Variable frequency on future large aircraft

Citation

(1999), "Variable frequency on future large aircraft", Work Study, Vol. 48 No. 6. https://doi.org/10.1108/ws.1999.07948fab.006

Variable frequency on future large aircraft

Variable frequency on future large aircraft

While variable frequency has been proven for small turbofan-powered aircraft, large aircraft pose greater challenges. To cater for the difference in speed range of turboprop and turbofan engines and for fitment to aircraft that can differ greatly in size, the requirements for variable frequency power have been split into two sections: narrow range of 360 hertz to 600 hertz, which covers turboprops and turbofans with narrow speed ranges (narrow range variable frequency has exactly the same power quality limits as constant frequency) and wide range of 360 hertz to 800 hertz which covers all present and foreseen turbofans. The wide range also covers the effect of future large passenger aircraft. "The sheer power requirements of large aircraft results in the need for very large generators," explains Crouchley. "Now, not only do we have to maintain power quality with much higher power and higher speed over a wider speed range, we also have to deal with the mechanical integrity inherent in large, high-speed equipment, such as generators."

Crouchley adds that as the lengths of the electrical power feeder cables and their associated impedance increases, their detrimental effect on power quality increases. He points out that the lengths of cable envisioned on future large aircraft will require action to reduce these effects. He also points out that future utilisation equipment design should include functions to reduce harmonic distortion effects on the power supplies. Harmonic distortion levels have been increasing as a consequence of the increases in the number of electrical loads and the relative number of power electronics loads such as in-flight entertainment.

Lucas Aerospace is currently involved in a number of demonstration programmes testing the viability of variable frequency systems for large aircraft applications. The company has more than two years of successful testing behind it. Lucas Aerospace is also actively engaged in developing this technology for future military applications where the same benefits of reliability and cost of ownership apply. Crouchley believes the challenges the company faces in moving its success from small turbofan engines to large engines is definitely achievable and that the technology will be fully proven by the time the next generation of future large aircraft and military programmes enter service.