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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 a 20 HP starter motor.

Identifier  ExFiles\Box 35\4\  scan 156
Date  4th July 1928
  
X.4384

4.7.28

X.4384. NEW ASPECT OF CALCULATIONS FOR
20 H.P. STARTER MOTOR, DATED 1.7.21.

Engine 500 lb-inches.
96.5 r.p.m.

On conductors:-

610 lb-inches at 96.5 r.p.m.
or at 8.4 : 1 ratio
72.7 lb-inches at 810 r.p.m.

Power .935 H.P. = 697 watts
Elec. eff. assumed .55
Power supplied = 1266 watts.
137 a at 9.26 volts.
Back E.M.F. = 5.10 volts.
Volts = 5.1
Speed = 810 r.p.m.
Torque = 72.7 lb-inches.

Resis = 4.16 / 137 = .0304 ohm

ØZ = (60x10^8 x 2 x 5.10) / (810 x 4) = 18.9 x 10^6

ØZ = (10^8 x 2 x 72.7) / (1.41 x 4 x 137) = 18.9 x 10^6

Ø = 161,000. Z = 116

EFC.
  
  


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