Design and Excitation Control of a Dual Three-Phase Zero-Sequence Current Starting Scheme for Integrated Starter/Generator

A harmonic excitation brushless synchronous machine has higher power density compared with the traditional aerial three-stage generator due to removing the axial exciter and is suitable for use as an integrated starting-generator (ISG). However, the field current cannot always be generated in the field windings at different rotor positions in the starting mode. A new excitation scheme employing double three-phase phase orthogonal zero-sequence armature currents is proposed in this article. Harmonic electromotive forces and field current can be produced by the zero-sequence rotating excitation magnetic field and almost has no relationship with the rotor positions. In addition, in order to obtain a constant field current during the starting process, a control method of the generated field current is also presented. The rotating speed and field intensity of the zero-sequence excitation magnetomotive forces are both feedback controlled by the zero-sequence current in the whole starting process. Comparing with the original harmonic excitation machine, a higher and more stable field current can be generated in the proposed ISG whenever the rotor is at any position or at a low speed. The prototype machine is manufactured and experiments are carried out together with the excitation control method to verify the effectiveness of the theoretical and finite element analysis results.

I. Introduction

The integrated starting-generator (ISG) has the capacity of starting the engine in the starting process, as well as operating as a generator after the engine reaches the steady running speed. Therefore, it is the key equipment of the modern more-electric or all-electric aircraft with the characteristics of high power density [1]–[4]. Various machines can be used as starting-generator, such as synchronous machines [4], [5], double salient pole machines [6], [7], and induction machines [8], [9]. Permanent magnet (PM) synchronous machines have high power density and high efficiency. However, the PM flux is difficult to be regulated when the speed or the load current of the machine is changing. Besides, quick demagnetizing is also cannot be realized at fault state (such as turn-to-turn short-circuit fault) when this kind of ISG is working as a generator. Because of the features of simple structure and higher reliability, the reluctance machine will be a good option for using as ISG. However, problems of larger torque ripple and relatively lower efficiency that exist in this kind of machine should also be improved. For the induction machine, the excitation control is complex while working in generator mode. The three-stage brushless synchronous generator has the advantages of reliable demagnetizing ability, high reliability, and low maintenance cost, so it has always been the main machine type of aircraft generator.