Analytical determination of the double star synchronous machine commutating reactance

The double star synchronous motor is used for high power traction drives. It allows division into two of the power of the inverters and reduction of torque ripple. A precise knowledge of the commutating reactance of the machine is required in order to know the electric stresses imposed on the static converter during the commutation and so to design inverters perfectly adapted to the machine. The commutating reactance of the double star motor is more difficult to determine because of the coupling between the two stars. As the double star machine has two three-phase windings shifted by 30 electrical degrees, the current distribution in a slot is different, it is possible to have coil sides of different stars in the same slot. The aim of this paper is to present an analytical method in order to obtain good approximation formulas of the double star synchronous machine commutating reactance. This study is based on the works by P.L. Alger and L.A. Kilgrore which deal with leakage inductances of the single star synchronous machines.