Multiphase machines with more than two independent currents

Historical and economic context: the number of phases as a design parameter

Historically, the economically efficient transmission of electrical energy led to the adoption of three-phase sinusoidal alternating current systems. The windings of alternating current electric machines, directly connected to the grid, had to be three-phase and optimized for sinusoidal alternating quantities. For applications requiring high-performance variable speed and torque drives, power electronics and digital vector control naturally and gradually (from 1970 to 2010) enabled these same three-phase machines with sinusoidal alternating quantities to prevail over direct current machines.

With the widespread electrification of transportation and in the context of energy transition, electric machines should ideally consume fewer raw materials and have less complex manufacturing processes, particularly for the winding stage: having more design parameters is welcome. However, for a machine powered by an inverter, three phases are no longer an absolute necessity. Increasing the number of phases becomes a design parameter. It therefore seems sensible to master not only the design of windings for these machines with more than three phases, but also the control of their currents, which are no longer, in principle, subject to the sinusoidal form.

Functional context: fault tolerance and consistent torque quality with fewer constraints

The windings of electrical machines, whose function is to create a rotating magnetic field within the air gap using alternating currents, are the cornerstone of electrical machines.

With two independent currents and three wires to connect, conventional three-phase machines are the minimum solution required to achieve this objective. The simplest solution for ensuring pulsation-free torque through vector control is therefore to supply sinusoidal currents to a winding characterized by a winding function that is itself sinusoidal, but in space.

With a three-phase machine, there are therefore constraints both during power supply and during the design stage. With an increase in the number of phases, the same torque quality can be obtained by vector control with several harmonics present temporally and spatially, as the permitted number of harmonics increases with the number of phases. As a result, constraints are reduced both during power supply (non-sinusoidal currents) and during design (non-sinusoidal winding function). The resulting freedom can be used to simplify the winding manufacturing process (e.g., concentric dental winding or "juxtaposed phases") or to implement new features such as changing the polarity of the machine without modifying any connections....