Q1. Synchronous motors for rotary kilns run at: ?
Synchronous motors for rotary kilns run at ultra-low speeds Rotary Kilns A rotary Kilns is used in a cement factory. rotary Kilns usually rotate at 0.5 to 2 rpm, but sometimes as fast as 5 rpm. The Kilns of most modern cement plants are running at 4 to 5 rpm.
एक सीमेंट कारखाने में एक रोटरी भट्टों का उपयोग किया जाता है। रोटरी भट्टी आमतौर पर 0.5 से 2 आरपीएम पर घूमती हैं, लेकिन कभी-कभी 5 आरपीएम तक चलाई जाती है । अधिकांश आधुनिक सीमेंट संयंत्रों के भट्टी को 4 से 5 आरपीएम पर चलायी जा रही हैं।
2. What is the force of attraction between two electric charge of opposite polarity having 1 Coulomb each when placed at a distance of 1 meter ?
Coulomb low:
the principle that the force between two point charges acts in the direction of the line between them and is directly proportional to the product of their electric charges divided by the square of the distance between them.
F∝q1×q2
F∝1/r2
F = K q1*q2/r2 F = K (-1)*1/ 1 = K Newton Where k is a constant whose value is 1 / 4πε0
The value of this constant is dependent upon the medium that the charged objects are immersed in.
In SI units, in the case of vacuum, it is equal to approximately 8987551788.7 N. m. C−2 or 8.99×109 N.
कूलॉम का नियम:
दो बिंदु आवेशों के बीच लगने वाला बल दोनों आवेशों के गुणन फल के समानुपाती यानि directly proportional होता है
और उन दोनों आवेशों की बीच की दूरी के वर्ग के व्युत्क्रमानपाती होता है अब वह बल आकर्षी या प्रतिकर्षि हो सकता है
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3. As per double field revolving theory, a sinusoidal alternating flux can be looked upon as the combination of two revolving fluxes ?
Double-field revolving theory:
Double-field revolving theory, which is based on Ferraris Principal sinusoidal varying flux (φ ) is the sum of two rotating fluxes or fields, the magnitude of which is equal to half the value of the alternating flux (φ / 2), and both the fluxes rotating synchronously at the speed, in opposite directions. डबल-फील्ड परिक्रामी सिद्धांत:
डबल-फील्ड परिक्रामी सिद्धांत के अनुसार एक प्रत्यावर्ती फ्लक्स दो रोटेटिंग कॉम्पोनेन्ट में विघटित किया जा सकता है।
जिसमे प्रत्येक कॉम्पोनेन्ट सिन्क्रोनस स्पीड से एक दूसरे से विपरीत दिशा में रोटेट होता है।
Total flux φr = -φm/2 + φm/2
4. In the given circuit, the value of load resistance for which the power delivered is maximum is: ?
The maximum power transfer theorem states that in a linear , bilateral DC network , maximum power is delivered to the load when the load resistance is equal to the internal resistance of a source
Load resistance RL = Rth Source internal resistance
The efficiency is only 50% when maximum power transfer is achieved
अधिकतम शक्ति हस्तांतरण प्रमेय में कहा गया है कि एक रैखिक, द्विपक्षीय डीसी नेटवर्क में, अधिकतम शक्ति लोड को वितरित की जाती है जब लोड प्रतिरोध का मान , स्रोत के आंतरिक प्रतिरोध के बराबर होता है।
अधिकतम शक्ति हस्तांतरण प्राप्त होने पर दक्षता केवल 50% है
Now we Calculate Rth on this circuit.
1 Open the load resistor.
2-independent voltage source make short circuit and independent current source make open circuit.
5. In the case of an ideal transformer, the primary supply voltage and current are: ?
Ideal transformer:
The ideal transformer model is developed by considering the windings of the transformer are purely inductive and the core of the transformer is loss free. Also there is zero leakage reactance of transformer.
That means, 100% flux passes through the core and links with both the primary and secondary winding of transformer. Although every winding must have some inherent resistance in it which causes voltage drop and I2R loss in it. In such ideal transformer model, the wingdings are considered as ideal(fully inductive), that means resistance of the winding is zero.
Now if an alternating source voltage V1 is applied in the primary winding of that ideal transformer, there Current I0 in the primary winding which is purely 90o in phase lagging with supply voltage V1.
6. The force on each of the conductors infinitely long and carrying a current of 1 A separated by a distance of 1m in vacuum is: ?
Force between two long and parallel current carrying conductor
7. In a parallel operation of 1-f transformers, a dead short circuit can happen if: ?
In a parallel operation of 1-f transformers, a dead short circuit can happen if
there is a difference in the transformation ratios of the transformers.
For parallel operation of the transformer, the conversion ratio must be the same.
