DC Generator

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Quizzes Created: 2 | Total Attempts: 1,236

Questions: 76 | Attempts: 637 | Updated: Mar 21, 2023

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Questions and Answers

1.

In D.C. generators, the cause of rapid brush wear may be
- A.
  
  Severe sparking
- B.
  
  Rough commutator surface
- C.
  
  Imperfect contact
- D.
  
  Any of the above
Correct Answer
D. Any of the above

Explanation
The correct answer is "Any of the above". This means that the cause of rapid brush wear in D.C. generators can be any of the mentioned factors: severe sparking, rough commutator surface, or imperfect contact. Each of these factors can contribute to increased friction and wear on the brushes, leading to their rapid deterioration.

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2.

The insulating material used between the commutator segments is normally
- A.
  
  Graphite
- B.
  
  Paper
- C.
  
  Mica
- D.
  
  Insulating varnish
Correct Answer
C. Mica

Explanation
Mica is commonly used as an insulating material between the commutator segments. Mica is a mineral with excellent electrical insulation properties, high heat resistance, and good mechanical strength. It can withstand high temperatures and does not conduct electricity, making it an ideal choice for insulating the segments of a commutator. Mica is also resistant to chemicals and moisture, which further enhances its suitability for this application. Overall, mica provides the necessary insulation and protection to ensure the proper functioning of the commutator in electrical devices.

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3.

Eddy currents are induced in the pole shoes of a D.C. machine due to
- A.
  
  Oscillating magnetic field
- B.
  
  Pulsating magnetic flux
- C.
  
  Relative rotation between field and armature
- D.
  
  All above
Correct Answer
C. Relative rotation between field and armature

Explanation
Eddy currents are induced in the pole shoes of a D.C. machine due to the relative rotation between the field and armature. When the field and armature rotate relative to each other, it creates a changing magnetic field which induces eddy currents in the pole shoes. These eddy currents can cause power losses and heating in the machine.

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4.

Which of the following statement about D.C. generators is false?
- A.
  
  Compensating winding in a D.C. machine helps in commutation
- B.
  
  In a D. C. generator interpoles winding is connected in series with the armature winding
- C.
  
  Back pitch and front pitch are both odd and approximately equal to the pole pitch
- D.
  
  Equalizing bus bars are used with parallel running of D.C. shunt generators
Correct Answer
D. Equalizing bus bars are used with parallel running of D.C. shunt generators

Explanation
Equalizing bus bars are not used with parallel running of D.C. shunt generators. Equalizing bus bars are used with parallel running of D.C. series generators to ensure equal voltage distribution among the generators. In parallel running of D.C. shunt generators, the generators are connected in parallel and their outputs are added together to meet the load demand. There is no need for equalizing bus bars in this configuration.

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5.

In a D.C. generator the ripples in the direct e.m.f. generated are reduced by
- A.
  
  Using conductor of annealed copper
- B.
  
  Using commutator with large number of segments
- C.
  
  Using carbon brushes of superior quality
- D.
  
  Using equalizer rings
Correct Answer
C. Using carbon brushes of superior quality

Explanation
Using carbon brushes of superior quality can help reduce the ripples in the direct e.m.f. generated in a D.C. generator. Carbon brushes are used to make electrical contact with the commutator, which is responsible for converting the alternating current produced in the armature windings into direct current. By using carbon brushes of superior quality, the contact between the brushes and the commutator is improved, resulting in a more stable and consistent direct e.m.f. output with reduced ripples.

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6.

Satisfactory commutation of D.C. machines requires
- A.
  
  Brushes should be of proper grade and size
- B.
  
  Brushes should smoothly run in the holders
- C.
  
  Smooth, concentric commutator properly undercut
- D.
  
  All of the above
Correct Answer
D. All of the above

Explanation
To achieve satisfactory commutation in D.C. machines, all of the mentioned factors are important. Brushes should be of proper grade and size to ensure good contact with the commutator. They should also smoothly run in the holders to avoid any friction or sparking. The commutator should be smooth and concentric, with proper undercutting to ensure effective switching of current between the armature coils. Therefore, all of the above factors are necessary for satisfactory commutation in D.C. machines.

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7.

In a D.C. generator the number of mechanical degrees and electrical degrees will be the same when
- A.
  
