Design Considerations for Aircraft Generator with Start Function

The paper explores the design implications of accommodating the engine start function in a large variable frequency starter-generator. Using data from an existing 225kVA starter-generator the trade off between the stator current, which impacts the size and rating of the start inverter, and the field current, which affects the size, rating, and complexity of the excitation system is explored. The suitability of a 3-phase AC main exciter is assessed as a direct replacement for the conventional main exciter. An optimization of the 3-phase configuration is presented, aimed at achieving minimum input VA to the exciter for a given field excitation requirement during start. Results from this prototype start excitation system indicate high levels of field current are possible from a design which could be retrofitted within the mechanical envelope of the conventional exciter.