अर्थार्त दोनों ट्रांसफार्मर का रूपांतरण अनुपात भित्र- भित्र है। ट्रांसफार्मर के समानांतर प्रचालन के लिए रूपांतरण अनुपात समान होना जरुरी है।
8. From the given Lissajous patterns obtained while measuring frequency using a cathode-ray oscilloscope (CRO), select the pattern which
indicates the relationship fv =1/2 fh ?
indicates the relationship fv =1/2 fh ?
9. Which of the following is NOT a method of earth resistance measurement?
4 Important Methods of Ground Resistance Testing:
1-2-point (dead earth) method
2-3-point (Fall-of-potential) method
3-4-point method
4-Clamp-on method
but Potier method is not used in earth resistance measurement
Potier Triangle or Zero Power Factor Method, The Potier triangle determines the voltage regulation of the Synchronous Generator .
10. The de-ionisation of the medium in current zero method of arc extinction is NOT achieved by: ?
In an a.c. system, current drops to zero after every half-cycle. At every current zero, the arc extinguishes for a brief moment. Now the medium between the contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltage known as restriking voltage. If such a breakdown does occur, the arc will persist for another halfcycle. If immediately after current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and the current will be interrupted.
The rapid increase of dielectric strength of the medium near current zero can be achieved by :
Causing the ionized particles in the space between contacts to recombine into neutral molecules. Sweeping the ionised particles away and replacing them by un-ionised particles The Deionization of the medium can be achieved by following methods:
Lengthening of the gap. The dielectric strength of the medium is proportional to the length of the gap between contacts. Therefore, by opening the contacts rapidly, higher dielectric strength of the medium can be achieved.
High pressure. If the pressure in the vicinity of the arc is increased, the density of the particles constituting the discharge also increases. The increased density of particles causes higher rate of de-ionization and consequently the dielectric strength of the medium between contacts is increased.
Cooling. Natural combination of ionised particles takes place more rapidly if they are allowed to cool. Therefore, dielectric strength of the medium between the contacts can be increased by cooling the arc.
Blast effect. If the ionised particles between the contacts are swept away and replaced by unionized particles, the dielectric strength of the medium can be increased considerably. This may be achieved by a gas blast directed along the discharge or by forcing oil into the contact space..
11. Which of the following is NOT an advantage of having a stationary armature in a synchronous machine: ?
The stationary armature-rotating field construction has many advantages
1-It is easier to insulate stationary winding for high voltages for which the alternators are usually designed. It is because they are not subjected to centrifugal forces and also extra space is available due to the stationary arrangement of the armature.
2-The stationary 3-phase armature can be directly connected to load without going through large, unreliable slip rings and brushes.
3-Only two slip rings are required for d.c. supply to the field winding on the rotor. Since the exciting current is small, the slip rings and brush gear required are of light construction.
4-Due to the simple and robust construction of the rotor, the higher speed of rotating DC field is possible. This increases the output obtainable from a machine of given dimensions.
5-stationary armature is easily cooled .
12. Which of the following is NOT a static compensation equipment for transmission lines ?
Static compensation equipment are those that do not have rotating parts.
like Shunt reactors, Shunt capacitors,Series capacitors
A static VAR compensator is a set of electrical devices for providing fast-acting reactive power on high-voltage electricity transmission networks.
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13. Which of the following is a commonly used circuit breaker in India, for rural outdoor applications ranging from 22 KV to 66 KV?
In India, rural areas where distances are large and remote areas are difficult to reach, the installation of such external, maintenance-free CBs should be a proof of a definite advantage. Vacuum CBs are employed for outdoor applications ranging from 22 to 66 kV
VCB-
A vacuum circuit breaker is a kind of circuit breaker where the arc quenching takes place in vacuum medium. The operation of switching on and closing of current carrying contacts and interrelated arc interruption takes place in a vacuum chamber in the breaker which is called vacuum interrupter Advantages of VCB 1-Vacuum offers the utmost insulating strength. So it has extreme superior arc quenching properties than any other medium.
2-The vacuum circuit breaker has a long life.
3-Unlike Oil Circuit Breaker (OCB) or air blast Circuit Breaker (ABCB), the explosion of VCB is avoided. This enhances the safety of the operating personnel.
4-No fire hazard
5-The vacuum CB is fast in operation so ideal for fault clearing. VCB is suitable for repeated operation.
6-Vacuum circuit breakers are almost maintenance free.
7-No exhaust of gas to the atmosphere and Noiseless operation.
Disadvantages of VCB 1-The main disadvantage of VCB is that it is uneconomical at voltages exceeding 38 kVolts.
2-The cost of the breaker becomes excessive at higher voltages. This is due to the fact that at high voltages (above 38 kV) more than two numbers of the circuit breaker are required to be connected in series.
3-Moreover, the VCBs production is uneconomical if produced in small quantities.
Applications of VCB
1-The vacuum circuit breaker is today recognized as the most reliable current interruption technology for medium voltage switchgear. It requires minimum maintenance compared to other circuit breaker technologies.