  R.p.m. is more than 300
- B.
  
  r.p.m. is less than 300
- C.
  
  Number of poles is 4
- D.
  
  Number of poles is 2
Correct Answer
D. Number of poles is 2

Explanation
In a D.C. generator, the number of mechanical degrees and electrical degrees will be the same when the number of poles is 2. This means that for every mechanical degree the rotor moves, the electrical degrees also change by the same amount. This is because in a 2-pole generator, each pole represents 180 electrical degrees. Therefore, when there are 2 poles, the mechanical and electrical degrees will be in sync.

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8.

The following constitute short-circuit in the armature winding.
- A.
  
  Insulation failure between two commutator bars
- B.
  
  Insulation failure between two turns of a coil
- C.
  
  Two of more turns of the same coil getting grounded
- D.
  
  All of the above
Correct Answer
D. All of the above

Explanation
The correct answer is "All of the above." This means that all of the given options - insulation failure between two commutator bars, insulation failure between two turns of a coil, and two or more turns of the same coil getting grounded - can cause a short-circuit in the armature winding.

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9.

In a D.C. machine stray loss is the sum of
- A.
  
  Total copper loss and mechanical loss
- B.
  
  Armature copper loss and iron loss
- C.
  
  Shunt field copper loss and mechanical loss
- D.
  
  Iron loss and mechanical loss
Correct Answer
D. Iron loss and mechanical loss

Explanation
In a D.C. machine, stray loss refers to the losses that occur due to the leakage of magnetic flux in the machine. These losses are primarily caused by two factors - iron loss and mechanical loss. Iron loss occurs due to the hysteresis and eddy current losses in the iron core of the machine. Mechanical loss, on the other hand, refers to the losses that occur due to friction and windage in the moving parts of the machine. Therefore, the correct answer is "Iron loss and mechanical loss."

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10.

Which of the following generator will have negligible terminal voltage while running on no-load?
- A.
  
  Series generator
- B.
  
  Shunt generator
- C.
  
  Compound generator
- D.
  
  Separately excited generator
Correct Answer
A. Series generator

Explanation
A series generator will have negligible terminal voltage while running on no-load because in a series generator, the field winding and armature winding are connected in series. When there is no load connected to the generator, the armature current is very small, causing a small voltage drop across the armature resistance. As a result, the terminal voltage of the generator becomes negligible.

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11.

In a separately excited generator supplying rated load the armature reaction:
- A.
  
  Is always present
- B.
  
  Is always absent
- C.
  
  May be sometimes present
- D.
  
  None of the above
Correct Answer
A. Is always present

Explanation
The armature reaction in a separately excited generator is always present. Armature reaction refers to the magnetic field produced by the armature windings, which interacts with the main magnetic field and affects the voltage output of the generator. In a separately excited generator, the field winding is supplied by a separate source, which creates a magnetic field independent of the armature current. Therefore, the armature reaction is always present in a separately excited generator, regardless of the load.

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12.

. When two D.C. series generators are running in parallel, an equalizer bar is used
- A.
  
  To increase the speed and hence generated e.m.f.
- B.
  
  To increase the series flux
- C.
  
  So that two similar machines will pass approximately equal currents to the load
- D.
  
  Efect of armature reaction To reduce the combined of both machines
Correct Answer
C. So that two similar machines will pass approximately equal currents to the load

Explanation
When two D.C. series generators are running in parallel, an equalizer bar is used so that two similar machines will pass approximately equal currents to the load. This is important to ensure that the load is shared equally between the generators and to prevent one generator from carrying a significantly higher load than the other. The equalizer bar allows for the adjustment of the field current in each generator, ensuring that they both contribute equally to the overall load.

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13.

A D.C. generator works on the principle of
- A.
  
  Lenz's law
- B.
  
  Ohm's law
- C.
  
  Faraday's law of electromagnetic induction
- D.
  
  None of the above
Correct Answer
C. Faraday's law of electromagnetic induction

Explanation
A D.C. generator works on the principle of Faraday's law of electromagnetic induction. This law states that when a conductor is moved through a magnetic field or when there is a change in the magnetic field through a conductor, an electromotive force (EMF) is induced in the conductor. In a D.C. generator, a coil of wire is rotated within a magnetic field, causing the magnetic field to change through the coil. This change in magnetic field induces an EMF in the coil, which results in the generation of a direct current. Therefore, the correct answer is Faraday's law of electromagnetic induction.

