Provided by www.YuvaJobs.com - ONGC Placement Paper Electrical Engineering 19 October 2012 ONGC :Graduate Trainees selection Process ONGC GT (Graduate Trainess) Time Limit 03 Hours ONGC GT (Graduate Trainess) ELECTRICAL ENGINEERING-208-2009 questions 1 A single phase transformer is connected to a supply of voltage V and frequency f and is supplying a load, connected across its secondary winding. If the supply voltage is reduced from V to V/2, the flux established in the core will decrease by nearly 50% increase by nearly 50% remain unchanged be affected by amount of load and not by the applied voltage 2 Under magnetic saturation of the core of a transformer, the reactance parameters of its equivalent circuit are affected such that X1 and X2 increase while Xm does not change X1 and X2 do not change while Xm decreases X1 and X2 do not change while Xm increases X1 and X2 decreases while Xm does not change 3 A single phase 2000/200 V, 10 kVA, two winding transformer is reconnected as a step-up autotransformer with 2000 V winding as primary. The kVA rating of the autotransformer shall be 10 kVA 11 kVA 110 kVA 220 kVA 4 The power factor at which power is drawn by a transformer is very low under O.C. test but high under S.C. test high under O.C. test but low under S.C. test low both under O.C. and S.C. tests high both under O.C. and S.C. tests 5 The O.C.C. of a shunt d.c. generator is represented by E = 12+66I. If the field resistance is 70 ohms, the no-load voltage of the generator will be 144 V 210 V 220 V 288 V 6 In d.c. shunt motors, the developed torque is proportional to induced e.m.f. armature current output power line current 7 The speed of a separately excited d.c. motor can be controlled below base speed by (a) armature resistance control and (b) supply voltage control. What happens to the slope of modified speed-torque characteristics under the two methods? It changes in both types of control It remains unchanged in both types of control It remains unchanged in armature resistance control while changes in supply voltage control It remains unchanged in supply voltage control while changes in armature resistance control 8 The polarity of interpoles used in d.c. machines is same as the main pole ahead in generators while opposite of the main pole ahead in motors same as the main pole ahead ion motors while opposite of the main pole ahead in generators same as the main pole ahead in both motors and generators opposite of the main pole ahead in both motors and generators 9 In synchronous generators, examine the following statements about voltage regulation: Synchronous impedance method gives pessimistic results MMF method gives optimistic results In unsaturated machine both methods (synchronous impedance and MMF) give same result Which of the statements are correct? 1, 2 and 3 Only 1 and 2 Only 3 Only 2 and 3 10 Under symmetrical short circuit of a synchronous generator, the armature reaction is magnetizing demagnetizing cross-magnetising mainly magnetising 11 In synchronous generators, the fifth harmonic in induced emf can be eliminated by using coils show chorded by an angle 9o 18o 27 o 36o 12 In induction motor operation, a slip greater than 1 refers to regenerative braking motoring in reverse direction plugging dynamic braking 13 As the applied voltage to an induction motor, operating under no-load, is reduced to half its speed reduced and stator current increases its speed reduced and stator current also reduces its speed reduces but there is not change in stator current there is no change in speed but stator current reduces 14 A 3-phase induction motor is delivering a load of 10 kW at a lagging p.f. of 0.5. If a 3-phase capacitor bank of 7.3 kVAR is connected at its input, the resulting kVA drawn from supply shall be (approximately) 26.5 24.5 20.0 14.0 15 An electric motor developing a starting torque of 10 Nm starts with a load of 6 Nm at its shaft. If the acceleration at start is 2 rad/sec2 , the moment of inertia of the system must be 2 kg-m2 2 N-m2 0.5 kg-m2 0.5 N-m2 16 A synchronous motor works as a synchronous condenser if underexcited overexcited excited with 100% excitation unexcited 17 In relation to damper bar winding in a synchronous motor, examine the following statements: Damper bars are provided in slots on poles Damper bars improve excitation Damper bars help the motor self start Damper bars minimize hunting. Which of the statements are correct? 1, 2, 3 2, 3 1, 4 1, 3, 4 18 In large power ratings, synchronous motor is considered superior to induction motor, since it is less expensive its efficiency is better it can be operated at better power factor its speed regulation is superior. 19 An induction motor drive is designed for two speed operation. For this purpose, the motor is provided with two windings on its stator corresponding to 4 poles and 8 poles. While the motor was running on 4 pole winding, a changeover to 8 pole winding is made. The motor will experience dynamic braking experience regenerative braking experience no braking but will slow down due to its inertia draw dangerously high current 20 A d.c. shunt motor drive of 100 h.p. is run (a) at half the rated speed by armature voltage control and (b) at 1.5 times the rated speed by field control. Its power rating in the two cases shall be modified as 100 h.p. in both cases 50 h.p. in case (a) and 150 h.p. in case (b) 50 h.p. in case (a) and 100 h.p. in case (b) 100 h.p. in case (a) and 150 h.p. in case (b) 21 A single phase full controlled rectifier can work as inverter, if the firing angle is between 0o and 90o 60o and 90o 90o and 180o 0o and 60o 22 When d.c. voltage is converted to a.c. by an inverter, PWM technique is used to control only the output voltage from within the inverter control both the output voltage and frequency from within the inverter control the output current reduce losses in the inverter 23 A thyristor which is in ON state is to be switched OFF. For this purpose a reverse bias is applied across it. Examine the following statements regarding flow of current from anode to cathode (forward current): Forward current can continue to flow if the circuit is purely resistive Forward current can continue to flow if the circuit is dominantly inductive Forward current can continue to flow if the circuit is dominantly capacitive Forward current will immediately stop flowing. 24 Which of the above statements are correct? Only 4 Only 1 Only 2 and 3 Only 2 25 In a 3-phase full controlled thyristor bridge rectifier operating under continuous conduction mode, each thyristor conducts for 180o and two thyristors conduct at a time 120o and three thyristors conduct at a time 180o and three thyristors conduct at a time 120o and two thyristors conduct at a time 26 A single phase fully controlled rectifier is feeding a d.c. motor. At a = 0o, the armature voltage is 200 V and motor speed is 1000 rpm. Neglecting armature resistance and considering continuous conduction, the armature voltage and speed at a = 60o will be 100 V and 500 rpm 100 V and 1000 rpm 200 V and 500 rpm 100 V and speed will depend upon load 27 The grading capacitors are provided across the series connected interrupters in EHV circuit breakers for reducing short circuit current improving power factor equalizing distribution of recovery voltage across the interrupters preventing inductive current chopping. 28 The minimum cost operating condition for a thermal plant is that the larger units should generate more power as compared to smaller units the smaller units should generate more power as compared to larger units the incremental cost rate of the units be proportional to their rating the incremental cost rate of all the units be equal 29 The effect of earth on a three-phase transmission line is to increase its capacitance and decrease inductance decrease its capacitance and increase inductance increase its capacitance without affecting inductance increase its inductance without affecting capacitance. 30 In case of a transmission line if the voltage regulation is negative, then the load power factor is zero lagging unity leading Provided by www.YuvaJobs.com - ONGC Placement Paper Electrical Engineering 19 October 2012