Design of DC Machines Interview Question Answer - 2

Which type of conductor used for inter pole winding?

Round or square for small rating machine and copper strip for large rating machine

What should be thickness of lamination for armature in the DC machine?

0.4 – 0.5 mm

Why single lamination for armature is not used in large DC machines?

Economy point of view

What should be shape of armature coil in the DC machine?

Diamond shape

Define : Space factor

Space factor
  • The space factor is defined as the ratio of net conductor area to the gross conductor area. 
  • The space factor for round conductor is

           Sf = 0.8 ( d / d1 )2
  • Where d is diameter of conductor and d1 is diameter of conductor with insulation

Describe the function of commutator.

Function of commutator
  • The function of the commutator is to convert alternating current of the armature into direct current.

Whether the commutator is placed on both side of armature?

Only one side, driving end or load end

Describe the function of risers. Which material is used for risers?

Function of risers
  • The connection of armature is made with commuatator with the help of risers.
  • The risers are made of copper material.
  • The risers are not necessary for small motors.
  • The armature conductors are soldered to commutators by lugs in small motors.

Why the shape of the commutators are made wedge type?

Shape of commutator
  • The commutators are made wedge shaped in order to forms a circular shape similar to that of armature.
  • The centrifugal force during rotation of armature does not affect commutators due to its shape.

Which material is used for manufacture of commutator?

Hard drawn copper but now silver copper

Describe the advantage of silver copper commutator over hard drawn copper.

Silver copper commutator
  • The temperature withstand capability during flood soldering of silver copper at armature coils to risers  is much higher              ( 3000 C ) than that of hard drawn copper.

Describe the importance of mica sheet insulation between two commutator segments.

Function of mica insulation between commutator segments
  • The mica sheet insulation of 0.8 mm thick is used between two commuatator segments because of good thermal stability and consistent mechanical properties during commutation.
  • It also protects the whole commuator against short circuit during flash over between two individual segments.

What is pig tail?

Pig tail
  • A flexible conductor connecting brush is called as pig tail.

Which material is used for brush in the DC machine?

Material for brushes
  • Natural graphite
  • Hard carbon
  • Electro graphite
  • Metal graphite

Why the natural graphite brushes are used in the small DC machines?

Graphite brushes
  • The natural graphite brushes have good lubricating effect at high speed resulting low noise level therefore they are used in small DC machines.

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