Armature Reaction
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Gujarat Power Engineer ing & Research Institute By- -Karan Thawani(58) Contents:- - Armature Reaction - Compensating winding - Interpoles
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This presentation contains animative understanding of Armature reaction,compensating winding and interpoles
Transcript of Armature Reaction
Gujarat Power Engineering & Research Institute
By--Karan Thawani(58)Contents:-
- Armature Reaction-Compensating winding- Interpoles
Armature Reaction
- Armature Reaction is the effect of “Armature field” on the “Main Field”.
-Armature field is the field which is produced by the armature conductors due to current flowing through them.
- Main field is the field which is produced by the poles which is necessary for the operation.
(1) De- Magnetisation-It weakens /reduces the main flux.
(2) Cross- Magnetisation-It distorts the main flux.
Effects Of Armature Reaction
Terminology Related To Armature Reaction
-MNA(Magnetically Neutral Axis)
It is the axis along which no E.M.F is produced, hence brushes are kept on this axis.
-GNA(Geometrically Neutral Axis)
It is the axis which divides the armature core in two equal parts.
-Polar Axis It is the imaginary line which joins the center of NS poles.
o Consider a two pole D.C Generator.
o For the sake of simplicity, the brushes are shown directly touching the armature conductors, but practically they touch commutator segments.
o Assume that there is no current flowing through armature conductors, Hence..
(1)The flux is distributed symmetrically with respect to polar axis.
(2) The M.N.A coincides the G.NA.
NS
Pole
Armature
Brush
Main FieldArmature conductors
MNA/GNA
o Now assume that there is current flowing through the armature conductors and the direction of rotation is clock-wise.
o According to Fleming’s right hand rule, the direction of current is inwards in conductors which are influence of N pole and outwards which are influence of S pole.
o Inward flow of current is represented by “ ” whereas the outward flow is represented by “ ” .
o Now due to the current flowing through the armature conductors, magnetic flux is produced around the armature conductors.
o According to Fleming’s right hand rule, the direction of this flux is clock-wise in conductors which are influence of N pole and Anti-clock wise in conductors which are influence of s pole.
o Now, the main flux is no more longer symmetrical about the pole axis.
o Hence the main field is distorted.
o Now, the resultant flux is the vector sum of the main field and the armature field.
o Hence the position of M.N.A changes(which is always perpendicular to the resultant flux).
o Due to change in position of M.N.A, the position of brush also changes which in known as “Brush Lead”.
o Brush lead is in the direction of rotation in the case of D.C. Generator, while in the opposite direction of rotation in case of D.C. Motor.
NS
Direction of rotation
Current going insideCurrent coming outside
Main FieldArmature Field
o Now due to brush shift, the armature conductors and hence armature current is redistributed.
o Some armature conductors which were earlier under the influence of N-pole come under the influence of S-pole and vice-versa.
o According to Fleming’s right hand thumb rule, armature conductors in 2(where = brush lead) produces De-magnetizing effect and remaining produces cross-magnetizing effect.
NS
Main FieldDe-magnetizing
Cross-magnetizing
New MNAGNA
=Brush Lead
o Magnetic flux density increases over one half of the core and decreases over the other half.
o The flux wave is distorted and there is a shift in the position of M.N.A.
o It causes the commutation problem.
Effects Of Armature Reaction
Compensating winding
o Compensating windings are used to nullify the cross-magnetizing effect.
o These windings are kept in slots of pole faces.
o It carries current in opposite direction to the current of armature winding just below the pole faces.
o It is connected in series with the armature winding.
NS
Direction of rotation
Assuming this as pole face
Main FieldArmature Field
Field by Comp. winding
Drawbacks:
o There is only one disadvantage of compensating winding and that is it is costly.
Advantages:
o It is used in large machines heaving heavy fluctuation.
o It is used when same output is required at low voltage.
o As the compensating windings are too costly, inter-poles are used to neutralize the Cross-magnetizing effect of armature reaction.
o These are small poles fixed to the yoke and spaced in between the main poles.
o They are wound with comparatively few heavy gauge copper wire turns and are connected in series with the armature so that they carry full armature current.
o Their polarity is same as the “Leading pole” in the case of D.C. Generator and “Trailing pole” in d case of D.C. Motor considering the direction of rotation.
Inter-poles:
N
N
S SN
NS
S
D.C. Generator
Main Pole
Inter-Pole
N
N
S S
N
N
S
S
D.C. Motor
Main Pole
Inter-Pole
N
S
NS
Main Field
Field by inter poles
Armature Field
Main Field
Field by inter poles
o There are mainly two functions of inter-poles:-
1. As their polarity is the same as that of the leading pole, they induce an e.m.f in the coil(under commutation) which helps the reversal of current. The e.m.f induced by the inter-poles is known as reversing-commutating e.m.f. This e.m.f neutralizes the reactance e.m.f and makes commutation sparkless.
2. Another function of the inter-poles is to neutralize the cross-magnetizing effect of armature reaction. Hence, brushes are not be shifted from the original position.
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