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).
Dynamo field fuse sizing and testing to prevent damage under open circuit conditions for 20 HP and 40/50 models.
| Identifier | ExFiles\Box 70\3\ scan0258 | |
| Date | 23th November 1926 | |
| Contd. -3- EFC2/T23.11.26. If it so happens that the dynamo is run at one moderate speed just within this condition, it is not impossible for damage to occur to the dynamo in these open circuit conditions. Had the dynamo been cold and suddenly accelerated to a high speed under these open circuit conditions, there is little doubt but that the field fuse would have gone, but it appears to be an unlucky case in which conditions have been such that the field fuse could not save the dynamo. As a matter of fact, the size of this field fuse was decided by experiments on the 20 HP. dynamo, and system, and was progressively put up by two stages from 40 SWG. via. 39 SWG. to 38 SWG. until absolute security in normal running was obtained, and in the case of the 20 HP. the risk of the abnormal conditions in which, though on open circuit, the fuse is not melted, is very much less than is the case on the present 40/50. The reason is that the field excitation current on the 20 HP. is larger than that on the 40/50, and it would probably be quite O.K. to use field fuses for the 40/50 of finer gauge, say 39 or even 40 SWG. Although it is perhaps undesirable to have fuses of two different ratings, nevertheless perhaps it would be advisable for us to make experiments in this direction to offer some security against burn-outs on the 40/50, without interfering with the normal running. It would be necessary for the proposed size to be very thoroughly tested before | ||