Rr310204-power Electronics

RR

Code No: RR310204 III B.Tech I Semester(RR) Supplementary, May 2009 POWER ELECTRONICS Time: 3 hours

(Electrical & Electronics Engineering)

Answer any FIVE questions All questions carry equal marks ?????

Max Marks: 80

1. (a) Draw and explain the necessity dynamic equilising circuit for series connected SCRs. Derive relations used for determining the values of capacitor C. (b) For the class C commutation circuit, the d.c. source voltage Vs =120V and current through R1 and R2 is 20A. The turn off time of both the SCRs is 60 µs. Calculate the value of commutation capacitance C for successful commutation. 2. (a) Explain the following ratings of SCR and their significance. i. Voltage safety factor. ii. Latching current. iii. Holding current. iv. Working peak OFF state forward voltage (VDW M ). v. Repetitive peak OFF state forward voltage (VDRM ). vi. Repetitive peak off state forward voltage (VDRM ). (b) The voltage and current ratings in a particular circuit are 3KV and 750A. SCRs with a rating of 800V and 175A are available. The recommended minimum derating factor is 15 percent. Calculate the number of series and parallel units required. Also Calculate the required values of R and C to be used in the static and dynamic equalizing circuits if the maximum forward leakage current for the SCRs is 10mA and ∆Q=20 microcoulombs. 3. (a) Obtain expressions for performance parameters input power factor, input harmonic factor, voltage ripple factor and active power input in case of single phase fully controlled bridge converter assuming ripple free load current. (b) A single phase fully controlled bridge converter is used for obtaining a regulated DC output voltage. The RMS value of the AC input voltage is 230V, and the firing angle is maintained at 450 so that load current is 5A. Calculate the DC output voltage, and the active and reactive power input. 4. (a) Explain the operation of three phase, semi converter with inductive load with the associated waveforms for continuous conduction. (b) A three phase half wave converter is operated from a three-phase star connected 208V, 50Hz supply and the load resistance is R=10Ω. If it is required to obtain an average output voltage of 50% of the maximum possible output voltage, calculate. (i) The delay angle. (ii) The rms and average output currents. (iii) The average and rms thyristor currents and (iv) The rectification efficiency. 5. (a) Discuss the basic principle of working of a single-phase to single-phase cycloconverter for continuous conduction for mid-point type step-down cycloconverter. (b) A single phase full wave voltage controller has a resistive load of R=10Ω and the input voltage is 120V, 50Hz. The delay angle of thyristors is 900 . Determine. i. The rms value of output voltage. ii. The input pf and. iii. The rms thyristor current and. iv. The average value of a thyristor current. 6. (a) Explain the principle of operation of a step-up chopper. Also mention different applications of a d.c chopper. (b) A dc chopper of input voltage 200 V remains on for 25 msec and off for 10 msec. Determine the average voltage which appears across the load. If the load is assumed to be resistive (R=10 Ohms), then find the RMS value of output voltage and power delivered to the load. 7. (a) Discuss in detail the working of Morgan’s chopper. (b) Draw and explain the operation of single phase McMurray-Bedford full- bridge inverter. 8. (a) Explain the operation of a single phase bridge inverter for RL and RLC loads with the help of load voltage and current waveforms. (b) Discuss various methods for the control of output voltage of single phase inverters. ?????