QUESTION 1 a. How can a silicon controlled rectifier (SCR) be turned off? (Total 10 marks) (1 mark) b. What is required in Figure 1 to turn off the LASCR and de-energized the relay? (3 marks) c. Write five features of an IGBT. (2 marks) d. Determine the gate trigger current and the anode current when the switch, SW1 is momentarily closed in Figure 2. Assume VAK 0.2V, Vox=0.7V and IH-5mA. (2 marks) e. Draw SCR characteristic curves for three IG values and explain briefly the role of holding current during forward conduction region. (2 marks) Voc VTRIG 3V Ra www 5.6 k Alarm, door opener, etc. FIGURE 1 330 SW2 2N5060 SWI FIGURE 2 Voc QUESTION 2 - (a) The buck dc-dc converter of Figure 3 has: 60V input 0.45 duty ratio 150uF capacitor (Total 10 marks) 530uH inductor 10kHz frequency 200 load resistance Assuming ideal components, calculate 1. The output voltage of the buck converter. (2 marks) ii. The maximum and minimum inductor currents. (4 marks) iii. The output voltage ripple. (4 marks) + www FIGURE 3 ww QUESTION 2 - (b) (Total 10 marks) Design a dc-dc boost converter that will have an output of 60V from a 24V source. Design for continuous inductor current and an output ripple voltage of less than one percent. The load is a resistance of 250. Assume ideal components for this design. You have to calculate the following: i. Duty ratio. ii. Minimum value of the inductor (Lmin). iii. Inductor current (IL). iv. Maximum and minimum currents (Imax, Imim). (1 mark) (2 marks) (2 marks) (3 marks) V. Output capacitor value. (2 marks) (Total 15 marks) QUESTION 2 - (c) The bridge rectifier circuit of Figure 4 has an ac source with Vm 100 V at 60 Hz and a series RL load with R-100 and L = 10 mH. i. Determine the average current in the load. ii. (2 marks) Estimate the peak-to-peak variation in load current based on the first ac term in the Fourier series. (4 marks) iii. Determine the power absorbed by the load and the power factor of the circuit. (3 marks) iv. Determine the average and rms currents in the diodes. (1 mark) D₁₂ D₁ v,(1)- V sin cor D₁₂ ŹD₂ BRIDGE RECTIFIER WITH AN RL LOAD Figure 4 QUESTION 2 -(d) (Total 10 marks) For the dc-ac inverter in Figure 5, Input dc voltage is 100V with 100 load. Use inductor value of 25 mH and 50 Hz frequency. i. Determine the amplitudes of the Fourier series terms for the square wave load voltage. (4 marks) ii. The amplitudes of the Fourier series terms for the load current. (4 marks) iii. Power absorbed by the load. Q1 DI D3 Q3 Vdc Q4 D4 D2 Q2 FULL-BRIDGE INVERTER USING IGBTS Figure 5 (2 marks) QUESTION 2 -(e) i. What is a Cyclo-converter? ii. How can the speed of an induction motor be reduced? (Total 5 marks) iii. The single-phase ac voltage controller of Figure 6 has a 120 Vrms, 60 Hz source. The load resistance is 15 Q. (1 mark) (1 mark) Determine the delay angle required to deliver 500 W to the load. (3 marks) FIGURE 6 QUESTION 3 - (a) (Total 10 marks) i. Silicon control rectifier (SCR) is extensively used in power control applications. Figure 6 show how the load current through R₁ can be controlled using the SCR gate voltage. Draw the voltage waveform (VAK) across the SCR in Figure 6 from anode to cathode (ground) in relation to the load current (IL) for 180°, 45° and 90° conductions. Assume an ideal SCR. (7 marks) ii. What is the voltage across the SCR if it is never triggered? (3 marks) R₁ R₁ 10 ΚΩ 100 120 V R₁₂ 500 2 FIGURE 6 QUESTION 3 - (b) (Total 10 marks) i. Design a duty cycle control circuit to switch ON and OFF alternatively gates of the IGBTs for the full-bridge dc-ac inverter. Two resistors (R1=2.2k2, R2-4.7KQ) and a capacitor (C=0.022uF) can be used to control the frequency as well as the duty cycle. Calculate the frequency (fr) and duty cycle. (5 marks) ii. Modify your circuit such that capacitor (C-0.022uF) charges through only R1 and discharges through R2. (5 marks) QUESTION 4 - (a) (Total 10 marks) i. Draw a block diagram of hybrid photovoltaic/wind/battery/grid system with arrows showing the direction of energy flow. (4 marks) ii. Explain a method to calculate the size of a photovoltaic (PV) based energy system for residential house with four occupants in Auckland. (2 marks) iii. Explain the role of a central controller for small-scale (residential) renewable energy systems. (2 marks) iv. List five disadvantages of PV and wind energy generation systems. (2 marks) QUESTION 4 - (b) 1. Explain the following 1. Two types of solar collectors? (Total 10 marks) (1 mark) ii. Types, advantages and disadvantages of energy storage systems? iii. Concentrated Solar Power (CSP). (2.5 marks) (1 mark) (1 mark) iv. Types of concentrated solar power. 2. Explain, how maximum power can be extracted from a Photovoltaic (PV) array under variable solar radiation condition? Explain with diagram. (4.5 marks)