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).
Technical analysis of magneto primary current, considering electrical lag, ampere-turns, and eddy currents.
| Identifier | ExFiles\Box 4\5\ 05-page074 | |
| Date | 10th June 1920 guessed | |
| -5- Contd. the material. The number of turns makes no difference so long as this product is unaltered. (In a magneto, given the primary winding space, the number of turns will be fixed by considerations of a suitable current value to be broken at contacts of convenient size and a suitable voltage to exist across the gap, also of suitable quantity and voltage to be taken up by a condenser of practically convenient dimensions). It is important to realise, that, under these circumstances, as the electrical lag of the alternating primary current approaches 90°, either due to increase of speed or reduction of primary resistance, the ampere turns of the current approach a limit of such a value as/set up an M.M.F. exactly opposing at every instant the M.M.F. of the permanent magnet on the armature, so that practically no flux actually gets through the armature, i.e. only just the very small alternating flux required to maintain the current against the resistance of the primary circuit. Note that the existence of eddy currents in this case cannot cause the primary current to be again reduced at very high speeds. Fifthly. The actual case of the primary current of a magneto differs from the eddy currents and from the short-circuited alternator armature in that the circuit is periodically made and broken. The rise of current in the primary between the points of make and break will now be considered for different degrees of advance and duration of make, and for Contd. | ||