(Solved): The figure shows the change in the speed of the DC shunt motor when a resistance is added to the ar ...

The figure shows the change in the speed of the DC shunt motor when a resistance is added to the armature circuit, provided that the flux $\Phi$ remains constant. Nominal values $P=5\mathrm{kW},\mathrm{U}=200\mathrm{V}$, nominal speed $1500\mathrm{rpm}$, shunt excitation winding $\mathrm{Rf}=100\Omega$, total armature resistance $\mathrm{Ra}=0.25\Omega$ When loaded at the nominal power of the DC shunt motor, its efficiency is $80\%$. a) Calculate the armature current at rated operating condition. b) Calculate the ideal number of revolutions (rom) in the idle state of the engine. c) While the motor is rotating at $750\mathrm{rpm}$, the armature current is half of its nominal value. Find the value of the R1 resistor (connected in series) that must be included in the armature circuit to run the motor at $750\mathrm{rpm}$.