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).
Page of calculations for motor resistance, including armature, field, and brush resistances.
| Identifier | WestWitteringFiles\T\November1928\ Scan058 | |
| Date | 27th November 1928 guessed | |
| -3- Contd. Forward terminal P.D. = 8.40 volts. Hence E.M.F. to overcome ohmic resistance = 4.62 volts Amperes = 180. Total motor ohms = .0256 = R Let field resistance = RF and armature " = RA Then R = 1.45 RA + RF to allow for brush and commutator resistance. The quantity 1.45 RA for total armature resistance with brushes is justified from estimations of motor resistances while on brake test. Suppose RF = .85 RA then R = 2.3 RA or RA = .0111 ohm. and RF = .0095 ohm. brushes being = .005 ohm. Total = .0256 ohm. These resistances are considered to be their values at 20°C in accordance with assuming engine cold for the value of its turning torque. Resistance of one armature turn at 20°C = .0111 x 4 / 37 = .00120 ohm. Length of armature turn = 14.5" approx. Resistance per 1000' = (12000 / 14.5) x .00120 = 1.00 ohm. But ohms of copper per 1000' at 20°C = .00815 ÷ cross sectional area in sq. ins. | ||