# Adoption of Multiphase and Variable Flux Motors in Automotive Applications
## 2 Literature Review
### Variable Flux Motors
#### 2.2.3 Mechanically Flux-Varying Motors  
Adjusting the flux linkage mechanically is the remaining option to obtain a variable flux motor. One way is to use externally controlled mechanical actuators, and the other is to take advantage of the centrifugal force at high speed to somehow change the flux. Different flux-weakening mechanisms are proposed in the literature.  
Mechanisms that use the centrifugal force of the rotor as actuator normally use springs to activate the flux-weakening mode. The motors of [70; 71; 72; 73; 70] use springs in the end-plates of the rotor to rotate the magnets in the rotor and in this way vary the linked flux. In [74], the magnetic circuit of a spoke-type motor is short-circuited in the inner part of the rotor thanks to a clutch that is activated at a given speed. When it comes to [75], a flux-weakening mechanism is presented for a double-rotor axial flux motor. An actuator with springs in one of the rotors allows it to shift the angle with respect the other rotor, which is equivalent to skewing a rotor, which allows it to obtain the flux-weakening effect. Although additional mechanisms are needed in all the configurations, their main advantage is that no external input is needed to activate the actuators.  
The mechanisms that use the centrifugal force to activate the flux-weakening mode are located in the rotor, as this is the moving part. When it comes to externally activated actuators, they can be located in the stator, in the rotor, or even in the airgap.  
[PERSON] et al. suggested in [76] a novel flux-switching motor where the magnets are located in an iron ring in the airgap. When this permanent magnet iron ring is rotated, the flux linkage can be adjusted. In [77], a similar iron ring mechanism is used for a spoke-type motor. By aligning the magnetic conducting parts of the ring with the stator teeth, the maximum flux linkage is obtained, and vice versa.  
An example of a flux-varying motor with the actuator in the stator is presented in [78; 79]. The same as in some other cases, the basis of the mechanism consists of short-circuiting the magnet flux to reduce the flux linkage. In this case, a flux-switching motor is presented, where the magnets are located in the stator, and so is the actuator. By placing a yoke at the top of the magnets, the magnet flux is deviated, and less flux is linked.  
Figure 3: Series, parallel and hybrid magnetic configurations in VFMM rotor. Based on [67]. (**a**) Magnet arrangements. (**b**) LCM load curves.  
Moreover, the number of flux-weakened magnets can be controlled to obtain different magnetisation levels.  
The remaining possibility to mechanically vary the flux is to act in the rotor. In [80,81], an actuator is used to control the skew in a two-segment rotor, which can obtain a total magnetisation manipulation range from 0% to 100%. When it comes to [82], an iron plate is placed in the rotor end-plates, deviating the flux through it and achieving the desired result.  
In summary, the main drawback of mechanically flux-varying motors is the complexity of the manufacturing process, the associated costs, and the maintenance of the mechanical parts.