2-The technology is mainly suitable for mainly medium voltage applications. For higher voltage vacuum technology has been developed, but it is not commercially feasible. Vacuum circuit breakers are used in metal-clad Switchgear and also in porcelain housed circuit breakers.
ACB-
There are mainly two types of ACB are available.
Plain air circuit breaker.The air circuit breaker, operated within the voltage level 1 KV, does not require any arc control device. Air blast Circuit Breaker.These types of air circuit breaker were used for the system voltage of 245 KV, 420 KV and even more, especially where faster breaker operation was required.
SF6
A typical SF6 circuit breaker consists of interrupter units each capable of dealing with currents up to 60 kA and voltages in the range of 50—80 kV. A number of units are connected in series according to the system voltage. SF6 circuit breakers have been developed for voltages 115 kV to 230 kV, power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles.
plain oil circuit breakers The plain oil circuit breakers were widely employed on low voltage AC and DC circuits upto 250 MVA at 11 kV. .
14. In the case of an alternator connected to an infinite bus, the active power can be varied by: ?
Reactive power :: The nature of reactive power drawn by the Synchronous generator is of leading when the motor is under-excited. On the other hand when the alternator is over excited the reactive power drawn will be of lagging in nature.
Active power :: In the case of an alternator connected to an infinite bus, the active power can be varied by changing the prime mover because active power depend upon the speed of prime mover and reactive power depend upon field excitation. .
15. During discharging of a capacitor of C = 10 µf through a resistance of 1 K? applied with 50 V, the voltage at the time of the it's time constant is:?
During discharging of a capacitor at time constant = 36.8% of applied voltage = 36.8% * 50 V = 18.4 V = 18.5V ans
Time constant:-
The time constant of a resistor-capacitor series combination is defined as the time it takes for the capacitor to deplete 36.8% (for a discharging circuit) of its charge or the time it takes to reach 63.2% (for a charging circuit) of its maximum charge capacity given that it has no initial charge. The time constant also defines the response of the circuit to a step (or constant) voltage input. Consequently, the cutoff frequency of the circuit is defined by the time constant. To calculate the time constant of a capacitor, the formula is τ=RC. This value yields the time (in seconds) that it takes a capacitor to charge to 63% of the voltage that is charging it up. After 5 time constants, the capacitor will charged to over 99% of the voltage that is supplying.
Therefore, the formula to calculate how long it takes a capacitor to charge to is:
Time for a Capacitor to full Charge= 5RC
After 5 time constants, for all extensive purposes, the capacitor will be charged up to very close to the supply voltage. A capacitor never charges fully to the maximum voltage of its supply voltage, but it gets very close.
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16. During the starting of a slip ring induction motor using rotor resistance starter, the insertion of resistance in the rotor circuit causes: ?
Slip ring-rotor motor:- A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased. The resistance in the rotor circuit greatly influences the characteristics of an induction motor. This is because the speed at which the maximum torque occurs depends on the rotor resistance Effect of rotor resistance :- A high rotor resistance will provide a high starting torque, leading to a rapid acceleration of the mechanical load system. This is also desirable for a low starting current and a high power factor during starting. However a high rotor resistance results in a high slip during the normal running operation. This causes increased rotor loss and reduced efficiency. Therefore, during a normal operation, a low rotor resistance is desirable. This results in a lower slip and thereby a higher efficiency.
17. In the method of synchronization of alternator to the bus-bar, a synchrono scope indicates correct speed when: ?
synchrono scope
In AC electrical power systems, a synchro scope is a device that indicates the degree to which two systems (generators or power networks) are synchronized with each other.
For two electrical systems to be synchronized, both systems must operate at the same frequency, and the phase angle between the systems must be zero (and two poly phase systems must have the same phase sequence). Synchro scopes measure and display the frequency difference and phase angle between two power systems. Only when these two quantities are zero is it safe to connect the two systems together. Connecting two unsynchronized AC power systems together is likely to cause high currents to flow, which will severely damage any equipment not protected by fuses or circuit breakers
18. In nodal analysis, for a network of N nodes, the number of simultaneous equations to be solved to get the unknowns is: ?
(N− 1) independent node equations using KCL.
(E -N+1)independent loop equations using KVL.
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19. The direction of rotation of a R-split phase single-phase induction motor may be reversed: ?
Split Phase Induction Motor
In addition to the main winding or running winding, the stator of single phase induction motor carries another winding called auxiliary winding or starting winding. A centrifugal switch is connected in series with auxiliary winding. The purpose of this switch is to disconnect the auxiliary winding from the main circuit when the motor attains a speed up to 75 to 80% of the synchronous speed. We know that the running winding is inductive in nature. Our aim is to create the phase difference between the two winding and this is possible if the starting winding carries high resistance
The direction of rotation of a R-split phase single-phase induction motor may be reversed by reversing either the auxiliary terminals or the main terminals .
