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Past Questions — 2016
For ceiling fans generally the single phase motor used is
A)
split phase type
B)
capacitor start type
C)
capacitor start and run type
D)
permanent capacitor type
Ceiling fans generally use a single-phase capacitor-run motor, also called a permanent split capacitor (PSC) motor or permanent capacitor type. The capacitor remains in the circuit during running to improve power factor, efficiency, and starting/running performance. Split-phase motors and capacitor-start motors are more common in applications needing high starting torque, while ceiling fans need smooth, quiet, continuous operation rather than high starting torque. Hence, the correct answer is permanent capacitor type.
A 3-phase 50 Hz induction motor runs at speed of 940 RPM; the speed of the rotating magnetic field will be:
A)
940 RPM
B)
1000RPM
C)
1050RPM
D)
1100RPM
For a 3-phase induction motor, the rotating magnetic field moves at synchronous speed: Ns = 120f / P. The rotor speed is 940 RPM, which must be slightly less than synchronous speed due to slip. Among the given options, 1000 RPM is the only plausible synchronous speed. It corresponds to a 6-pole, 50 Hz motor: Ns = 120 × 50 / 6 = 1000 RPM. The other options are not suitable as the field speed must be higher than rotor speed.
The maximum value of torque that a synchronous motor, can develop without losing its synchronism, is known as
A)
breaking torque
B)
synchronizing torque
C)
pull out torque
D)
slip torque
In a synchronous motor, the maximum torque it can develop while still remaining in step with the supply frequency is called the pull-out torque. If the load exceeds this value, the motor loses synchronism.
- Breaking torque: not the standard term for this limit in synchronous motors.
- Synchronizing torque: the torque that tends to restore synchronism after a small disturbance, not the maximum permissible torque.
- Slip torque: associated with induction motors, not synchronous motors.
Hence, the correct answer is pull out torque.
Which of the following conditions is necessary for triggering system for thyristors?
A)
It should be synchronized with the main supply
B)
It must use separate power supply
C)
It should provide a train of pulses
D)
None of these
A thyristor triggering system must deliver gate pulses in a reliable manner, and in practice a train of pulses is used so that the device can turn on even if the first pulse is missed or the device is not ready at that instant. Synchronization with the main supply is generally required for phase control, but it is not the essential triggering condition itself. A separate power supply is not necessary as a general condition. Therefore, the correct answer is that it should provide a train of pulses.
Maximum wind energy available is proportional to:
A)
Air density
B)
Cube of wind velocity
C)
Square of the rotor diameter
D)
All of these
The maximum wind power available is given by the expression P = (1/2) × ρ × A × V^3, where ρ is air density, A is the swept area of the rotor, and V is wind velocity. Since swept area A is proportional to the square of rotor diameter, the available wind energy is proportional to air density, cube of wind velocity, and square of rotor diameter. Therefore, all the given factors are correct.
Chopper control for DC motors provides variation in
A)
Input voltage
B)
Frequency
C)
Both (a) and (b)
D)
None of these
A chopper is a DC-to-DC converter used in DC motor speed control. It varies the average DC voltage supplied to the motor by switching the input on and off rapidly. The output frequency is not the main controlled quantity here; the control is essentially by changing the input voltage level. Hence, the correct answer is input voltage.
In a 3-phase controlled bridge rectifier, with an increase of an overlap angle, the output dc voltage
A)
Decreases
B)
Increases
C)
Does not change
D)
Depends upon load inductance
In a 3-phase controlled bridge rectifier, overlap occurs because of source inductance during commutation. As the overlap angle increases, more of the input supply is effectively shorted/shared between outgoing and incoming devices, reducing the average dc output voltage. Therefore, the output dc voltage decreases. The other options are incorrect because the voltage is not unchanged, does not increase with overlap, and while load inductance can affect current continuity, the effect of overlap angle on average dc output voltage is a general reduction.
