The Armature Reaction happens in both dc generator and motors. Consider the dc machine is act as a generator for better understanding.
What is Armature Reaction ?
It is the Effect of armature flux on the main field flux. when the armature carries current Ia produces armature flux.
Consider the dc generator is rotating in clockwise direction and it has 2 poles. The main field flux Փm from north to south. According to the flemming right hand rule, the armature current is flowing in the conductor.
We can define the direction of armature flux using right hand thumb rule. The physical mean point between the north and south poles called as Geometrically Neutral Axis GNA. The axis which passes the zero crossing of resultant magnetic field on the air gap is called as Magnetic Neutral Axis MNA.
The Armature flux Փa is perpendicular to the main field flux Փm and the Resultant flux is Փr. The Armature flux Փa crosses the main field flux causes Cross Magnetization effect.
The Variation of armature current Ia results in variation of armature flux Փa. The resultant flux shifts the magnetic neutral axis. There will be poor commutation in that machine. The angle between the Geometric Neutral axis and magnetic neutral axis is Ɵ.
The main field flux is opposed by demagnetizing flux Փd causes demagnetizing effect. The conductors in the armature which are responsible for demagnetization effect is 4Ɵ in terms of angle.Where Ɵ is the electrical angle.
Analysis of Armature reaction in terms of Ampere Turns
Let zbe the total number of armature conductors. I be the current in the armature.
demagnetization per pole = 2Ɵe
pole pitch = 180 electrical degree
cross magnetization per pole = 180-2Ɵe
Number of conductors = 2*Number of turns
Total ampere turns = ZI/2A
A- number of parallel paths
Total ampere turns per pole = ZI/2AP
P-number of poles
In order to reduce the effect of armature reaction, Compensating winding is provided in between the poles called inerpoles.
For example, The 8 pole generator has a output current of 200A and 500v having lap winding. The armature with 1280 conductors and 160 commutator segments. If brushes are advanced by 4 segments find out demagnetizing ampere turns and cross magnetizing ampere turns.
In lap winding, Number of parallel paths = Number of poles
A=P=8
Z=1280 conductors and 160 commutator segments.
160 commutator segments = 360 degree
1 commutator segments = 2.25 degree
4 commutator segments = 4*2.25 = 9
1 degree electrical = p/2 mech degree
so, 36 degree electrical
Demagnetizing ATd/Pole = 800 Ampere Turns
Cross magnetizing ATc/Pole = 1200 Ampere Turns
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