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 on the design and performance considerations for starter motors.
| Identifier | ExFiles\Box 179b\1\ img147 | |
| Date | 7th November 1932 | |
| HS{Lord Ernest Hives - Chair}/WST. FROM R.{Sir Henry Royce} C. to SC. WOR.{Arthur Wormald - General Works Manager} BY.{R.W. Bailey - Chief Engineer} E.{Mr Elliott - Chief Engineer} ORIGINAL STARTER MOTORS. R2/W7.11.32. S-4 X6134. You give us some particulars of improvements made, and it would seem in reading the report that a cut and try, rule of thumb method, had been employed. The magnetic circuit of the motors and dynamos should be very carefully proportioned. I do not know the present practice, but I have in mind about 15,000 lines per sq. c/m.{Mr Moon / Mr Moore} being used in the armature core below the teeth, and the field magnet limbs, but the yoke rather lower than this, say 13,000., and the teeth at the point of smallest area might go up to 20,000. Assuming a total flux on these lines an estimate would be made of the ampere turns required to magnetise each part of the circuit. This analysis would shew whether the various sections are well proportioned - i.e. no part taking an excess of magnetising force. The next point will be to assume a certain battery current available, say a current that will reduce the battery electromotive force at its terminals from 12, to 8 or 9. The section of the copper would then be chosen according to the possibilities of space. It never pays to have one part abnormally high and another part abnormally low, but the current density in the armature conductors would naturally be more than in the field conductors. Care must be taken to watch the fall of potential between the brushes and commutator, and over the rest of the circuit, that is, outside the motor and battery. Derby formally sent me a complete analysis on these lines so that I should know what was happening, and be able to make any suggestions which came to my mind. I think that the amount of current taken from the battery will not be found to vary very much; if we take a larger current we take it for a less time, and if we take a small current we may want it for a long time. Stalling torque and current is of the very utmost importance, and should be given. Probably the battery terminal voltage will then have got down to 5 or 6 when this occurs, and the whole of the iron of the motor will be as saturated as economy permits. Naturally we should like to make the maximum current in both stalling and running conditions as low as possible for many reasons, including that of burning the brushes and contacts. We can wind for high speed and good efficiency, or for lower speed and less efficiency, and naturally a stalled motor shews no efficiency. R.{Sir Henry Royce} | ||