Answer Happy

Q4. (a) Briefly describe how inverters create a sinusoidal waveform using Pulse Width Modulation (PWM). Draw the relevant diagram. [4 marks] (b) You are part of a team trying to restore a historic electric locomotive, which is driven by a large induction motor. During the restoration process, the induction motor was refurbished and, due to its age, its rated torque has been reduced to half of its initial rated torque. Before refurbishment, you tried to test it, and applied a mechanical load with inertia JL = 600 kg.m2. You measured the inertia of the motor's own rotor, and found it to be JM = 1,500 kg.m2. The motor managed to accelerate to 906 rpm within 50 seconds. The synchronous speed is 960 rpm. Determine how much time it would take for the motor to accelerate to the rated speed, as well as the rotor actual speed after the refurbishment. - [10 marks] (c) Draw the approximate phasor diagrams of a synchronous machine for the following load currents. The different values do not need to be calculated, only the shape of the phasor diagram is sufficient. (1) 1 = 20 20° [2 marks] (ii) 1 = 142-35° [2 marks] (iii) 1 = 33222° [2 marks]