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Aviation History
1958
1958 - 0115.PDF
24 January 1958 117 system subject to the availability of a practicable constant-speeddrive. In a paper surveying aircraft radio power supplies Dr. B. J. O'Kane urged that a constant-frequency supply best suited the requirements of the radio designer. He stressed the advantages of operating valve-heaters in parallel and eliminating rotary trans- formers. "This is no new idea," he said, "on the contrary, it is so old that there must be some good reason, not obvious to the radio engineer, against the adoption of A.C. supplies. No doubt the absence of a constant-speed drive ... has something to do with the lack of enthusiasm shown by aircraft designers, but there would now appear to be several types. . . ." He went on to refer to the "differences of opinion on the relative weights of D.C. and A.C. systems, but even on the assumption that an A.C. system is slightly heavier, the reduction of brush and commutator main- tenance, particularly in high-flying jet aircraft, would seem to favour alternating current." In replying to the discussion on his paper Dr. O'Kane pointed out that Mr. R. H. Woodall had stated in a paper given to the Rj\e.S. that a constant-frequency system was lighter than a D.C. system, whereas Mr. W. J. Scon in another R.Ae.S. paper had stated that design estimates had shown a slight disadvantage in weight and size of the A.C. radio power-supply equipment as compared with that of the corresponding D.C.-operated equip- ment. He added: "Although this apparently does not refer to the English Electric Type 2502 starter/genera- tor. Rating ii 400 A at 30V between 5J0OO and 10,500 r.p.m. Altitude limit SOMOrt complete electrical system, I have heard it argued that there is a saving with D.C. systems." And argued it was in the discussion following papers dealing with (1) a 112 volt D.C. system; (2) rectified A.C. systems; and (3) a constant-frequency A.C. system. These papers had been presented, respectively, by Messrs. A. Ball and R. N. Lake; Mr. P. L. Cronbach; and Mr. P. C. Finucane. In the same session Dr. D. C. Flack in a paper about "future trends" did not seem to think that any sort of system could be standardized. Summing up he said: "The primary generation in any case will be alter- nating current; the D.C. supply, which will always be a require- ment, will be provided by static rectifiers, and on large aircraft will possibly be at 56 volts. Rotary conversion equipment will not be used." He concluded by saying: "There is often a natural choice of supply for certain items of utilization equipment, and for this reason D.C. supplies and constant-frequency A.C. supplies will be necessary. It is essential, however, that where alternatives are possible the equipment designers should co-operate with the aircraft design team to arrive at the best overall results." Which statement suggested that every installation would be virtually tailor-made, incorporating special bits and pieces which in the opinion of the designers were necessary to ensure "the best overall results." Opening the discussion, Mr. V. A. Higgs said that: "At every meeting where the question of alternating versus direa current arises mere is always an extremely lively discussion. . . ." He noted that: "One paper gives ten reasons for using alternating current; there is only really one reason, a quite positive reason, which is that A.C. equipment is electrically more simple." He went on to discuss certain snags, although he thought that the "new high-performance aircraft will have to use A.C. power because it will not be possible to design D.C. equipment to survive in these aircraft." Mr. J. F. Lewis referred to Messrs. Ball and Lake having "cast doubt on the standards for alternating current in aircraft. The standard for military aircraft is that main supplies shall be 400 c/s 200 volts 3-phase, and that has been so for some years." Later he declared: "We must find some way of letting the radio people know that there is a standard, and has been one for some time. The fact that we are still making 115-volt inverters does not mean that we want to go on doing so for ever." Mr. L. G. Hill, in a communication included in the published version of the discussion, said: "There appears to be a body of engineers to whom 3-phase alternating current seems the panacea. Nothing could be farther from the truth." After emphasizing that the "main objection to direct current appears to be brush wear," and arguing that the real solution to the problem was better design and manufacture of commutators and slip-rings, with light brushes of adequate area, he maintained that the "adoption of 3-phase alternating current would introduce more problems than it would solve in the limited space available in aircraft. Descriptions of experimental systems carefully avoid reference to these additional problems." Arguing that the choice of a system was dictated by circum- stances, Mr. R. J. C. Tarr pointed out that: "Rectification has Rotax SOkVA vari- able-r.p.m., constant- frequency alternator (see "Flight," August 30. 1957, page 347). been the only way to use our alternators for many years because we could not drive them at constant speed. A civil or transport aircraft with a lot of non-frequency-sensitive loads . . . represent- ing a large proportion of the output direct from the variable-speed alternator, can use such a system economically. Most military air- craft, on the other hand, have a high proportion of frequency- sensitive loads, and the necessity to transform and rectify most of the alternator output increases the weight and cooling drag of such a system as to make it uneconomical. Circumstances change, however, and new rectifiers of higher efficiency, less weight and requiring less cooling could affect one's choice of system." He noted that the use of silicon rectifiers could result in weight saving on equipment. Replying to the discussion, Mr. P. C. Finucane declared: "there appears to exist in this country a selection of diehards who try to discourage every attempt to introduce A.C. systems into aircraft. Judging by the discussion, these are in the minority and most of the objections can be put down to ignorance of the facts. I would like to make it quite clear that the aircraft electrical engineer will put in the best system, whether it be A.C. or D.C., to suit the role of the aircraft. If the demand at constant-frequency warrants it, then let us have A.C. systems and let us get down to the business of solving the minor problems which confront us in the same way that the problems of the 112-volt D.C. system have been solved —but a little quicker and a little better." In reply to a criticism Plessey rotary in- verter, rated at 3kVA and weigh- ing less than 601b. that A.C. motors were not suitable for applications requiring a high starting torque, Mr. Finucane pointed out that in a particular installation there were 53 A.C. motors which did not require high starting torques and eliminated "212 potential trouble-makers (brushes). . . ." He added that by a redesign of some existing equipments it was possible to reduce the continuous 28-volt D.C. demand to about 2 per cent of the total in large aircraft. At another session of the Convention, Messrs. R. H. Woodall, P. J. Daglish and V. A. Higgs presented a paper on trends in the development of airborne electrical equipment, with particular refer- ence to constant-frequency A.C. systems. From the discussion that followed a few deficiencies of the A.C. system emerged—in particular, the requirement for an A.C. motor developing a high starting torque. Mr. H. Zeffert took exception to Mr. Daglish's "statements on
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