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 explanation of the Smith Constant Current Dynamo, detailing magnetic fields and armature reactions.
| Identifier | ExFiles\Box 3\5\ 05-page178 | |
| Date | 8th March 1920 guessed | |
| THE SMITH CONSTANT CURRENT DYNAMO. This tends to distort the main magnetic field by sweeping it in the direction of rotation of the armature, and thus crowding the magnetism into one side of the pole shoe. This effect is known as distortion. (2) The magnetising force which acts directly to oppose or to assist the main magnetic field. This either weakens or strengthens the main magnetic field, and is known as “demagnetisation” or “magnetisation” as the case may be. In order to understand how the armature reaction regulates the output of the machine it will be well to consider the current distribution in the armature conductors and the resistances for the four most important speeds. Referring to Figs. 3, 4, 5 and 6, it will be seen that the armature conductors are divided by the brushes into four sections ab, bc, cd and da.{Bernard Day - Chassis Design} Two of these sections, ab and cd, are shown by full lines, and behave, as far as their magnetic effect is concerned, like a single coil of wire inclined at 45° to the main field poles. The other two sections, bc and da{Bernard Day - Chassis Design}, shown dotted, also behave like another coil inclined at 45° to the main poles on the other side. When the current in the dotted sections, bc and da{Bernard Day - Chassis Design}, is greater than that in the full line sections, ab and cd, it can be shown that the combined effect of the two coils tends to strengthen the magnetic field from the poles of the machine, and also to sweep the main field in the direction of rotation of the armature, the effects being those of “magnetisation” and “distortion”. When the current in sections ab and cd is equal to that in sections bc and da{Bernard Day - Chassis Design}, the joint effect of the two coils is almost entirely one of distortion, tending to move the magnetic field around in the direction of rotation of the armature. Finally, when the current in sections ab and cd is greater than that in sections bc and da{Bernard Day - Chassis Design}, the effect of the full line coil is greater than that of the dotted coil, and the resultant effect is one of “demagnetisation” and “distortion,” i.e., the main field is weakened and also swept along in the direction of rotation. Consider now Fig. 3, which shows the current distribution at point C in Fig. 2, when the dynamo is connected to the battery but has not commenced to supply any current. There is then no current from the main brushes, but a current of 4 amps is flowing in the resistances. From what we have already seen Page 3 | ||