7 March 2020

DC Generator Interview Question Answer - 3


41
Describe the relation between brush angle θ in mechanical and electrical?

                    
θ ( Mechanical ) = θ ( Electrical ) / Pole pairs                       
                           = θ ( Mechanical ) × 2 / P
            Where P = Number of poles


42
Describe the function of compensating winding? Where it is located?


Function of compensating winding
  • The function of the compensating winding is to neutralize the cross magnetizing effect of the armature reaction. 
  • These windings are embedded in the slots of the pole shoes.


43
Why the compensating winding is used in the DC machine which subject to fluctuation of load?


Role of compensation winding for variable load
  • When there is large fluctuation of load, the flux may move backward or forward according of change in load. 
  • The shifting of this flux set up statically induced emf in the armature coils. 
  • The magnitude of this emf depends upon amount of change in load. 
  • It may be so high to produce arc between consecutive commutator segment and resulting armature may be short circuited. 
  • The compensating winding neutralizes statically induced emf and reduces the flash over across commutator segments.


44
What is direction of current in the compensating winding with reference to armature winding?


The direction of the current in the compensating winding is opposite to the direction of the current flows in the armature winding.

45
Explain : Commutation and Commutation time


Commutation
  • The current in the short circuit coil is reversed while it crosses the magnetic neutral axis is called as commutation.

Commutation time
  • The time period during armature coils remain short circuited is called as commutation time.


46
What is meaning of the ideal commutation? What is its effect?


Ideal commutation
  • The change of current from + I to zero and then from zero to – I by the end of commutation period is known as ideal commutation.  
  • There are no sparking between brushes and commutator due to the ideal commutation.


47
Why sparking occurs on the brushes during commutation?


The main reason for sparking on the brushes is due to reactance voltage in the armature coil during commutation.

48
How to decide commutation time in the DC Generator?


Commutation time
  • The commutation time in the DC Generator is given by        Tc = ( Wb – Wm ) / v

 Where
       Tc = Commutation time
     Wb =  Brush width in cm
    Wm  =  Width of mica insulation in cm
        v =  Velocity of commutator segment cm / sec

49
Describe the different methods to reduce sparking on the brushes?


There are following methods to reduce sparking on the brushes
  • Resistance commutation and
  • Emf commutation


50
Describe the advantages and disadvantages of carbon brushes over copper brushes?


Advantages
  • Self lubricating
  • Help to obtaining sparkless commutation
  • They damage commutator less than that of copper brushes in the case of sparking.

Disadvantages
  • High contact resistance
  • They are not suitable for small machines due to higher (approx. 2 volt) voltage drop across brushes.
  • Large brush holder requires due to low current density as compared to copper brushes
  • The size of commutator should be larger than that of copper brushes in order to dissipate heat and prevent temperature rise.


51
Why carbon brushes are not used in small rating DC Generator?


Carbon Brushes
  • The carbon brushes have high contact resistance as compared to copper brushes therefore it has more voltage drop which is undesirable for small voltage rating DC Generator. 
  • Therefore the carbon brushes are not used in the small rating DC Generator.


52
How to improve the commutation condition in the DC Generator?


The commutation condition is improved in the DC Generator by
  • Emf commutation method
  • Resistance commutation


53
How the reversing emf is produced in the armature of the DC Generator?


The reversing emf is produced in the armature of the DC Generator by
  • Using inter – poles and
  • Giving brushes a forward lead such that to bring the short circuited coil under the influence of the next pole of opposite polarity


54
At which place the inter – poles are located in the DC Generator?


The inter – poles are located mid way between main poles at the geometrical neutral axis in the DC Generator.

55
How the inter – poles are connected with polarity?


Connection of inter poles
  • The inter – poles are connected is in series with the armature winding. 
  • The polarity of the inter – poles is the same as that of main pole ahead in the direction of the rotation.



56
Describe the functions of ( a ) Inter-poles ( b ) Compensating winding


Function of inter – poles
  • The main function of the inter-poles is neutralize the reactance voltage thereby making sparkles commutation
  • The inter – poles are used to neutralize cross – magnetizing effect of the armature reaction.


Function of compensating winding
  • The function of the compensating winding is to neutralize the effect of armature reaction.


57
At what percentage of load, the inter – poles neutralize the cross-magnetize effect of armature reaction?


The inter – poles neutralize the cross – magnetize effect of armature reaction at any load i.e. 0% to 100% load

58
In which winding equalizing connections are used? Lap winding or wave winding? Why?


Equalization connection
  • The equalizing connections are used only in the lap winding.
  • The characteristic of the lap winding is such that all conductors lie in any parallel path under pair of the poles.
  •  If the flux under all the poles is same, the emf induced in each parallel path is also same and each path carries the same current. 
  • There is always unbalance of emf in the two parallel paths due to magnetic property of the material and slight variation of the air gap. 
  • Therefore the conductors under stronger poles generate higher emf resulting higher current than that of conductors under weaker poles. 
  • This will resulting current distribution becomes unequal at the brushes and sparking occurs.


59
At which place in the armature, the equalizer rings are connected?


The equalizer connection is a copper ring connected at the armature back at the equal potential point.

60
Describe the function of equalizing ring in the lap winding.


Function of equalizing ring
  • The function of the equalizing ring is to avoid unequal distribution of the current at the brushes and thereby help to improve sparking at the brushes and getting the sparkles commutation. 





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