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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).
Analysis of dynamo temperature rise, performance limitations, and failure modes due to heat.

Identifier  ExFiles\Box 62\3\  scan0336
Date  2nd March 1933
  
(2)
E.1/HP.2.3.33.

Temp. rise by resistance.
Armature.................. 129°C. 72.5°C 103°C.
Field..................... 102°C. 41°C 73°C.

(A) Clamped to a wooden bench; no cooling wind.
(B) Mounted on cylinder block immediately behind fan.
(C) Mounted on side of engine close to cylinder block and shielded by distributor.

We have always assumed that the armature will be 40° hotter than the yoke and this largely agrees with the Lucas figures.

In the case of a dynamo failure from heat we find invariably that the breakdown is caused by the commutator soldered connections melting. The commutator undoubtedly runs hotter than the armature but since the solder used has a melting point of 190°C and the cotton insulation of the armature windings begin to deteriorate shortly after 100°C has been reached it will be seen that there is no margin for protecting the insulation.

By long experience RR. have established a standard for setting their dynamos to a 65°C rise in temperature above atmosphere for limiting the performance, but we have never been able to get enough output from the dynamo to balance the full lamp load when running at night particularly at high speed, and the outputs we are expecting from vibrator controlled dynamos will still fall short of this desirable condition unless the lamp bulbs used are restricted in capacity below what is considered necessary on a fast car.

The aforementioned data has been put down to show how far we are up against temperature on this question, and to show that some improvement in cooling would be definitely useful.

We accept that RR. bonnet temperatures will be on the average higher than most cars on account of the absence of louvres but as the air passing through the radiator is only raised about 10 to 12° in temperature it is the conducted and radiated heat which will affect the dynamo in addition to its own internal heating.

RR. dynamos always have been cradle mounted alongside of the crankcase and in the case of the present 20/25 about
  
  


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