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).
Technical memorandum discussing the design of the iron core for an ignition coil.
| Identifier | ExFiles\Box 179\3\ img079 | |
| Date | 8th March 1932 | |
| SECRET. To R.{Sir Henry Royce} from Hs{Lord Ernest Hives - Chair}/Wst. C. SG.{Arthur F. Sidgreaves - MD} C. HOR. C. EV.{Ivan Evernden - coachwork} Hs{Lord Ernest Hives - Chair}/Wst.3/AMB.8.3.32. K6058 JAPAN 3. AND PARAGRID IGNITION COIL. With reference to H2/M24.1.32 we have been further considering the proposals outlined therein with regard to the iron core of the ignition coil, and would suggest that with this type of coil where the magnetic circuit is partly completed with an iron sheath round the coil, "hedgehogging" is unnecessary. A laminated iron sheath is necessary round the outside of these types of coils in order to confine the flux patch within the case so that no excessive damping can occur due to flux interlinking with the circular tin case. To complete the iron circuit any further would tend to increase the iron losses and consequently the damping on the secondary e.m.f. volt wave. Copper losses in the windings as compared with iron losses are small and the small advantage gained by increasing the winding inductance due to decreasing the magnetic circuit reluctance is out weighed by the reduction in the secondary, H.T. voltage caused by increased iron losses. There would appear to be an optimum condition for the reluctance in an ignition coil magnetic core circuit as compared with the winding quantities for a maximum secondary e.m.f. output and we believe with the outside secondary type of coil we were very near it so that with our new design of coil the proportion of air to iron should be kept approximately the same. This is all the more necessary as the inductance of the primary winding is automatically considerably increased due to the extra length of the mean turn. As a matter of interest we carried out a test some time ago increasing the iron circuit of our standard coil and although the primary inductance was increased, no increase in the secondary e.m.f. was noted. Another important point is that if we bend over the ends of the core strips the insulation problem becomes serious as with this type of coil the core is at a high potential; also the cost of assembly is increased. We should be pleased to have R's remarks on the above Hs{Lord Ernest Hives - Chair}/Wst. | ||