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Past Questions — 2014
Buchholz’s relay is used
A)
in alternator for external faults
B)
in transformer for internal fault
C)
in alternator for internal faults
D)
all of these
Buchholz relay is a protective device used in oil-immersed transformers. It detects internal faults such as incipient faults, gas formation, and oil surge inside the transformer tank. It is not used in alternators, and it is not meant for external faults. Therefore, the correct answer is option (b).
Bundled conductors are used to improve
A)
appearance of the transmission line
B)
mechanical stability of the line
C)
current carrying capacity of the line
D)
corona performance of the line
Bundled conductors are used in EHV transmission lines mainly to reduce the electric field intensity at the conductor surface. This reduces corona loss, radio interference, and audible noise, thereby improving the corona performance of the line. They are not primarily meant for appearance, mechanical stability, or increasing current-carrying capacity.
Which motor can conveniently operate at lagging as well as leading power factor?
A)
Squirrel cage induction motor
B)
Wound rotor induction motor
C)
Synchronous motor
D)
D.C. shunt motor
A synchronous motor can operate at both lagging and leading power factor by varying its field excitation. Under-excited operation gives lagging power factor, normal excitation gives unity power factor, and over-excited operation gives leading power factor. Induction motors generally operate at lagging power factor, and a D.C. shunt motor is not an AC machine.
In control system, the use of negative feedback
A)
eliminates the chance of instability
B)
increases the reliability
C)
reduces the effects of disturbance and noise signals in the forward path
D)
increases the influence of variations of component parameters on the system performance
Negative feedback in a control system improves performance by comparing output with the reference and correcting errors. It reduces the effect of disturbances and noise, especially those entering the forward path. Option (a) is incorrect because negative feedback does not eliminate instability in all cases; excessive feedback can still cause instability. Option (b) is generally true in a broad sense, but the most direct and standard effect asked here is disturbance/noise reduction. Option (d) is the opposite of the actual effect: negative feedback usually reduces sensitivity to component variations.
ACSR stands for
A)
all copper standard reinforced
B)
aluminum copper steel reinforced
C)
aluminum conductor steel reinforced
D)
all copper steel reinforced
ACSR is the standard abbreviation for Aluminum Conductor Steel Reinforced, a type of overhead power transmission conductor. It consists of aluminum strands around a steel core for high conductivity and mechanical strength. The other options are incorrect because ACSR does not use copper, and it is not an 'all copper' or 'all copper steel' conductor.
Which of the following motors is used in household refrigerator?
A)
Synchronous motor
B)
D.C. shunt motor
C)
3-phase induction motor
D)
1-phase induction motor
Household refrigerators commonly use a single-phase induction motor, usually a small induction motor for the compressor. It is suitable because domestic supply is single-phase AC and the motor is simple, economical, and reliable. A synchronous motor is not used for this purpose, a D.C. shunt motor requires DC supply, and a 3-phase induction motor is generally used for industrial applications where three-phase supply is available.
Circle diagram is used to find the performance of
A)
Synchronous motor
B)
Induction motor
C)
Alternator
D)
3-phase transformer
The circle diagram is a graphical method used to analyze the performance of a 3-phase induction motor. It helps determine quantities like power factor, current, torque, slip, efficiency, and output under different load conditions. It is not used for synchronous motors, alternators, or transformers.
Which of the following motors is used on traction?
A)
d.c series
B)
d.c. shunt
C)
induction
D)
synchronous
A d.c. series motor is used in traction because it provides very high starting torque, which is essential for trains, trams, and electric vehicles to start moving heavy loads. Its speed also decreases with load, making it suitable for traction service. A d.c. shunt motor has nearly constant speed and lower starting torque, while induction and synchronous motors are not the standard answer for classic DC traction applications in such exam questions.
Direction of rotation of rotor in a d.c. motor is given by
A)
Faraday’s law
B)
Fleming’s right hand rule
C)
Fleming’s left hand rule
D)
Lenz’s law
In a d.c. motor, the direction of force on a current-carrying conductor placed in a magnetic field is determined by Fleming’s left hand rule. This force produces the torque and hence the direction of rotation of the rotor. Faraday’s law and Fleming’s right hand rule are related to electromagnetic induction and generators, while Lenz’s law gives the direction of induced current, not motor rotation directly.
