# Adoption of Multiphase and Variable Flux Motors in Automotive Applications
## 2 Literature Review
### Variable Flux Motors
#### 2.2.2 Variable Flux Memory Motors  
Variable flux memory motors (VFMMs) make use of low-coercivity magnets (LCMs) together with high-coercivity magnets (HCMs) to adjust the PM flux. HCMs remain magnetised in the whole working range the same as in traditional permanent magnet motors, whereas LCMs are demagnetised and magnetised by applying \(d\)-axis current pulses to manipulate the magnetisation state (MS), in this way extending the operating range and efficiency of the motor [60]. The main advantage of these motors is that they do not need any extra elements such as field coils, which is beneficial from manufacturing point of view.  
With the magnetisation state manipulation, higher efficiencies are achieved. [PERSON] et al. observed a maximum of 17% of efficiency improvement at high speed and low torque compared to the Nissan Leaf IPM motor. [PERSON] et al. evaluated motors between 120 kW and 275 kW at different drive cycles, and achieved power loss reductions between 14% and 28%. Additionally, the survey paper by [PERSON] et al. [61] demonstrates the possibility to extend the maximum torque-speed envelope of the motor.  
One of the main drawbacks is the high demagnetising and magnetising currents required to change the MS of the LCMs. In fact, [PERSON] and [PERSON] state that it is the biggest challenge to obtain competitive VFMMs [62]. The inverter must be oversized to provide these currents, which are typically several times the working current [63]. [PERSON] et al. report that 2.5 p.u. and 6.5 p.u. currents are needed to demagnetise and magnetise LCF magnets, respectively [64]. Similarly, the work [65] states that 800 A and 1600 A are required for a 325 A motor. Reducing these currents limits the range of manipulation of the MS, and a compromise must be made according to [66]. Moreover, decreasing the demagnetising current can lead to unwanted demagnetisation.  
To balance the magnetising and remagnetising currents, the rotor topology and the magnet arrangement are the key points according to [PERSON] et al. [67]. They state that the magnetising current is very high in the parallel configuration, and it can suffer from unwanted demagnetisation due to the low demagnetising current (confirmed in [64]). Meanwhile, the demagnetising current is increased higher in the series one, avoiding unwanted demagnetisation, but requiring larger currents for MS manipulation (confirmedin [68]). Therefore, [PERSON] et al. propose a hybrid configurations to balance both currents. Figure 3, based on [67], shows three possible magnet configurations, series, parallel, and hybrid, together with the load curves of the LCMs.  
To address the unwanted demagnetisation problem, some of the authors opt for flux-intensifying configurations. The idea is that, in these motors, the MTPA curve is located in the first quadrant of the \(I_{d}\)-\(I_{q}\) plane as \(L_{d}>L_{q}\), thus operating with positive \(d\)-axis current and avoiding the risk of demagnetisation in normal operation. In [69], the motor can operate until 90% of full load condition without unwanted demagnetisation, and the MS control current needed is 4 p.u.  
All in all, the VFMM is a potential candidate for electric vehicles as they substantially improve the efficiency, especially at high speeds. However, the problem of large MS control currents has to be solved as the oversizing of the inverter is too high in all cases.