Multiphase machines with more than two independent currents

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

Historically, the need for cost-effective transmission of electrical energy led to the adoption of three-phase sinusoidal alternating current systems. The windings of AC electric machines, which were directly connected to the grid, therefore had to be three-phase and optimized for sinusoidal alternating currents. For applications requiring high-performance drives with variable speed and torque, 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 DC machines.

With the widespread electrification of transportation and against the backdrop of the energy transition, electric motors should ideally require fewer raw materials and have less complex manufacturing processes, particularly during the winding stage: having more design parameters is a welcome development. 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 makes sense 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, limited to a sinusoidal waveform.

"Functional" context: fault tolerance and consistent torque output even under reduced load

The windings of electric machines, whose function is to generate a rotating magnetic field within the air gap using alternating currents, form the cornerstone of electric machines.

With two independent currents and three wires to connect, conventional three-phase motors represent the simplest solution for achieving this goal. The simplest way to ensure 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 in terms of power supply and during the design phase. As the number of phases increases, the same torque quality can be achieved through vector control with multiple harmonics present both temporally and spatially, as the permissible number of harmonics increases with the number of phases. Consequently, constraints are reduced both in terms of power supply (non-sinusoidal currents) and design (non-sinusoidal winding function). The resulting design flexibility can be used to simplify the winding manufacturing process (e.g., concentric tooth winding or “juxtaposed phases”) or to implement new features such as changing the machine’s polarity without modifying...