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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
