Describe a Method - Power Electronics - Past Exam Paper, Exams of Power Electronics

Download Describe a Method - Power Electronics - Past Exam Paper and more Exams Power Electronics in PDF only on Docsity! Semester II Examinations 2008 / 2009 Exam Code(s) 4BN Exam(s) B.E. Degree (Electronic Engineering) Module Code(s) EE411 Module(s) Power Electronics Paper No. I External Examiner(s) Professor G. Irwin Internal Examiner(s) Professor G. Ó Laighin Professor W.G. Hurley Instructions: Answer 3 questions from 4. All questions will be marked equally (20 marks). Duration 2 hrs No. of Pages 3 Department(s) Electrical & Electronic Engineering Requirements None Q1. Figure 1 shows the circuit diagram of a voltage fed bridge inverter with the load voltage and current waveforms. Label the operation of the switches to give the correct output current waveform. [4 marks] (a) Calculate the value of delta that would eliminate the third harmonic completely; [5 marks] (b) For Vdc=100 V, R=10 Ω, L=25 mH, δ=30, and a switching frequency of 50 Hz, calculate the power delivered to the load [take up to the 7th harmonic into account]; [6 marks] (c) Describe a method whereby the fifth harmonic could be eliminated. [5 marks] + - + - V dc i a L R Vo Figure 1 Q2. Derive an expression for K (in terms of duty cycle) in the equation Vrms = KfNBmAc in the case of a forward converter. [8 marks] A forward converter has the following specifications: Input voltage 36 V, output voltage 24 V, frequency 75 kHz Calculate the number of turns in the primary winding and in the reset winding for Bm = 0.2 T and Ac = 1.0 cm 2. [12 marks] Q3. Draw the circuit diagram of a three-phase, phase-controlled full converter; sketch the shape of the output voltage waveforms. [6 marks] The speed of a 4 hp, 400 V, 1500 R.P.M. shunt excited dc motor is controlled by a three-phase full converter drive. The ac input is a 3-phase Y- connected 350V (line- line), 50 Hz supply. The armature resistance is 5 Ω. The machine constant is 2.1 V- s/r. Calculate the phase angle  at rated power and speed. [14 marks] 1 hp = 746 W Q4. (a) Explain the role of a snubber in a power electronic circuit; [5 marks]