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).
Permanent magnetisation, flux leakage, and the differences between magneto and electro-magnet circuits.
| Identifier | ExFiles\Box 35\1\ scan 179 | |
| Date | 11th August 1920 | |
| R.R. 258a/100 T.(S.H. 159, 11-8-20) G.{Mr Griffiths - Chief Accountant / Mr Gnapp} 2820 -9- Contd. (1) It tends to remove the permanent magnetisation. (2) It causes leakage of induction through the space between the poles other than through the armature. The leakage may be taken as approx. proportional to the air gap for small air gaps. Note that if we start with 100% flux of magnetisation and a leakage of 10% with a certain width of air gap, so that there is 90% through the armature, then if we double the air gap the leakage is 20% and the useful flux 80%. There is this important difference between the magneto cases and the electro-magnet. In an electro-magnet circuit the air gap is the chief reluctance and doubling the air gap may nearly halve the useful flux. The difference is this, that the flux in this case, is maintained by an impressed magneto motive force whereas in the magneto case there is no resultant M.M.F. round the whole circuit. There is forward M.M.F. across the air gaps and armature and a reverse demagnetising M.M.F. of equal amount in the substance of the magnet. In the magneto case, we rely upon the property of permanent magnetisation for our flux. Concerning the magnetisation of permanent magnets, it would seem that we ought to magnetise these in such a manner as to impress the magnetisation which we wish to be permanent, in a stream line formation of magnetic molecules of the same distribution as we wish it tohave when actually supplying flux to the armature. Magnetisation with an electro magnet Con... | ||