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Past Questions
To start and run ceiling fans, generally we use
A)
inductor
B)
resistor
C)
transistor
D)
capacitor
Ceiling fans generally use a single-phase capacitor motor. The capacitor provides the necessary phase shift to create a rotating magnetic field, which helps the motor start and run smoothly. Inductors, resistors, and transistors are not the usual starting/running component for ceiling fan motors.
The power factor of a squirrel cage induction motor is
A)
low at light load only
B)
low at heavy load only
C)
low at light and heavy load both
D)
low at rated load only
In a squirrel cage induction motor, the magnetizing current is required even when the load is small. At light load, this magnetizing component dominates the stator current, so the power factor is low. As the load increases, the active component of current increases and the power factor improves, becoming better near rated load. Therefore, the power factor is low at light load only.
If the field of a synchronous motor is under excited, the power factor will be
A)
leading
B)
lagging
C)
more than unity
D)
unity
In a synchronous motor, if the field is under-excited, the motor draws more magnetizing current from the supply. This makes the armature current lag behind the supply voltage, so the power factor becomes lagging. If it were over-excited, the power factor would become leading; at normal excitation, it can be unity.
A salient pole synchronous motor is running at no load. Its field current is switched off. The motor will
A)
come to stop
B)
continue to run at synchronous speed
C)
continue to run at a speed slightly more than the synchronous speed
D)
continue to run at a speed slightly less than the synchronous speed
A salient pole synchronous motor needs DC field excitation to produce rotor magnetic poles and maintain synchronism with the rotating stator field. If the field current is switched off at no load, the motor loses synchronizing torque. Since there is no load, it may momentarily keep rotating due to inertia, but it cannot remain at synchronous speed and will gradually slow down and come to rest. Hence, the correct answer is (a).
Slip of the induction machine is 0.02 and the stator supply frequency is 50 Hz. What will be the frequency of the rotor induced emf?
A)
10 Hz.
B)
50 Hz.
C)
1 Hz.
D)
2500 Hz.
For an induction motor, rotor induced emf frequency is given by f_r = s × f_s, where s is slip and f_s is stator supply frequency. Here, s = 0.02 and f_s = 50 Hz, so f_r = 0.02 × 50 = 1 Hz. Hence, the correct option is (c).
Embarrassed is to humiliated as frightened is to
A)
reckless
B)
agitated
C)
courageous
D)
terrified
The relationship is of synonyms or near-synonyms: "embarrassed" and "humiliated" both express a similar feeling of shame or discomfort. Likewise, "frightened" and "terrified" are both words for being very afraid. The other options do not match this synonym relationship.
While running, a synchronous motor is compelled to run at synchronous speed because of
A)
damper winding in its pole faces
B)
magnetic locking between stator and rotor poles
C)
induced e.m.f in rotor field winding by stator flux
D)
compulsion due to Lenz’s law
A synchronous motor runs at synchronous speed because the rotor magnetic field locks with the rotating stator magnetic field. This magnetic locking keeps the rotor in step with the stator field, so the speed remains equal to synchronous speed. Damper winding is mainly used for starting, not for maintaining synchronous speed. Induced e.m.f. in the rotor field winding is not the reason for synchronous running, and Lenz’s law is not the direct cause here.
A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is
A)
1500 r.p.m.
B)
1425 r.p.m.
C)
25 r.p.m.
D)
75 r.p.m.
For a 3-phase, 4-pole, 50 Hz induction motor, synchronous speed is Ns = 120f/P = 120 × 50 / 4 = 1500 r.p.m. With slip s = 0.05, the rotor runs at Nr = (1 − s)Ns = 0.95 × 1500 = 1425 r.p.m. The rotor m.m.f. rotates relative to the rotor structure at slip speed, which is sNs = 0.05 × 1500 = 75 r.p.m. Hence the correct answer is 75 r.p.m. Option (b) is the rotor speed, not the relative speed of rotor m.m.f.
In a transformer, with closed magnetic coupling between primary and secondary it will improve
A)
Efficiency
B)
All day efficiency
C)
Regulation
D)
None of these
Closed magnetic coupling means the primary and secondary are tightly linked by the magnetic flux, so leakage flux is reduced. Lower leakage reactance improves the voltage drop under load, which improves voltage regulation. Efficiency is not the main direct effect here, and all-day efficiency depends on load pattern, not magnetic coupling alone.
A fluorescent lamp basically consists of a long glass gas discharge tube. Its inner surface is coated with
A)
Zinc
B)
Copper
C)
Phosphorous
D)
Sodium
A fluorescent lamp has a glass discharge tube whose inner surface is coated with a phosphor material. In many exam options this is written as 'phosphorous', though the correct technical term is phosphor. The coating converts ultraviolet radiation produced by the discharge into visible light. Zinc, copper, and sodium are not used as the inner coating for this purpose.
Two transformers of identical voltages but different capacities are operated in parallel, for satisfactory load sharing
A)
Impedances should be equal
B)
Per unit impedance must be equal
C)
Per unit impedance and X/R ratio must be equal
D)
Impedance and X/R ratio must be equal
For two transformers to share load satisfactorily in parallel, their percentage (per unit) impedances should be equal so that the load divides in proportion to their ratings. Since the transformers have different capacities, equal actual impedances are not required; what matters is equal per unit impedance. Also, their X/R ratios should be equal so that the load current division remains proper and circulating currents are minimized. Hence, the correct condition is equal per unit impedance and equal X/R ratio.
