No abstract

Electric motors equipped with multiple three-phase windings are often used for the advantages they offer in terms of reliability, performance, and inverter power segmentation. When the windings are fed by independent voltage-source inverters (VSIs), circulation harmonic currents can occur in stator phases. If the motor is an induction machine, circulation currents are known to originate form transient imbalances in the applied voltages due to inverter switching. This paper investigates the problem when a synchronous machine is used and shows how additional (and possibly major) sources for circulation currents can arise in this case (even with a round-rotor design) due to the nonsinusoidal air-gap flux distribution. The phenomenon is illustrated for a 45-MW quadruple three-phase synchronous motor supplied by four medium-voltage (MV) multilevel VSIs. Its circulation currents are predicted with two alternative methods, i.e., analytically and from time-stepping finite-element simulation. The results obtained in both ways are shown to well match measurement results collected on the actual motor during full-load system testing.

No abstract

Load commutated inverters (LCIs) are still widely used for their robustness and reliability in high-power synchronous motor drives, in either single or multiple three-phase configurations. A restriction to their use in high-speed applications is due to the criticalities of thyristor operation at high switching frequencies. The upper frequency limits of LCIs are usually addressed in the existing literature as something independent of the drive architecture. On the contrary, this paper highlights how the maximum frequency that can be safely attained closely relates to the number of LCIs that are used to supply the synchronous motor. In fact, moving from a single to multiple three-phase arrangements is proved to introduce more stringent frequency constraints due to the mutual interaction between stator windings during commutations. Frequency limits for safe operation of single- and multiple-LCI drives are derived in quantitative terms\ud and experimentally assessed on a 2-MW synchronous motor fed by a couple of LCIs. Practical implications of such limits in the\ud design of high-power high-speed drives are finally discussed

Salient-pole synchronous generators are conventional electric machines which have been widely studied over decades. Nevertheless, some aspects still constitute a challenge for designers and need to be approached with state-ofthe-art modeling and computation techniques. An example is illustrated in this paper dealing with a 14 MVA synchronous generator. Different design solutions for its damper cage are studied and implemented on full-scale machine prototypes. Transient finite-element analyses are used to predict the influence of damper design on machine sub-transient reactances. Comparisons are presented between experimental results and finite element simulations and a physical interpretation is provided of the relationship found between damper structure and machine sub-transient reactances.