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).
Calculations for an E.A.C. starter motor.
| Identifier | ExFiles\Box 64\1\ scan0117 | |
| Date | 30th April 1923 | |
| X.9620 - E.A.C. STARTER MOTOR. (Calculations based on data given). Torque required to turn E.A.C. engine - 900 lb-inches (say) Speed reduction ratio = 32 Taking 85% gearing efficiency effective torque ratio = .85 x 32 = 27.2 Motor torque when rolling engine is therefore required to be 33.1 lb-inches Engine speed required to be 55 R.P.M. Motor " " = 1760 R.P.M. Motor power (rolling condition) is therefore .925 H.P. 1.05 Assuming 65% efficiency electrical H.P. becomes 1.42 Watts = 1060 Amperes = 108 Volts = 9.84 Static torque at this current must be 10% greater (say) than the running torque, i.e. must be 36.4 lb-inches. Take a useful polar flux density of 5600 lines per cm², reckoned on minimum pole face. Armature diameter = 3.100" Armature circumference corresponding to 69% polar embrace = 69/400 x π x 3.100 = 1.68" = 42.7 mm. | ||