnetic fields, namely the excitation field and the armature field. The electromagnetic torque is generated through the interaction of these two fields. The armature field can only be excited with the armature winding current, while the excitation field can be produced in various ways. When field winding is used to generate the excitation field, which is referred to as electrically excited machine (EEM), the air-gap field can be easily
regulated by controlling the field current and therefore the EEM can operate over a wide speed range. However, since the field current will inevitably introduce additional copper loss, the efficiency of the EEM is reduced and the heat generated by the copper loss may cause a problem of heat dissipation. Permanent magnet (PM) machines can solve the aforementioned problems, in which the excitation field is generated by the PMs. High torque density and high efficiency can be achieved in PM machines when high-magnetic-energy-density PM materials are used. The major drawback of the PM machines is that the air-gap flux is difficult to control due to their fixed excitation and their speed range is limited accordingly. In order to achieve high torque density and high efficiency while still maintaining good flux regulating capability, hybrid excited machines (HEMs) are proposed these can be regarded as combinations of EEMs and PM machines (Wang and Niu 2017). The excitation field in the HEM is provided by a primary PM excitation and a secondary field coil excitation source. Since HEMs theoretically have good overall performances, they are being widely studied by researchers.
