From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Letter from The British Thomson-Houston Co. Ltd discussing magneto flux curves and voltage waves.
| Identifier | ExFiles\Box 4\5\ 05-page060 | |
| Date | 20th May 1920 | |
| X 430 THE BRITISH THOMSON-HOUSTON CO.LTD LOWER FORD STREET, COVENTRY. May 20th 1920. Mr. H.I.F.Evernden, B.Sc., Expl. Drawing Office, Messrs. Rolls-Royce Ltd., Derby. Dear Sir, RE MAGNETO FLUX CURVES. X430 In reply to your letter of yesterday's date, it is obvious that if a magneto were built to give a "square shouldered" voltage curve, the low speed sparking performance would be very bad as compared with the results that could be obtained by using exactly the same armature core flux, and arranging the design to give the peaky voltage wave commonly adopted. When running at low speeds one can neglect the primary self-induction and assume that the current during the period of closure builds up in the primary in conformity with the primary voltage wave. With a peaky wave it is clear that the current would reach a considerably higher value (roughly equal to R/r) at the moment of break, than would be obtained if the voltage curve had a flat top and gave the same total area as the peaky wave. This point should be made clear by the rough curves given below. When operating at high speeds a "square shouldered" voltage wave should give good results, although it would still be at a disadvantage as compared with a peaky wave, for the reason that with the latter it is possible to trap a much bigger area during the period of primary closure per degree of angular movement of the armature. In an ordinary magneto the primary circuit is closed when the voltage is zero, and it is opened again after a period of about 100°. During this angular movement I suppose that something like 80% of the voltage wave is trapped and utilised to build up current in the primary circuit. If a "square shouldered" wave were employed, the percentage would be very little in excess of 50, and the value of the current broken at the contacts would be correspondingly reduced. Of course the amount of wave trapped in the latter case could be increased by extending the period of closure, but there is a limit to the possibilities of doing this, owing to the fact that after the contacts have opened | ||