SSC JE 2015 Electrical question paper with solution

Biasing of the transistor means setting up a fixed level of current which should flow on the transistor with the fixed level of the voltage drop across the transistor.

For proper working of the transistor, the base-emitter junction is forward biased and the collector-base junction is reversed biased.

By making the base-emitter junction forward biased the resistance of the base-emitter become very low as compared with the resistance of the collector-base junction.

A forward-biased semiconductor junction has low resistance while the reversed biased junction has high resistance. And the large difference in junction resistance makes the transistor capable of power gain.

By connecting the 3 lamps in parallel the total resistance of a parallel is less than the resistance of the smallest resistance. Hence the overall resistance is decreased.

The power dissipated in a circuit is equal to

P = V²/R

So by reducing the resistance, we increase the overall power. Therefore to realize the full rated power of the lamps they are to be connected in parallel.

The 3-phase power is given by

P = 3VICosΦ

For star connection V_L = √3V

For Delta connection I_L = √3I

(3V_LI_LCosΦ)/√3

= √3V_L I_L  cosΦ

Since the base is connected to +ve probe it has a higher potential than other terminals so it makes the transistor in forward bias mode for both terminals and as we know in forward bias resistance is very low.

A secondary coil is wound over the primary coil in a transformer to have maximum flux linkages such that the maximum efficiency is obtained.By doing this, the distance between the coils is reduced and thus lower flux leakages.

Wattmeter measure the active power P in the circuit. In AC circuit the active power at any instant is given by

P = VI

And the average power is given by which can be measured by wattmeter only is given as

P = VICosΦ

Where V and I are RMS values of voltage and current

Lissajous Pattern with different Ratios is given as

Fy/fx =hx/hy

Fy: unknown frequency

Fx: known frequency

hy: tangencies at the y-axis

hx: tangencies at the x-axis

Fy/1000 = 5/2

Fy = 2500 Hz

In case if the supply is 3-phase, 4 wire system, then for the unbalanced system or loads the two wattmeters can’t be used as there is current flowing through neutral. The three-wattmeter method of power measurement will work regardless of whether the load is balanced or unbalanced, delta or wyne connected.

The squirrel cage induction motor has a very rugged construction, the three-phase squirrel-cage motor is capable of handling the starting current without any damage to winding. However very large motors may cause line drops, which may affect other equipment operating from the same system For such installations, reduced voltage starters a used. The reduced voltage limits the starting current to a lower value.

Starting a squirrel cage induction motor under reduced voltage may be reduced the starting voltage considerably. From the torque equation of a three-phase induction motor, the starting torque is approximately proportional to the square of the applied voltage i.e T ∝ V²

By reducing the voltage by 50% ( reducing voltage by 1/2) than

T = (1/2)²

T = 1/4 = 25%

Hence by reducing the voltage by 50%, the torque is reduced to 25% and 75% of the full load value.

Voltage = 250 V

Load current (I_L) = 100 A

Shunt field (Rf) = 130Ω

Armature field (Ra) = 0.1Ω

Series field  (Rse) = 0.1Ω

Brush voltage Drop = 1V

Now voltage drop in the field winding = I_LR_se

= 100 x 0.1 = 10 V

For short shunt compound generator, the voltage drop across series field winding gets added with voltage drop across armature winding

Voltage drop across field winding  + Voltage drop across armature winding

V₁ = 250 + 10 = 260 V

Field current If = V1/Rf = 260/130 = 2 A

So armature current

Ia = I_L + I_f = 100 + 2 = 102 A

Generated EMF

E = V₁ + IaRa

= 260 + 102×0.1

E = 270.2 Volt