← Junior Engineer
Past Questions — 2015
In a transformer the voltage regulation will be zero when it operates at
A)
Unity p.f.
B)
Leading p.f.
C)
Lagging p.f.
D)
Zero p.f. leading
Voltage regulation of a transformer becomes zero when the secondary terminal voltage does not change between no-load and full-load. This happens at a certain leading power factor, because the reactive drop across leakage reactance is compensated by the leading current component. At unity p.f. or lagging p.f., regulation is generally positive, and at zero p.f. leading it is not necessarily zero.
Refrigeration air conditioning equipment usually reduces the absolute
A)
water flow
B)
humidity
C)
airflow
D)
evaporation
Refrigeration air-conditioning equipment cools air below its dew point, causing moisture to condense out. This reduces the absolute humidity of the air. The other options are not correct because air-conditioning does not directly reduce water flow, airflow, or evaporation as the primary effect.
Energy band in which free electrons exists?
A)
Valence band
B)
Conduction band
C)
First band
D)
Second band
Free electrons in a solid are those that can move easily and contribute to electrical conduction. These electrons are found in the conduction band. The valence band mainly contains bound electrons, not free carriers. Therefore, the correct answer is conduction band.
When an AC voltage is applied to a tube light fixture, it happens in the following sequence
A)
choke, starter, filaments of the tube
B)
starter, filaments of the tube, choke
C)
filaments of the tube, choke, starter
D)
none of these
In a fluorescent tube light, when AC is applied, the starter first closes the circuit and heats the filaments. Then the starter opens, causing a high voltage to be induced by the choke (ballast), which strikes the arc in the tube. So the sequence is: starter → filaments of the tube → choke. Hence, option (b) is correct.
The efficiency of solar cells is about
A)
25%
B)
15%
C)
40%
D)
60%
The efficiency of commonly used solar cells is about 15%. This is the standard textbook/exam value for photovoltaic cells. 25%, 40%, and 60% are much higher than the usual efficiency of conventional solar cells used in basic engineering questions.
When a synchronous motor is running at synchronous speed, the damper winding produces
A)
damping torque
B)
eddy current torque
C)
torque aiding the developed torque
D)
no torque
In a synchronous motor, once the rotor reaches synchronous speed, there is no relative motion between the rotating magnetic field and the damper winding. Since induced current in the damper bars depends on relative motion, no emf is induced and therefore no torque is produced by the damper winding. Damper winding mainly helps during starting and in damping oscillations, not at synchronous speed.
Vent plug is provided in each lead acid cell to
A)
Pour water or electrolyte when needed
B)
Check the electrolyte level
C)
Allow escape of gases during charging
D)
All of these
In a lead-acid cell, the vent plug is provided to let the gases formed during charging escape safely and to prevent pressure build-up inside the cell. It is not meant for pouring water/electrolyte or checking electrolyte level. Therefore, the correct option is (c).
Which of the following areas is preferred for solar power plants?
A)
Coastal areas
B)
Hot arid zones
C)
Mountain tops
D)
High rainfall zones
Solar power plants are best suited to areas with high solar insolation, clear skies, low cloud cover, and minimal rainfall. Hot arid zones provide these conditions, so they are preferred for solar power plants. Coastal areas, mountain tops, and high rainfall zones are less ideal due to humidity, cloudiness, or weather constraints.
A diac is ____ switch.
A)
an AC
B)
a mechanical
C)
a dc
D)
both ac and dc
A DIAC is a bidirectional trigger device used mainly in AC circuits. It conducts only after the breakover voltage is reached and can conduct in both directions, so it is commonly described as an AC switch. It is not a mechanical switch, and it is not used as a DC switch because it needs alternating polarity for normal operation.
Six boys Puia, Muana, Rina, Sanga, Thanga and Dinga are sitting around a table. Puia is second to the left of Dinga. Muana is between Rina and Sanga. Thanga is second to the left of Sanga. Which one is sitting opposite to Puia?
A)
Rina
B)
Muana
C)
Thanga
D)
Sanga
Let us arrange the six boys around a circular table.
1. Puia is second to the left of Dinga.
2. Thanga is second to the left of Sanga.
3. Muana is between Rina and Sanga, so the three sit consecutively as Rina–Muana–Sanga or Sanga–Muana–Rina.
A consistent circular arrangement is:
Dinga, Sanga, Muana, Rina, Puia, Thanga.
Here, Puia sits opposite Rina.
So the correct answer is Rina.
An auto transformer is preferred over an ordinary two winding transformer due to
A)
Safety of operation
B)
Availability of taps on secondary
C)
Copper savings
D)
All of these
An autotransformer uses a single continuous winding, so part of the winding is common to both primary and secondary. Because of this, it requires much less copper than an ordinary two-winding transformer. That is the main reason it is preferred in suitable applications.
(a) Safety of operation is not a reason to prefer it; in fact, autotransformers provide less electrical isolation and are generally less safe than two-winding transformers.
(b) Availability of taps on secondary is a feature, but not the primary advantage asked here.
(d) All of these is incorrect because safety is not an advantage.
The harmonic currents in a power transformer does not cause increase in
A)
Secondary voltage
B)
Core loss
C)
Copper loss
D)
Magnetic interference with protective relays
Harmonic currents in a transformer increase losses and cause interference, but they do not directly cause an increase in secondary voltage. Harmonics raise core loss due to additional hysteresis and eddy-current effects, increase copper loss because RMS current becomes higher, and can create magnetic interference with protective relays. Therefore, the correct answer is secondary voltage.
In generation of wind power, which of the following is not related?