20. The series field of a long-shunt compound generator is excited by a: ?
Types of compound generators:
1-Short Shunt Compound Generator
2-Long Shunt Compound Generator
Short-shunt compound generator in which only shunt field winding is connected in parallel with the armature, as shown in Fig. In short shunt connection series field carries no current at no-load and thus its open-circuit characteristics are same as that of a shunt generator.
The current relations of the short-shunt connection of a d.c. compound generator is
(Armature current)la = If (Field current) + IL (Load current)
Long-shunt compound generator in which the shunt field is connected in parallel will both the series winding and the armature, as shown in Fig. In long shunt connection, the series field winding carries the current same as that of shunt field winding at no-load.
The current relations of the long-shunt connection of a d.c. compound generator is
Armature current, Ia = series field current,
la = If + IL
Two fields of the compound generators may be cumulative or differential wound. If it is cumulative the two fields are additive and the two fields are subtractive if it is differential wound.
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21. For the given circuit, the maximum power in the load can be: ?
Maximum power p = $Vth^2$/4Rth
condition for maximum power dissipation across the load is RL=RTh
so find the Rth
Rth = 2 + 2 + (6 || 6) = 7Ω
Calculate Vth from voltage division Rule
Vth = 10 × 6 ⁄ (6 + 6) = 5 V
maximum power p = $Vth^2$/4Rth
P = $5^2$/4*7 = 25/28 = 0.893 W = 893 mW
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22. Intrinsic semiconductors at room temperature have: ?
At room temperature (300 Kelvin)(300Kelvin),
the electrons in the valence band are moved to the conduction band. When an electron leaves the valence band it creates a vacancy known as Hole.
A hole attracts electrons as it is positively charged.
In intrinsic semiconductor number of free electrons is equal to number of holes
The conductivity of the intrinsic semiconductor become zero at room temperature while the extrinsic semiconductor is very little conductive at room temperature
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23. What is the composition of the alloy 'Kanthal' (used as a heating element in electrical heating systems)? ?
Kanthal is the trademark for a family of iron-chromium-aluminium (FeCrAl) alloys used in a wide range of resistance and high-temperature applications. Kanthal FeCrAl alloys consist of mainly iron, chromium (20–30%) and aluminium (4–7.5 %).
The first Kanthal FeCrAl alloy was developed by Hans von Kantzow in Hallstahammar, Sweden. The alloys are known for their ability to withstand high temperatures and having intermediate electric resistance. As such, it is frequently used in heating elements.
24. In 3-phase power measurement for a balanced load using the two-wattmeter method, the reactive power is given by: ?
Always remember for any unbalance load
minimum number of watt meter required = no of conductor which current flowing.
For any balance load
minimum number of watt meter required= 1 no
√3(W1 - W2) = √3VL ILsinφ
the reactive volt-amperes of a 3 phase circuit can be obtained by multiplying the difference of two watt meter reading by √3
Some Other point about watt meter readings:
25. In a balanced 3-phase system, the zero phase sequence currents are: ?
Zero sequence component :-
it represents three equal and parallel phasors with zero degree phase displacement.
It specifies that current is flowing from source to Neutal .
NOTE:-IN A BALANCED THREE PHASE SYSTEM OPERATING IN NORMAL CONDITION, ONLY POSITIVE SEQUENCE COMPONENT EXISTS. .
26. A solenoid having N turns and I amperes will have an inductance of..............henry ?
L = NØ/I.
This expression can also be defined as the magnetic flux linkage, ( NΦ ) divided by the current, as effectively the same value of current flows through each turn of the coil. Note that this equation only applies to linear magnetic materials.
27. The term 'Permeance' in a magnetic circuit is analogous to............... in an electric circuit ?
MAGNETIC CIRCUIT ------- ELECTRIC CIRCUIT
Flux = mmf/reluctance ------------- Current = emf/ resistance
Flux φ is measured in weber (wb)--------Current I is measured in amperes
Permeability ------------------ Conductivity
Reluctivity ------------------- Resistivity
Permeance = 1/reluctance--------------- Conduction = 1/ resistance
Magnetic intensity H = NI/l------------ Electric density E = V/d
Flux density B = φ/a (wb/m2)----------- Current density J = I/a (A/m2)
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28. In a salient pole synchronous machine, the MMF acting along the d-axis is: ?
Two Reaction Theory
Two Reaction Theory was proposed by Andre Blondel.
According to this theory, the armature m.m.f. can be divided into two components as,
1. The component acting along the pole axis called direct axis.
2. The component acting at right angles to the pole axis called quadrature axis.
In a salient pole synchronous machine, the MMF acting along the d-axis is Only field MMF and MMF acting along the q-axis Only armature MMF
29. In the two-watt meter method of 3-phase power measurement, if the phase sequence of the supply is reversed: ?
the two watt-meters used for measuring the three-phase power will interchange their readings after they have been subjected to phase sequence reversal. However, the magnitudes of the currents and the component voltages in these balanced systems will not be affected by the phase sequence reversal.
