Thermal Design of an Integrated Inductor for 45kW Aerospace Starter-Generator

This paper presents a combined electromagnetic and thermal design of integrated inductor, for the application of a 45kW aircraft starter-generator. The inductor is designed at high current density by using the apporach of area product, followed by the finite element analysis, which validates the electromagnetic performance of integrated inductor. The total power losses at 8, 20, and 32kRPM are evaluated in order to investigate the thermal design of a combined starter-generator and integrated inductor system, whilst, achieving the full integration from a thermal management point of view. As both starter/generator and inductor share common cooling configuration; performance of a direct and an indirect cooling options are compared. The direct cooling configuration, based on a semi-flooded design, can lead to a temperature reduction up to 90°C in the most critical components.

I. Introduction

A compact motor drive system is required, from physical and functional integration point of view, in order to achieve higher power density and efficiency. Such power dense systems are mandatory in aircraft, marine and transportation applications. Passive elements which are introduced after the drive components have been defined lead into discrete sub-systems [1]. To overcome this, the integration of passives need to be introduced both from functional and physical point of view [2, 3]. There are many possibilities in the aircraft motor-drive systems to integrate the passive components. Pasives integration in such systems offer many advantages such as increased energy densities, reduced price, weight, space and eases the construction task. Thus, applications where high energy densities are required, integrative approach appears to be the best solution [4, 5]. In the past, the integration of passives has been a focus in electric motor drive market that has resulted in an overall compact design of a power system. In [2, 3], a new integration methodology of the inverter output filter inductor is presented for permanent magnet synchronous motor drive systems. Integrated motor uses the main motor inductance as filter inductance instead of sizing a separate inductor between the inverter and the motor, which yields to the elimination of ohmic losses and its associated mass and occupied space. The author of [4–7] introduced two new options for passive filter inductors integrated within the common housing of the motor as shown in Fig. 1. It includes: motor-shaped rotational inductor and motor-shaped rotor-less inductor. Both integrated inductors are mounted axially on to the rotor shaft, resulting in a shared cooling system. Thus, eliminating the requirement of an external cooling system. The rotor of the rotational inductor rotates with the fundamental frequency of the stator magnetic field which minimises the rotor iron losses. On the other hand, integrated rotor-less inductor has the identical structure, but without rotor, which makes it appropriate for smoothing inductors for DC-Link applications, grid-side input filters and isolation transformers. In comparison, the rotational inductor can only be employed for high speed motor drive systems. In [8], author has presented a design of motor-shaped rotor-less inductor adopted for 45kW aircraft starter-generator. This integrated inductor was designed and sized at current density of 18A/mm² which uses the existing cooling system of the starter-generator. A substantial weight and space reduction of 55.4% and 52.7 respectively is achieved, when compared with the standard EE-core inductor which had come at the expense of higher ohmic losses, where, heat generated in the windings is assumed to be taken out by the cooling system available for the starter-generator, designed for aircraft applications.