A)
Kinetic energy
B)
Potential energy
C)
Mechanical energy
D)
Electrical energy
Wind power is generated from the kinetic energy of moving air. This kinetic energy is converted into mechanical energy by the turbine blades, and then into electrical energy by the generator. Potential energy is not directly related to wind power generation.
Which device does not have a gate terminal?
A)
Triac
B)
SCR
C)
FET
D)
Diac
A DIAC is a two-terminal bidirectional trigger device, so it does not have a gate terminal. In contrast, a TRIAC, SCR, and FET all have a gate terminal used for control. Therefore, the correct answer is DIAC.
If a transformer primary is energised from a square wave voltage source, its output voltage will be
A)
A square wave
B)
A sine wave
C)
A triangular wave
D)
A pulse wave
For an ideal transformer, the secondary voltage is proportional to the primary voltage by the turns ratio. So if the primary is supplied with a square wave voltage, the secondary output voltage will also be a square wave (scaled in magnitude). A sine wave would require sinusoidal excitation, and triangular or pulse waveforms are not the direct transformer output for square-wave input.
Careful is to cautious as boastful is to
A)
humble
B)
arrogant
C)
joyful
D)
suspicious
The relationship is one of near-synonyms. "Careful" is similar in meaning to "cautious". Likewise, "boastful" is similar in meaning to "arrogant". The other options do not match the meaning of "boastful".
Sodium-vapor lamps cause less
A)
light pollution
B)
noise pollution
C)
air pollution
D)
none of these
Sodium-vapor lamps emit light in a narrow wavelength band and produce less scattered glare compared with many other lamps. Hence, they cause less light pollution. They are not related to noise pollution, and air pollution is not the intended effect here.
Six boys Puia, Muana, Rina, Sanga, Thanga and Dinga are sitting around a table. Puia is second to the left of Dinga. Muana is between Rina and Sanga. Thanga is second to the left of Sanga. Which of the following are the neighbours of Puia?
A)
Dinga & Puia
B)
Thanga & Rina
C)
Dinga & Rina
D)
Data inadequate
Let Dinga be fixed. Puia is second to the left of Dinga, so Puia’s position is determined. Thanga is second to the left of Sanga, and Muana is between Rina and Sanga. Arranging the six boys consistently around the circle gives the order such that Puia sits next to Dinga on one side and Rina on the other. Therefore, the neighbours of Puia are Dinga and Rina. Hence, option (c) is correct.
Piezo-electric transducers are
A)
Digital transducer
B)
Inverse Transducer
C)
Active transducer
D)
Passive transducer
Piezo-electric transducers generate an electrical output directly from the applied mechanical stress, without requiring an external power source for the sensing action. Hence, they are classified as active transducers. They are not digital transducers, and they are not passive transducers because passive transducers need external excitation. 'Inverse transducer' refers to a device that converts electrical energy into a non-electrical form, which is not the usual classification here.
The instrument having its deflection depends upon average value is
A)
Movingiron
B)
Hot wire
C)
Rectifier type
D)
Induction type
A rectifier-type instrument first converts AC into a unidirectional current using a rectifier, and the deflection is proportional to the average value of the rectified current/voltage. Moving iron instruments respond to RMS value, hot-wire instruments also indicate RMS value due to heating effect, and induction type instruments work on alternating magnetic fields and are not based on average value.
The size of a conductor used in power cables depends on the
A)
operating voltage
B)
power factor
C)
current to be carried
D)
type ofinsulation used
The size (cross-sectional area) of a conductor in a power cable is mainly decided by the current it must carry, because the conductor must safely handle the load current without excessive heating and voltage drop. Operating voltage affects insulation thickness, not conductor size directly. Power factor influences current indirectly for a given power, but the direct design basis is the current. Type of insulation also affects insulation thickness and cable construction, not the conductor size itself.
A megger indicates infinity when test terminals are open-circuited. This is because
A)
No current flows through the current coil
B)
No current flows through the pressure coil
C)
No current flows through the compensating winding
D)
Current does not flow through current coil and pressure coil
A megger measures insulation resistance. When the test terminals are open-circuited, the resistance is effectively infinite, so no current flows through the current coil. In a megger, the deflection to infinity corresponds to zero current in the current coil. The pressure coil is not the reason for this indication, and the compensating winding is not relevant here.
Which of the following devices is employed in substation to limit the short circuit current in the power system?
A)
Shunt condenser
B)
Reactor
C)
Series capacitor
D)
Shunt capacitor
A reactor is used in substations to limit short-circuit current by adding inductive reactance in series with the system. This increases the impedance during fault conditions and reduces the fault current. A shunt condenser, series capacitor, and shunt capacitor are used mainly for power factor improvement, voltage support, or reactive power compensation, not for limiting short-circuit current.
Poles suitable for lines upto 66 kV is
A)
Wood Poles
B)
R.C.C. Poles
C)
Steel tubular poles
D)
Steel towers
For distribution and sub-transmission lines up to about 66 kV, steel tubular poles are commonly used because they provide adequate mechanical strength and durability. Wood poles are generally used for lower-voltage lines, while steel towers are preferred for higher-voltage transmission lines above 66 kV. R.C.C. poles are also used in distribution, but steel tubular poles are the standard suitable choice for lines up to 66 kV.
Earthing conductivity is affected by:
A)
Moisture content in the soil
B)
Chemical composition
C)
Concentration of salts in the soil
D)
All ofthese
Earthing conductivity depends on the soil’s ability to conduct current. This is influenced by moisture content, chemical composition, and the concentration of salts in the soil. Moist soil and higher salt content generally reduce resistance and improve conductivity. Therefore, all the given factors affect earthing conductivity.