The lower the rating of the single phase motors
A)
the lower is the efficiency
B)
the higher is the efficiency
C)
the lower is the frequency
D)
both (b) & (c)
In single-phase motors, efficiency generally decreases as the motor rating becomes smaller. Small motors have proportionally higher losses such as friction, windage, and core losses compared to their output power, so their efficiency is lower. Hence, the lower the rating of the single-phase motor, the lower is the efficiency. Options (b) and (c) are incorrect because efficiency does not increase with lower rating, and frequency is not determined by motor rating.
The law that finds application in electrolysis is
A)
Faraday’s law
B)
Coulomb’s law
C)
Ohm’s law
D)
Lenz’s law
Electrolysis is governed by Faraday’s laws of electrolysis, which relate the amount of chemical change at an electrode to the quantity of electric charge passed. Coulomb’s law deals with electrostatic force, Ohm’s law with the relation between voltage, current, and resistance, and Lenz’s law with the direction of induced current in electromagnetic induction.
The current from the stator of an alternator is taken out to the external load circuit through
A)
sliprings
B)
commutator segments
C)
solid connections
D)
carbon brushes
In an alternator, the armature winding is on the stator, so the output current is directly available at the stationary terminals. In machines where a rotating connection is needed, carbon brushes are used with slip rings, but for an alternator the stator current is taken out through the terminal connections. Among the given options, the best answer is carbon brushes only if referring to the general current-collecting arrangement; however, technically the stator output is taken out through solid connections. Since the question asks specifically about the stator current being taken out to the external load circuit, the correct option is solid connections.
The choke of tube light is
A)
a capacitor
B)
a resistor
C)
an inductor
D)
a diode
In a tube light, the choke is used to limit the current after the lamp starts operating. A choke is essentially an inductor, which offers inductive reactance to AC and helps in starting the tube by producing a high voltage when the circuit is interrupted. A capacitor stores charge, a resistor limits current by resistance, and a diode allows current in one direction only—none of these are the choke in a tube light.
The capacity of lead acid battery is adversely affected by increase in
A)
Discharge rate
B)
Charge rate
C)
Temperature
D)
Specific gravity of the electrolyte
The capacity of a lead-acid battery decreases when the discharge rate increases. This is because at higher discharge currents, the chemical reactions inside the battery cannot keep up efficiently, so the usable capacity falls. Charge rate and specific gravity do not directly cause this adverse effect in the same way, and temperature is not the standard answer here.
Two winding transformer transfers electric power from one circuit to another
A)
without electrically isolated
B)
without a change of frequency
C)
without mutual inductive influence to each other
D)
Both (a) and (b)
A transformer transfers AC power from one circuit to another by electromagnetic induction. In a two-winding transformer, the frequency of the output remains the same as the input frequency; only voltage and current levels change. Option (a) is incorrect because transformer windings are electrically isolated from each other. Option (c) is incorrect because the transfer occurs through mutual inductive coupling, not without it. Therefore, the correct answer is (b) without a change of frequency.
An SCR is a ____ triggered device.
A)
current
B)
power
C)
voltage
D)
noise
An SCR (Silicon Controlled Rectifier) is a current-triggered device because it is turned ON by applying a small gate current. Once triggered, it continues conducting until the anode current falls below the holding current. The other options are incorrect: it is not voltage-triggered, power-triggered, or noise-triggered.
In synchronous motor, damper winding is provided in order to
A)
stabilise rotor motion
B)
suppress rotor oscillations
C)
suppress rotor oscillations
D)
both (b) & (c)
In a synchronous motor, the damper winding helps in damping out hunting, i.e., rotor oscillations. So it is used to suppress rotor oscillations and stabilize rotor motion. Since options (b) and (c) are the same, the intended correct choice is the option meaning both.
Solar cells are made of
A)
Aluminium
B)
Germanium
C)
Silicon
D)
Cadmium
Solar cells are commonly made of silicon because it is a semiconductor material that can efficiently convert sunlight into electrical energy. Aluminium is a conductor, not a semiconductor. Germanium can be used in semiconductors but is not the standard material for solar cells. Cadmium is used in some thin-film solar technologies, but the usual answer in basic exam questions is silicon.
Earth resistance of service connection should not exceed
A)
1.0ohms
B)
2.0 Ohms
C)
3.5 Ohms
D)
5.0 Ohms
For a service connection, the earth resistance is required to be kept low for safety and proper fault current dissipation. The standard value commonly specified in such exam questions is not more than 2 ohms. Hence option (b) is correct. The other values are either too low to be the usual prescribed limit in this context or too high to ensure safe earthing.
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.
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 which is cheapest for dc measurement is
A)
Movingiron
B)
PMMC
C)
Hot-wire
D)
Electro-dynamo
For DC measurement, the cheapest instrument among the given options is the moving iron instrument. It is rugged, simple in construction, and generally less expensive than PMMC instruments. PMMC is more accurate for DC but costlier. Hot-wire instruments are mainly used for both AC and DC but are not the cheapest. Electro-dynamo (electrodynamometer) instruments are more complex and expensive.