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14.

Two D.C. shunt generators, each with armature resistance of 0.02 ohm and field resistance of 50 ohm run in parallel and supply a total current of 1000 amperes to the load circuit. If their e.m.fs. are 270 V and 265 V, their bus bar voltage will be
- A.
  
  270 V
- B.
  
  267.5 V
- C.
  
  265 V
- D.
  
  257.4 V
Correct Answer
B. 267.5 V

Explanation
The bus bar voltage can be calculated using the formula:

Bus bar voltage = (Total current * Total resistance) + (e.m.f.1 + e.m.f.2)

Total resistance = (Armature resistance + Field resistance) = (0.02 + 50) ohm = 50.02 ohm

Total current = 1000 amperes

e.m.f.1 = 270 V
e.m.f.2 = 265 V

Substituting the values into the formula:

Bus bar voltage = (1000 * 50.02) + (270 + 265) = 50020 + 535 = 50555 V

Rounding off to the nearest decimal place, the bus bar voltage is 267.5 V.

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15.

In a D.C. generator the critical resistance can be increased by
- A.
  
  Increasing its field resistance
- B.
  
  Decreasing its field resistance
- C.
  
  Increasing its speed
- D.
  
  Decreasing its speed
Correct Answer
C. Increasing its speed

Explanation
Increasing the speed of a D.C. generator can increase its critical resistance. The critical resistance refers to the minimum resistance that should be connected to the generator's armature circuit to avoid excessive sparking at the commutator. By increasing the speed, the back EMF (electromotive force) produced by the generator also increases. This higher back EMF allows for a larger voltage drop across the armature resistance, which in turn allows for a higher critical resistance.

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16.

For both lap and wave windings, there are as many commutator bars as the number of
- A.
  
  Slots
- B.
  
  Armature conductors
- C.
  
  Winding elements
- D.
  
  Poles
Correct Answer
C. Winding elements

Explanation
For both lap and wave windings, the number of commutator bars is equal to the number of winding elements. In lap winding, the winding elements are connected in parallel, while in wave winding, the winding elements are connected in series. The commutator bars are used to connect the armature conductors to the external circuit, and the number of commutator bars must match the number of winding elements to ensure proper functioning of the winding. Therefore, the correct answer is "Winding elements".

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17.

In case of D.C. machine winding, number of commutator segments is equal to
- A.
  
  Number of armature coils
- B.
  
  Number of armature coil sides
- C.
  
  Number of armature conductors
- D.
  
  Number of armature turns
Correct Answer
A. Number of armature coils

Explanation
The number of commutator segments in a D.C. machine winding is equal to the number of armature coils. The commutator segments are used to connect the armature coils to the external circuit, allowing for the conversion of alternating current to direct current. Each armature coil is connected to a separate commutator segment, and the number of commutator segments is therefore equal to the number of armature coils.

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18.

A D.C. welding generator has
- A.
  
  Lap winding
- B.
  
  Wave moving
- C.
  
  Duplex winding
- D.
  
  Any of the above
Correct Answer
A. Lap winding

Explanation
A D.C. welding generator typically uses a lap winding. Lap winding is a type of armature winding where the end of one coil overlaps with the start of the next coil. This arrangement allows for a better distribution of current and reduces the chances of short circuits. It also provides a higher voltage output and better control over the welding process. Therefore, a D.C. welding generator commonly utilizes a lap winding.

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19.

DC shunt generator has terminal voltage versus load current characteristic which is
- A.
  
  Constant
- B.
  
  Slightly drooping
- C.
  
  Slightly rising
- D.
  
  Highly drooping
Correct Answer
D. Highly drooping

Explanation
The terminal voltage versus load current characteristic of a DC shunt generator is highly drooping. This means that as the load current increases, the terminal voltage of the generator decreases significantly. This characteristic is due to the internal resistance of the generator, which causes a voltage drop across the generator itself as current flows through it. As the load current increases, the voltage drop across the internal resistance also increases, resulting in a decrease in the terminal voltage.

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20.

