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 use of Bakelite for impregnating coils, armatures, and condensers.
| Identifier | ExFiles\Box 39\3\ Scan021 | |
| Date | 21th December 1920 | |
| Qy2/G21.12.20. -2- Contd. Bakelite impregnation of coils. X.753. X.3343. X.3878. X.3966. X.3331. X.543. The Westinghouse motor armatures are varnished with Bakelite varnish and baked, the result aimed at being to make the coil practically solid so that banding is unnecessary. Other makers do this also and banding even of large armatures is being gradually discontinued (with the exception of steam-turbine generator armatures) as the banding is looked upon as introducing additional risks of failure, while it is found that a properly impregnated covering of Bakelite will hold the coils in place equally well. This may explain to Mr. Clark why the American electrical machines are generally not banded, a fact which he has criticised. As he has pointed out, if for any reason the varnishing or baking is improperly done, the coils may lift out of the slots. The small number of rejections or defective machines at the Westinghouse factory on an output of something like 2000 motors a day, shows that this does not often occur. Motor field windings are also generally coated with Bakelite and baked solid in the same way. In small generators, in order to get a higher copper efficiency, the field coils are generally wound with enamelled wire, a special non-cracking baked enamel being used and each coil carefully checked for shorts. If Bakelite were used on these it would dissolve the enamel, so that cotton taping would be necessary. For this reason small generator field coils are not Bakelite impregnated. The enamel used generally is Tung oil enamel of Chinese origin made from cotton seed. In small generator armatures, which are hand wound, Bakelite impregnating would make it impossible to replace a shorted coil and is therefore not used. With motors the risk of a short is less because stiff copper strip is used, form wound. It was explained that a proper Bakelite impregnation would make it quite impossible to remove a coil without wrecking the armature. X.3648. It will be remembered that in my descriptions of the Frazer electric transmission last year I referred to the inventors use of two hollow armatures which were made up of taped copper strip impregnated with Bakelite and baked, and that the same outer armature was also the flywheel of the engine and transmitted the fluctuating torque through the copper strips and impregnating material. Mr. Frazer said that, after coating and baking, his armatures were as solid as metal for all practical purposes. Condensers: X.2512. Condensers are habitually made over here by Westinghouse, Bosch and others, not only for ignition but for wireless outfits and larger work, without Mica, by building the tinfoil with strips of paper or Bakelite-Micarta in between and impregnating with Bakelite varnish and then baking. (Mica is still used for magneto condensers, however, because of its higher dielectric strength.) | ||