Ques 42. An isolator is used in series with an air blast circuit Breaker employed at UHV lines because:
CB life is enhanced with the use of isolator
Current to be interrupted will be large
Gap between CB contacts is small so an isolator is used to switch off voltage✔
Gap between CB poles is small
Air blast circuit breaker is small in size, because of the growth of dielectric strength is so rapid (which final contact gap needed for arc extinction is very small) so isolator is used to switch off the voltage.
Ques 43. Diversity factor has a direct effect on the
The operation cost of the unit
Fixed cost of the unit generated✔
The variable cost of the unit generated
Both variable and fixed cost of unit generated
Diversity factor = Sum of individual maximum demand/ Station maximum demand.
Diversity helps to improve the load factor and economic operation of the power plant.
Load factor = Average load/ Maximum Demand
Both load factor and diversity factors are inversely proportional to maximum demand. Higher load factor and diversity factor means steady consumption or supply which means less maximum demand thus the initial and running cost will be low. Therefore the cost per unit of power generation will be less.
The fixed or standing charge to cover the first cost is divided into all the units used during that period. The more units used (and the higher the load factor), the less will be the fixed cost per unit.
Ques 44. Regulation of an alternator supplying resistive or inductive load is
Infinity
Always Negative
Always Positive✔
Zero
The voltage regulation of an alternator is defined as the change in its terminal voltage when the full load is removed, keeping field excitation and speed constant, to the rated terminal voltage.
Where Vph = Rated terminal voltage
Eph =No load-induced e.m.f
An increase in the load current in a pure resistive load causes a decrease in the output voltage. For an inductive load an increase in the load current causes a greater voltage drop as compared to the resistive load. Therefore for inductive and resistive load conditions there is always drop in the terminal voltage hence regulation values are always positive.
In the case of leading load which means capacitive load, the effect of armature flux on main field flux is magnetizing i.e, the armature flux is adding up with the main field flux. Since it is adding up, the total induced emf(Vph) will also be more than the induced emf at no load(Eph).Hence the regulation is negative
Ques 45. The highest transmission A.C voltage in India is
1750 kV
132 kV
220 kV
400 kV
Wrong option
The highest transmission A.C voltage in India is 750 KV till now.
According to the latest report Power Grid Corporation of India (PGCIL) has operationalized its ultra-high voltage 1200 kV National Test Station (NTS) at Bina in Madhya Pradesh and it is the highest operational transmission voltage level (including ac and dc operational voltage level) in the world.
Ques 46. Point out the WRONG statement
The magnetizing force at the center of a circular coil varies
Inversely as its radius
Directly as the number of its turns
Directly as the current
Directly as its radius✔
At the centre of a current-carrying coil, the magnetic field intensity is directly proportional to the current and inversely proportional to the radius of the coil.
Ques 47. The rotor slots, in an induction motor, are usually not quite parallel to the shaft because it;
Improve the power factor
Improves the efficiency
Help the rotor teeth to remain under the stator
Help in reducing the tendency of the rotor teeth to remain under the stator teeth✔
The slots are not made parallel to each other but are bit skewed as the skewing prevents magnetic locking of stator and rotor teeth and makes the working of the motor more smooth and quieter.
Ques 48. If a 10 μF capacitor is connected to a voltage source with v(t) 50sin2000tV, then the current through the capacitor is _________ A
106 cos 2000t
5 x 10-4 cos 2000 t
cos 2000 t✔
500 cos 2000 t
The current flowing through the capacitor is given by
I(t) =Cdv(t)/dt
Where I(t) is instantaneous current
C = capacitance in farad
dv/dt =instanaeous rate of voltage changed
Ques 49. In a series resonance circuit, the impedance at half power frequencies is:
2R
R/√2
√2 R✔
R/2
In series resonance circuit Z = R = V/Im
Half power frequency is the frequency when the magnitude of voltage or current is decreased by the factor of 1/√2 from its maximum value. (Also known as cut-off frequency)
Therefore at half power frequency
I = Im/√2
Now impedance at half power frequency
= V/Im/√2 = √2V/Im =√2R
Ques 50. A 10 Ω resistive load is to be impedance matched by a transformer to a source with 6250Ω of internal resistance. The ratio of primary to secondary turns of transformer should be
25✔
10
15
20
R1 = 6250Ω and R2 = RL = 10Ω
Then referring the RL =10Ω on the primary side for matching the source impedance
