Ques 31. If the speed of a universal motor in AC operation is Nac and in DC operation is Ndc (and I = current, V = supply voltage, r = resistance, X = reactance, cosΦ = power factor) then the speed ratio
Nac/Ndc = (cosΦ – IX/V)/(1 – IR/V)
Nac/Ndc = (1 – IR/V)/ (cosΦ – IX/V)
Nac/Ndc = (1 – IR/V)/ (cosΦ – IR/V)
Nac/Ndc = (cosΦ – IR/V)/(1 – IR/V)✔
For AC supply,
EAC = V – I(R + jX)
For DC supply,
EDC = V – IR
From both the above equation
EAC/EDC = (V – IZ)/(V – IR)
Where power factor cosφ = R/Z
Since In universal motor EMF is directly proportional to the motor speed hence
EAC = NAC & EDC = NdC
Ques 32. Sheaths are used in underground cables to
Provide proper insulation
Provide mechanical strength
Protect the cable from moisture✔
None of the above
The underground cable is required to be protected from moisture, gases, or other liquids of the soil and atmosphere. Therefore, the metallic sheath of aluminum, lead, or alloy is used over the insulation for protection. A metal sheath is essential because no organic material is adequately moisture resistant. The main advantages of the lead sheath include easy-to-make, high-corrosion resistance, high flexibility, etc.
Recently, aluminum is also used as a sheath because of its low weight, low cost, ease to manufacture, and higher mechanical strength compared to the lead alloy. Aluminum eliminates the use of armor usually required in lead sheath cables. The aluminum sheath can withstand the required gas pressure Without reinforcement.
Ques 33. If the frequency of a transmission system is changed from 50 Hz to 100 Hz, the string efficiency
Will increase
Will Decrease
Remain Unchanged✔
May increase or decrease depending on the line parameters
The string efficiency is defined as the ratio of voltage across the string to the product of the number of strings and the voltage across the unit adjacent string.
String efficiency= Operating phase voltage(Vph)/(n x voltage across the disc nearest to the conductor)
Hence string efficiency does not depend upon the frequency so if the frequency is increased string efficiency remains unchanged.
Ques 34. The power station where coal is used as a fuel is called
Terrestrial power station
Thermal power station✔
Solar power station
Nuclear power station
A fossil fuel power station (Thermal power station) is a power station which burns fossil fuel such as coal, natural gas, or petroleum to produce electricity.
Ques 35. Making capacity of a circuit breaker is equal to
2.55 times symmetrical breaking current✔
1.5 times symmetrical breaking current
√2 times symmetrical breaking current
Symmetrical breaking current
The making current of circuit breaker is the maximum peak value of the current that the breaker can interrupt without any damage if the breaker is closed at fault. The making current of the circuit breaker is always more than the breaking current.
Making current of circuit breaker = 2.55 × Breaking current
Ques 36. A power insulator has 4 units. The voltage across the bottom-most unit is 33.33% of the total voltage. Its string efficiency is
25%
33.33%
66.67%
75%✔
The string efficiency is given as
String efficiency = Voltage across the string / (number of discs x voltage across the disc nearest to the conductor).
= Vo/(N x Vc)
Voltage of disc nearest to the conductor = 0.333 x Vo
String efficiency = Vo/(4 x 0.333 x Vo)
= 0.75
%String efficiency = 0.75 x 100 = 75%
Ques 37. The load factor for the load duration curve shown below is
1.0
0.7✔
0.6
0.5
From the curve
Ques 38. An alternator has a rated field current of 3 A. It is developing 180 V at a field current of 1.5 A at the rated speed. If its field current is made 3A rated speed, then the generated voltage would be
360 V
180 V
Less than 360 V✔
270 V
Given,
If1 = 1.5 A
Voc = 180 V
If2 = 3 A
From the OCC and SCC, it is clear that,
Voc ∝ If
180 ∝ 1.5 …. (1)
V ∝ 3 …. (2)
From equation (1) and (2)
V = 360 volt
SinceOCC is not linear, therefore generated voltage will be slightly less than 360 volts.
Ques 39. Which one is the disadvantage of oil when used as an arc extinguishing medium in the oil circuit breaker
During arcing, oil produces carbon particles✔
During arcing, oil produces hydrogen gas
Oil provides insulation to the live exposed contacts from the earthed parts of the breaker
Oil provides good insulation between the fixed and moving contacts
Oil circuit breakers are the oldest type of circuit breakers that used oil as a dielectric or insulating medium for arc extinction. In an oil circuit breaker, the contacts of the breaker are made to separate within an insulating oil which has better insulating properties than air.
Disadvantages of the oil circuit breaker are
Due to the decomposition of oil in the arc, the carbon particles are generated which polluted the oil, and hence the dielectric strength of the oil decreases.
The oil used in the oil circuit breaker is inflammable and hence, causes a fire hazard.
Ques 40. If the discharge voltage of a thyrite arrester is 373 kV RMS and the rated voltage is 211 kV the discharge factor of the arrester is
1.25
0.80
1.77
2.5✔
Discharge factor is given as
Discharge factor = Discharge voltage(crest value)/Rated voltage (rms value)
