← MES Jr. Grade
Past Questions
Transformer cores are laminated to reduce
A)
eddy current loss
B)
hysteresis loss
C)
both hysteresis and eddy current loss
D)
copper loss
Transformer cores are laminated into thin insulated sheets to break the path of circulating currents in the core. This greatly reduces eddy current loss. Hysteresis loss depends on the magnetic properties of the core material, not on lamination. Copper loss occurs in the windings, not the core.
The transfer function of a system is 10/(1+s). The steady state error to unit step input when operated as a unity feedback system is
A)
10
B)
0
C)
1/11
D)
For a unity feedback system, the steady-state error to a unit step input is e_ss = 1 / (1 + Kp), where Kp is the position error constant. Here the open-loop transfer function is G(s) = 10/(1+s). So Kp = lim(s→0) G(s) = 10. Therefore, e_ss = 1/(1+10) = 1/11. Hence the correct option is (c).
Which three-phase connection can be used in a transformer to introduce a phase difference of 30° between its output and corresponding input line voltages?
A)
Star – star
B)
Star – Delta
C)
Delta – delta
D)
Delta – Zigzag
A star–delta transformer connection introduces a 30° phase shift between primary and secondary line voltages. In a star connection, line voltage and phase voltage differ by 30°, and in a delta connection the line voltage equals phase voltage. This combination produces the required 30° displacement. Star–star and delta–delta connections do not introduce a 30° phase shift, while delta–zigzag is not the standard answer for this specific phase-shift purpose.
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.
Bulk power transmission over very long HVDC lines are preferred on account of
A)
low cost of HVDC terminals
B)
no harmonic problems
C)
minimum line power losses
D)
simple protection
HVDC transmission is preferred for very long-distance bulk power transfer mainly because it has lower transmission losses than AC over long lines. In HVDC, there is no reactive power flow, no charging current, and no skin effect, so the line losses are minimized. Option (a) is incorrect because HVDC terminals are actually expensive. Option (b) is incorrect because HVDC converters do create harmonics, so filters are needed. Option (d) is incorrect because protection of HVDC systems is not simpler than AC systems.
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.
A cylindrical rotor synchronous motor is switched on to the dc supply with its field windings shorted on themselves. 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 cannot self-start on DC supply. With the field windings shorted, there is no rotating magnetic field as in AC operation, so no starting torque is produced. Hence the cylindrical rotor synchronous motor will not start. Option (c) applies to a synchronous motor started on AC with damper windings, not on DC supply.
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 output is controlled by the field excitation. Increasing excitation makes the generator supply more reactive power (lagging VARs), while decreasing excitation reduces it or can make it absorb reactive power. Prime mover input mainly controls real power, not reactive power. Direction of rotation does not control reactive power, and speed is fixed by system frequency in normal operation.
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.
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.
Distributed winding is preferred over concentrated winding as it
A)
reduces noise
B)
reduces the machine size
C)
reduces the amount of copper required
D)
improves the generated waveform
A distributed winding spreads the coil sides over several slots per pole per phase, so the induced EMF from different coils adds more smoothly. This makes the air-gap flux and generated voltage waveform closer to a sine wave and reduces harmonics. It does not primarily reduce machine size or copper required; in fact, distributed windings usually need more end-turn copper than concentrated windings. Noise reduction can be a secondary benefit, but the main reason is improved waveform.
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.
Direction of rotation of d.c. motor is reversed by
A)
Reversing armature connection
B)
Interchanging armature and field connection
C)
Adding resistance to field circuit
D)
Reversing supply connection
In a d.c. motor, the direction of rotation depends on the relative directions of armature current and field flux. To reverse rotation, either the armature current or the field current must be reversed, but not both. Interchanging the armature and field connections reverses one of them relative to the other, so the motor rotates in the opposite direction. Reversing only the armature connection or only the supply connection is not the standard complete method here, and adding resistance to the field circuit only changes speed/torque, not direction.
The leakage flux in a transformer depends on
A)
the applied voltage
B)
the frequency
C)
the load current
D)
the mutual flux
Leakage flux in a transformer is the part of the magnetic flux produced by a winding that does not link with the other winding. It is mainly proportional to the current flowing in the winding, so it varies with load current. When load current increases, leakage flux increases. Applied voltage and frequency mainly affect the mutual flux, not the leakage flux directly.
Zero-sequence currents can flow from a line to transformer bank if the windings are in
A)
grounded star/delta
B)
delta/star
C)
star/grounded star
D)
delta/delta
Zero-sequence currents require a return path through the neutral/ground. In a transformer bank with one side grounded star (Yg) and the other side delta, zero-sequence current can flow from the line into the transformer because the grounded star provides the neutral return path, while the delta allows circulating currents to accommodate the zero-sequence component. Therefore, grounded star/delta is the correct connection.
Why others are incorrect:
- delta/star: the delta side blocks zero-sequence current from entering the line side.
- star/grounded star: zero-sequence may exist, but the standard line-to-transformer-bank flow of zero-sequence current is classically associated with a grounded star–delta connection.
- delta/delta: no neutral grounding path, so zero-sequence current cannot flow from the line into the bank.
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 relative motion between the rotating magnetic field and the stator windings. If the direction of rotation is reversed while the field current remains the same, the order in which the induced EMFs reach their maxima also reverses, so the phase sequence reverses. Reversing the field current alone does not reverse phase sequence; it only changes the polarity of the field. Doubling the number of poles changes frequency, not the phase sequence.
Which of the following type of motor is not self starting?
A)
Synchronous motor
B)
DC series motor
C)
Induction motor with medium slip
D)
Induction motor with high slip
A synchronous motor is not self-starting because at standstill the average starting torque is zero. It needs an auxiliary starting method such as damper windings, pony motor, or variable frequency starting. In contrast, a DC series motor is self-starting, and induction motors are generally self-starting due to the rotating magnetic field produced by the stator.
A d.c. series motor should always be started with some load on its shaft, because otherwise
A)
it will draw a very high current from the supply
B)
it will run at an excessively high speed
C)
it will not be able to develop any torque
D)
all of these
In a d.c. series motor, the field winding is in series with the armature, so the flux decreases greatly when the load is light. At no-load, the back emf becomes high and the motor speed can rise dangerously to a very large value. Hence a series motor must always be started with some load on its shaft. Option (a) is not the main reason, and (c) is incorrect because the motor can still develop torque.
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.
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.
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.
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.
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.
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.