SSC JE 2015 Electrical question paper with solution

• 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.

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.

Disadvantages Of Electric drive

1. In the case of the failure of the supply, the electric drive comes to the rest position which may paralyze the whole system.
2. 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.
3. 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.

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)

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.

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

The voltage gain or current gain of amplifier is defined as

Gain = 20logAv

where Av is the gain in decibel

=20log1000

= 80 

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.