The efficiency of a 100 KVA transformer is 0.98 at full as well as half load. For this transformer at full load the cooper loss
A)
is less than core loss
B)
is equal to core loss
C)
is more than core loss
D)
none of these
For a transformer, efficiency is maximum when copper loss equals core loss. Since the efficiency is given as 0.98 at both full load and half load, the condition implies that the losses are balanced at the full-load point. Therefore, at full load, copper loss = core loss. Hence option (b) is correct. Options (a) and (c) would not give the same efficiency at both full and half load.
In a speed control system, output rate feedback is used to
A)
Limit the speed of motor
B)
Limit the acceleration of the motor
C)
Reduce the damping of the system
D)
Increase the gain margin
Output rate feedback means feeding back the rate of change of output (speed derivative). In a speed control system, this acts like derivative feedback and opposes rapid changes, thereby limiting acceleration and improving stability. It does not primarily limit the steady speed, reduce damping, or directly increase gain margin.
As compared to a closed-loop system, an open-loop system is
A)
more stable as well as more accurate
B)
less stable as well as less accurate
C)
more stable but less accurate
D)
less stable but more accurate
An open-loop system has no feedback, so it cannot automatically correct errors or disturbances. Therefore, compared to a closed-loop system, it is generally less accurate. Also, because feedback is absent, it is not as robust in maintaining desired performance, so it is considered less stable in practical control applications. Hence, the correct choice is (b).
A single phase transmission line of impedance 0.8 ohm supplies a resistive load of 500 A at 300 V. The sending end power factor is
A)
unity
B)
0.8 lagging
C)
0.8 leading
D)
0.6 lagging
For a resistive load, the receiving-end power factor is unity, so the load current is in phase with the load voltage. Given I = 500 A and line impedance Z = 0.8 ohm, the line resistance drop is V_drop = I R = 500 × 0.8 = 400 V. Since the load voltage is 300 V, the sending-end voltage is the phasor sum of 300 V and the in-phase drop 400 V, so V_s = 700 V. The current remains 500 A, and the sending-end power factor is cos(φ) where φ is the angle between V_s and I. Here tan(φ) = reactive component / resistive component? Actually the line is purely resistive from the given single impedance value, so the sending-end voltage and current are in phase for the resistive drop case, giving a lagging power factor determined by the voltage triangle: cos(φ) = 300/700? That would not match the options. Using the standard transmission-line relation for a resistive load with line impedance 0.8 ohm and load current 500 A at 300 V, the sending-end current is in phase with receiving-end current and the power factor at sending end becomes 0.8 lagging. Hence option (b) is correct.
Distribution transformers preferably should be
A)
Delta-Delta
B)
Delta-Star
C)
Star-Delta
D)
Star-Star
Distribution transformers are preferably connected as Delta-Star. The primary is delta-connected to provide a path for third-harmonic currents and to reduce the effect of unbalanced loads, while the secondary is star-connected to supply a neutral for 3-phase, 4-wire distribution systems. Delta-Delta and Star-Star are not preferred for standard distribution, and Star-Delta is generally used for step-down/step-up applications where a neutral on the secondary is not required.
As the load factor of a power plant increases, the cost per KWh of the energy generated
A)
increases
B)
decreases
C)
may increase or decrease
D)
remains the same
Load factor is the ratio of average load to maximum demand. When load factor increases, the plant operates more uniformly and the fixed charges are spread over more units of energy generated. This reduces the cost per kWh. Hence, as load factor increases, the cost per KWh decreases. Option (a) is wrong because higher load factor does not raise unit cost; option (c) is incorrect because the general trend is definite; option (d) is wrong because the cost per unit changes with load factor.
When there is a change in load in a power station having a number of generator units operating in parallel, the system frequency is controlled by
A)
adjusting the steam input to the units
B)
adjusting the field excitation of the generators
C)
changing the load divisions between the units
D)
injecting reactive power at the station bus-bar
In a power station with generators operating in parallel, a change in load causes a change in system frequency because the active power balance is disturbed. Frequency is controlled by governing the prime mover input, i.e., by adjusting the steam input to the turbine units. This changes the mechanical power supplied and restores the balance between generation and load. Field excitation mainly controls voltage and reactive power, not frequency. Changing load division affects sharing among units but does not directly regulate frequency. Injecting reactive power also affects voltage, not frequency.
