Revolutionizing Automotive Traction: A Comprehensive Review of Multiphase Drives for Next-Generation Vehicles
In recent years, the revolutionary development of power electronics and converters paved the way for more scope for research in multiphase drives. Preliminary phases like the three-phase drives showed more torque fluctuations than the new five-phase drives by 67 percent. This encouraged researchers to gain more knowledge about six-phase modulation as MPDs have the feature to segregate power across several phases, therefore reducing power rating per phase. Moreover, as new phases are added, the magneto-motive force distribution is improved, reducing torque ripples in the machine's air gap, and increasing the degrees of freedom.
MPDs are primarily used in Electric vehicles and marine applications, for reducing cost and weight by integrating chargers with contemporary machines, moreover, in ships MPDs are used in place of mechanical poles to control machine speed. However, despite having better features in terms of performance, power density, and fault-tolerant capacity, traditional three-phase drives are still dominant in industries. One of the main aspects of studying MPDs is the modeling and power conversion processes like AC-DC-AC which is fed via a convertor that is based upon pulse-width modulation.
Prominent features of Multiphase Drives
Traditionally used, three-phase drives are not as efficient and cost-effective as Multiphase Drives, mainly due to unique and dominating features like lower power rating per phase, better fault-tolerant operation, and lower DC link current ripples. The highlighting aspect of MPDs is their ability to divide power across a number of phases as shown in Figure 1 which are supplied via converter legs without using parallel components. This property itself tackles issues faced by three-phase drives like poor heat dissipation, different load sharing, and differences in turning on/off timings.
Figure 1: Phase current VS. Number of phases.
Conventional three-phase drives are renowned for having faults that generally arise due to machine winding faults, making them more prone to inefficiency and failure. Thereby, fault tolerance is one of the major aspects to consider when choosing a drive. The new multiphase drive has an increased number of phases which provides more degree of freedom whilst in faulty conditions. This makes their fault tolerance much higher as compared to their counterpart's three-phase drive. One particular MPD, namely the six-phase drive has amassed a lot of attention for having a high fault tolerance due to its higher space harmonics in the air gap flux of the machine.
On the other hand, the dc link bank of capacitors is another important aspect worth considering in the powertrain of electric vehicles. Current generation drives have higher Dc link current ripples in capacitors which affect lifetime and severely degrade the shelf life of components. However, recent research showed that the different number of phases have different Dc link capacitances as shown in Figure 2.
Figure 2: Dc link capacitance VS. Number of phases.
It is observed that a five-phased drive is most suited as the Dc link capacitance almost stays constant with more phases, moreover, this helps in packing more power than conventional three-phase drives.
A Short Review of the Mathematical Modeling of Multiphase Drives
Six-phase machines exhibit varied winding configurations, which hinge on the angle between their two three-phase windings. Symmetrical six-phase machines, which feature a 60° space angle, differ from their asymmetrical counterparts which possess a space angle difference of 30°. In comparison to the dual three-phase machine, the asymmetrical six-phase machine boasts superior performance in terms of the space harmonics of the 5th and 7th harmonic in the airgap flux when compared to its symmetric counterparts. Thereby it is important to create a model of the system to showcase high-performance and superior control techniques for MPDs.
Vector Space Decomposition and Double-DQ Model are two such unique ways to extract maximum performance out of MPDs, moreover, these models are based on the configuration and alignment of different drives. The Double-DQ Model uses two-three phases of windings in a machine with six phases while applying two three-phase Clarke transformations. Therefore, a general equation as shown in Figure 3 depicts how different phases are related to different windings sets.
Figure 3: A general equation for Double-DQ Model.
Here, f is a general symbol for different machine variables and k depicts the subscripts of values 1 or 2 to discern variables of the two three-phase windings. In contrast to DDM, VSD uses a completely different method to model multiphase drives, by decomposing machine currents and voltages from six-dimensional space into new subspaces. This method helps to showcase complete machine dynamics and assists to design a controller without the alterations in the structure and has a higher resistance to tolerance.
New and Efficient Control Methods Used in Multiphase Drives
In order to achieve stellar performance in MPDs, better control methods are a must to achieve higher performance. In the latest studies, there are 2 mostly used control methods called current control method, and model predictive control.
Harmonic current control method
The currents that are produced in the xy subspace contribute to losses, as these arise due to voltage harmonics resulting from deadtime or modulation models, therefore they don't have any contribution to the production of torque. However, by using the Harmonic current control method, controllers apply and track a reference of zero value to nullify these currents. Other proposed ideas like the application of resonant or dual PI controllers in the case of asymmetrical six-phase induction motors are still in the process to be researched.
Model Predictive Control
The concept behind MPC is to control asymmetrical six-phase drives which is providing a fast dynamic response. MPC achieves such an impressive feat by choosing an optimum switching pattern for each sampling period based on the cost function of the model in the discrete-time domain as depicted in Figure 4.
Figure 4: A block diagram representing the implementation of MPC to six-phase drives.
All types of cost functions are applied and tried to choose the function that meets the criteria of a certain THD of the output waveform, removing all circulating currents in the xy subplane.
Conclusion
MPDs, or multiphase drives, possess noteworthy characteristics that make them a viable option for electric vehicle (EV) applications. These features include the use of lower-rated semiconductor devices, efficient operation even under faulty conditions with optimal current control, the ability to enhance torque density by harmonic injection, and a smaller DC-link size. All of these factors contribute to making MPDs a competitive solution in the realm of EVs. Moreover, modulation and control methods are one other reason for the superior performance of MPDs which enhanced efficiency, power density, and overall performance when compared to traditional three-phase drives.
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