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).
Modifications to dynamo design to improve commutation and performance, including a new armature.
| Identifier | ExFiles\Box 166\1\ img099 | |
| Date | 20th April 1937 | |
| -2- Most of our work recently has been to try out different grades of brushes but without success as although we can produce a brush which wears less than the standard F.C.C. it is invariably much nosier. We are faced therefore with fundamental modifications to dynamo design to achieve better results. The easiest and most straightforward modification which will improve commutation is to use in this dynamo the experimental armature produced for the P.III. dynamo to RR.Sch.911, which has 36 slots and 72 comm. bars and which gives proportionately much better commutation. Our tests with this armature on the P.III. dynamo shewed an increase of approximately 50% in the permissible output at high speeds. The present cost of the Bentley and Wraith dynamo armature is £1. 17. 1. and this new armature will cost 9/9 more. Against this the D.O. have suggested modifications to the manufacture of the dynamo as outlined in R.R. scheme 1072 which should more than compensate for this increase. We strongly recommend the new armature as being the only way to directly improve the commutation on our high speed machines. It should be remembered that we ask for very low 'cutting in' speeds on our machines which at present 'cut in' at about 12 -13 m.p.h. on the Bentley, involving the dynamo being driven at 1 1/2 engine speed. Other high speed car manufacturers do not expect such low 'cutting in' speeds and therefore gear the dynamo to run slower. For instance, both Alvis and Lagonda dynamos run at engine speed. If we ran the Bentley dynamo at engine speed our 'cutting in' speed would be increased to 18 m.p.h. but the high speed performance would be very much improved. For continuous work in London traffic in the Winter 18 m.p.h. has always been considered to be too high although this has never been proved with Bentley chassis. To maintain the attractive feature of the low 'cutting in' on our dynamos therefore and still obtain good high speed commutation we must be prepared for our dynamos to cost more than other peoples. We are accordingly instructing Bentley dynamos to be made up for test on Experimental cars with the modifications referred to above. Rm{William Robotham - Chief Engineer}/Wst. | ||