← MES Jr. Grade
Past Questions
The most useful modulation technique for high fidelity audio broadcasting on radio in current practice is
A)
amplitude modulation
B)
frequency modulation
C)
pulse amplitude modulation
D)
pulse code modulation
For high-fidelity audio broadcasting, frequency modulation (FM) is preferred because it is much less affected by amplitude noise and interference than amplitude modulation. FM also provides better sound quality and higher immunity to static, which is why it is widely used for radio music and audio broadcasting. Amplitude modulation is more susceptible to noise, while pulse amplitude modulation and pulse code modulation are not used for conventional radio broadcasting in this context.
The width of the depletion layer of a junction
A)
Decreases with light doping
B)
Increases with heavy doping
C)
Is independent of applied voltage
D)
Is increased under revered bias
In a p-n junction, the depletion layer width depends on both doping level and applied bias. Under reverse bias, the potential barrier increases and the depletion region widens. Hence option (d) is correct. Option (c) is wrong because depletion width is not independent of applied voltage. Options (a) and (b) are incorrect as stated: light doping actually increases the depletion width, while heavy doping reduces it.
Eight memory chips of 32×4 bit size have their address buses connected together. What is the size of this memory system?
A)
512×2 bits
B)
256×4 bits
C)
64×16 bits
D)
32×32 bits
Each chip is 32×4 bits, meaning it has 32 memory locations and each location stores 4 bits. If 8 such chips have their address buses connected together, they all respond to the same 32 addresses. Since their data buses are combined, the data width adds up: 8 × 4 = 32 bits. So the overall memory organization is 32 locations of 32 bits each, i.e. 32×32 bits. The other options do not match the combined word size.
The number of diodes required in a bridge rectifier circuit is
A)
one
B)
two
C)
three
D)
four
A bridge rectifier uses four diodes arranged in a bridge configuration to convert AC into pulsating DC. During each half-cycle, two diodes conduct and the other two block, ensuring full-wave rectification. Therefore, the number of diodes required is four. Option (a), (b), and (c) are incorrect because a bridge rectifier specifically needs four diodes.
Instruction RET (Return) of a 8085 microprocessor
A)
Stack Pointer and Program counter
B)
Stack Pointer only
C)
Program counter only
D)
Set Interrupt Mask and Acc
In the 8085, the RET instruction returns from a subroutine by restoring the saved address from the stack into the Program Counter (PC). This operation uses the Stack Pointer (SP) to access the top of the stack and then updates the PC with the return address. Hence, both the Stack Pointer and Program Counter are involved. Option (b) is incomplete, (c) ignores the stack access, and (d) is unrelated.
A modulator is a system to
A)
separate two frequencies
B)
impress the information on to a radio frequency carrier
C)
extract information from the carrier
D)
amplify the audio frequency signal
A modulator is used to place the information signal onto a high-frequency carrier wave for transmission. This process is called modulation. Option (c) describes a demodulator, which extracts the information from the carrier. Option (a) is not the purpose of modulation, and option (d) refers to amplification, not modulation.
A Z-diode is normally operated in
A)
Forward bias mode
B)
Reverse bias mode
C)
Rectifier mode
D)
Converter mode
A Zener diode is designed to operate in the reverse-bias region, where it maintains an almost constant voltage after breakdown. That is why it is used for voltage regulation. In forward bias, it behaves like an ordinary diode, so option (a) is incorrect. Rectifier mode and converter mode are not the normal operating modes of a Z-diode, so (c) and (d) are incorrect.
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.
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.
Synchronous generators used in thermal power plants have
A)
Cylindrical rotors
B)
Salient pole rotors
C)
Stator slots in multiple of 6
D)
All of these
Synchronous generators in thermal power plants are typically turbo-alternators driven by steam turbines. Steam turbines run at very high speeds, so the generator rotor must be strong, smooth, and capable of withstanding high centrifugal forces. Hence, a cylindrical (non-salient pole) rotor is used. Salient pole rotors are used in low-speed hydro generators, not thermal power plants. The number of stator slots being in multiples of 6 is not a defining feature for thermal power plant synchronous generators, so option (d) is incorrect.
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.
Which compensator is used to increase the damping of a badly under-damped system?
A)
Phase-lag
B)
Phase-lead
C)
Phase lag-lead
D)
None of these
A badly under-damped system has low damping and tends to oscillate. A phase-lead compensator adds positive phase and improves transient response, increasing damping ratio and reducing overshoot. A phase-lag compensator mainly improves steady-state accuracy and usually does not increase damping significantly. A lag-lead compensator can do both, but the standard compensator used to increase damping is the phase-lead compensator.
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.
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.
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.
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.
In pneumatic systems, electrical capacitance is analogous to
A)
restriction to flow
B)
volume of air
C)
filled helical tube
D)
none of these
In the electrical-pneumatic analogy, capacitance represents the ability to store energy. In pneumatic systems, this storage effect is analogous to the compressibility and storage of air in a volume. Hence, electrical capacitance corresponds to the volume of air. Restriction to flow is analogous to resistance, not capacitance. A filled helical tube is not the standard analogy for capacitance.
Which three-phase connection can be used in a transformer to introduce a phase difference of 30 degrees between its output and corresponding input time voltages?
A)
Star-Star
B)
Star-Delta
C)
Delta-Delta
D)
Delta-Zigzag
In a three-phase transformer, a Star-Delta (Y-Δ) connection produces a phase displacement of 30° between primary and secondary line voltages. This is the standard connection used when a 30° phase shift is required. Star-Star and Delta-Delta connections do not introduce a 30° phase shift, and Delta-Zigzag is not the usual answer for this standard transformer phase-displacement question.
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 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.
The transfer function of a tachometer is of the form
A)
KS
B)
k/s
C)
k/(S+1)
D)
k/S(S+1)
A tachometer is a velocity-measuring device. Its output voltage is proportional to angular speed, so in Laplace form the transfer function is of the form G(s) = K s when expressed as a differentiating action in control-system representation. Among the given options, this matches KS. The other options represent an integrator (k/s), a first-order lag (k/(s+1)), and an integrator with lag (k/s(s+1)), which are not tachometer characteristics.
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.
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.