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Design of DC Machines Interview Question Answer - 4

Describe the advantages of laminated structure of DC machine.

Advantages of laminated structure
  • To reduce eddy current loss
  • To reduce time constant of winding

Describe the importance of time constant in the DC machine.

Lower the time constant of the machine, faster its dynamic response

Describe the effect of lamination on the time constant of DC machine.

The time constant of DC machine reduces as the thickness of lamination decreases and vice versa.

Which are the main factors affecting reactance voltage in the DC machine?

Factor affecting reactance voltage
  • The reactance voltage is affected by armature winding inductance, armature current per parallel path and time of commutation.
  • The reactance voltage is directly proportional to armature winding inductance and armature current whereas it is inversely proportional to commutation time.

How the numbers of armature turns affect the reactance voltage?

Armature turns – Reactance voltage
  • The inductance of the winding is directly proportional to square of number of turns therefore higher number of turns result in high inductance of winding.
  • As the number of armature turns are small, lower the reactance voltage.

Why the SCR fed DC motor has more armature coil than the pure DC?

The SCR fed DC motor is de – rated due to higher copper losses, higher iron losses and heating condition therefore SCR fed DC motor has more armature coil than when it is fed through pure DC.

How the large diameter of commutator affects the stability of the DC machine?

Diameter of commutator
  • As the diameter of commutator increases, inertia of moving part increases which also increases time constant of the machine.
  • This will result in poor stability condition.

Which are the factors to be considered before selecting average flux density in the DC machine?

Average flux density in air gap
  • Size of machine
  • Flux density in the teeth
  • Frequency

Why the flux density at the root of teeth is limited up to 2.2 weber / meter2?

Flux density in the teeth
  • The flux density in the air gap should be selected such that the flux density at the root of the teeth does not exceed  2.2 weber / meter2 otherwise mmf required for the teeth would be excessively large it means that the field mmf will have to large.
  • There are higher field copper loss and higher cost of copper due to large field mmf.
  • The iron losses in the teeth is depends upon flux density the in the air gap.

Describe the effect of average flux density on the size of DC machine.

Relation between size of machine and average flux density
  • As the diameter of the DC machine increases, the width of tooth also increases.
  • This will allows higher value of average flux density without causing saturation of the machine.
  • Therefore the high value of average flux density allows as the size of the machine increases.

Why the semi closed machine has high value of ampere conductors per meter than that of closed machines?

Ampere conductor per meter
  • The totally enclosed DC machine has poor ventilation as compared to semi closed machine therefore it is prefer to adopt high value of ampere conductors per meter in the semi closed machine than that of closed machines.

Give reason : The higher value of ampere conductors should used for high speed machines.

Relation between ampere conductors and speed
  • The ventilation condition for the high speed machine is better than the low speed machine therefore it is recommended that the higher value of ampere conductor should be used for high speed machines.

Give reason : The low value of ampere conductors should be used for high voltage machines.

Relation between ampere conductor and voltage rating of machines
  • As the large space occupies by the insulation in the high voltage machine, the space left for the armature conductor is less.
  • Therefore the low value of armature conductors should be used for high voltage machines.

Which are the factors to be considered while selecting ampere conductor per meter of the DC machine?

Ampere conductor per meter
  • Voltage
  • Speed of machine
  • Size of machine
  • Temperature rise

Describe the effect of ampere conductors on the size of the DC machine.

Ampere conductors – Size of machine
  • As the specific electric loading is directly proportional to diameter of the armature, the higher value of specific electric loading is preferred for large diameter machine and vice versa.
  • However the relation between ampere conductors and size of machine is not linear due to increase in temperature with increase in linear dimensions of the machine.

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