The material for commutator brushes is generally
- A.
  
  mica
- B.
  
  Copper
- C.
  
  Cast iron
- D.
  
  carbon
Correct Answer
D. carbon

Explanation
Carbon is the most commonly used material for commutator brushes. This is because carbon has excellent electrical conductivity, low friction, and good wear resistance. It can effectively transfer electrical current between the commutator and the stationary parts of the motor. Carbon brushes also have self-lubricating properties, which reduce friction and heat generation during operation. Additionally, carbon brushes are relatively inexpensive and easy to replace, making them a popular choice in various electrical applications.

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21.

The insulating material used between the commutator segments is normally
- A.
  
  graphite
- B.
  
  Paper
- C.
  
  Mica
- D.
  
  insulating varnish
Correct Answer
C. Mica

Explanation
Mica is commonly used as an insulating material between the commutator segments because of its excellent electrical insulation properties. It has a high dielectric strength, which allows it to withstand high voltage without conducting electricity. Mica is also resistant to heat, chemicals, and moisture, making it a durable and reliable choice for insulating the commutator. Additionally, mica is a good thermal conductor, allowing it to dissipate heat effectively and prevent overheating of the commutator. Overall, mica is a suitable material for insulating the commutator, ensuring proper functioning and longevity of the electrical device.

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22.

3.In D.C. generators, the brushes on commutator remain in contact with conductors which
- A.
  
  lie under south pole
- B.
  
  Lie under north pole
- C.
  
  Lie under interpolar region
- D.
  
  are farthest from the poles
Correct Answer
C. Lie under interpolar region

Explanation
In D.C. generators, the brushes on the commutator remain in contact with conductors that lie under the interpolar region. The interpolar region is the area between the north and south poles of the generator. This region is important because it helps in reducing the sparking and improves the efficiency of the generator. By having the brushes in contact with the conductors in this region, the commutation process is improved, ensuring a smooth flow of current and reducing any potential issues with sparking or arcing.

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23.

4.If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be
- A.
  
  demagnetisation only
- B.
  
  Cross magnetisation as well as mag¬netisation
- C.
  
  Crossmagnetisation as well as demagnetising
- D.
  
  cross magnetisation only
Correct Answer
C. Crossmagnetisation as well as demagnetising

Explanation
When the brushes of a DC generator are moved to the magnetic neutral axis, it results in both cross magnetization and demagnetization. Cross magnetization occurs because the brushes are now aligned with the magnetic field, causing some of the magnetic flux to pass through the armature conductors. Demagnetization occurs because the brushes are no longer in contact with the commutator segments, which reduces the magnetic field strength. Therefore, the correct answer is cross magnetization as well as demagnetizing.

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24.

Armature reaction of an unsaturated D.C. machine is
- A.
  
  crossmagnetising
- B.
  
  Demagnetising
- C.
  
  Magnetising
- D.
  
  none of above
Correct Answer
A. crossmagnetising

Explanation
Armature reaction of an unsaturated D.C. machine refers to the magnetic field produced by the armature current that interacts with the main magnetic field of the machine. In the case of crossmagnetising armature reaction, the armature current produces a magnetic field that is perpendicular to the main magnetic field, causing a distortion in the overall magnetic field. This can result in a reduction in the effective magnetic field strength, affecting the performance of the machine. Therefore, the correct answer is crossmagnetising.

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25.

D.C. generators are connected to the busbars or disconnected from them only under the floating condition
- A.
  
  to avoid sudden loading of the primemover
- B.
  
  To avoid mechanicaljerk to the shaft
- C.
  
  To avoid burning of switch contacts
- D.
  
  all above
Correct Answer
D. all above

Explanation
The correct answer is "all above". D.C. generators are connected to the busbars or disconnected from them only under the floating condition to avoid sudden loading of the prime mover, to avoid mechanical jerk to the shaft, and to avoid burning of switch contacts. By keeping the generators in a floating condition, these potential issues can be prevented, ensuring the smooth operation and longevity of the generator system.

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26.

In D.C. generators, current to the external circuit from armature is given through
- A.
  
  commutator
- B.
  
  Solid connection
- C.
  