30. The principal of single resonating atom may be used in the measurement of: ?
Frequency measurement is very important in many applications of alternating current, especially in AC power systems designed to run efficiently at one frequency and one frequency only. Method of Frequency Measurement 1-Using the Principle of Mechanical Resonance 2-Using a form of Tank Circuit 3-Using Quartz Crystal .
31. The magneto motive force experienced by a unit N-pole at any point in a circle of 'r' meters away from the center of a bunch of 'N' conductors carrying a current of I amperes each is: ?
The magneto motive force experienced by a unit N-pole at any point in a circle of 'r' meters away from the center of a bunch of 'N' conductors carrying a current of I amperes each is NI/2πr AT/m .
32. Which of the following materials can be used as an arc quenching medium in High Rupturing Capacity (HRC) fuse ?
The body is made up of cylindrical ceramic material and is closed by metallic end caps to which the fuse elements are fixed. The fuse elements are surrounded by silica,Plaster of Paris which acts as the arc quenching medium.
Advantages of HRC fuses
1-When compared to other circuit interrupters of same capacity HRC fuses are the cheaper one.
2-Simple and easy to install.
3-No maintenance required.
4-High breaking capacity.
5-They are consistent in performance.
6-Their inverse time characteristic makes them much suited for overload protection.
T7-hey are capable of clearing high as well as low currents.
Quick operation.
8-They are capable clearing high as well as low currents.
9-During severe faults, an HRC fuse breaks the circuit before the peak current of the fault is reached.
Disadvantages
1-These fuses once blown out cannot be reused.
2-Causes over heating of adjacent contacts.
3-The possibility of interlocking is greater.
33. Two identical coils X and Y of 500 turns each lie in parallel planes such that 80% of flux produced by one coil links with the other. If a current of 5 A flowing in 'X' produces a flux of 10 mWb in it, the mutual inductance between X and Y is: ?
Formula used M = N2Φ1/I1 H
Flux produced in X = 10 mWb = 10 × 10−3 Wb
Flux linked with Y = 10 × 10−3 × 0.8 = 8 × 10−3 Wb
M = (500 x 8 x 10-3 )/5 = 0.8 H
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34. The function of brushes in a DC generator is to: ?
A brush is a device which conducts current between stationary wires and moving parts, most commonly in a rotating shaft. Typical applications include electricmotors, alternators and electric generators
The function of brushes in a DC generator is to collect current from the commutator
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35. For zero power factor leading loads, the effect of an armature reaction in an alternator is: ?
Armature reaction in an alternator The armature reaction flux is constant in magnitude and rotates at synchronous speed.
The armature reaction is cross magnetising when the generator supplies a load at unity power factor.
When the generator supplies a load at lagging power factor the armature reaction is partly demagnetising and partly cross-magnetising.
When the generator supplies a load at leading power factor the armature reaction is partly magnetising and partly cross-magnetising.
Armature flux acts independently of main field flux.
For zero power factor leading loads, the effect of an armature reaction in an alternator is magnetising
For zero power factor lagging loads, the effect of an armature reaction in an alternator is demagnetising
For Unity power factor loads, the effect of an armature reaction in an alternator is cross-magnetising
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36. The type of electrode used in seam welding is: ?
Like spot welding, seam welding relies on two electrodes, usually made from copper, to apply pressure and current. The electrodes are often disc shaped and rotate as the material passes between them. This allows the electrodes to stay in constant contact with the material to make long continuous welds
37. According to Tellegen's Theorem, the sum of instantaneous powers for the n branches in a network is always: ?
According to Tellegen theorem, the summation of instantaneous powers for the n number of branches in an electrical network is zero. .
38. Phase voltage and current of a 3 phase, three-wire star-connected system, with an inductive load of power factor 0.707 (lag), is 150 V and 30 √30 A. If the power in the system is being measured using two watt meters, the difference in meter readings is: ?
Vph = 150 volt
Iph = 30 √30
Cosθ =0.707
θ = 45
Now reading of watt meter 1st W1 = VL IL Cos(30-θ) W1 = √3 150 30√30 Cos(30-45) W1 = 3 x 4500 Cos(-15) W1 = 3x4500x.965 W1 = 13039.65 W = 13.039KW Now reading of watt meter 2st W2 = VL IL Cos(30-θ) W2 = √3 150 30√30 Cos(30+45) W2 = 3 x 4500 Cos(75) W2 = 3x4500x.258 W2 = 3493.8 W = 3.49KW the difference in meter readings is W1-W2 = 13.039-3.49 =9.54 KW .
39. In steam power stations, the condenser creates a............at the exhaust of the turbine. ?
The steam condenser in a steam power plant is used to convert the exhaust steam from the turbine into water. so that the condensed water can be used again as the feed water. In steam power stations, the condenser creates a very low pressure at the exhaust of the turbine ..
40. In busbar protection, what is the method of providing an earthed metal barrier surrounding a bus bar throughout its length called ?
