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 of correspondence discussing methods of varnish impregnation for coils, comparing dipping, vacuum, and vacuum-pressure systems.
| Identifier | ExFiles\Box 24\2\ Scan293 | |
| Date | 19th March 1928 guessed | |
| -6- Contd. "I have read with interest the recent correspondence on this subject and have been disappointed to note that it has been conducted in the main by correspondents interested more particularly in the manufacture of the plant rather than in its use." It is difficult to avoid being impressed with the fact that the authors of recent books on insulation, as well as of articles in your journal, have unanimously agreed that excellent results are obtained by introducing oil varnish into coils by a dipping process after preliminary heating and drying in a suitable oven. Some writers go so far as to maintain that vacuum impregnation is cumbrous and unnecessary, to say nothing of the initial expenditure, and their view is supported by the representatives of varnish manufacturers, who on more than one occasion have confidently stated that equal results are obtained by the dipping process; moreover, it is common knowledge that a number of well known firms adopt this simple and straightforward method, although there can be no question but that they could afford vacuum impregnating plants if that method were conclusively proved to be the best. It will, of course, be understood that I am not at present referring to Bakelite varnish. On theoretical grounds it is probably easy to prove that a combined vacuum pressure system of varnish impregnation is the best, but there are several points which your correspondents have not dealt with fully, but which are of primary importance; for example, one of your correspondents indicates that the pump should be capable of producing and maintaining a vacuum of about 30 inches, but on referring to makers' catalogues I find that none indicate that their pumps or plant are capable of such results or that this high vacuum is necessary. Another of your correspondents insists upon the importance of pressure, and on theoretical grounds I believe most engineers would agree, but only a user fully appreciates the inconvenience and annoyance which result when the application of such air pressures introduces moisture into his vacuum chamber. Every engineer who has had experience with compressed air realises that moisture will always be produced by condensation in the air receiver, and this takes place when compressed air is used in an impregnator, yet how many manufacturers of impregnating plant have realised this point and provided means for drying the compressed air before applying it to the surface of the varnish in the impregnating chamber? | ||