Ques 41. In a motor starter, the electromechanical contractor provides inherent protection against
Over-current
Short-circuit
Single Phasing
Under Voltage
Answer 4. Under Voltage
Explanation:
Undervoltage occurs when the average voltage of a three-phase power system drops below the intended levels. Electromechanical devices, including three-phase motors and pumps, are designed to be operated at very specific voltage levels. If these devices are allowed to operate at reduced voltage levels they will draw higher currents which can damage the motor.
A three-phase monitor relay, with Undervoltage protection such as an electromechanical relay, is used for this purpose. The Undervoltage relay may either alarm or trip voltage-sensitive loads, such as induction motors, whenever the line voltage drops below the calibrated setting. For electromechanical relays, an instantaneous Undervoltage relay with its contacts connected in series with the time Undervoltage relay contacts may also be used to provide a fast reset time.
Ques 42. In the circuit, v is the input voltage applied across the capacitor of 2 F. Current through the capacitor is
Answer. D
The given triangle output can be changed into the square output
The instantaneous value of capacitor current is
i(t) = cdv/dt
where C is the capacitance in farad
dv/dt = = instantaneous rate of voltage change(Volt/second)
For charging rate of voltage change is 10 and for discharging rate of voltage change is -11 therefore
Ques 43. In a semiconductor, the resistivity
Depends on temperature
Depends on the voltage
Depends on the current through it
None of the above
Answer.1. Depends On Temperature
Explanation:
In a semiconductor, there is a bandgap and at absolute zero temperature, all the electrons are below the gap, and thus not available for conduction.
At finite temperature, some electrons are thermally excited into the conduction band, and thus you get conduction when an electric field is applied. As the temperature is increased covalent bonds breaks and more electron-hole pairs are available for conduction. Thus conductivity increases and resistivity, the reciprocal of conductivity decreases.
Ques 44. A geyser is operated from 230 V, 50 c/s mains. The frequency of the instantaneous power consumed by the geyser is
25 c/s
50 c/s
100 c/s
150 c/s
Answer 3. 100 c/s
Explanation:
Frequency of instantaneous power consumed = 2f = 2 x 50 = 100 c/s
Ques 45. Ampere-second is the unit of
E.M.F
Power
Electric Charge
Energy
Answer.3. Electric Charge
Explanation:
The SI unit of charge, the coulomb, “is the quantity of electricity carried in 1 second by a current of 1 ampere
Ques 46. Two lossy capacitors with equal capacitance values and power factors of 0.01 and 0.02 are in parallel, and the combination is supplied from a sinusoidal voltage source. The power factor of the combination is
0.03
0.015
0.01
0.0002
Answer.2. 0.015
Explanation:
Since capacitor are connected in parallel therefore
PF = PF1 + PF2 / 2
= 0.01 + 0.02 /2
= 0.015
Ques 47. A voltmeter when connected across a dc supply, reads 124 V. When a series combination of the voltmeter and unknown resistance X is connected across the supply, the meter reads 4 V. If the resistance of the voltmeter is 50 kΩ, the value of X is
1550 kΩ
1600 kΩ
1.6 kΩ
1.5 MΩ
Answer 4. 1.5Ω
Explanation:
Current through voltmeter
Iv = V/R = 4/50000 = 8 x 10-5 A
Dc supply voltage Vdc = 120 V
Then the value of unknown resistance is given as
R = V/I
R = 120/8 x 10-5 = 1.5 MΩ
Ques 48. The purpose of providing a choke in the tube light is
To eliminate the corona effects
To avoid radio interference
To improve the power factor
To limit current to the appropriate value
Answer.4. To limit current to the appropriate value
Explanation:
There are mainly two functions of the choke coil
Limit the current.
Produce high voltage across the tube light.
Limit the current
In a gas discharge, such as a fluorescent lamp, current causes resistance to decrease. This is because as more electrons and ions flow through a particular area, they bump into more atoms, which frees up electrons, creating more charged particles.
In this way, the current will climb on its own in a gas discharge, as long as there is an adequate voltage (and household AC current has a lot of voltage).
If the current in a fluorescent light isn’t controlled, it can blow out the various electrical components.
Produces High voltage Across Tubelight
Choke is nothing but the coil/ballast ( inductor) which is used to induce the high voltage across it. as we know that inductor has the property to induce high voltage for a brief period of time, this high voltage is required to ionize the gases in the starter.
Ques 49. In a 3-phase 400 V, 4-wire system, two incandescent lamps, one having 230 V, 100 W specification and the other 230 V, 200 W are connected between R phase-neutral and Y phase-neutral respectively. If the neutral wire breaks
100 W lamp will fuse first
200 W lamp will fuse first
Both the lamp will fuse together
Both the lamp will glow
Answer.1. 100 W lamp will fuse first
Explanation:
The 100W lamp will fail first, Although the voltage across the 100 w lamp will be higher (293V), it will fail because of the effective current (0.489A) which is more than the 100 w lamp’s normal current (0.4A).
An increase in current causes more heat and the heating element will fail.
Ques 50. A solenoid of inductance 250 mH and resistance 10 Ω is connected to a battery. The time taken for the magnetic energy to 1/4th of its maximum value is
