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).
Weakness of the third brush control system in 20 HP dynamos, leading to overheating.
| Identifier | ExFiles\Box 51\1\ Scan248 | |
| Date | 27th September 1923 | |
| To R & E from EFC. {E. Fowler Clarke - Electrical Engineer} c. CJ. & PN. {Mr Northey} c. EY. WOR. {Arthur Wormald - General Works Manager} EP. {G. Eric Platford - Chief Quality Engineer} DA. {Bernard Day - Chassis Design} c. Mr. Brock. X4383 EFC {E. Fowler Clarke - Electrical Engineer} 4/T27.9.23. X. 4383 - RE 20 H.P. DYNAMOS. X4383 A weakness of the third brush system of control, (in which protection is not afforded by a field fuse of small capacity) and which may not perhaps so far have been fully realised, is that a partial sticking or jamming of the main brush, which is connected only to the external circuit (in our case the negative brush) is likely to lead to overheating of the dynamo by self-excitation, in the same way as if the battery were disconnected, even though there may be sufficient partial or intermittent contact of the brush with the commutator to cause a 'charge' reading on the ammeter. At the same time the heating of the machine due to self-excitation would be augmented by the heating of the commutator, due to flashing at this brush. It is also possible that an excessive output may be due to the same cause, as excessive E.M.F. existing in the armature may cause in the battery circuit, on the average, i.e. through the intermittent contact, a larger current than if this E.M.F. were properly held down by satisfactory contact at the brush. In any case it is known that the insertion of resistance in the external circuit, which is in some degree equivalent to this, would cause the dynamo to run with a higher E.M.F. Cont.d | ||