The Transformer core laminations are insulated from each other by:
A)
Paper
B)
Thin varnish coating
C)
Mica strip
D)
All the above can be used for insulation
Transformer core is made of thin steel laminations to reduce eddy current loss. Each lamination is insulated from the next by a thin insulating coating, commonly varnish or oxide layer. Paper and mica are not the standard insulation used between core laminations, so 'Thin varnish coating' is the correct answer.
The installed capacity of wind energy in India is about
A)
8000 MW
B)
1500 MW
C)
6000 MW
D)
4000 MW
The installed wind energy capacity in India around the period of this exam is approximately 8,000 MW, which matches option (a). The other values are too low for India’s wind-energy capacity at that time.
The normal way to close a SCR is by approximate
A)
Gate current
B)
Cathode current
C)
Anode current
D)
Forward current
An SCR is turned ON by a gate pulse, but it is normally turned OFF by reducing the anode current below the holding current. In other words, the normal way to close (switch off) an SCR is by removing/reducing the anode current. Gate current is used only for triggering, not for turn-off. Cathode current and forward current are not the standard term for the normal turn-off method.
The current density of a photo voltaic cell ranges from
A)
10-20 mA/cm2
B)
40-50 mA/cm2
C)
20-40 mA/cm2
D)
60-100 mA/cm2
A photovoltaic cell typically produces a current density in the range of about 20 to 40 mA/cm² under standard illumination conditions. This is the commonly accepted range for solar cells in basic renewable energy/electrical engineering MCQs. Option (a) is generally too low, while (b) and (d) are higher than the usual current density range for a PV cell.
The rating of transformer may be expressed in
A)
kw
B)
kVAR
C)
kVA
D)
Horse power
A transformer is rated in kVA because its capacity depends on voltage and current, not on the power factor of the load. Since core loss depends mainly on voltage and copper loss depends on current, the transformer rating is based on apparent power (V × I), i.e., kVA. kW would depend on power factor, kVAR is for reactive power, and horsepower is used for mechanical machines, not transformers.
The maximum torque in a 3-phase induction motor occurs at a slip
A)
R/X
B)
X/R
C)
R2/X
D)
R2/X
For a 3-phase induction motor, the condition for maximum torque is when rotor resistance per phase equals slip reactance per phase: R2 = sX2. Hence slip at maximum torque is s = R2/X2, often written simply as R/X in simplified MCQ form. Among the given options, (a) R/X matches this relationship. Options (b), (c), and (d) do not represent the correct slip expression.
The friction losses in Real Transformers are
A)
0%
B)
5%
C)
25%
D)
50%
A transformer is a static electrical device with no moving parts, so there is no mechanical friction. Therefore, friction losses in a real transformer are 0%. The other options are not correct because they suggest mechanical losses that do not occur in transformers.
In a dc motor, if the field coils get opened, the speed of the motor will
A)
Decrease
B)
Come to a stop
C)
Increase
D)
None of these
In a DC motor, speed is approximately proportional to back EMF divided by field flux. If the field coils open, the field current becomes zero, so the flux weakens drastically. With very weak flux, the motor tries to speed up sharply. In a shunt motor this may cause dangerous overspeeding. Therefore, the speed will increase. The other options are incorrect because the motor does not normally stop immediately, and it certainly does not decrease in speed when field flux is lost.
The chemical used in the breather is:
A)
Sodium chloride
B)
Silica sand
C)
Silica gel
D)
Copper silicate gel
In a transformer breather, the chemical used is silica gel. It absorbs moisture from the air entering the transformer tank, keeping the insulating oil dry. Sodium chloride and silica sand are not used for this purpose, and copper silicate gel is not the standard desiccant in transformer breathers.
What is the maximum possible output of a solar array?
A)
300 W/m2
B)
100 W/m2
C)
250 W/m2
D)
500 W/m2
A solar array’s practical maximum output is typically taken as about 300 W/m² under very good operating conditions. This is the highest among the given options. 100 W/m² and 250 W/m² are lower than the usual peak practical value, while 500 W/m² is unrealistically high for a standard solar array output in this exam context.
