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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 and specifications for a Japan Sequence Starter Motor.

Identifier  ExFiles\Box 62\1\  scan0100
Date  9th December 1932
  
X6091
9.12.32.

JAPAN SEQUENCE STARTER MOTOR.

Engine torque when turning cold (0°C.) estimated at 600 lb.inches.

Engine speed at this torque estimated at 68 R.P.M. approx.

Gear ratio 12.7 : 1.00

Motor speed = 880 R.P.M.

Assuming an overall mechanical efficiency of 76.5% the required motor torque will be 60 lb. ins.

The motor power is therefore :-
.01184 x 60 x 880 = 668 watts.

Assuming an electrical efficiency of .40, under the above conditions, the applied power would be 1670 watts or 220 amperes at 7.60 volts.

Assume a useful flux density reckoned at the pole face of 6500 lines per sq.cm.

Armature diameter = 2.800" (average)
Armature circumference corresponding to 70% Polar arc
= (70/400) x 3.142 x 2.800 = 1.54" or 3.91 cm.

Armature length 3.000" or 7.62 cm.

Flux per pole = 3.91 x 7.62 x 6500
= 194 x 10³ lines

Number of armature conductors required therefore :-
= (Tb x 10⁸) / (1.41 x P x ΦC)

= (60 x 2 x 10⁸) / (1.41 x 4 x 194 x 10³ x 220)

= 50

Suggest trying a 29 slot armature with 29 coils, one turn per coil.

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