  Slip rings
- D.
  
  none of above
Correct Answer
B. Solid connection

Explanation
In D.C. generators, current to the external circuit from the armature is given through a solid connection. This means that the armature winding is directly connected to the external circuit without the use of any intermediate components such as commutators or slip rings. This ensures a reliable and efficient transfer of electrical current from the generator to the external circuit.

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27.

Iron losses in a D.C. machine are independent of variations in
- A.
  
  speed
- B.
  
  Load
- C.
  
  Voltage
- D.
  
  speed and voltage
Correct Answer
B. Load

Explanation
Iron losses in a D.C. machine are independent of variations in load. Iron losses, also known as core losses, occur in the iron core of the machine due to hysteresis and eddy current losses. These losses are constant and do not change with variations in load. The load only affects the copper losses in the machine, which are caused by the resistance of the windings and vary with the load. Therefore, variations in load do not impact the iron losses in a D.C. machine.

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28.

Brushes of D.C. machines are made of
- A.
  
  carbon
- B.
  
  Soft copper
- C.
  
  Hard copper
- D.
  
  all of above
Correct Answer
A. carbon

Explanation
The brushes of D.C. machines are made of carbon. Carbon brushes are commonly used in D.C. machines because carbon is a good conductor of electricity and has low friction characteristics. It can withstand high temperatures and is durable, making it suitable for the constant sliding contact with the commutator in D.C. machines. Soft copper and hard copper are not typically used for brushes in D.C. machines.

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29.

If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by
- A.
  
  Blv
- B.
  
  Blv2
- C.
  
  Bl2v
- D.
  
  Bl2v2
Correct Answer
A. Blv

Explanation
The correct answer is Blv. According to Faraday's law of electromagnetic induction, the induced electromotive force (emf) is directly proportional to the magnetic flux density (B), the length of the conductor (l), and the velocity of the conductor (v). Therefore, the correct equation for the induced emf is Blv.

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30.

In case of a 4-pole D.C. generator provided with a two layer lap winding with sixteen coils, the pole pitch will be
- A.
  
  4
- B.
  
  8
- C.
  
  16
- D.
  
  32
Correct Answer
B. 8

Explanation
In a two-layer lap winding, each coil spans two poles. Since there are 16 coils, there must be 8 pole pairs. Therefore, the pole pitch, which is the distance between the centers of two adjacent poles, will be 8.

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31.

For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher e.m.f. ?
- A.
  
  Lap winding
- B.
  
  Wave winding
- C.
  
  Either of and above
- D.
  
  Depends on other features of design
Correct Answer
B. Wave winding

Explanation
The wave winding will give the higher e.m.f. in a D.C. generator when the number of poles and the number of armature conductors is fixed. This is because in a wave winding, each coil spans across two poles, resulting in a higher number of parallel paths for the current to flow through. This increases the e.m.f. generated compared to a lap winding where each coil spans across only one pole. Therefore, the wave winding is more efficient in generating a higher e.m.f. in this scenario.

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32.

Copper brushes in D.C. machine are used
- A.
  
  where low voltage and high currents are involved
- B.
  
  Where high voltage and small cur-rents are involved
- C.
  
  In both of the above cases
- D.
  
  in none of the above cases
Correct Answer
A. where low voltage and high currents are involved

Explanation
Copper brushes in D.C. machines are used where low voltage and high currents are involved. This is because copper is an excellent conductor of electricity and can handle high currents without overheating or causing excessive resistance. Additionally, copper brushes have good wear resistance and can maintain good contact with the commutator, ensuring efficient transfer of current. Therefore, copper brushes are suitable for applications where low voltage and high currents are present, such as in electric motors or generators.

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33.

In a four-pole D.C. machine
- A.
  
  all the four poles are north poles
- B.
  
  Alternate poles are north and south
- C.
  
  All the four poles are south poles
- D.
  
  two north poles follow two south poles
Correct Answer
B. Alternate poles are north and south

Explanation
In a four-pole D.C. machine, the correct answer is "alternate poles are north and south." This means that the machine has two north poles and two south poles, with each pole alternating in polarity. This arrangement allows for the creation of a magnetic field that is essential for the operation of the machine. By having alternating north and south poles, the machine can generate the necessary electromagnetic forces required for its functioning.

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34.

In case of D.C. machines, mechanical losses are primary function of
- A.
  
  current
- B.
  
