# Adoption of Multiphase and Variable Flux Motors in Automotive Applications
## 2 Literature Review
### Multiphase Motors
#### 2.1.2 Performance of MP Motors  
Multiphase motors have several benefits over the three-phase ones. Many authors confirmed the higher torque capability of multiphase motors due to an increase in the winding factor. [PERSON] et al. adapted the Toyota Prius 2010 motor from three-phase to DTP, obtaining a 3.53% higher winding factor and a 2.45% higher maximum torque [27]. In [28], a fractional 12 slot and 10 pole motor was analysed and 3.5% higher torque was obtained.  
In terms of torque ripple, increasing the number of phases reduces torque ripple [6; 29]. In [27], torque ripple was reduced more than 60%, and in [28], around 18%. Moreover, as analysed in [30], the order of the first torque ripple harmonic increases. In the case of DTP motors with phase displacement of 30\({}^{\circ}\), the order of the first torque ripple harmonic increases from 6 to 12 [31].  
When it comes to efficiency and power losses, the more sinusoidal magnetomotive force of multiphase motors reduce iron losses. [PERSON] et al. observed 24%, 44%, and 60% reductions in stator iron, magnet, and rotor iron losses, respectively, in the DTP motor compared to the equivalent three-phase one [28]. Alternatively, [PERSON] et al. noticed a higher continuous power capability in an automotive motor and a 3% higher efficiency over the WLTC driving cycle [12], which was partly thanks to the higher winding factor.  
On the other hand, the higher number of phases brings the opportunity to use the additional degrees of freedom for different purposes, some of which are listed and explained below:  
* Current harmonic injection (CHI): To increase torque in MP motors, higher-order current harmonics can be used. The idea is to use the higher-order EMF harmonics to produce torque by injecting the corresponding current harmonics, the same way it is achieved with the fundamental one. The torque capability of five-phase [29; 32; 33], DTP [34; 35], seven-phase [36], and nine-phase [37] motors is addressed in the articles, while in [38] asymmetric motors were analysed. In [39] and [40], the permanent magnet shape of a surface permanent magnet DTP motor was analysed. By changing the magnet shape and injecting current harmonics (3 rd, 5 th, and 7 th), torque was improved by 30%. Additionally, [PERSON] et al. [36] designed a seven-phase motor with simultaneously 2 and 6 pole pairs taking advantage of this characteristic.
* Fault-tolerant controls: Fault tolerance is interesting for automotive motors. After a fault, the additional degrees of freedom can be used to continue operating. Controls with different objectives can be distinguished in the literature, such as the minimisation of copper losses [41], the balancing of current waveform amplitudes [8], and torque ripple minimisation [42]. Moreover, according to [43], robust controllers such as the Fuzzy Logic Controller can deal with motor faults even without fault detection. This was demonstrated in [44] for a nine-phase induction motor.
* Multimotor operation: Various motors can operate with a single inverter, referred to in the literature as multimotor operation [21; 45]. This is achieved by phase transposition, which allows each motor to operate in different \(\alpha\beta\) planes as the phase sequence of the motors is different. However, this operating mode is not interesting for electric vehicles, as copper losses would be double.