The two most commonly used schemes for busbar protection are : 1. Differential Protection: The basic method for busbar protection is the differential scheme in which currents entering and leaving the bus are totalised. During normal load condition, the sum of these currents is equal to zero. When a fault occurs, the fault current upsets the balance and produces a differential current to operate a relay. 2. Fault Bus Protection: It is possible to design a station so that the faults that develop are mostly earth-faults. This can be achieved by providing earthed metal barrier (known as fault bus) surrounding each conductor throughout its entire length in the bus structure. With this arrangement, every fault that might occur must involve a connection between a conductor and an earthed metal By directing the flow of earth-fault current, it is possible to detect the faults and determine their location. This type of protection is known as fault bus protection Protection of Lines: The protection of lines presents a problem quite different from the protection of station apparatus such as generators, transformers and Busbar Protection. While differential protection is ideal method for lines, it is much more expensive to use. The two ends of a line may be several kilometers apart and to compare the two currents, a costly pilot-wire circuit is required. This expense may be justified but in general less costly methods are used. The common methods of line protection are : Time-graded over current protection Differential protection Distance protection .
41. In the process of Nickel plating of iron articles, iron will first be applied with a film of............to ensure good quality ?
Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish. .
42. In a transformer, the load component of a primary current may be calculated as: ?
N2/N1 = V2/V1 = I1/I2 = K constant..
43. Solid state power supplies are protected from high voltage surges due to lightning using: ?
The Varistor is a passive two-terminal solid state semiconductor device that is used to ... When a high voltage surge is applied to a circuit, the outcome is usually ... Transient surges originate from a variety of electrical circuits and sources ... to overcome some of the limitations associated with their silicon carbide cousins..
44. Calculate the length of a wire required for an electric radiator to dissipate 1 kW when connected to a 230 V supply, if the coils of the radiator are made of wire 0.5 mm in diameter having resistivity of 60 µ ?
R = $V^2$/P=230*230/1000 = 52.9 Ohm
R = pL/A = ( 60 * $10^-6$ *L )/π ($.25^2$ *$10^-4$) = 305.28*L *$10^-4$ ohm
52.9 =305.28*L *$10^-4$ ohm
L = 52.9/(305.28*$10^-4$) =1732.8 cm
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45. In case of an electromechanical generator, the frequency is: ?
The formula for calculating synchronous speed is: Sync. Speed (in RPM) = 120 x Frequency (Hz) / poles.
Ns= (120 f) / P
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46. In the method of speed control of induction motor by inducing emf in the rotor circuit, if the injected voltage is in phase opposition to the induced motor emf, then: ?
By injecting EMF in rotor circuit If we inject emf which is in opposite phase with the rotor induced emf, rotor resistance will be increased. If we inject emf which is in phase with the rotor induced emf, rotor resistance will decrease. Thus, by changing the phase of injected emf, speed can be controlled.
47. Demand factor of a power plant is: ?
Demand Factor = Maximum demand / Total connected load
Demand factor is always < =1..
48. Which of the following can be used as a dopant to make N-type semiconductor ?
An N-type semiconductor is a type of material used in electronics. ... The purpose of doing this is to make more charge carriers, or electron wires available ... The pentavalent impurity that is added is called a 'dopant' and the process of addition
Doping means the introduction of impurities into a semiconductor crystal to the defined modification of conductivity. Two of the most important materials silicon can be doped with, are boron (3 valence electrons = 3-valent) and phosphorus (5 valence electrons = 5-valent).
The examples of pentavalent impurities are Phosphorus (P), Arsenic (As), Antimony (Sb)
Trivalent impurity atoms have 3 valence electrons. The various examples of trivalent impurities include Boron (B), Gallium (G), Indium(In), Aluminium(Al).
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49. Which of the options is INCORRECT for the following statement? Three resistances are said to be parallel when: ?
Resistors are in parallel if their terminals are connected to the same two nodes. The equivalent overall resistance is smaller than the smallest parallel resistor
And
the total current is the sum of the three separate currents
potential difference across all resistances is the same
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50. In torque-slip characteristics of an induction motor, at normal speeds close to synchronism the torque is: ?
In starting time torque is inversely proportional to the slip.
and when motor running at normal speeds than torque is directly proportional to the slip
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51. The cable used for high voltage applications is: ?
The cable used for high voltage applications is Elastomer insulated cable
A high-voltage cable (HV cable) is a cable used for electric power transmission at high voltage. A cable includes a conductor and insulation, and is suitable for being run underground or underwater. This is in contrast to an overhead line, which does not have insulation. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, and alternating current (AC) and direct current (DC) power transmission.
ELASTOMER CABLES
The Elastomer cables are popularly known as Rubber cables and probably the first type insulation in the world. Originally it was with natural rubber for both insulation & Sheath, however, in late 70s, it has been replaced by synthetic rubber for insulation & sheath
Elastomer cables are preferred for flexible application and in congested locations where the bending radii are very small. Elastomer cables are available from low voltage upto 33 kV grade.
