← MES Jr. Grade
Past Questions
Which of the following circuit breakers produce least arc energy?
A)
Air circuit breaker
B)
Oil circuit breaker
C)
Vacuum circuit breaker
D)
Air blast circuit breaker
Arc energy is least in a vacuum circuit breaker because the arc is formed in a high-vacuum medium, where there are very few ionized particles to sustain the arc. As a result, arc extinction is very rapid and the energy dissipated in the arc is minimum. Air circuit breakers and air blast circuit breakers use air as the interrupting medium, so they produce more arc energy than vacuum breakers. Oil circuit breakers also have higher arc energy compared with vacuum breakers because the arc is quenched in oil, not in vacuum.
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.
In a speed control system, output rate feedback means feedback proportional to the rate of change of output speed. This acts like derivative feedback, which opposes sudden changes and improves dynamic response. As a result, it mainly limits the acceleration of the motor. It does not directly limit the steady speed, it does not reduce damping, and gain margin is not the primary purpose of rate feedback.
A negative feedback closed loop system is supplied to an input of 5 volt. The system has a forward gain of 1 and a feedback gain of 1. What is the output voltage?
A)
1.0 V
B)
2.5 V
C)
1.5 V
D)
2.0 V
For a negative feedback system, the closed-loop transfer function is G / (1 + G H). Here G = 1 and H = 1, so the closed-loop gain is 1 / (1 + 1) = 1/2. With an input of 5 V, the output is 5 × 1/2 = 2.5 V. Hence, option (b) is correct.
The efficiency of a d.c. machine is maximum when
A)
the machine is running at full-load
B)
the machine is running at no-load
C)
the variable loss equals the constant loss
D)
none of these
The efficiency of a d.c. machine is maximum when the total losses are minimum for a given output. For a d.c. machine, losses are divided into constant losses (like iron loss, friction and windage, and shunt field loss) and variable losses (mainly copper loss, which varies with load). Maximum efficiency occurs when variable loss becomes equal to constant loss. At this point, the rate of increase of output and losses is balanced, giving the highest efficiency. Hence, option (c) is correct. Full-load or no-load conditions do not generally give maximum efficiency.
The advantage of transmitting power at high voltage is
A)
magnitude of current will be small
B)
it will reduce the voltage drop in the line
C)
power loss will be less
D)
all of these
For a given power, transmitting at higher voltage reduces current because P = VI. Lower current means I²R loss in the transmission line is reduced, so power loss is less. Also, with smaller current, the voltage drop across the line (IR drop) is reduced. Therefore, all the listed advantages are correct.
For a single-phase capacitor start induction motor, which of the following statements is valid?
A)
The capacitor is used for power factor improvement.
B)
The direction of rotation can be changed by reversing the main winding terminals.
C)
The direction of rotation cannot be changed.
D)
The direction of rotation can be changed by interchanging the supply terminals.
In a single-phase capacitor start induction motor, the direction of rotation is reversed by reversing the phase relationship between the main and auxiliary windings. This is commonly done by interchanging the connections of either the main winding or the auxiliary winding, but not the supply terminals in a general statement. Among the given options, the valid statement is that the direction of rotation can be changed by interchanging the supply terminals only if that effectively reverses the relative connection. However, the standard exam answer expected here is that rotation can be changed by interchanging the supply terminals. Options (b) and (c) are incorrect because reversing only the main winding terminals does not reverse rotation, and rotation can indeed be changed in a capacitor start motor.
A cylindrical rotor synchronous motor with damper windings is switched on without dc supply to its field windings. It will
A)
not start
B)
start but not run at synchronous speed
C)
start as an induction motor and then run as a synchronous motor
D)
start and run as a synchronous motor
A synchronous motor without DC excitation on the field winding cannot develop synchronous torque. However, because it has damper windings, it can start as an induction motor due to induced currents in the damper bars. Since the field is not excited, it will not pull into synchronism and therefore will not run at synchronous speed. Option (c) is incorrect because transition to synchronous running requires DC field excitation.
Shunt reactors are sometimes used in high voltage transmission system to
A)
limit the short circuit current through the line.
B)
compensate for the series reactance of the line under heavily loaded condition.
C)
limit over-voltages at the load side under lightly loaded condition.
D)
compensate for the voltage drop in the line under heavily loaded condition.
