A 12.5-kW, 230-V shunt motor has 2400 shunt-field turns per pole, an armature resistance (including brushes) of 0.18Ω, and a commutating-field resistance of 0.035Ω. The shunt-field resistance (exclusive of rheostat) is 375Ω. When the motor is operated at no load with rated terminal voltage and varying shunt-field resistance, the following data are obtained:

The no-load armature current is negligible. When the motor is operating at full load and rated terminal voltage with a field current of 0.468 A, the armature current is 58.2 A and the speed is 1770 r/min.

a. Calculate the full-load armature reaction in equivalent demagnetizing ampere-turns per pole.
b. Calculate the electromagnetic and load torques and the rotational loss at the given operating condition.
c. What starting torque will the motor produce with a field current of 0.555 A if the starting armature current is limited to 85A? Assume that the armature reaction under these conditions is equal to 175 ampere-turns per pole.
d. Design a series field winding to give a speed of 1575 r/min when the motor is loaded to an armature current of 58.2 A and when the shunt field current is adjusted to give a no-load speed of 1800 r/min. Assume the series field will have a resistance of 0.045 Ω.