Ques 12. The area of the hysteresis loop will be the least for one of the following materials.
Wrought Iron
Silicon Steel✔
Hard Steel
Soft Iron
The material used for the Core in a transformer is selected on the basis of its magnetic permeability and its characteristic hysteresis loop. A material with a low hysteresis loop area and high permeability need to be selected for the higher efficiency of the transformer.
Steel, silicon alloys have a very narrow hysteresis loop. Silicon steel alloy has high permeability and low hysteresis losses, therefore, these materials are used for transformer core and armature core which are subjected to rapid reversal of magnetization.
Ques 13. A 150-volt moving iron voltmeter of accuracy class 1.0 reads 75 V when used in a circuit under standard conditions. The maximum possible percentage error in the reading is
2
0.5✔
4
1
percent error = [experimental value – theoretical value] / theoretical value x 100%
75 -150 = -75
|-75| = 75
%error = 75/150
= 0.5%
Ques 14. The magnitude of AT required to establish a given value of flux in the air gap will be much greater than that required for iron part of a magnetic circuit because
Air in Gas
Air is a good conductor of magnetic flux
Air has the lowest relative permeability✔
Iron has the lowest permeability
The reluctance of a uniform magnetic circuit can be calculated as:
R = l/μo μr A
R ∝ 1/μo μr
Where l = length
μo = permeability of vacuum, equal to 4π × 10−7
μr = relative magnetic permeability of the material
A = cross-sectional area of the circuit
Reluctance of air > Reluctance of iron
So air has the lowest relative permeability
Ques 15. The unit of luminous flux is
Candela
Lumen✔
Lux
Steradian
The SI unit of luminous flux is the lumen (lm). One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian.
Ques 16. In bipolar junction transistors, the value of configuration which will give both voltage gain and current gain is
CC
CB
CE✔
None of the above
The CE (common Emitter ) configuration is the only configuration that provides both voltage gain and current gain greater than unity.
In the case of CB (common Base) current gain is less than unity and in the case of CC (common collector) voltage gain is less than unity.
Ques 17. The locked rotor current of a shaded pole motor is
Less than the full load current
Equal to full load current
Several times the full load current
Slightly more than the full load current✔
The shaded-pole motor is rugged, inexpensive, small in size, and requires little maintenance. Its stalled locked-rotor current is only slightly higher than its normal rated current so that it can even remain stalled for short periods without harm.
Ques 18. The voltage across the 1 kΩ resistor of the network shown in the given figure is
4 V
6 V
1 V
2 V✔
Ques 19. For painful shock what is the range of electric shock current at 50 Hertz?
0-1 mA
3-5 mA✔
0-3 mA
5- 10 mA
The damage caused by electric shock depends on the current flowing through the body — 1 mA can be felt; 5 mA is painful.
Above 15 mA, a person loses muscle control, and 70 mA can be fatal.”
The amount of damage done by the electric shock depends not only on the magnitude of the current but also on which portions of the body that the electric current is flowing through. The reason for this is that different parts of the body have different resistances, which can lead to an increase in the current
Ques 20. The purpose of starting winding in a single-phase induction motor is to?
Produce rotating flux in conjunction with the main winding✔
Limit temperature rise of the machine
Reduces Losses
Increase losses
In single-phase motors, there is only one main winding and a single-phase supply. So there is no RMF (rotating magnetic flux) generated inside the stator of such machines. So such machines are not inherently self-starting.
The auxiliary winding or starting winding is connected to the ac mains via a capacitor in order to generate a two-phase supply, which can generate RMF when the coils are spatially placed at 90° with respect to the main winding. This RMF is responsible for the operation of single-phase induction motors. Adding the capacitor makes the motor self-starting.
