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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).
Comparing the features of Watford EW6A magnetos against the standard EW6 type.

Identifier  ExFiles\Box 65\3\  scan0384
Date  20th July 1929
  
R.{Sir Henry Royce} from EFC.
c. Sg.{Arthur F. Sidgreaves - MD}
c. By
c. E.{Mr Elliott - Chief Engineer} DA.{Bernard Day - Chassis Design} Wor.{Arthur Wormald - General Works Manager}

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EFC1/AD20.7.29.

WATFORD EW6A MAGNETOS.

x7791
x7792

The principal features of the above magnetos as compared with the present standard EW6 type are
(a) Still better low speed performance.
(b) Increased range to 66°.

It will be remembered that from time to time we have approached Messrs. North regarding the better application of principles in the design of the armature windings by which better low speed performance would be secured. As reported at the time in our EFCl/T8.5.28 we had prepared a group of curves of percentage short circuit primary current on a speed base for various magnetos, and found that the curves of the Watford magnetos were not very good, and in particular fell a good deal short of a similar curve taken off an R.Bosch magneto. This set of curves was sent to you under cover of the note above referred to; another set was shewn to a representative of Messrs. North with a verbal explanation of the principles involved. A little while later there arose from experimental road tests a demand for an increased range of operation of the magneto up to 66°. (present standard range about 59°).

During a later visit of representatives of
  
  


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