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).
Investigation into dynamo cut-out failures due to insufficient contact pressure and proposed design improvements.
| Identifier | ExFiles\Box 168a\3\ img142 | |
| Date | 25th January 1935 | |
| Ha/Wat.5/WA.{Mr Wallis}25.1.35. R.R. DYNAMO CUT-OUT. Owing to complaints having been received of cut-out points oxidising and also of the red warning light coming in at certain high speeds we came to the conclusion that the cause of the trouble was insufficient contact pressure on the points. This trouble is accentuated on the constant voltage controlled system due to the fact that at high speeds when the battery is charged and there is no appreciable load on the system, the charging current is very small. This reduces considerably the series coil excitation and therefore reduces the available magnetic force holding the contacts together. An investigation into the electro-magnetic design of the cut-out was accordingly carried out and it was found that the iron magnetic circuit was very much over saturated causing the total magnetic pull on the cut-out to be disproportionately small in comparison with the electro magnetic force as expressed in ampere-turns on the windings. An analysis made of the magnetic circuit shewed that the two factors causing most of the inefficiency were the iron cross section of the armature and the excessive flux leakage caused by using iron flanges on the coil bobbins. The contact pressure of the standard cut-out with 12.0 volts across the shunt coil and a current of 15 amperes flowing through the series coil was 9 1/2 ozs. We consider this very inadequate. By deleting the iron coil flanges this pressure was increased, for the same currents in the coils, 13 1/2 ozs. By the fitting of a new M.C. armature having a cross section of .735 sq.cm. as compared with .4375 sq.cm. standard, the pressure was increased to 20 ozs. The attached curve shews graphically the increased pull obtained against ampere-turns. | ||