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 a dynamo burn-out, focusing on the functionality and limitations of the field fuse.
| Identifier | ExFiles\Box 70\3\ scan0257 | |
| Date | 23th November 1926 | |
| Contd. -2- EFC2/T23.11.26. We think the burn-out has resulted from running on open circuit at a moderate speed and voltage only, for the reasons :- (a) that the shunt cutout coil, though obviously having been hotter than normal, had not been excessively hot. (b) that the field fuse apparently had not, (.i.e. if properly in place) fused and relieved the situation. It may now be asked as to why the field fuse did not function. In regard to this, we think we have emphasized in several earlier reports that the size of the field fuse is rather critical, because there is not sufficient difference between the maximum normal current and the minimum abnormal current which it has to carry. The field fuse has to be such that it cannot become oxidised in normal running; otherwise it will, with alternate heating and cooling, progressively get weaker and eventually fuse. This means that it is not a complete protection to the dynamo in circumstances in which it will just carry a heavy field current continuously of one value, say 5.5 amperes, just below its fusing value, and in this connection I would like to draw your attention to the fact that we have made a definite point of this in the Instruction Book that we virtually do not guarantee this field fuse to be an infallible protection against damage to dynamos resulting from failure of the emergency fuse from any cause. Contd. | ||