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 explaining the effects of eddy-currents and flux changes in an armature core and pole pieces.
| Identifier | ExFiles\Box 4\5\ 05-page072 | |
| Date | 11th August 1929 | |
| -3- Contd. change of flux in the armature core and pole piece, for suppose a very small change to take place, E.M.F's will be set up in the various conducting paths, which will cause currents, and these currents will be of such magnitudes and directions as to cause M.M.F's (magneto-motive forces) almost exactly opposing the M.M.F. of the permanent magnet at every instant. Thus it will be seen that the effect of eddy-currents in the extreme case is to prevent any great change of flux ever taking place in the armature and pole pieces, in spite of rotation in the magnetic field, even at a very slow speed. The case is that of a short-circuited-alternator of negligible armature resistance. The alternating armature current lags nearly 90° in phase behind the E.M.F. due to the main flux, (i.e. open circuit E.M.F.), and the demagnetising effect of the armature on the field is a maximum and nearly complete. The same condition results, even if the armature resistance be not negligible at slow speed, provided the speed be sufficiently increased as to again render the armature resistance relatively negligible. If the armature resistance were considerably increased, or the speed reduced, the armature current would only lag say 10° behind the open circuit E.M.F.* A demagnetising effect would still exist, of a much smaller value, but there would be a cross magnetising effect tending to distort the main flux. Thus the effect of increasing the eddy current Contd. * Max.value would then occur roughly at position given by Fig.3). | ||