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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).
The operation of an auto switch for a dynamo system, including testing, potential disadvantages, and future modifications.

Identifier  ExFiles\Box 59\1\  Scan256
Date  12th December 1927
  
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taken in regard to the obtaining of a patent for this piece of apparatus.

In our EFCl/T2.12.27 we mentioned a point which we thought at that time might be a small snag, viz. that when the auto switch operates at a fairly high speed, the cutout might not come out owing to insufficient reverse current back through the machine against its E.M.F. By testing on the road we have found that this does not occur as the auto switch only operates within a range of comparatively moderate speeds when the output of the dynamo is in the neighbourhood of its peak output. We do not see that the fact that the auto switch will not operate at high speeds when the dynamo output is smaller is any particular disadvantage. After all, the auto switch is a voltmeter and any type of voltmeter, indicating or automatic, would be subject to the same fall of voltage due to the generator output being reduced. In due course, however, we shall have under consideration a modification of the dynamo output curbe to keep this up more in the later stages than was expedient with the system without the auto switch. For the time being we are experimenting on experimental cars with out dynamos set for larger output than normal - in fact set up to what we call the "maximum permissible output", i.e. maximum permissible output when not subject to the action of the auto switch.

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