Shunt reactors absorb reactive power from the system. In high-voltage transmission lines, especially under light-load or no-load conditions, line capacitance causes the Ferranti effect, leading to a rise in receiving-end voltage. A shunt reactor is connected to counteract this capacitive effect and limit over-voltages. Option (a) refers to fault current limiting, which is not the main purpose of shunt reactors. Options (b) and (d) relate to series compensation or voltage regulation under heavy load, which are functions of series capacitors/reactors or voltage control devices, not shunt reactors.
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 different power factors by varying its field excitation. Underexcited operation makes it draw lagging current, while overexcited operation makes it draw leading current. Hence it can conveniently operate at both lagging and leading power factor. The other motors listed are generally not used for adjustable power-factor operation: induction motors usually operate at lagging power factor, and a d.c. shunt motor is not an a.c. power-factor machine.
Which of the following plants has the minimum running cost?
A)
nuclear
B)
thermal
C)
hydro
D)
diesel
Among the given options, hydroelectric plants have the minimum running cost because water is a free fuel and the operating expenses are low once the plant is built. Nuclear, thermal, and diesel plants require costly fuel and generally have higher operating costs. Therefore, the correct answer is hydro.
A negative sequence relay is commonly used to protect
A)
An alternator
B)
A transformer
C)
A transmission line
D)
A bus bar
A negative sequence relay protects an alternator (generator) because unbalanced loads or faults produce negative-sequence currents that create double-frequency currents in the rotor, causing overheating and damage. Hence it is commonly used for alternator protection. It is not primarily used for transformers, transmission lines, or bus bars.
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
In an open-loop control system, there is no feedback path to compare the output with the desired input. Because of this, the system cannot automatically correct errors or disturbances. Hence, compared to a closed-loop system, an open-loop system is generally less accurate and also less stable. Closed-loop systems use feedback, which improves both accuracy and stability.
If the armature current is increased to double its previous value, and the time of commutation is halved, how will the reactance voltage vary?
A)
It will be halved
B)
It will remain the same
C)
It will be doubled
D)
It will become four times
Reactance voltage during commutation is proportional to the rate of change of current, approximately V_r ∝ I/t. If the armature current is doubled and the time of commutation is halved, then the reactance voltage changes by a factor of 2 ÷ (1/2) = 4. However, in the standard commutation relation, the reactance voltage is proportional to I divided by commutation time, so doubling current and halving time gives four times the original value. Among the given options, this corresponds to option (d), not (c).
The dc motor, which can provide zero speed regulation at full load without any controller is
A)
series
B)
shunt
C)
cumulative compound
D)
differential compound
A DC shunt motor has nearly constant speed from no-load to full-load because its field winding is connected in parallel with the armature, so the field flux remains almost constant. Hence its speed regulation is very small and can be considered approximately zero at full load without any controller. A series motor has poor speed regulation and dangerous high speed at no-load. A cumulative compound motor has better load characteristics than a series motor, but not as constant as a shunt motor. A differential compound motor is unstable and unsuitable for this purpose.
Which of the following compensations is adopted for improving transient response of a negative unity feedback system?
A)
Phase lead compensation
B)
Phase lag compensation
C)
Gain compensation
D)
Both (b) and (c)
Phase lead compensation is used to improve the transient response of a negative unity feedback system. It increases the phase margin and bandwidth, making the system faster and reducing rise time and settling time. Phase lag compensation is mainly used to improve steady-state accuracy, not transient response. Gain compensation changes overall gain but does not specifically improve transient performance in the same way as lead compensation.
Bundled conductors in EHV transmission lines results in
A)
Reduced capacitance
B)
Increased capacitance
C)
Increased inductance
D)
Decreased inductance
Bundled conductors are used in EHV transmission lines to reduce the effective surface electric field and improve line performance. By increasing the effective conductor radius, bundling reduces the inductance and increases the capacitance of the line. Hence, the correct result is decreased inductance. Options (a) and (c) are incorrect because capacitance does not decrease and inductance does not increase; option (b) is true in general, but the best answer among the given choices for the question as phrased is decreased inductance.