Gas-filled cables ordinarily have a common metal sheath containing solid or packed segmented cores covered with several layers of insulating material. They are made in single-core and triple-core types with a lead or aluminum sheath and in triple-core types with a steel conduit. The advantages of gas-filled cables are the ease of supplying a cable line with additional gas and the convenience of producing a cable of great length with preimpregnated insulation, which is especially important in laying underwater cables. However, their insulation has a relatively low dielectric strength, which is dependent to a considerable extent on variations in temperature and gas pressure.
high-voltage (35-275 kilovolts)
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52. When two alternators are in exact synchronism, their terminal voltages are: ?
ACHIEVING SYNCHRONIZATION
To synchronize AC generators, several important factors must be checked.
1. The phase rotation of both generator systems must be the same. Check this with lights as described later or use a phase rotation meter to determine ABC or ACB rotation.
2. The AC voltages of both generators should be equal. In practice the voltage of the on-coming generator is usually 1-2 volts higher than that of the other operating generator.
3. The frequencies of the on-coming generators must match when synchronized. In practice the frequency of the on-coming generator is 1-2 hertz higher than that of the on-line generator. This can be observed with lights or by using a synchroscope.
The speed and output voltage of the on-coming generator are slightly higher to pre vent it from becoming a load to the system when it's connected..
53. A fluorescent tube of 20 W has a luminous flux of: ?
The brightness of a light bulb is measured in light output, or lumens. Depending on the type of lamp and efficiency, fluorescent bulbs are rated initially between about 45 and 100 lumens per watt. So a 20 Watt tube should produce between 900 - 1000 lumens. .
54.What is the principle behind the working of phase sequence indicators for 3-f unbalanced 3-wire loads? ?
Uttaar ..
55. The one-wattmeter method of 3 Phase power measurement can only be used for: ?
The one-wattmeter method of 3 Phase power measurement can only be used for Balanced load.
for Balanced load power measure one-wattmeter = Vph. Iph. cosφ
ln this method one wattmeter will measure only the power of one phase
Total Power in three phase = 3 × Vph. Iph. cosφ
56. While measuring the insulation resistance of complete wiring installation to earth using Megger, the resistance measured must not be less than: ?
Types of Megger This can be separated into mainly two categories:
Electronic Type (Battery Operated)
Manual Type (Hand Operated)
Advantages of Electronic Type Megger
Level of accuracy is very high.
IR value is digital type, easy to read.
One person can operate very easily.
Works perfectly even at very congested space.
Very handy and safe to use.
Advantages of Hand Operated Megger
Still keeps important in such high-tech world as it’s an oldest method for IR value determination.
No external source required to operate.
Cheaper available in market.
But there are another types of megger which is motor operated type which does not use battery to produce voltage it requires external source to rotate an electrical motor which in turn rotates the generator of the megger.
Insulation Resistance or IR Test is carried out by maintenance engineers to ensure the healthiness of overall insulation system of an electrical power transformer. It reflects the presence or absence of harmful contamination, dirt, moisture and gross degradation. IR will be usually high (several hundred Mega ohms) for a dry insulation system. Maintenance engineers use this parameter as an index of dryness of the insulation system.
This test is performed at or above rated voltage to determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration. The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer.
This test should be conducted before and after repair or when maintenance is performed. The test data should be recorded for future comparative purposes. The test values should be normalized to 20°C for comparison purposes.
The general rule of thumb that is used for acceptable values for safe energisation is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ..
57. In common-collector configuration of bipolar junction transistor (BJT), the output voltage is: ?
The Common Collector (CC) Configuration
In the Common Collector or grounded collector configuration, the collector is now common through the supply so the collector is common to both the input and the output. The input signal is connected directly to the base terminal, while the output signal is taken from across the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit.
Characteristic Common Base Common Emitter Common Collector
Input Impedance Low Medium High
Output Impedance Very High High Low
Phase Shift 0 degree 180 degree 0 degree
Power Gain Low Very High Medium
Voltage Gain High Medium Low
Current Gain Low Medium High
..
58. Select the motor with the least starting torque: ?
Shaded pole induction motor produces very small starting torque about 50% of full load torque. Efficiency of shaded pole motor is low because of continuous power loss in shading coil. Used for small fans and small appliances..
59. Which of the following motors can be used as a part of a control circuit in robot applications ?
Servo Motor Applications
Servos are used in radio-controlled airplanes to position control surfaces like elevators, rudders, walking a robot, or operating grippers. Servo motors are small, have built-in control circuitry and have good power for their size.
60. A room measuring 12 × 20 ft is illuminated by 10 lamps rated 100 W each with an efficiency of 12 luminous/watt. Assuming a depreciation factor of 1.5 and coefficient of utilisation as 0.5, the illumination at the plane of the room is: ?
