Electrical Machines - short answer questions from AMIE exams (Winter 2020)

 Answer the following, in brief: (2 x 10)

The lap winding is used for low-voltage and high current, whereas the wave winding is employed to satisfy the requirements of high voltage and low current in de machine, justify.

Lap winding is suitable for low voltage high current machines because of more number of parallel paths.

Comment on the shunt field winding has many turns of fine wires, while a series field winding has comparatively fewer turns of heavy wire.

Series Field winding carries high current  so conductor size increases(thick  wire) whereas  shunt field winding carries  less current hence  (thin wire).

The circular section of armature of a dc machine is made of thin, highly permeable, and electrically insulated steel laminations (stacked), why?

  • The process of lamination involves dividing the core into thin layers held together by insulating materials.
  • Due to lamination effective cross-section area of each layer reduces and hence the effective resistance increases.
  • As effective resistance increases, the eddy current losses will get decrease.

The induced emf does not start at zero when the field current is zero, but at some value somewhat greater, why?

Even after the field current is reduced to zero, the machine is left out with some flux as residue so emf is available due to residual flux.

Interpole winding in a dc machine has only a few turns of comparatively heavy wire, justify.

They are wound with comparatively few heavy gauge Cu wire turns and are connected in series with the armature so that they carry full armature current.

The axial length of stator core is comparatively short for slow-speed large diameter hydro electric alternators while this length can be many times its diameter for high-speed turbo alternators, why?

As the diameter is proportional to the peripheral speed, the diameter of the high speed machines has to be kept low. For a given volume of the machine when the diameter is kept low the axial length of the machine increases. Hence a turbo alternator will have small diameter and large axial length.

The terminal voltage of an ac generator may actually be higher than the generated voltage, when the power factor is leading, why?

  • Because a leading-load forces the regulator to 'chase-its-tail,' i.e., it over-compensates the voltage-output, as it uses the [out-of-phase] current as the reference for voltage!
  • The voltage tends to rise. The causes of a change in terminal voltage with load change are (1) armature resistance (2) armature reactance, and (3) armature reaction.
  • At higher leading loads, the terminal voltage is greater than the induced EMF and the voltage regulation is negative.

The external characteristics of an alternator vary largely, as a function of load with unity, lagging, and leading power factor, justify.

  • At unity and lagging power factor loads, the terminal voltage is always less than the induced EMF and the voltage regulation is positive.
  • At higher leading loads, the terminal voltage is greater than the induced EMF and the voltage regulation is negative.
  • The lower the leading power factor, the greater the voltage rise from no load (Eg0) to full load (Vt)
  • The lower the lagging power factor, the greater the voltage decrease from no load (Eg0) to full load (Vt)

Generally a step-down transformer is used as a wielding transformer, why?

The step down transformer is less than the primary and the current of the secondary is more than the primary current. Principally, the power of the primary and secondary is equal. This larger secondary current is very useful in welding process. That is why step down transformer is used for welding.

The core of a transformer is built up of thin laminations of highly permeable ferromagnetic material (silicon sheet steel), why?

The central iron core of most transformers is made of a highly permeable material, which is typically thin silicon steel laminations. These thin laminations are joined together to provide the necessary magnetic path while minimizing magnetic losses.

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