The speed of a separately excited motor is controlled by a single-phase semiconverter in Figure 14.12. The field current is also controlled by a semiconverter and the field current is set to the maximum possible value. The ac supply voltage to the armature and field converter is one phase, 208 V, 60 Hz. The armature resistance is Ra = 0.11 Ω, the field current is If = 0.9 A and the motor voltage constant is Kv = 1.055 V/A rad/s. The load torque is TL = 75 N # m at a speed of 700 rpm. The viscous friction and no-load losses are negligible. The armature and field currents are continuous and ripple free. Determine (a) the field resistance Rf; (b) the delay angle of the converter in the armature circuit αa; and (c) the input power factor (PF) of the armature circuit.

Figure 14.12

$ La Ra M (a) Circuit (b) Quadrant 0 da aa 17 77 #+0 #+a (c) Waveforms 2T 21 Gof 60/ cof