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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 describing the operation of an automatic dynamo charging system and relay unit.

Identifier  WestWitteringFiles\O\2April1926-June1926\  Scan211
Date  12th June 1926
  
EFC1/T12.6.26. -11- Contd.

are reversely effective as compared with that of the volt
coil. There is of course the usual automatic cut-in and
cut-out in addition to this relay unit.
Supposing for the moment that the system is
unloaded, except for starter motor load at starting, every
time the chassis engine is started up we find the electro-
magnetic switch closed, so that the dynamo proceeds to cut in
and charge at the full normal rate. Such charging will
continue until by rise of back E.M.F. of the battery the
voltage of the system has risen to a pre-determined amount,
say about 17 volts. The automatic switch then opens and
the field resistance is inserted. The output of the dynamo
falls to this reduced output curve for varying speeds and
the switch remains in the open position so long as running is
continued, whatever the duration of the run, the battery
therefore receiving a relatively short initial 'boost' and
afterwards a reduced rate of charge continuously. On
stopping or slowing down sufficiently for the cutout to come
out, the relay switch is restored to the "on" position, and
then upon again starting or reaccelerating, the same process
is repeated.
If, now, during any run a load is switched on
to the system, this, if a comparatively light load, owing
to its reverse effect on the electro-magnetic switch, necess-
itates a voltage rise to a slightly higher value before the
Contd.
  
  


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