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).
Theoretical analysis of magneto operation, discussing eddy current loss, flux, and primary coil windings.
| Identifier | ExFiles\Box 4\5\ 05-page073 | |
| Date | 10th June 1920 | |
| -4- Contd. loss in the armature core and pole pieces of a magneto, either by reducing the resistances of eddy current paths or by increasing the speed, is to reduce the flux in the direction of rotation, and ultimately to choke it back altogether. Or we can put it this way, that any change of flux in core and pole pieces is hindered by eddy current loss, and the more so, the quicker we require these changes to be. The backward pulls, due to this cause on the armature are longer and larger than the forward, and the work done on the resultant drag appears as heat due to the currents overcoming the ohmic resistances of the conducting paths. Fourthly, suppose the armature of the magneto to be wound with a primary coil only. If this is continuously short circuited the case lies between the 90° and 10° considered above for the short-circuited alternator, except for very slow speeds, and approaches the 90° case closely for high speeds. That is to say, for high speeds, the current (or nett E.M.F.) lags nearly 90° behind the open circuit E.M.F. (gross E.M.F.), the nett E.M.F. being the small difference between the nearly exactly opposed open circuit E.M.F. and the E.M.F. of self induction of the current in the primary winding. The exact phase of the primary current at any given speed depends upon what may be called the total cross sectional conductance of the winding, a quantity proportional to cross sectional area of one wire multiplied by the number of turns, and to the specific conductance of Contd. | ||