Emil Levi

Presentations: 

Opportunities for Multiphase Electric Machine Utilisation in Electric Vehicles

Although multiphase (more than three phases) machines have been known for practically half a century, it is only in recent times that they are becoming more wide-spread in industrial applications. In addition to the obvious advantage of reducing the required power-per-phase and hence required semiconductor rating, multiphase systems offer a number of other advantages that make them suitable for specific niche applications. These all stem from the fact that, regardless of the number of stator phases, independent flux and torque control of an ac machine always requires only two independently controllable currents (two degrees of freedom). The remaining degrees of freedom can then be used for other purposes. The keynote will explore the purposes that are of the highest relevance for electric vehicle applications.

Three specific issues will be covered by the presentation. These are: i) the realisation of fully integrated on-board fast (three-phase) and slow (single-phase) battery charging systems in electric vehicles, with the emphasis on topologies that require no or minimum hardware reconfiguration; ii) fault-tolerant drive operation in propulsion mode, enabling the realisation of the ‘limp-home’ mode; and, iii) use of different sub-windings in a single multiphase machine for uneven power sharing including simultaneous motoring/generating operation of the said sub-windings. The last one is of high relevance for electric vehicles with multiple electric energy sources and it in essence can enable re-charging of the battery during high-speed driving of the vehicle. Illustrative examples, based on different stator winding phase numbers, will be used and experimentally obtained illustrations of the operation will be provided throughout.

Integrated On-board Battery Chargers for Electric Vehicles

To reduce the CO2 emissions and achieve the 80% reduction target by 2050, it is essential that the electric vehicle (EV) development and their adoption by the public are accelerated. This requires removal of one of the major obstacles, which is the so-called range anxiety. The first and the most obvious approach to relieving EV driver’s range anxiety is to provide the vehicle with an on-board charger, so that the driver can charge the battery at any electric grid connection point. Such an on-board battery charger should provide means for both slow charging (from a single-phase grid) and fast charging (from a three-phase grid).

A preferable solution for this purpose is the use of integrated on-board chargers: the idea is to reuse the existing components of an EV, which are already in place for the propulsion mode of operation. This lecture looks at the currently available solutions that integrate some or all of the already existing EV components into the charging process and then concentrates on the solutions based on machines and converters with more than three phases (multiphase systems). A whole range of entirely novel solutions, based on using machines with different phase numbers, have been developed in the last few years. All are characterised with the zero torque development during charging process. A further very important property of these solutions is that, in addition to the charging mode, vehicle-to-grid (V2G) operation is also possible without any modifications whatsoever. The stator winding configurations encompassed by the presentation are the symmetrical six-phase, asymmetrical six-phase, symmetrical and asymmetrical nine-phase and five-phase and all topologies enable connection to either three-phase or single-phase grid.

Multi-phase Drive and Generation Systems

Although multiphase (more than three phases) machines have been known for almost half a century, it is only in recent times that they are becoming more wide-spread in industrial applications. In addition to the obvious advantage of reducing the required power-per-phase and hence required semiconductor rating, multiphase systems offer a number of other advantages that make them suitable for specific but important niche applications. These all stem from the fact that, regardless of the number of stator phases, independent flux and torque control of an ac machine always requires only two independently controllable currents (two degrees of freedom). The remaining degrees of freedom can then be used for other purposes and this will be the subject of this lecture.

The lecture is tutorial in nature and will commence with an introduction to the types of multiphase machines, principles of multiphase machine modelling, vector control, and multiphase voltage source inverter PWM schemes. ‘Classical’ (i.e. older) uses of additional degrees of freedom will be addressed next, including the multi-motor multiphase series-connected drive systems with reduced-switch-count inverter supply, use of the additional degrees of freedom for the purposes of achieving fault-tolerant operation, and torque enhancement by low order stator current harmonic injection. Next, more recent applications of the additional degrees of freedom will be considered. This encompasses capacitor voltage balancing in machines with multiple three-phase windings and multiple three-phase converters connected in series, realisation of fully integrated on-board fast (three-phase) and slow (single-phase) battery charging systems in electric vehicles, power sharing between three-phase windings in a multiphase machine with a multitude of said sub-windings, a braking method for induction motor drives with diode front-end rectifier, and advanced testing opportunities available in the machines with multiple three-phase windings. Basic concepts will be explained and illustrative examples will be provided throughout.