← MES Jr. Grade
Past Questions — 2015
Signal flow graph is used to obtain
A)
stability of a system
B)
transfer function of a system
C)
controllability of a system
D)
absorbability of a system
A signal flow graph is mainly used to determine the overall transfer function of a linear system using Mason’s gain formula. It represents the cause-and-effect relationship between variables in a control system. It is not primarily used to directly find stability, controllability, or absorbability. Therefore, the correct answer is transfer function of a system.
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.
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 insulation of modern EHV is designed based on
A)
Lightening voltage
B)
Switching voltage
C)
Corona voltage
D)
Radio Interference
In modern EHV (Extra High Voltage) systems, insulation design is mainly governed by switching surges rather than lightning surges. As voltage levels increase, switching overvoltages become more severe and important for insulation coordination. Therefore, EHV insulation is designed based on switching voltage. Lightning voltage is more critical for lower voltage systems, corona voltage is not the design basis, and radio interference is an effect of EHV lines but not the insulation design criterion.
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.
HVDC transmission lines are more economical for
A)
Short distance transmission
B)
Long distance transmission
C)
Any distance transmission
D)
Interconnected systems
HVDC lines are generally more economical for long-distance transmission because they have lower line losses, no reactive power issues, and lower conductor requirements for the same power transfer over very long distances. For short distances, the converter station cost makes HVAC more economical. Interconnected systems can use HVDC links, but the standard economical advantage asked here is for long-distance transmission.
MHO relay is normally used for protection of
A)
Short line
B)
Long line
C)
Medium line
D)
None of these
A Mho relay is a distance relay commonly used for the protection of long transmission lines. It is suitable because it is less affected by power swings and performs well when the line impedance is high, which is typical of long lines. Short lines are usually protected by simpler overcurrent or impedance relays, while medium lines may use other distance relay characteristics.
Time response for a second order system depends on value of zeta. If zeta = 0 then the system is called as
A)
Un-damped system
B)
Under damped system
C)
Critically damped system
D)
Over damped system
For a standard second-order system, the damping ratio is denoted by ζ (zeta). If ζ = 0, there is no damping at all, so the system is called an un-damped system. Under-damped systems have 0 < ζ < 1, critically damped means ζ = 1, and over-damped means ζ > 1.
The function of commutator in a d.c. generator is
A)
to collect current from armature conductors
B)
to improve commutation
C)
to conduct current to the brushes
D)
to change a.c. to d.c.
In a d.c. generator, the emf induced in the armature conductors is alternating in nature. The commutator acts as a mechanical rectifier: it reverses the connections of the armature coils at the right instant so that the current at the brushes becomes unidirectional (d.c.). Option (a) is incorrect because the brushes collect current from the commutator, not directly from armature conductors. Option (b) refers to commutation quality, which is helped by brushes/interpoles, not the basic function of the commutator. Option (c) is also incorrect because the commutator, not the brushes, is the part that helps transfer current from the rotating armature to the stationary circuit.
Which portion of the power system is more prone to faults?
A)
Alternators
B)
Transformers
C)
Overhead lines
D)
Underground cables
Overhead lines are more prone to faults because they are directly exposed to wind, lightning, trees, pollution, birds, and other environmental effects. In contrast, alternators and transformers are housed and protected in substations or power stations, and underground cables are generally less exposed to external disturbances, so they experience fewer faults.
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.
A Buchholz relay is used for
A)
Protection of induction motor
B)
Protection of synchronous motor
C)
Protection of transformer against external fault
D)
Protection of transformer against internal fault
A Buchholz relay is a gas-actuated protective device used in oil-immersed transformers. It operates when internal faults such as insulation failure, winding short-circuit, or overheating generate gas and oil surge inside the transformer. Therefore, it is used for protection of the transformer against internal faults. It is not used for induction motors or synchronous motors, and external faults are generally protected by other relays.
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 dissipating heat generated during operation. It is not primarily used for lubrication. Therefore, the correct answer is both insulation and cooling.
A d.c. series motor should not be started without a load on it because
A)
it will not run at no load
B)
it may develop excessive speed at no-load and thus damage itself
C)
excessive sparking will occur at commutator
D)
the staring current will be very high
In a d.c. series motor, the field winding is in series with the armature, so the flux decreases greatly when the load is removed. At no-load, the armature current becomes very small, the flux weakens, and the motor speed rises dangerously high. This can cause mechanical damage to the motor. Therefore, a series motor should never be started without load. Option (d) is true for starting current in general, but it is not the main reason for avoiding no-load operation.
In the protection scheme, relay functions as a
A)
switching device
B)
sensing device
C)
breaking device
D)
none of these
In a protection scheme, a relay continuously monitors electrical quantities such as current, voltage, or frequency and senses abnormal conditions. When it detects a fault, it sends a trip signal to the circuit breaker. Therefore, the relay works as a sensing device, not a switching or breaking device. The circuit breaker performs the actual breaking action.
The function of starter in d.c. motors is
A)
to avoid excessive current at starting
B)
to control the speed
C)
to avoid armature reaction
D)
to avoid excessive heating
In a d.c. motor, at the moment of starting the back emf is zero. If full supply voltage were applied directly, the armature would draw a very large current because armature resistance is very low. A starter inserts resistance initially to limit this starting current and protect the motor. It is not mainly used for speed control, armature reaction, or heating, though limiting current also helps prevent overheating.
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.
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.
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.
How can the reactive power delivered by a synchronous generator be controlled?
A)
By changing the prime mover input
B)
By changing the excitation
C)
By changing the direction of rotation
D)
By changing the prime mover speed
In a synchronous generator, reactive power is controlled by the field excitation. Increasing excitation makes the generator deliver more reactive power (lagging VARs), while decreasing excitation reduces or can even absorb reactive power. The prime mover input mainly controls real power, not reactive power, so options (a) and (d) are incorrect. Direction of rotation does not control reactive power, so (c) is also incorrect.
The phase sequence of a three phase alternator will reverse if
A)
The field current is reversed keeping the direction of rotation same
B)
The field current remains the same but the direction of rotation is reversed
C)
The field current is reversed and the number of poles is doubled
D)
The number of poles is doubled without reversing the field current
In a three-phase alternator, the phase sequence depends on the direction of rotation of the rotor relative to the stationary armature windings. If the direction of rotation is reversed while the field current remains unchanged, the induced emf sequence reverses from R-Y-B to R-B-Y (or vice versa). Reversing the field current only changes the polarity of the magnetic field, not the phase sequence. Doubling the number of poles does not reverse the phase sequence by itself.
Salient pole type rotors are generally used with prime movers of
A)
high speed
B)
low speed
C)
medium speed
D)
both high and low speed
Salient pole type rotors have projecting poles and a large diameter, so they are suitable for low-speed prime movers such as diesel or water turbines. They are not preferred for high-speed operation because the construction is mechanically weak at very high speeds. Hence, the correct answer is low speed.
The characteristic equation of a system is given as: 2s^4 + s^3 + 3s^2 + 5s + 10 = 0. How many roots does the system will have?
A)
one
B)
two
C)
three
D)
four
The characteristic equation is a 4th-degree polynomial: 2s^4 + s^3 + 3s^2 + 5s + 10 = 0. The degree of the characteristic equation gives the number of roots of the system. Since the highest power of s is 4, the system has 4 roots. Therefore, the correct option is (d) four.