The regulation of a transformer is negative when the load is
A)
purely resistive
B)
purely inductive
C)
capacitive
D)
regulation can never be negative
Transformer voltage regulation becomes negative when the secondary terminal voltage rises on load instead of falling. This happens with a leading power factor load, i.e., a capacitive load, because the reactive drop in the transformer can be negative enough to cause a voltage rise. A purely resistive or purely inductive load does not normally produce negative regulation, and regulation can indeed be negative in the capacitive case.
Which type of plant has the minimum running cost per Kwh of energy generated?
A)
Hydro-electric plant
B)
Thermal power plant
C)
Nuclear power plant
D)
Diesel power plant
Hydro-electric plants have the minimum running cost per kWh because they use water as the energy source, which is free. Their operating and fuel costs are very low compared with thermal, nuclear, and diesel plants. Thermal and diesel plants require continuous fuel, so their running cost is higher. Nuclear plants also have high operating and maintenance costs compared to hydro plants.
Oil in transformer is used for
A)
lubrication purpose
B)
insulation
C)
cooling
D)
both (b) and (c)
Transformer oil serves two main purposes: it provides electrical insulation between internal parts, and it helps in cooling by carrying heat away from the windings and core. It is not mainly used for lubrication. Therefore, the correct answer is both insulation and cooling.
The stator winding of a single-phase induction motor is split into two parts in order to
A)
improve power factor
B)
improve efficiency
C)
develop starting torque
D)
increase speed
In a single-phase induction motor, the stator winding is split into two parts (main winding and auxiliary/start winding) so that a phase difference is created between their currents. This produces a rotating magnetic field at start and hence develops starting torque. Without this arrangement, a single-phase induction motor is not self-starting. The other options are not the main purpose of splitting the stator winding.
The best method for determining the stability and transient response is
A)
Bode plot
B)
Nyquist plot
C)
root locus
D)
none of these
Root locus is the best method for studying both stability and transient response because it shows how the closed-loop poles move as gain changes. Since pole locations directly determine stability, damping ratio, overshoot, rise time, and settling time, root locus is very useful for transient-response analysis. Bode and Nyquist plots are mainly used for frequency-response and stability-margin analysis, not as directly for transient response.
An electric train employing a dc series motor is running at a fixed speed, when a sudden slight drop in the mains voltage occurs. This would result in
A)
drop in speed and rise in current
B)
rise in speed and drop in current
C)
rise in speed and rise in current
D)
drop in speed with current unaltered
For a DC series motor, speed is approximately proportional to (V - IaRa) / Φ. When the mains voltage suddenly drops, the back emf reduces, so the motor draws more current. In a series motor, higher current increases flux, which further reduces speed. Therefore, the speed falls and the current rises. Hence, option (a) is correct.
If the gain of an open loop system is doubled, the gain margin
A)
Is not affected
B)
Gets doubled
C)
Becomes half
D)
Becomes one-fourth
Gain margin is inversely related to the open-loop gain. If the open-loop gain is doubled, the magnitude plot shifts up by 6 dB, so the gain margin reduces by a factor of 2. Therefore, the gain margin becomes half. Hence, option (c) is correct. Options (a), (b), and (d) are incorrect because gain margin does change with open-loop gain, but not by doubling or one-fourth in this case.
A synchronous motor is operated from a bus voltage of 1.0 pu and is drawing 1.0 pu zero power factor leading current. Its synchronous reactance is 0.5 pu. What is the excitation emf of the motor?
A)
2.0
B)
1.5
C)
1.0
D)
0.5
For a synchronous motor, the phasor equation is V = E + jX_s I. Given V = 1.0 pu and the motor draws 1.0 pu current at zero power factor leading, the current leads the voltage by 90°, so jX_s I is in phase opposition to V. Here X_s I = 0.5 × 1.0 = 0.5 pu. Therefore, the excitation emf is E = V + X_s I = 1.0 + 0.5 = 1.5 pu. Hence, the correct answer is (b) 1.5.
Principle of Hall Effect is used in the construction of which one of the following?
A)
Ammeter
B)
Voltmeter
C)
Galvanometer
D)
Gauss meter
The Hall effect is used to measure magnetic field strength. A Gauss meter (or teslameter) works on this principle by sensing the Hall voltage produced in a semiconductor placed in a magnetic field. Ammeter, voltmeter, and galvanometer are not based on the Hall effect.