Tom Feehally

The Doubly Fed Induction Machine as an Aero Generator

Modern aircraft require a robust and reliable supply of electrical power to drive a growing number of high-power electrical loads. Generators are driven by a mechanical offtake from the variable speed gas turbine (GT), while a constant frequency ac network is preferred. Here, doubly fed induction machines are advantageous since they can be controlled, through a fractionally rated converter, to decouple electrical frequency from the mechanical drive speed, making control of the network frequency possible. However, the converter must be suitably rated, according to drive speed range, electrical voltage and frequency regulation, and power requirements. This paper develops and validates a simulation model of the doubly fed induction generator (DFIG) system, which is applied to find the power flow through the machines stator and rotor connections over a wide mechanical speed range in order to size the converter. A field-orientated control scheme is implemented, to provide stand-alone voltage and frequency regulation across a drive range of ±40% synchronous speed, on a purpose-built 6.6-kW hardware test platform. Based on the mechanical speed range of an aero GT and the identified converter sizing, the suitability of a DFIG for aero applications is appraised. It is shown that a converter rated at 18% of full system rating can be used to meet the aircraft electrical specifications and offers a potential improvement in aircraft performance, with no additional mechanical components.

Real-time insulation lifetime monitoring for motor windings

This paper investigates the development and implementation of a real-time thermal ageing model for polymer-based electrical wire insulation using the classical Arrhenius relationship for chemical reaction rates. The paper presents the theoretical development and implementation of the method for predicting the insulation lifetime based on real-time temperature measurements using fibre-optic sensors embedded inside copper-wound coils. The performance of the presented lifetime model in delivering consistent results for winding insulation lifetime predictions is then assessed and validated using real-time steady-state and transient thermal experiments on a wound test coil mounted into a purpose built motorette test rig.

Analysis of electro-mechanical interaction in aircraft generator systems

The supply of electrical power requires a generator, which must be driven from a prime mover, by some form of mechanical drivetrain. Such an electro-mechanical system will have natural resonant modes in both the electrical and mechanical subsystems. These subsystems are interfaced through the electrical generator for the useful transfer of power, but may also couple unwanted disturbances between the electrical and mechanical domains resulting in interaction. The interaction can lead to lifetime reduction in the mechanical components and instability in the electrical network, resulting in poor reliability for the wider system. Predicting the occurrence of interaction, through simulation, is challenging, requiring multi-domain models, operating with different time scales. This paper analyses an aircraft auxiliary power offtake to produce a reduced-order mechanical drivetrain model, allowing the modal frequencies to be predicted and cross-domain interactions to be modelled. A purpose-built electro-mechanical test platform is used to validate the model and demonstrate how electrical disturbances are passed through to the generator to the mechanical system. Future research will use the test bed to demonstrate strategies for avoiding or suppressing unwanted interactions.

The doubly-fed induction machine as an aero generator

Modern aircraft require a robust and reliable supply of electrical power to drive a growing number of high power electrical loads. Generators are driven by a mechanical offtake from the variable speed gas turbine, while a constant frequency AC network is preferred. Here doubly-fed induction machines are advantageous since they can be controlled, through a fractionally rated converter, to decouple electrical frequency from the mechanical drive speed, making control of the network frequency possible. However, the converter must be suitably rated, according to drive speed range, electrical voltage and frequency regulation, and power requirements. This paper develops and validates a simulation model of the doubly-fed induction generator system, which is applied to find the power flow through the machines stator and rotor connections over a wide mechanical speed range in order to size the converter. A field orientated control scheme is implemented, to provide standalone voltage and frequency regulation across a drive range of �40% synchronous speed, on a purpose-built 6.6kW hardware test platform. Based on the mechanical speed range of an aero gas turbine and the identified converter sizing, the suitability of a doubly-fed induction generator for aero applications is appraised.

Can Trained Runners Effectively Attenuate Impact Acceleration During Repeated High-Intensity Running Bouts?

The purpose of this study was to investigate the effects of prolonged high-intensity running on impact accelerations in trained runners. Thirteen male distance runners completed two 20-minute treadmill runs at speeds corresponding to 95% of onset of blood lactate accumulation. Leg and head accelerations were collected for 20 s every fourth minute. Rating of perceived exertion (RPE) scores were recorded during the third and last minute of each run. RPE responses increased (P < .001) from the start (11.8 ± 0.9, moderate intensity) of the first run to the end (17.7 ± 1.5, very hard) of the second run. Runners maintained their leg impact acceleration, impact attenuation, stride length, and stride frequency characteristics with prolonged run duration. However, a small (0.11-0.14g) but significant increase (P < .001) in head impact accelerations were observed at the end of both first and second runs. It was concluded that trained runners are able to control leg impact accelerations during sustained high-intensity running. Alongside the substantial increases in perceived exertion levels, running mechanics and frequency domain impact attenuation levels remained constant. This suggests that the present trained runners are able to cope from a mechanical perspective despite an increased physiological demand.