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).
The switchbox and cutout arrangement for a dynamo charging circuit.
| Identifier | ExFiles\Box 65\2\ scan0059 | |
| Date | 11th November 1930 | |
| X7730 AD. 70 11.11.30. X.5730 X.7730. SWITCHBOX AND CUTOUT ARRANGEMENT X.7740. The standard arrangement we have adopted for the charging switch operation and cutout shunt coil connection is the third of the four cases described in EFC {E. Fowler Clarke - Electrical Engineer} 4/T16.5.28 and AD.28 of 23.2.29 (X7730, 7740) in which the shunt coil is connected to the field side of the dynamo and the switch breaks the field circuit first. Experiments showed that breaking the field circuit when the dynamo is charging normally produces a less destructive spark than breaking the armature circuit in spite of its higher inductance, owing to the much smaller current involved. With the cutout shunt connected to the field, moreover, on breaking the field the voltage falls away very rapidly owing to the field inductance, in fact, momentarily reversing, so that the cutout falls out, breaking the armature circuit which is of low inductance, by the time the armature current is somewhere near zero value so that the cutout spark is very slight. On the production test rig, Mr. Watson has used the second of the arrangements in the above report in which, except at low speeds, there is no spark at the cutout points. He has found also that the switch used has stood up to some thousands of operations without failure. This arrangement, however, is not the great advantage to the cutout that might at first appear, as in any case a moderately large spark usually occurs at the contacts on closing if switched on with the dynamo running, and this is the same whatever arrangement is adopted. A small spark at break, therefore, is of minor consequence. In addition, it is undesirable to have the cutout coil connected to the armature side of the dynamo, as in the event of No.1 fuse failing, or anything electrically similar, if the dynamo is running at above a moderate speed, the field is then fed by the battery against part of the E.M.F. of the armature, which E.M.F. now becomes abnormal and holds in the cutout. This condition | ||