11. The change in DC excitation of a synchronous motor wil also change the ?
Changes in the field current affect the strength of the magnetic field developed by the revolving rotor.
Variations in the rotor field strength don't affect the motor which continues to operate at a constant speed.
However, changes in the dc field excitation do change the power factor of a synchronous motor.
Effect of change in excitation of synchronous motor
One of the most important features of a synchronous motor is that by changing the field excitation, it can be made to operate from lagging to leading power factor.
Under Excitation -The p.f. cosφ decreases and becomes more and more lagging in nature.
Over Excitation -In this case, the power factor is leading in nature.
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12. In the salient pole of the phase synchronous machine the rotor will have ?
Rotor is that part of synchronous machine which can rotate. It carries the field winding.
From construction point of view, there are two types of rotor:
1-Salient Pole Rotor
2-Cylindrical Pole Rotor
Cylindrical pole rotor is also known as round rotor or non-salient pole rotor.
1-Salient Pole Rotor
#-Salient pole rotors have large diameter and shorter axial length.
#-They are generally used in lower speed electrical machines, say 100 RPM to 1500 RPM.
#-Salient pole rotors generally need damper windings to prevent rotor oscillations during operation.
#-Salient pole synchronous generators are mostly used in hydro power plants.
#-The rotors of salient-pole rotor machines have concentrated winding on the poles and a nonuniform air gap.
2-Cylindrical Pole Rotor #-The air gap is uniform due to the smooth cylindrical periphery.
#-Small diameter and the large axial length is the feature.
#-Mechanically strong.
#-Preferred for high-speed alternators
#-Prime movers used are steam turbines, electric motors.
#-For the same size, the rating is higher than Salient pole type.
#-This rotor is also called as non-salient or non-projected pole type.
#-This is preferred for high-speed alternators ranging from 1500 r.p. m to 3000 RPM.
#-Separate damper winding is not provided.
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13. The 2’s complement of 1010101 is ?
1’s complement of a binary number is just an inversion of individual bits.
1’s complement of (1010101)2 is (0101010)2
2’s complement of the number add 1 to the least significant bit (LSB). So
0101010
+ 1
0101011
The least significant bit (LSB) is the right-most bit in a string. It is called that because it has the least effect on the value of the binary number, in the same way as the unit digit in a decimal number has the least effect on the number's value .
14. Under no-load conditions in transmission lines, the receiving end voltage is greater than the sending end voltage. This effect is called ?
Ferranti effect
In electrical engineering, the Ferranti effect is the increase in voltage occurring at the receiving end of a very long (> 200 km) AC electric power transmission line, relative to the voltage at the sending end, when the load is very small, or no load is connected.
Definition: The effect in which the voltage at the receiving end of the transmission line is more than the sending voltage is known as the Ferranti effect. Such type of effect mainly occurs because of light load or open circuit at the receiving end.
15. ___Earthing is the best form of earthing and is very cheap in cost ?
Pipe Earthing
Pipe Earthing is the best form of earthing and is very cheap in cost.
In this method of earthing, a galvanized and perforated pipe of approved length and diameter is placed upright in a permanently wet soil.
16. The names of the four materials have been given, select the one which has the least resistivity at 20°C ?
The resistivity of iron at 20°C is 9.7 × $10^-8$
The resistivity of Glass at 20°C is 1.1 × $10^11$
The resistivity of silver at 20°C is 1.59 × $10^-8$
The resistivity of nichrome at 20°C is 1.1 × $10^-6$
17. A diode appears as in forward region of its characteristics ?
Diode
A P-N junction acts as a voltage-controlled switch.
During forward biasied the p-n junction diode acts like an ON switch.
When reverse biased, it acts like an OFF switch.
18. The force experienced by a current-carrying conductor lying parallel to a magnetic field is ?
Force experienced by a current-carrying conductor lying parallel to a magnetic field is
BIL sinφ
When the conductor is perpendicular to the magnetic field, the force will be maximum.
When it is parallel to the magnetic field, the force will be zero.
19. Observe the given figure. Find Thevenin’s resistance as seen from open-circuited terminals ?
Explanation will provide as soon as possible.
20. In a suspension type insulator the voltage ?
The disc nearest to the conductor has maximum voltage across it
#-Each unit operates the voltage of about 11kV and hence depending upon the voltage the appropriate number of discs are connected in series with the string.
#-If one of the units is damaged, then it is replaced by the new one and hence no need of replacing the whole string.
#-The string is free to swing in any direction and, therefore, great flexibility is provided to the transmission line.
#-The conductors are placed below the suspension insulators and hence it partly protects the conductor from lightning.
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