Important MCQ Question Of DC Generator | Objective Type Question Of DC Generator

Important MCQ Question Of DC Generator | Objective Type Question Of DC Generator

 

Ques 1.  The armature of DC generator is laminated to

  1. Reduce Hysteresis loss
  2. Insulate the Core
  3. Reduce eddy current loss
  4. Provide air cooling passage

Ans 3. Reduce eddy current loss

Explanation:

Why is Armature of DC Generator Laminated?

  • Eddy Current is the form of power loss and magnetic loss.
  • Eddy current loss produce heat and decrease motor efficiency.
  • Eddy current losses are directly proportional to the area of the armature or more precisely the path of motion.
  • Consider an armature with a single piece of iron.

    Eddy current losses without Lamination
  • In this case, the eddy current losses are represented by Red lines.
  • Now consider lamination armature, a complete lamination is made up of four or six or even eight segment lamination.

    Eddy current losses with Lamination
  • These laminations are insulated from each other by the thin coating of varnish or any other insulating material.
  • These individual, thin pieces have lesser area and higher resistance than one solid piece, and therefore, produce fewer eddy currents.
  • With laminated armature, eddy current losses are reduced to very less
  • NOTE = Thinner the laminations, greater is the Resistance offered to induced e.m.f. Therefore smaller the current hence lesser eddy current loss. 
  • That is why armature of DC machines (either motor or generator) is laminated. 

 

Ques 2. The commutator segments are connected to the armature conductor by means of

  1. Copper Lugs
  2. Insulation Pads
  3. Resistance wires
  4. Brazing

Ans 1. Copper Lugs

Explanation:

What is copper Lugs?

  • Copper lugs or copper cable lugs provide the electrical connection between power source to other power cables.

    Copper lug
  • They are connected to other power electrical equipment such as fuse blocks, starter etc.
  • Copper lugs are highly conductive and can carry high current.

    DC Commutator
  • Therefore commutator segment is connected to armature conductor by means of copper lugs as shown in the figure.

 

Ques 3. In D.C Generators the pole shoes are fastened to the pole core by

  1. Rivets
  2. Resistance wire
  3. Copper Lugs
  4. Counter sunk screw

Ans 4. Counter sunk screw

Explanation:

  • A counter sunk screw is a screw that is “sunk” into a piece of material.
  • It makes a very strong, tight joint and is used in places where another joinery is not practical.

    Counter Sunk Screw
  • The pole core itself may be a solid piece made out of either cast iron or cast steel but the pole shoe is laminated and is fastened to the pole face by means of counter sunk screws.

 

Ques 4. The bearing used to support the rotor shafts are generally

  1. Ball Bearing
  2. Magnetic bearing
  3. Bush bearing
  4. None of the above

Ans 1. Ball Bearing

Explanation:

  • Generators use coils of wires which rotate through a magnetic field to create voltage, which ‘pushes’ the current through the wires thus creating heat.
  • Bearing are used to reduce frictional losses, ensure smooth running of the rotor.
  • Therefore ball bearing is used in D.C generator to support rotor shaft.
  • NOTE ⇒ For small machine ball bearing is used.
    For larger machine or heavy duty, D.C generator roller bearings are used.

 

Ques 5. In Lap winding, the number of brushes is always

  1. Double the number of poles
  2. Same as number of poles
  3. Half the number of poles
  4. Multiple of number of poles

Ans 1. Same as number of poles

Explanation:

  • In a lap winding, the number of parallel paths, a, is always equal to the number of
    poles, p, and also to the number of brushes.
  • The number of parallel paths in a DC machine depends on the type of armature winding used.
  • There are two types of windings
    ⇒Lap winding
    ⇒Wave Winding

Lap Winding

  • If the connection is started from slot 1, then connections overlap each other as winding proceeds till starting point is reached again.

    Lap winding
  • Due to such connection, the total number of conductors get divided into P number of parallel paths where P = number of poles.
  • A large number of parallel paths indicate high current carrying capacity of the machine, hence lap winding is preferred for high current rating generators.

 

Ques 6. In case of wave wound generators, the average pitch is

  1. Must be Odd
  2. May be Odd or Even
  3. Must be Even
  4. None of the above

Ans 2. May be odd and even

Explanation: 

  • Where
    YB = Back Pitch
    ⇒YF = Front Pitch
    ⇒Z = Number of conductor
  • Note = Average Pitch must be an integer which may be odd or even but it should not be in a fraction in case.

 

Ques 7. The material for commutator Brushes is generally

  1. Mica
  2. Cast Iron
  3. Copper
  4. Carbon

Ans 4. Carbon

Explanation:

  • A commutator is a moving part of a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit.
  • Two or more electrical contacts called “brushes” made of a soft conductive material like carbon.
  • The brushes are press against the commutator, making sliding contact with successive segments of the commutator as it rotates.
  • The windings (coils of wire) on the armature are connected to the commutator segments.
  • Commutators are relatively inefficient, and also require periodic maintenance such as brush replacement.
  • Carbon has a very high melting point (~3500degC) compared with other conductors.

 

Ques 8. For Generating large current on D.C generators which winding is generally preferred

  1. Progressive wave winding
  2. Lap Winding
  3. Retrogressive Lap winding
  4. Current depends on design

Ans 2. Lap winding

Explanation:

  • A large number of parallel paths indicate high current carrying capacity of the machine, hence lap winding is preferred for high current rating generators.
  • For more detail information visit Qus 5.

 

Ques 9. The insulating material used for the commutator segment is generally

  1. Graphite
  2. Carbon
  3. Mica
  4. Insulating varnish

Ans 3. Mica

Explanation:

  • A commutator consists of a set of contact bars fixed to the rotating shaft of a machine and connected to the armature windings.
  • As the shaft rotates, the commutator reverses the flow of current in a winding.
  • Mica is useful for its unique electrical and thermal properties and its mechanical properties.
  • Mica is a good electrical insulator (bad conductor of electricity) at the same time as being a good thermal conductor.
  • Therefore mica is used as insulating material for commutator.

 

Ques 10. The purpose of providing dummy coils in a generator is

  1. To provide mechanical balance for the rotor
  2. To reduce eddy current loss
  3. To reduce hysteresis loss
  4. To increase efficiency of Generator

Ans 1. To provide mechanical balance for the rotor

Explanation:

What is dummy coil?

  • Dummy coil is used with wave winding when the requirement of the winding is not met by the standard armature.
  • Dummy coil  is not connected to the commutator so they do not influence the electrical characteristics of the winding
  • Dummy coil ends are cut short and taped.
  • Their main use is to provide mechanical balance for the rotor because rotor having some slots without winding would be out of balance mechanically. 

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Dc generator explain

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