In my last post i have explained about armature reaction and how it effects voltage on dc generator.In this post i want to explain how to reduce armature reaction effect in dc generator.
In dc generator the effect of armature reaction has been resolved into two components
In dc generator the effect of armature reaction has been resolved into two components
- demagnetising effect of armature reaction
- cross magnetising effect of armature reaction
Before compensating we have to know the number of conductor which effects armature reaction so we have to calculate the number of turns which cause armature reaction.
Demagnetising ampere turns per pole:
The exact conductors which produce demagnetising effects are shown in fig. below
The brush axis has been given a forward lead of 𝚹 so as to lie on the new position of M.N.A. All conductors lying within angles AOC = BOD = 2𝚹 at top and bottom of armature are carrying current in such a direction as to send the flux through the armature from right and left. It is these conductors which act in direction opposite to the main field and are hence called the demagnetising armature conductors.
From this conductors we have demagnetising effect so to neutralize this effect we have to use extra turns to main field windings. To add extra turns we have to know the turns that are causing the effect.
Let Z = total number of armature conductors
I = current in each armature conductors
= Ia/2 for simplex wave winding
= Ia/P for simplex lap winding
𝚹m = forward lead in mechanical or geometrical or angular degrees
Total number of armature conductors in angles AOC and BOD is (4𝚹m✕Z)/360
total number of armature turns in these angles = (2𝚹m✕ZI)/360
demagnetising ampere turns per pair of poles = (2𝚹m✕ZI)/360
demagnetising ampere turns per pole = ZI✕(𝚹m)/360
For neutralizing demagnetising effect of armature reaction,We have to use extra number of turns on each pole.
no.of extra turns/pole = ATd/Ish (for shunt generator)
= ATd/Ia (for series generator)
where ATd = Demagnetising ampere turns per pole
Ish = current in shunt winding of shunt generator
Ia = current in armature winding of series generator
Cross-magnetising ampere turns per pole:
The exact conductors which produce cross-magnetising effects are shown in fig. below
The conductors which are lying between AOD and COB are responsible for cross-magnetising effect. These conductors carry current in such a direction as to produce a combined flux pointing vertically downwards i.e at right angles to the main flux. This results in distortion of main field. This conductors are called as cross magnetising conductors.
The number of cross magnetising conductors can be calculated as follows.
Total armature-conductors/pole both cross and demagnetising = Z/P
Demagnetising conductors/pole = Z (2𝚹m/360)
cross magnetising conductors/pole = (Z/P)-Z(2𝚹m/360)
cross magnetising conductors/pole = ZI [(1/P)-(2𝚹m/360)]
cross magnetising amp-turns/pole = ZI [(1/2P)-(𝚹m/360)]
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The number of cross magnetising conductors can be calculated as follows.
Total armature-conductors/pole both cross and demagnetising = Z/P
Demagnetising conductors/pole = Z (2𝚹m/360)
cross magnetising conductors/pole = (Z/P)-Z(2𝚹m/360)
cross magnetising conductors/pole = ZI [(1/P)-(2𝚹m/360)]
cross magnetising amp-turns/pole = ZI [(1/2P)-(𝚹m/360)]
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