RR
Code No: RR310205
III B.Tech. I Semester(RR) Supplementary Examinations, May 2009 ELECTRO MECHANICS-III Time: 3 hours
(Electrical & Electronics Engineering)
Answer any FIVE questions All questions carry equal marks ?????
Max Marks: 80
1. (a) Derive the emf equation for an alternator taking into consideration the pitch and distribution factors. (b) A 3-phase, star-connected alternator has the following data: Voltage required to be generated on open circuit is 4000V at 50Hz, speed=500rpm, stator slots/pole/phase/conductors/slot=12. Calculate the useful flux/pole. Assume all conductors per phase to be connected in series and coil to be full pitch. 2. (a) What is meant by voltage regulation? Explain in detail the synchronous impedance method for obtaining voltage regulation of 3-phase alternator. (b) A 3-phase 1000KV A, 11KV star connected alternator has resistance of 1.6 ohms per phase and reactance of 26ohms/phase. Find the percentage regulation for a load of 800KW at 0.8 p.f lagging. 3. (a) Describe the slip test conducted for synchronous machines. (b) A 11KV , 1000KV A, 3-phase star connected alternator has a resistance of 2ohms/phase. The OCC and the characteristics with rated current at zero p.f are given in the following table. Find the voltage regulation of the alternator for full load current at 0.8 p.f leading. Field Current(A): 40 50 110 140 180 Line voltage O.C.C(Volts): 5800 7000 12500 13750 15000 Line voltage ZPF(Volts): 0 1500 8500 10500 1250 4. (a) Obtain the output power expression of an alternator connected to infinite bus bars. (b) Two similar 1-phase alternators together supply a load of 10+j5 ohm. Induced emfs of each are 250 |0 volts and synchronous impedance 0+j2 ohm. Find the terminal voltage, and also find the current power. 5. (a) Explain why synchronous motor is not self starting. (b) A 3-phase synchronous motor has 12-poles and operates from a 440V , 50Hz supply. Calculate its speed if it takes a line current of 100A at a p.f of 0.8 leading. Determine the torque developed by the motor. Neglect losses. 6. (a) Explain double revolving field theory of single phase induction motor. Draw the torque-slip characteristics. (b) Explain the constructional details of a shaded pole induction motor and explain how the torque is developed. 7. The excitation of a 415V , 3-phase, mesh-connected synchronous motor as such that the induced emf is 520V . The impedance per phase is 0.5+j4Ω.If the friction and iron losses are constant at 1000W , calculate the horse-power output, line current, power factor and efficiency for (a) Maximum power output. (b) Maximum power input. 8. Write short notes on the following: (a) Determination of sub-transient, transient and steady state reactances. (b) Suppression of Harmonics in Alternators. ?????