Even-phase multi-motor vector controlled drive with single inverter supply and series connection of stator windings

Vector control principles enable independent flux and torque control of an AC machine by means of only two stator d-q axis current components. This means that in AC machines with a phase number greater than three there exist additional degrees of freedom, which are nowadays used to enhance the overall torque production of a multi-phase machine through injection of higher stator current harmonics. However, these additional degrees of freedom can be used to control other machines independently within a multi-motor drive system. To do so, it is necessary to connect in series stator windings of all the multi-phase machines, with an appropriate phase transposition. A vector control algorithm is then applied to each machine separately, total inverter phase current references are created by summation of individual machine phase current references, and supply to the stator windings of the multi-machine set is provided from a single current controlled voltage source inverter (VSI). The concept is introduced in the paper using the general theory of electrical machines, and all the possible situations that may arise for an even number of phases are examined. Although an induction motor drive is considered throughout, the concept is equally applicable to all AC machines with sinusoidal stator and rotor flux distribution. Its main advantage is the potential for saving in the number of inverter legs, compared to an equivalent 3-phase motor drive system. The saving depends on the number of phases and, for an even phase number, comes into existence when the number of phases is ≥8. Various even phase numbers are considered in detail and appropriate connection diagrams are given. Verification of the proposed multi-phase multi-motor drive is provided by simulation of a 10-phase 4-motor system.