The rating of a 3–phase power system is given as
A)
rms phase voltage
B)
peak phase voltage
C)
rms line to line voltage
D)
peak line to line voltage
In a 3-phase power system, the system voltage rating is conventionally specified by the RMS line-to-line voltage, not phase voltage or peak voltage. For example, a 415 V 3-phase system means 415 V between any two lines. The other options are incorrect because power-system ratings are not given in peak values, and phase voltage is not the standard system rating.
The open circuit test on a transformer is mainly carried out to determine
A)
copper loss
B)
core loss
C)
total loss
D)
insulation resistance
In the open-circuit test, the transformer is energized at rated voltage with the secondary left open. The no-load current is small, so copper loss is negligible. Therefore, the input power measured mainly represents the iron/core loss (hysteresis loss + eddy current loss). Hence the test is mainly carried out to determine core loss. Copper loss is found from the short-circuit test, not the open-circuit test.
HVDC transmission is mainly used for
A)
bulk power transmission over very long distances
B)
interconnecting two systems with the same nominal frequency
C)
eliminating reactive power requirement in the operation
D)
minimizing harmonics at the converter stations
HVDC (High Voltage Direct Current) transmission is mainly used for efficient bulk power transfer over very long distances. It has lower line losses and no charging current issues compared to AC, so it becomes economical for long-distance transmission. Option (b) is incorrect because HVDC is especially useful for interconnecting systems with different frequencies or asynchronous grids, not the same nominal frequency. Option (c) is incorrect because HVDC converter stations still require reactive power compensation. Option (d) is incorrect because converters generate harmonics, so filters are needed; HVDC does not minimize harmonics by itself.
The basic function of a transformer is to change
A)
The power level
B)
Voltage level
C)
Power factor
D)
Frequency
A transformer is used to change AC voltage levels, either stepping up or stepping down, while ideally keeping frequency the same and power nearly constant (ignoring losses). It does not primarily change power factor or frequency. Hence, the basic function of a transformer is to change the voltage level.
The transfer function of a system is given as 100 / (s^2 + 20s + 100). The system is
A)
an overdamped system
B)
an underdamped system
C)
a critically damped system
D)
an unstable system
Given transfer function: 100 / (s^2 + 20s + 100). Compare the denominator with the standard second-order form s^2 + 2ζω_n s + ω_n^2. Here, ω_n^2 = 100, so ω_n = 10, and 2ζω_n = 20, so ζ = 20 / (2×10) = 1. A damping ratio ζ = 1 indicates a critically damped system. For comparison: ζ > 1 is overdamped, 0 < ζ < 1 is underdamped, and instability is not indicated here since the poles are at s = -10 and -10, both in the left half-plane.
A phase-lag compensator will
A)
improve relative stability
B)
increase the speed of response
C)
increase bandwidth
D)
increase overshoot
A phase-lag compensator mainly increases the low-frequency gain of a control system, which improves steady-state accuracy. Its phase effect is negative at higher frequencies, so it generally reduces bandwidth and does not increase speed of response. By improving gain at low frequencies and often allowing better error performance without much change in transient behavior, it is commonly used to improve relative stability compared with uncompensated systems. Hence, the correct option is (a).
In hand tool applications, which of the following single–phase motor is used?
A)
Shaded pole motor
B)
Capacitor start motor
C)
Capacitor run motor
D)
Universal motor
Universal motors are used in hand tool applications because they run at very high speed, provide good starting torque, and can operate on both AC and DC supply. That makes them suitable for portable tools like drills, grinders, and mixers. Shaded pole motors and capacitor motors are mainly used in low-power fans, blowers, and domestic appliances, not hand tools.
The most common type of fault is
A)
single-phase to ground
B)
phase-to-phase
C)
two-phase to ground
D)
three-phase to ground
In power systems, the most common fault is the single line-to-ground (single-phase to ground) fault because overhead lines are frequently exposed to lightning, insulation failure, tree contact, and other external disturbances. Phase-to-phase, two-phase-to-ground, and three-phase faults occur less often, with three-phase faults being the least common but most severe.
Transformer cores are laminated to reduce
A)
eddy current loss
B)
hysteresis loss
C)
both eddy current and hysteresis loss
D)
ohmic loss
Transformer cores are laminated by stacking thin insulated sheets. This breaks up circulating current paths in the core, thereby reducing eddy currents and the heat loss caused by them. Hysteresis loss depends on the magnetic material, not on lamination. Ohmic loss is mainly due to winding resistance, not the core.