  Voltage
- C.
  
  Speed
- D.
  
  none of above
Correct Answer
C. Speed

Explanation
In D.C. machines, mechanical losses primarily depend on the speed of the machine. This is because mechanical losses are caused by friction and windage, which increase as the speed of the machine increases. Therefore, the higher the speed of the D.C. machine, the greater the mechanical losses.

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35.

A separately excited generator as compared to a self-excited generator
- A.
  
  is amenable to better voltage control
- B.
  
  Is more stable
- C.
  
  Has exciting current independent of load current
- D.
  
  has all above features
Correct Answer
D. has all above features

Explanation
A separately excited generator is able to provide better voltage control compared to a self-excited generator. It is also more stable and has the advantage of having an exciting current that is independent of the load current. Therefore, it possesses all of the mentioned features.

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36.

While applying Fleming’s right-hand rule to And the direction of induced e.m.f., the thumb points towards
- A.
  
  direction of induced e.m.f.
- B.
  
  Direction of flux
- C.
  
  Direction of motion of the conductor if forefinger points in the direction of generated e.m.f.
- D.
  
  direction of motion of conductor, if forefinger points along the lines of flux
Correct Answer
D. direction of motion of conductor, if forefinger points along the lines of flux

Explanation
When applying Fleming's right-hand rule, the thumb represents the direction of the induced e.m.f. The forefinger points in the direction of the generated e.m.f. The middle finger represents the direction of the magnetic field or lines of flux. Therefore, if the forefinger points along the lines of flux, the thumb will point in the direction of the motion of the conductor. This is because the induced e.m.f. is generated due to the relative motion between the conductor and the magnetic field.

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37.

The bearings used to support the rotor shafts are generally
- A.
  
  ball bearings
- B.
  
  Bush bearings
- C.
  
  Magnetic bearmgs
- D.
  
  needle bearings
Correct Answer
A. ball bearings

Explanation
Ball bearings are generally used to support the rotor shafts. Ball bearings consist of small metal balls that are held in a ring and allow for smooth rotation with minimal friction. They are commonly used in various applications where high radial and axial loads need to be supported. In the case of rotor shafts, ball bearings provide a reliable and efficient means of supporting the shafts and facilitating smooth rotation.

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38.

In D.C. generators, the cause of rapid brush wear may be
- A.
  
  severe sparking
- B.
  
  Rough commutator surface
- C.
  
  Imperfect contact
- D.
  
  any of the above
Correct Answer
D. any of the above

Explanation
The cause of rapid brush wear in D.C. generators can be any of the mentioned factors: severe sparking, rough commutator surface, or imperfect contact. Severe sparking occurs when there is excessive arcing between the brushes and the commutator, leading to increased wear. A rough commutator surface can cause uneven contact with the brushes, resulting in accelerated brush wear. Imperfect contact refers to a condition where the brushes do not make proper contact with the commutator, causing friction and wear. Therefore, any of these factors can contribute to rapid brush wear in D.C. generators.

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39.

Fleming’s right-hand rule regarding direction of induced e.m.f., correlates
- A.
  
  magnetic flux, direction of current flow and resultant force
- B.
  
  Magnetic flux, direction of motion and the direction of e.m.f. induced
- C.
  
  Magnetic field strength, induced voltage and current
- D.
  
  magnetic flux, direction of force and direction of motion of conductor
Correct Answer
B. Magnetic flux, direction of motion and the direction of e.m.f. induced

Explanation
Fleming's right-hand rule states that when a conductor moves through a magnetic field, the direction of the induced electromotive force (e.m.f.) is perpendicular to both the direction of motion and the magnetic flux. This means that the induced e.m.f. will be in a direction that is perpendicular to both the direction of motion of the conductor and the magnetic field. Therefore, the correct answer is "magnetic flux, direction of motion, and the direction of e.m.f. induced."

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40.

In lap winding, the number of brushes is always
- A.
  
  double the number of poles
- B.
  
  Same as the number of poles
- C.
  
  Half the number of poles
- D.
  
  two
Correct Answer
B. Same as the number of poles

Explanation
In lap winding, the number of brushes is always the same as the number of poles. This is because in lap winding, each coil is connected to two adjacent commutator segments, and each commutator segment is connected to a brush. Therefore, the number of brushes required is equal to the number of poles.

