Ques 81. Which of the following motor is not self-starting?
DC series motor
Slip ring Induction motor
Synchronous motor✔
Squirrel cage induction motor
At first synchronous motor has Stator supplied with 3 phase supply and rotor with dc supply.
When dc supply is given to the rotor, alternate poles form on the rotor.
Because of the three-phase supply, a rotating Magnetic field will generate rotating torque at synchronous speed.
Consider during the positive half cycle (positive torque), the N pole of the stator and S pole of the rotor is in front of each other then they will experience the force of attraction, and the tendency of the rotor will be to rotate in an anti-clockwise direction.
Now consider, during the negative half cycle (Negative torque), the Pole of the stator will change to S. Then S pole of the stator and rotor will experience the force of repulsion and the tendency of the rotor will be to rotate in a clockwise direction.
So combine effect of the whole cycle will result in zero torque. Thus due to a cancellation of torque during positive and negative half cycle synchronous motor can’t be self-starting.
For the asynchronous motor to be self-starting we prefer damper winding. or mechanical input for starting.
Ques 82. A dc voltmeter has a sensitivity of 1000 Ω/volt. When it measures half full scale in the 100 V range, the current through the voltmeter will be?
50 mA
1 mA
0.5 mA✔
100 mA
Sensitivity is 1000 ohms / Volt
The full-scale current is going to be 1V/1000ohms = 1 mA.
That means 1/2 of the full scale is going to be 0.5 mA.
Hence correct answer is 0.5 mA.
Ques 83. The electric drives possess the following drawback:
Requires a continuous power supply✔
Requires hazardous fuel requirement
Not available with various rating
Not adaptable to various environments
Disadvantages Of Electric drive
In the case of the failure of the supply, the electric drive comes to the rest position which may paralyze the whole system.
The application of the electric drive system is limited to electric field area only i.e electric drives are not used where the supply is not available. They require a continuous power supply.
In the case of the faults like short circuits, and breakdown of overhead conductors the electric drive system may get damaged and lead to several problems.
Ques84. The mutual inductance between two coils is 4H. If the current in one coil changes at the rate of 2A/sec. then EMF induced in the other coil is?
8 V✔
2 V
0.5 V
5.0 V
Ques 85. The emf induced in a coil of N turns is given by
N(dΦ/dt)
-N(dΦ/dt)✔
(dΦ/dt)
N(dt/dΦ)
The emf induced in a coil is equal to the negative rate of the change of the magnetic flux linked with it i.e -N(dΦ/dt)
Ques86. To prepare a P-type semiconductor material the impurities to be added to silicon are?
Boron, Gallium✔
Arsenic, Antimony
Gallium, Arsenic
Gallium, Phosphorous
The term p-type refers to the positive charge of the hole. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. P-type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities (or doping an n-type semiconductor). A common p-type dopant for silicon is boron and Gallium.
Ques 87. A bar of Iron 1 cm2 in cross-section has 10-4 wb of magnetic flux in it. If μr = 2000. What is the magnetic field intensity in the bar?
796 × 103 AT/m
398 AT/m✔
398 × 10-4 AT/m
398 × 104 AT/m
Given area 1 cm2 = 10-4 m2
Flux Φ = 10-4 wb
Relative permeability μr = 2000
the magnetic permeability of free space μo = 4π × 10−7 N A−2
Ques 88. An amplifier has a gain of 10,000 expressed in decibels the gain is?
10
40
100
80✔
The voltage gain or current gain of amplifier is defined as
Gain = 20logAv
where Av is the gain in decibel
=20log1000
= 80
Ques 89. In batten wiring, the cables are carried on seasoned teak wood perfectly straight and well-varnished teak wood batten of the thickness not less than?
2 cm
1cm✔
3 cm
4 cm
All sheathed cables on brick, walls, stone, or plaster walls and ceilings steel joints, or any structural steelwork shall be run on Well-seasoned and varnished, straight teak wood battens of not loss than I0 mm finished thickness.
