SSC JE 2011 Electrical question paper with Solution

Alternators are connected primarily in the star to achieve the following motives:

1. The phase voltages in star connection are 57.7 % of the line voltages, i.e. the armature winding in star connection is less exposed to voltage as compared to the delta connection which in turn proves more economic if we consider insulation, breakdown strength, the requirement of conductor material
2. Easy protection: Neutral grounding is necessary to allow zero sequence currents to flow to the ground in case of a fault.
3. Elimination of harmonics: Star connection facilitates a neutral connection which is instrumental in eliminating triple harmonics.
4. No circulating currents: In star connection, we don’t have circulating parasitic currents like in Delta which lead to heating losses.

P_i = iron loss(core loss) = 400 Watt,

P_c = 800 Watt(copper loss),

Maximum efficiency occurs at fraction x of full load such that

x = √(P_i ⁄ P_c) = √(400/800)

=0.707 x 100 = 70.7 % 

We know that for D.C series Motor torque is directly proportional to the product of armature current and field flux, Ta ∝ ɸ.Ia. In DC series motors, the field winding is connected in series with the armature, i.e. Ia= If. Therefore, before the magnetic saturation of the field, flux ɸ is directly proportional to Ia. Hence, before magnetic saturation Ta ∝ Ia².

For DC shunt motor torque develop

T ∝ Ia

∴ 60 ∝ 10

For the second case

T” = 20

T” = (20 x 60)/10

T” = 120 Nm

A fixed connection of resistance in parallel with the contact space or arc is called resistance switching. The resistance switching is mainly used for reducing the restriking voltage and the transient voltage surge.

The air blast circuit breaker is very sensitive to the restriking voltage. Thus current chopping may occur which may be avoided by employing resistance switching.

With the increase in transmission voltage, for the same amount of power to be transmitted current in the line decreases which reduces I²R losses (or copper losses). This will lead to an increase in transmission efficiency. The efficiency of a High voltage transmission line is around 80 – 95%

Paper Industry use squirrel cage TEFC (totally enclosed fan cooled ) Motor

Total heat required = m[s(t₂ − t₁) + L]

Where

m = Mass = 100 × 10³

s = Specific Heat = 0.054

t₂ = Melting Temperature = 235°C

t₁ = Intital temperature = 35°C

L = Latent fusion = 13.3

= 100 × 10³[0.054(235 − 35) + 13.3

= 2.43 × 10⁶ Cal

Power output of the furnace

= 2.43 × 10⁶ Cal

= 2.43 × 10⁶ × 4.18/(60 × 60) J/sec

= 2.82 × 10³ W

= 2.82 kW

Suspension type insulator consists of a number of porcelain discs connected in series by metal links in the form of a string. The porcelain portion of each disc is between two metal links. Therefore, each disc forms a capacitor C as shown in Figure. This is known as mutual capacitance or self-capacitance.

If there were mutual capacitance alone, then the charging current would have been the same through all the discs and consequently, the voltage across each unit would have been the same i.e., V/3.But in actual practice, the capacitance also exists between the metal fitting of each disc and the tower or earth. This is known as shunt capacitance.

Due to shunt capacitance, the charging current is not the same through all the discs of the string. Therefore, the voltage across each disc will be different and the disc nearest to the line conductor will have the maximum voltage

A reaction turbine is a type of turbine that develops torque by reacting to the pressure or weight of a fluid; the operation of reaction turbines is described by Newton’s third law of motion (Every action has an equal and opposite reaction).

E.g of Reaction turbines are Kaplan turbine, Propeller turbine, Francis turbine, Bulb turbine, Straflo, Tube turbine

Pelton wheel is an Impulse type turbine. An impulse turbine is a turbine that is driven by high-velocity jets of water or steam from a nozzle directed onto vanes or buckets attached to a wheel.