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41.

The following magnet is used in large machines to create magnetic flux
- A.
  
  Permanent magnet
- B.
  
  Electro magnet
- C.
  
  Temporary magnet
- D.
  
  Any of the above
Correct Answer
B. Electro magnet

Explanation
Electro magnets are used in large machines to create magnetic flux. Unlike permanent magnets, electro magnets are made by passing an electric current through a coil of wire, which generates a magnetic field. This magnetic field can be controlled and adjusted by varying the amount of current passing through the coil, making electro magnets suitable for use in large machines where magnetic flux needs to be created and manipulated. Permanent magnets, on the other hand, have a fixed magnetic field and cannot be easily adjusted or turned on/off like electro magnets. Temporary magnets are also not commonly used in large machines for creating magnetic flux.

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42.

The special device which converts AC into DC and vice versa is known as
- A.
  
  Armature
- B.
  
  Slip rings
- C.
  
  Split rings
- D.
  
  Field magnets
Correct Answer
C. Split rings

Explanation
Split rings are a type of special device used in electric motors and generators to convert alternating current (AC) into direct current (DC) and vice versa. They are typically made of copper or other conductive materials and are used to control the flow of electrical current in the device. By rotating with the armature, the split rings help to change the direction of the current, allowing for the conversion between AC and DC.

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43.

The following is (are) the part(s) of a field magnet.
- A.
  
  Yoke
- B.
  
  Pole cores
- C.
  
  Pole shoes
- D.
  
  All of the above
Correct Answer
D. All of the above

Explanation
The correct answer is "All of the above." This means that all the options mentioned - yoke, pole cores, and pole shoes - are parts of a field magnet. A field magnet typically consists of these components, which work together to generate a magnetic field. The yoke provides a closed magnetic circuit, while the pole cores and pole shoes help concentrate and direct the magnetic field in a desired manner. Therefore, all of these parts are essential for the functioning of a field magnet.

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44.

The brushes are ____ in shape.
- A.
  
  Triangular
- B.
  
  Rectangular
- C.
  
  Cylindrical
- D.
  
  Square
Correct Answer
B. Rectangular

Explanation
The correct answer is rectangular because the word "brushes" implies that there are multiple brushes, and the shape of each brush is described as rectangular.

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45.

Function of _____ is to collect current from the commutator and supply it to the external load.
- A.
  
  Field magnet
- B.
  
  Armature
- C.
  
  Brushes
- D.
  
  Yoke
Correct Answer
C. Brushes

Explanation
Brushes are used in a DC motor to collect current from the commutator and transfer it to the external load. The commutator is a rotating device that switches the direction of the current in the armature coil, and the brushes make contact with the commutator segments to collect the current. The brushes are typically made of carbon or graphite, which have good electrical conductivity and can withstand the friction and heat generated during operation. By collecting the current from the commutator, the brushes ensure that the electrical power generated by the motor is transferred to the external load efficiently.

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46.

For larger machines
- A.
  
  Ball bearings are used at both driving and non driving ends.
- B.
  
  Ball bearings are used at driving end and roller bearings are used at non driving end.
- C.
  
  Roller bearings are used at driving end and ball bearings are used at non driving end.
- D.
  
  Roller bearings are used at both driving and non driving ends.
Correct Answer
C. Roller bearings are used at driving end and ball bearings are used at non driving end.
47.

The following is (are) keyed to the shaft
- A.
  
  Armature core
- B.
  
  Commutator
- C.
  
  Cooling fan
- D.
  
  All of the above
Correct Answer
D. All of the above

Explanation
All of the listed components (armature core, commutator, and cooling fan) are keyed to the shaft. This means that they are securely attached to the shaft using a key, which is a small piece of metal that fits into a groove on both the shaft and the component. This keying ensures that the components rotate along with the shaft, allowing for efficient and synchronized operation of the motor or machine.

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48.

Which of the following generators have two field windings?
- A.
  
  Series wound generator
- B.
  
  Shunt wound generator
- C.
  
  Compound wound generator
- D.
  
