Electrical Machines Questions and Answers

Electrical Machines Questions and Answers

[1] What is the function of an exciter?
The exciter is used to excite the field winding of synchronous machine by a dc supply.

[2] What are the functions of yoke? What is the choice of material for the yoke? [Anna University May-03]

  • It gives the mechanical support to the poles.
  • It acts as an outermost cover of the dc machine. Thus the insulating materials get protected from moisture, dust and flammable gases.
  • It forms a part of the magnetic circuit. It provides a path of low reluctance for magnetic flux.

Materials used for Yoke:

  1. It is prepared by using cast iron.
  2. For large machines rolled steel, cast steel and silicon steel is used. They provides high permeability

[3] Define coil span
It is defined as the distance on the periphery of the armature between two coil sides of a coil. Usually, it is expressed in terms of no of slots or degrees electrical

[4] Define coil span factor or pitch factor [Anna University May-04, Dec-12]
The factor by which there is a reduction in the emf due to short pitching of the coils is called coil span factor.
It is defined as the ratio of the resultant emf when coils are short pitched to the resultant emf when coils are full pitched.
KC = [ER when coil is short pitched]/[ER when coil is full pitched] = cos (α/2)

Here α = angel of short pitching

[5] Define distribution factor or winding factor [Anna University Dec -05, 11,12]
The factor by which there is a reduction in the emf due to distribution of coils is called distribution factor (Kd).
The distribution factor is defined as the ratio of resultant emf when coils are distributed to the resultant emf when coils are concentrated.
It is also known as winding factor or breadth factor.
Kd = sin(mβ/2) / msin(β/2)

[6] Explain the concept of Electrical Degree. Give the relation between electrical angle and mechanical angle.
The angle through which an induced alternating emf travels for mechanical rotation of conductors is called an electrical degree.
For 2 pole machine, electrical angle is same as mechanical angle of rotation.

For P  pole machine the relation between the electrical degree and mechanical angle is θe = P/2(θm)

[7] What is meant by Mechanical Angle?
The angle through which the rotor or conductors of the machine rotates physically is called Mechanical Angle.
While the angle through which is induced emf in the conductors travels is called Electrical Angle.

[8] Define synchronous speed [Anna University Dec-03]
The speed of the synchronous machine for which it produces alternating emf at a specified rated frequency for a fixed number of poles is called its synchronous speed.
It is denoted as NS.

NS = 120f/P
f  =  frequency
P = No of poles

[9] What is the nature of the induced emf in a DC generator? How it is converted to DC?
The basic nature of the induced emf in a dc generator is alternating.
It is converted to dc using commutator.

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