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).
Explanation of the Smith Constant Current Dynamo, focusing on armature reaction and output adjustment.
| Identifier | ExFiles\Box 3\5\ 05-page181 | |
| Date | 4th February 1920 guessed | |
| Page 7 THE SMITH CONSTANT CURRENT DYNAMO. the effect of armature reaction is one of "magnetisation" and distortion." The field poles are thus being strengthened by the armature reaction. Fig. 4 shows the current distribution when the currents in the resistances are just equal to the main output of the dynamo, as shown at D in Fig. 2. It will be seen that the dotted coil is still more powerful than the full line coil, so that the effect of the armature reaction is still to strengthen the main field and to distort it. This causes the output to rise very rapidly due to the strong main field. In Fig. 5 is shown the current distribution at the point E in Fig. 2, when there is no current in the resistance, the current from the dynamo being 5 amperes. At this point the auxiliary brushes could be removed without affecting the working of the machine, which is behaving exactly as an ordinary shunt wound dynamo. The only effect of the armature reaction is one of "distortion," the main field being neither weakened nor strengthened to any appreciable extent. The current distribution at full output is shown in Fig. 6, from which it will be seen that the full line coil is now more powerful than the dotted line coil, the effect of armature reaction is being "demagnetising" and "distorting." This means that the main field is much weakened, and also swept across the pole shoes to such an extent that the conductors in sections lc and da{Bernard Day - Chassis Design} are practically inoperative in generating any voltage. If the speed of the machine is still further increased, the current in sections dc and da{Bernard Day - Chassis Design} commences to increase again, but in the opposite direction. Both the full line and dotted coils now act to demagnetise and distort the field, thus effectually preventing any increase in the current output. Summarising the foregoing results, and referring to Fig. 2, it will be seen that between speeds of 0 and 725 revolutions per minute the armature reaction assists to magnetise the field, and at speeds above 725 revolutions per minute the armature reaction weakens and distorts the field. The result is that the machine begins to charge the battery, i.e., "cuts in" at a low speed and that the current output rises very rapidly to its final value, remaining constant when this value is attained. ADJUSTMENT OF OUTPUT OF DYNAMO. The output of any given dynamo of this type can be adjusted within limits by rotating the four brushes, so that while they | ||