  All of the above
Correct Answer
C. Compound wound generator

Explanation
A compound wound generator is a type of generator that has two field windings. One winding is connected in series with the armature, while the other is connected in parallel or shunt with the armature. This configuration allows the generator to have both the advantages of a series wound generator (high starting torque) and a shunt wound generator (good voltage regulation). Therefore, the correct answer is compound wound generator.

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49.

A motor converts
- A.
  
  Mechanical energy into electrical energy
- B.
  
  Chemical energy into electrical energy
- C.
  
  Electrical energy into Mechanical energy
- D.
  
  Electrical energy into chemical energy
Correct Answer
C. Electrical energy into Mechanical energy

Explanation
A motor is a device that uses electrical energy to produce mechanical energy. It does this by converting the electrical energy into rotational motion, which can be used to drive machinery or perform other mechanical tasks. This process is achieved through the interaction of magnetic fields and electric currents in the motor's components. Therefore, the correct answer is "Electrical energy into Mechanical energy."

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50.

To produce dynamically induced emf, the following is (are) necessary
- A.
  
  A magnetic field
- B.
  
  A conductor
- C.
  
  Motion of conductor with respect to the field
- D.
  
  All of the above
Correct Answer
D. All of the above

Explanation
For the production of dynamically induced emf, all three factors mentioned in the options are necessary. A magnetic field is required to create a flux, a conductor is needed to allow the flow of electrons, and the motion of the conductor with respect to the field is necessary to induce the emf. Therefore, all of the above options are essential for the production of dynamically induced emf.

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Mar 21, 2023

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Sep 20, 2018

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DC Generator

In D.C. generators, the cause of rapid brush wear may be

The insulating material used between the commutator segments is normally

Eddy currents are induced in the pole shoes of a D.C. machine due to

Which of the following statement about D.C. generators is false?

In a D.C. generator the ripples in the direct e.m.f. generated are reduced by ​

Satisfactory commutation of D.C. machines requires

In a D.C. generator the number of mechanical degrees and electrical degrees will be the same when

The following constitute short-circuit in the armature winding.

In a D.C. machine stray loss is the sum of

Which of the following generator will have negligible terminal voltage while running on no-load?

In a separately excited generator supplying rated load the armature reaction:

. When two D.C. series generators are running in parallel, an equalizer bar is used

A D.C. generator works on the principle of

Two D.C. shunt generators, each with armature resistance of 0.02 ohm and field resistance of 50 ohm run in parallel and supply a total current of 1000 amperes to the load circuit. If their e.m.fs. are 270 V and 265 V, their bus bar voltage will be

In a D.C. generator the critical resistance can be increased by

For both lap and wave windings, there are as many commutator bars as the number of

In case of D.C. machine winding, number of commutator segments is equal to

A D.C. welding generator has

DC shunt generator has terminal voltage versus load current characteristic which is

The material for commutator brushes is generally

The insulating material used between the commutator segments is normally

3.In D.C. generators, the brushes on commutator remain in contact with conductors which

4.If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be

Armature reaction of an unsaturated D.C. machine is

D.C. generators are connected to the busbars or disconnected from them only under the floating condition

In D.C. generators, current to the external circuit from armature is given through

Iron losses in a D.C. machine are independent of variations in

Brushes of D.C. machines are made of

If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by

In case of a 4-pole D.C. generator provided with a two layer lap winding with sixteen coils, the pole pitch will be

For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher e.m.f. ?

Copper brushes in D.C. machine are used

In a four-pole D.C. machine

In case of D.C. machines, mechanical losses are primary function of

A separately excited generator as compared to a self-excited generator

While applying Fleming’s right-hand rule to And the direction of induced e.m.f., the thumb points towards

The bearings used to support the rotor shafts are generally

In D.C. generators, the cause of rapid brush wear may be

Fleming’s right-hand rule regarding direction of induced e.m.f., correlates

In lap winding, the number of brushes is always

The following magnet is used in large machines to create magnetic flux

The special device which converts AC into DC and vice versa is known as

The following is (are) the part(s) of a field magnet.

The brushes are ____ in shape.

Function of _____ is to collect current from the commutator and supply it to the external load.

For larger machines

The following is (are) keyed to the shaft

Which of the following generators have two field windings?

A motor converts

To produce dynamically induced emf, the following is (are) necessary

In a D.C. generator the ripples in the direct e.m.f. generated are reduced by