To calculate the speed of the DC shunt machine when running as a motor taking 40 A at 250 V, we need to consider the armature reaction and its effect on the field.
Given:
Open-circuit voltage (V_oc) = 250 V
Speed (N_oc) = 1000 RPM
Armature resistance (R_a) = 0.5 Ω
Field resistance (R_f) = 250 Ω
Motor current (I_m1) = 4 A
Motor current (I_m2) = 40 A
Supply voltage (V) = 250 V
Armature reaction weakens the field by 4%.
First, let’s calculate the field current (I_f) and the field weakening percentage: