The Constant Speed Drive (CSD) in effect acts as an automatic gearbox , maintaining the generator shaft speed at a constant rpm which results in a constant frequency output of 400 Hz, usually within ∼ 10 Hz or less. The drawback of the hydro-mechanical CSD is that it needs to be correctly maintained in terms of oil charge level and oil cleanliness . Also to maintain high reliability frequent overhauls may be necessary. However the IDG is used to power the majority of civil transport aircraft today.
Modern AC Electrical Generation Types - Variable Frequency Variable Frequency (VF) power generation as shown in following figure is the simplest and most reliable form of power generation. In this technique no attempt is made to nullify the effects of the 2:1 engine speed ratio and the power output, though regulated to 115 VAC, suffers a frequency variation typically from 380 to 720 Hz . This wide band VF power has an effect on frequency sensitive aircraft loads, the most obvious being the effect on AC electric motors that are used in many aircraft systems. There can therefore be a penalty to be paid in the performance of other aircraft systems such as fuel, ECS and hydraulics. In cases of variations in motor/pump performance a motor controller may be needed to restore an easier control situation. VF is being widely adopted in the business jet community as their power requirements take them above the 28 VDC/12 kW limit of twin 28VDC systems. Recently Airbus A380 and Boeing 787 have also started using VF power.
Modern AC Electrical Generation Types - Variable Speed Constant Frequency (VSCF) ➢ Figure shows the concept of the VSCF converter . In this technique the variable frequency power produced by the generator is electronically converted by solid state power switching devices to constant frequency 400 Hz, 115 VAC power . Two options exist: DC link: In the DC link the raw power is converted to an intermediate DC power stage – the DC link – before being electronically converted to three phase AC power . DC link technology has been used on the B737, MD-90 and B777 but has yet to rival the reliability of CF or VF power generation Cycloconverter: The cycloconverter uses a different principle. S ix phases are generated at relatively high frequencies in excess of 3000 Hz and the solid state devices switch between these multiple phases in a predetermined and carefully controlled manner. The effect is to electronically commutate the input and provide three phases of constant frequency 400 Hz power. Though this appears to be a complex technique it is in fact quite elegant and cycloconverter systems have been successfully used on military aircraft in the US: F-18, U-2 and the F-117 stealth fighter.
Recent Civil and Military aircraft Power System Development The power generation types of developed and proposed for few civil and military (fighter) aircraft platforms are shown in the table below.
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- Fall '20
- Alternating Current, DC Generator, Electrical generator