A dc, separately excited motor is used to drive a constant-torque load. The field circuit is excited by a full-wave, ac/dc SCR converter. The armature circuit of the motor is connected to a constant dc voltage source of 160 V. The inductance of the field circuit is large, and the field current is always continuous. The ac voltage (input to the converter) is 120 V (rms), and the field resistance is 100 ?. The armature resistance is 2 ?. When the triggering angle of the SCRs is adjusted to zero, the motor speed is 1200 rpm, and the armature current is 10 A.

Question

a. Calculate the average current and dc power of the field circuit when the
triggering angle is equal to 20o.
b. Calculate the rms voltage across the field windings for the condition given in
(a). Explain how the rms voltage is dependent on the triggering angle.
c. Calculate the no-load speed of the motor. Ignore the friction and windage
losses.
d. Calculate the triggering angle to operate the motor at a speed of 1400 rpm.
e. Can you use the field converter to reduce the motor speed to 1000 rpm? How?

Answer 1

e. We can only reduce the field current using the full-wave rectifier. The motor

speed for the given constant load is 1200 rpm at the minimum triggering angle

(? = °0 ), which corresponds to the maximum flux. Thus, the same converter

cannot be used to reduce the speed to 1000 rpm for the given load unless the

voltage applied to the converter is increased. (This will result in over fluxing of

the machine, so care should be taken to avoid saturation.)