Area = 12 x 20 = 240$ft^2$
coefficient of utilisation = 0.5
depreciation factor = 1.5
Total lumens flux Ф = 10 x 100 x 12 = 12000 lumens
illumination at the plane of the room is = (lumens flux Ф x depreciation factor x coefficient of utilisation ) / Area
illumination at the plane of the room is = (12000 x 1.5 x 0.5) / 240 = 37.5 lumens/$ft^2$ .
61. Reactance electromotive force in the stator of an alternator: ?
The Voltage drop across armature resistance Ia Ra
The Voltage drop across synchronous reactance Ia Xs
these drops in phase quantity
Now drop Ia Ra is always in phase with Ia due to resistive drop And current Ia lag by 90 with respect to drop Ia Xs drop as it is a drop across purely induction reactance
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62. Power stations and sub-stations are protected against direct strokes of lightning using: ?
Earthing screen
Earthing screen is generally used over electrical substation. In this arrangement a net of GI wire is mounted over the sub-station. this this protect power station or substation against direct strokes of lightning
Overhead ground wires
Overhead ground wires are protect transmisson line.
This is similar as earthing screen. The only difference is, an earthing screen is placed over an electrical sub-station, whereas, overhead earth wire is placed over electrical transmission network..
63. Input power supplied during a short-circuit test on a transformer equals: ?
Open circuit test or no load test on a transformer is performed to determine 'no load loss (core loss)' and 'no load current I0'.
short-circuit test of a transformer is used to determine copper losses in the transformer at full load .
64. If during no-load test on an induction motor it takes 10 A current and 300 Watts of power at a line voltage of 200 V, the stator core loss will be: ?(Assume stator resistance/phase as 0.3 )
When motor running without load power = 300 watts current = 10 A Stator resistor = 0.3 ohm /phase Than Stator copper loss = 3 $I^2$R 3 × 10 x 10 × 0.3 = 90 W Stator core loss = power – Stator copper loss = 300 − 90 = 210 W .
65. For unity power factor loads, the effect of an armature reaction in an alternator is ?
Armature Reaction effect in a Synchronous Generator
1. Unity power factor - Armature Reaction, distorting effect.
2. Zero Leading power factor - Armature Reaction, magnetising Effect.
3.Zero Lagging power factor - Armature Reaction, de-magnetising effect
Armature Reaction effect in a Synchronous Motor
1. Field winding normal excitation - unity power factor - Armature Reaction, cross-magnetising effect.
2. Field winding over excitation - Zero Leading power factor - Armature Reaction, de-magnetising effect
3.Field winding under excitation -Zero Lagging power factor - Armature Reaction, magnetising effect.
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66. The circuit whose properties or characteristics change with the direction of its operation is ?
Unilateral Circuit These are the circuits whose properties and characteristics change with the direction according to its operation. A Diode Rectifier is a unilateral circuit because it cannot perform rectification in both directions
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67. The schematic arrangement given is of protection of alternator against ?
when earth fault occur on alternator relay cut the power supply of alternator .
68. Voltage regulation and efficiency of four transformers A, B, C & D are as given below:
A - 5% regulation & 94% efficiency
B - 2% regulation & 96% efficiency
C - 5% regulation & 97% efficiency
D - 2% regulation & 97% efficiency
The transformer that is in a good working condition is ?
A - 5% regulation & 94% efficiency
B - 2% regulation & 96% efficiency
C - 5% regulation & 97% efficiency
D - 2% regulation & 97% efficiency
The transformer that is in a good working condition is ?
The transformer that is in a good working condition is - 2% regulation & 97% efficiency
For working good condition transformer should have low Voltage regulation and high efficiency
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69. The magnitude of flux in a magnetic circuit may be calculated as ?
Flux φ = mmf / reluctance = NI / S
φ = NI / 1 /1l / (µ0 µr /a) = NIµ / (l/a) .
70. The reluctance of a straight magnetic path is: ?
reluctance S = l /(µ0 µr a) .
71. When a DC series motor is started with an open field winding connection, the motor will ?
Because you're dealing with a DC Series Motor, the Armature and Field windings are connected in series, so an open in the field windings will result in zero current through the field, and armature, so motor not start. .
72. In a conductor of length 1m moving in a magnetic field of constant flux density B Wb/m2 at an angle of ? with the direction of magnetic flux, the e.m.f. induced will be (assume the conductor travels 'x' meters in 't' seconds): ?
e = Bl sinθ dx/dt.
73. In case of a DC machine, the efficiency is maximum when ?
In dc machine condition for max efficiency copper loss = constant loss .
74. For the given circuit, current through 3 A. resistance is:?
Apply KVL on this circuit
6 - 3i + 4i - 4i = 0
i = 6/3 = 2A
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75. A DC generator having 2-layer lap winding is wound with 4-poles and 18 coils. The pole pitch in this case is: ?
Pole pitch = no of conductor / no of pole or Pole pitch = slots/pole
Number of conductors or coil side = 18 x 2 = 36
Pole pitch= 36/4 = 9 .
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