Ques 21. In a 3-phase synchronous generator, the stator winding is connected to the star because a delta connection would
Have circulating currents due to triple harmonics
Require more insulation and conductor material✔
Require a larger conductor and more core material
Result in a short circuit
Alternators are connected primarily in the star to achieve the following motives:
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
Easy protection: Neutral grounding is necessary to allow zero sequence currents to flow to the ground in case of a fault.
Elimination of harmonics: Star connection facilitates a neutral connection which is instrumental in eliminating triple harmonics.
No circulating currents: In star connection, we don’t have circulating parasitic currents like in Delta which lead to heating losses.
Ques 22. A 40 kVA transformer has a core loss of 400 W and a full load copper loss of 800 W. The fraction of rated load at maximum efficiency is
50%
62.3%
70.7%✔
100%
Pi = iron loss(core loss) = 400 Watt,
Pc = 800 Watt(copper loss),
Maximum efficiency occurs at fraction x of full load such that
x = √(Pi ⁄ Pc) = √(400/800)
=0.707 x 100 = 70.7 %
Ques 23. The torque developed in a DC series motor in unsaturated magnetic circuit conditions is
Almost practically constant at all load currents
Directly Proportional to the load current
Directly proportional to the square of the load current✔
Inversely Proportional to the square of load current
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.
Ques 24. A 220 V shunt motor develops a torque of 60 Nm at an armature current of 10 A. The torque developed when the armature current is 20 A is
30 Nm
240 Nm
84 Nm
120 Nm✔
For DC shunt motor torque develop
T ∝ Ia
∴ 60 ∝ 10
For the second case
T” = 20
T” = (20 x 60)/10
T” = 120 Nm
Ques 25. Resistance switching is normally employed in
All types of circuit breakers
Bulk oil circuit breakers
Minimum oil breakers
Airblast circuit breakers✔
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.
Ques 26. High voltage transmission efficiency is in the range of
30-45%
55-70%
5-10%
85-95%✔
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%
Ques 27. In which of the following industries, are all the motors totally enclosed and moisture-proof?
Iron and steel industry
Sugar industry
Paper industry✔
Textile industry
Paper Industry use squirrel cage TEFC (totally enclosed fan cooled ) Motor
Ques 28. 100 kg of tin is to be smelt for an hour in a smelting furnace. If the melting temperature of tin = 235°C specific heat = 0.054, latent heat of liquefication 13.3kcal/kg, the initial temperature of = 35°C, the value of J 4200J/kcal, then the capacity of the furnaces is
2.81 kW
1.26 kW
2812 kW
3032 kW
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
Ques 29. In suspension type insulator the potential drop is
Maximum across the lowest disc✔
Maximum across the topmost disc
Maximum across the disc at the midpoint of the string
Uniformly distributed across the string
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
Ques 30. Of the following water turbines, which is not a reaction turbine?
Pelton wheel✔
Kaplan turbine
Propeller turbine
Francis turbine
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.