The power factor of a 3-phase induction motor at no load is approximately
A)
0.2
B)
0.7
C)
0.85
D)
1
A 3-phase induction motor at no load draws a large magnetizing current to establish the air-gap flux, so its current is highly reactive. Hence the no-load power factor is very low, typically around 0.2 to 0.3. Among the given options, 0.2 is the best answer. Options 0.7, 0.85, and 1 correspond to much better power factors, which are not typical for no-load induction motors.
An Auto-transformer (which has only one winding) may be used as a
A)
Step-Up Transformer
B)
Step-Down Transformer
C)
Both Step-Up and Step-Down transformer
D)
None of these
An auto-transformer has a single continuous winding with taps, so it can operate in two ways: as a step-up transformer when the output is taken from a higher-voltage section, and as a step-down transformer when the output is taken from a lower-voltage section. Hence, it can be used as both step-up and step-down transformer. Therefore, option (c) is correct.
Choose the correct order, from left to right, in which these parts should be placed in order to form the object.
A)
3 1 4 2
B)
3 4 1 2
C)
3 2 4 1
D)
1 2 3 4
The correct left-to-right arrangement is 3 2 4 1, so option c is correct. This sequence matches the way the parts interlock to form a continuous object: part 3 provides the left end, part 2 fits next to it, part 4 continues the central shape, and part 1 completes the right end. The other options are incorrect because they place at least one part in a position where its connecting edges or contours do not align with the neighboring part, breaking the overall shape continuity.
In dc choppers, the waveforms for input and output voltages are respectively
A)
Discontinuous, continuous
B)
Continuous, discontinuous
C)
Both continuous
D)
Both discontinuous
In a DC chopper, the input is a DC supply, so the input voltage is continuous. The chopper switches this DC on and off to produce a pulsed output, so the output voltage is discontinuous. Hence, the correct pair is continuous for input and discontinuous for output.
Full-load copper losses in a 3-phase 50 Hz 4-pole induction motor running at 1455 rpm are 300 W. The rotor input is
A)
5kW
B)
10kW
C)
20kW
D)
50kW
For a 4-pole, 50 Hz induction motor, synchronous speed Ns = 120f/P = 120 × 50 / 4 = 1500 rpm.
Given running speed N = 1455 rpm, slip s = (Ns − N)/Ns = (1500 − 1455)/1500 = 45/1500 = 0.03.
Rotor copper loss = s × rotor input.
Given rotor copper loss = 300 W, so rotor input = 300 / 0.03 = 10,000 W = 10 kW.
However, since the question asks for the rotor input and the options include 10 kW, that is the correct value. The other options do not match the slip-loss relation.
Low voltage winding is placed next to the core in the case of concentric windings as__is reduced:
A)
Hysteresis loss
B)
Leakage fluxes
C)
Eddy current loss
D)
Insulation requirement
In a transformer with concentric windings, the low-voltage (LV) winding is placed next to the core because it is at a lower potential relative to the earthed core. This reduces the insulation needed between the winding and the core. The arrangement does not directly reduce hysteresis loss, eddy current loss, or leakage fluxes.
The starting torque in a single-phase induction motor is
A)
High
B)
Low
C)
Zero
D)
Very low
A single-phase induction motor is not self-starting. At standstill, the single-phase supply produces only a pulsating magnetic field, which can be resolved into two equal oppositely rotating fields. These produce equal and opposite torques, so the net starting torque is zero. Therefore, the motor needs an auxiliary starting arrangement to begin rotation. Hence, option (c) Zero is correct. The other options are incorrect because the starting torque is not high, low, or very low—it is theoretically zero at start.
In a Transformer, the primary flux is always the secondary (flux).
A)
Greater than
B)
Smaller than
C)
Equal to
D)
Equal in both step up and Step down Transformer
In an ideal transformer, the same mutual flux links both the primary and secondary windings. Therefore, the primary flux and secondary flux are equal. Option (a) and (b) are incorrect because one is not greater or smaller than the other. Option (d) is misleading because the equality of flux is true for all ideal transformers, not only step-up and step-down types.