Question

3. A controlled single-phase fiull-wave controlled rectifier is connected in a de motor drive in Figure 4. The de motor armature inductance can be assumed large enough such that any motor curent ripple is negligible. (a) Assuming negligible supply reactance, show that the average rectifier output voltage V, in tems of the supply ms voltage E and the firing angle a, is given by 2/2E (6 marks) cos a _ (b) Given that the supply voltage is 230V, 50HZ and that the motor armature resistance is 0.25 , determine the firing angle required to maintain a motor current of 20A when develops a back-emf, E, of 170V and sketch the waveform of the rectifier (8 marks) the motor output voltage vyy for this value of firing angle. (c) An internal combustion engine is now connected to the motor shaft to drive the motor generator. The motor is driven in the reverse direction to generate a back-emf, E of-150V. The drive fining angle is adjusted to obtain a motor curent, a, of 24A. as a O Draw the de equivalent circuit for the de side of the circuit, labelling carefully the motor current direction and the magnitude and polarity of each voltage /back-emf in the circuit (3 marks) ) Determine the firing angle required for this condition and sketch the waveform of the rectifier output voltage vyy for this value of firing angle (6 marks) (d) When the de machine is used as a generator, we have power transferred from the dc side to the ac side of the converter. Explain why even in this condition, the availability of the ac supply is essential for the corect operation of this circuit (4 marks) (e) Sketch the wavefom of the supply current, i and show its phase relationship to the supply voltage, v for the motoring condition in (b) and the generating condition in (c) (6 marks) R L i i. T14 T24 + Vm T34 T44 Figure 4

Answer 1

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