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).
Page discussing the characteristics of generators, particularly third brush machines, and their interaction with external circuits.
| Identifier | ExFiles\Box 61\2\ scan0217 | |
| Date | 12th December 1925 guessed | |
| -5- contd. where it has a characteristic of simultaneously rising volts and amperes, provided the characteristics of the external circuit are such as to prevent the volts and amperes from becoming excessive. Now this is what we do do with the third brush machines, but we do not do it directly for these reasons, obviously. By exciting the field winding from a portion only of the armature span between the main brushes we alter the family of characteristics of the generator in such a way that each one, except a few early ones, crosses its neighbour once as shown in diagram 2, these crossings taking place in the lower regions, where the simultaneous rise of volts and amperes on any one characteristic occurs. Now these crossings mean that so long as we are working on an external circuit with a characteristic confined to this lower region, a progressive rise of speed does not mean a progressive rise of volts and amperes, but an initial rise (see points 1 2 3 4 5 6 7 8) with a subsequent fall, and this is the speed ampere characteristic of such a generator, working upon an external(battery) circuit containing a back E.M.F. and a resistance. Clearly, the shape of the actual speed-ampere characteristic of a machine (always supposing the temper- ature and brush contact conditions to be reasonably, consistent), depends upon the exact characteristic of the external circuit, from which it follows that in comparing Contd - | ||