Answer Happy

A 220 V, 1200 rev/min, 100 A separately excited DC motor is considered for a traction drive for which load inertia is large and start/stop duty is frequent. It implies that regenerative braking is desirable. A single-phase, 240V, 50 Hz supply is available. The motor armature resistance Ra = 0.25 9. (a) Explain, with sketches of operating quadrants and converter output voltage expressions, the suitability of a half-controlled versus fully-controlled thyristor bridge converters for the drive. You may assume that the inductance in the armature circuit is large enough to keep the armature current continuous. (b) The motor is required to develop its full rated torque at 1000 rev/min. If a fully- controlled thyristor bridge converter is used, calculate the firing angle for this condition of operation. (c) Calculate the firing angle for this converter if a braking torque equal to twice the rated torque is to be applied when braking is initiated at 1200 rev/min. (d) The motor is connected to a load through a 10:1 reduction gear-box. The total system inertia and viscous friction referred to the motor shaft are 1.5 kgm² and 0.1 Nm/rad/sec respectively. The minimum load torque is 375 Nm. Calculate the maximum possible deceleration of the load. (e) The motor runs a load at a speed of 450 rev/min when the firing angle is set to 60°. Calculate the developed torque Tm. (f) When load torque reduces substantially from the condition of (e), the motor speed rises to 900 rev/min. The firing angle is not altered. Determine whether the armature current will be continuous or discontinuous for this condition of operation.