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).
Analysis of magnetic circuit design for energy storage, comparing closed versus open circuits and the impact of core size and windings.
| Identifier | WestWitteringFiles\V\December1930-February1931\ Scan189 | |
| Date | 28th January 1931 | |
| -2- RFC3/AD28.1.31 contd. energy has to be supplied and stored in a form in which it is again available, would render the use of a completely closed magnetic circuit out of the question owing to the phenomenon of hysteresis in the iron which results in a very large proportion of the supplied energy being stored in a form in which it is not again available. The locality of stored magnetic energy in the case of an ignition coil is far less in the iron itself than in the air space through which the magnetic lines pass in completing their magnetic circuit through the core and this is the case, providing the air space is not too small in distance measured round the circuit. The air space being of reasonable dimensions, the advantage of the nearly closed magnetic circuit would appear to lie more particularly in the better degree of mutual coupling between the primary and the secondary, which almost certainly results in a smaller proportion of the parasitic oscillations above referred to. Practically the same result would be brought about by using a much longer straight core with the present length of windings, as it may be considered proved (by voltage observations) that where the secondary windings extend to a relatively large diameter over a core that is not much longer than the windings themselves, the parasitic or surge effects which have been referred to are enhanced. Hence the reason for hedgehogging and also for turning round the core to form a more nearly closed magnetic circuit. From the point of view of the storage of energy, which must in any case be mainly in the air space, it is of not much consequence for the main effect whether it has to be stored in a more concentrated form in a small air space or in a much less concentrated form in the large air space which virtually would exist if the core extended to a greater length than at present beyond the winding at each end, in which case the result is similar to that of the more nearly closed magnetic circuit. This, of course, is on the assumption of the quantity of energy stored remaining the same in the two cases. The use of a larger core would clearly be an advantage in saving copper but perhaps would introduce augmented difficulties of insulation arising out of increased voltage per turn and particularly as the interlinking from a mutual coupling point of view would then be less good unless the core (only) were also simultaneously lengthened at each end. | ||