TITEL OF PRESENTATION :- ARMATURE REACTION PREPARATION NAME ENRL.ON :-  MR. KHUNT PANKAJ  MR. MAHALE PRADEEP  MR. PATEL SHUNIL  MR. PRAJAPATI PARTH  MR. RANA KETAN  MR. PATEL SOHAL  MR. TEJANI SUNIL BY GUIDED :- - Mr. ANIMESH D.TANDEL MAHATMA GANDHI INST. OF TECHNICAL EDUCATION & RESEARCH CENTRE, NAVSARI.

INTRODUCTION TO ARMATURE REACTION For the operation of any D.C. machine, presence of magnetic flux is essential. Whenever current flows through a field coil, it produces a flux which is called as field flux. Now suppose the D.C. machine is functioning as a generator, an e.m.f. will be induced in the armature when it is driven by a prime mover. If the generator is driving a load, the induced e.m.f. in the armature will cause current to flow through the load. Thus current will start flowing through the armature conductor. Now every current carrying conductor will set up its own magnetic field. All these armature conductor combine together to produce a flux which can be called armature flux. And the effect of this armature flux on the distribution of main flux is called armature reaction.

GEOMETRICAL AND MAGNETIC NEUTRAL AXIS :  GNA: The GNA is the axis that bisects the angle between the centre line of adjacent poles. Clearly it is the axis of symmetry between two adjacent poles.  MNA : MNA is the axis drawn perpendicular to the mean direction of the flux passing through the centre of the armature, clearly no emf is produced in the armature conductors along this axis, because then they cut no flux with no current in the armature condutors the MNA coincides with GNA.

WORKING: CASE : 1 – ONLY FLOWING FIELD CURRENT When the generator is on no-load, a small current flowing in the armature does not appreciably affect the main flux( F m ) coming from the main pole. Show the fig.1, the flux due to main poles, when the armature conductors carry no current. The flux across the air gap is uniform. The mmf producing the main flux is represented in magnitude and direction by the vector OF M.

CASE 2 :- ONLY FLOWING ARMATURE CURRENT : When the generator is loaded, the current flowing through armature conductors, the armature conductor to the left of GNA carry current ‘in side‘ [ × ], and those to the right carry current ‘out side’ [ ] of the direction of magnetic lines of force can be found by cork screw rule. It is directed downward parallel to the brush axis. The mmf producing the armature flux is represented in magnitude and direction by the vector OF A.

CASE 3 : COMBINED EFFECT OF TWO FIELDS : The flux due to the main poles and that due to current in armature conductors acting together. by superimposing Fm and Fa, we obtain the resulting flux Fo. It is clear that flux density at, the trailing pole tip is increased while at the leading pole tip it is decreased.

When MNA gets shifted through an angle , the brushes are also shifted along the new MNA. Due to this shift in brush position, the armature conductor and armature current gets redistributed. Some of the armature conductors which were under the N pole will now come under he S pole.The result is that armature mmf F A will no longer be vertically downward, but will be rotated in the direction of rotation through an angle  as shown in fig;. We can resolve the resultant mmf OF O in to two quadrature components namely OF D and OF C.

DISCUSS EFFECTS OF ARMATURE REACTION  Demagnetizes or weakens the main flux,  Cross- magnetizes or distorts the main flux. The component F D is in direct opposition to the mmf OF M due to main poles, it has a demagnetizing effect on the flux due to main poles, for this reason, it is called demagnetizing effect. The component F C is right angles to the mmf OF M due to main poles, it distorts the main field, for this reason it is called cross magnetizing effect. It may be noted that with the increase of armature current, both demagnetizing and cross magnetizing effects will increase.

CONCLUSIONS:  With brushes located along GNA ( = 0), there is no demagnetizing component of armature reaction F D = 0, there is only cross magnetizing effect of armature reaction.  With the brushes shifted from GNA armature reaction will have both demagnetizing and cross magnetizing effects their relative magnitudes depend on the amount of shift, this shift is directly proportional to the armature current,(  Ia ).  The demagnetizing component of armature reaction weakens the main flux, on the other hand the cross magnetizing component of armature reaction distorts the main flux.  The demagnetizing effect leads to reduced generated voltage while cross magnetizing effect leads to sparking at the brushes.