Design of DC Machines Interview Question Answer - 3

State the application of hard carbon brushes.

The hard carbon brushes are used in fractional HP slow speed machines.

Describe the advantages of metal graphite brushes.

Advantages of metal graphite brushes
  • Low contact voltage drop
  • Used for high current density
  • Applicable for low voltage, high current machines

Which methods are used for applying brushes to commutator?

Methods of applying brushes to commutator
  • Radial
  • Trailing
  • Reaction

What is meaning of the radial position of brushed with commutator?

Radial position of brushes
  • If the brushes are set such that their center line is radial to the commutator, the position of brushes is called as radial position.
  • The brushes are fitted into deep brush box in order to prevent wear during high speed.
  • They are used in reversible machines because it does not affect stability condition.

Describe the advantages and disadvantages of trailing position of brushes.

  • The spring, friction and radial forces in the trailing box have contact of brush with left side of brush box give good stability.

  • It increases frictional resistance to sliding of brushes due to irregular commutator surface.

What should be angle of inclination for applying reaction brushes to commutator?

300 to 400 with negative x axis

What should be angle of inclination for applying radial brushes to commutator?


What should be angle of inclination for applying trailing brushes to commutator?

100 to 150 to positive x axis

What is meaning of the Grooving of commutator?

Grooving of commutator
  • The raised part of solid (ridges) of commutator is called as grooving of commutator.

How to prevent grooving of commutator?

Staggering of brushes
  • The grooving of commutator is prevented by staggering ( positive and negative brushes on slightly different axial position ) of brushes.

Which type of bearing is used in the DC machine?

  • There are roller bearings used on both ends in the small rating machine.
  • The roller bearing is used at the driving end and ball bearing is used at the non driving end of the higher rating machines.

Describe the disadvantages of DC motor fed through bridge circuit.

Disadvantages of DC motor fed through bridge circuit
  • High copper losses
  • High iron loss
  • Poor commutation

What is form factor of Pure Direct current?


Why the DC motors are de – rated when it is fed through thyristor rectifier bridge?

DC Motor – De rated
  • When the DC motors are fed form thyristor rectifier bridge, the armature copper losses as well as core losses increases which produces heating.
  • Therefore the DC motors are de – rated while supply through thyristor rectifier bridge.

Give reason : The fluctuating armature current results in bad commutation.

Effect of armature current on commutation
  • The peak value of armature current is commutated therefore there is increase in the reactance voltage which causes delay commutation.
  • The inter pole flux, armature reaction and reactance voltage are proportional to armature current.
  • The inter pole flux neutralize the flux produced by the armature current but inter pole flux produces eddy current therefore it does not synchronism with armature reaction and reactance voltage.
  • As the reactance voltage does not neutralize completely, it causes sparking.

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