# Adoption of Multiphase and Variable Flux Motors in Automotive Applications
## 2 Literature Review
### Variable Flux Motors
#### 2.2.1 Hybrid Excitation Motors  
HE motors have extra field coils (FCs) apart from the stator winding and rotor permanent magnets (PMs) compared to traditional PM motors. These FCs enhance or weaken the permanent magnet flux by applying a DC current.  
A great amount of different topologies exist among the HE motors. One main separation is made in [50] according to the location of the magnets and field windings, which can be either in the rotor or in the stator. Figure 2 shows an example of each.  
Topologies with PMs and FCs in the rotor (Figure 2a) can be formed by mixing FCs and PMs in the rotor. This way, the stator remains the same, but electric energy has to be transmitted to the rotor, which usually requires slip rings, increasing maintenance cost and making the system less reliable. In [51], the flux-weakening capacity was notably improved by adding field coils in the rotor of an IPM motor. In addition, in [52], the torque density was increased by adding permanent magnets to an electrically excited rotor. Unlike other  
Figure 1: Torque–speed envelope and high-efficiency zone improvement with VF motors. (a) Envelope with different flux levels. (b) Overall operation envelope.  
Figure 2: HE motor types depending on magnet and field coil location [50]. (a) PM: rotor/FC: rotor. (b) PM: rotor/FC: stator. (c) PM: stator/FC: stator.  
alternatives, in [53], by changing the polarity of field coils, the motor could work as a 4-pole or as a 8-pole one, which was useful to widen the speed range.  
Using different flux paths for magnets and field coils, that is, using parallel magnetic circuits for magnets and field coils, it is possible to move the field coils to the stator as shown in Figure 2b. This way, there is no need to feed the rotor, eliminating the slip rings. In [54], some configurations with field coils in the stator are presented. It is stated that some of them needed extra yokes in the axial flux path, making the motor heavier, as the example in Figure 2b. The motor in [55], additionally, needed an extra stator yoke for the circulation of field coil flux, and part of the rotor was not used. Therefore, HE motors having the field coil in the stator do also have some drawbacks.  
Another option is to put the magnets as well as the FCs in the stator. Among these motors, the flux switching [56; 57] and flux reversal [58] topologies can be adopted, as they have PMs in the stator. By adding FCs in the stator, permanent magnet flux can be enhanced or weakened the same way it is accomplished with the other alternatives. Additionally, in [59], a hybrid claw pole motor was adopted, but the PM location was changed from the rotor to the motor front and back covers, to generate axial flux. This allowed it to avoid problems in the magnets due to the centrifugal force in high-speed motors, but increased the reluctance of the magnetic circuit of the field coil, as it was arranged in series with the PMs.  
To sum up, HE motors obtain a better flux-weakening capability than IPM motors but need extra elements such as field coils, power supply, and flux paths, making the manufacturing process more complex, increasing the mass of the motor, and making it more expensive. These are the main limiting points for not adopting them in market EVs.