Armature Reaction in DC machines

Definition

Armature reaction is the effect of magnetic field set up by the armature current on the distribution of flux under the main poles of a dc machine either generator or motor.

Effects of armature current on the distribution of flux under main poles 

For simplicity and convenience ,we consider an armature rotating in clockwise direction in the bipolar field.

Distribution of Flux Due to Main poles when generators is supplying no load

when the field winding is energized and the generator is supplying no load,the brushes will make contact with the conductors lying in the magnetic neutral plane(MNP) which coincides with geometrical neutral plane(GNP).vector OA represents main mmf both in magnitude as well as in direction,producing the main field flux.currents flowing in the armature conductors creates a magnetizing effect which acts as at right angles to main field flux.Thus,this effect of magnetizing action of the armature currents is called cross-magnetization.

magnetic neutral plane:The plane through the axis,along which no emf is induced in the armature conductors.

Fig:Distribution of Flux Due to Main poles when Generator is supplying No Load

Distribution of Flux Due to Armature current carrying conductors while Field coils carrying No current

When the field coils are not energized and generator is supplying load the direction of the current in armature conductors is same.The direction of current can be determined by applying Flemings right hand rule. Flemings right hand rule states that current in all the conductors under N-pole inward(shown by X) and outward in all the those conductors lying under s-pole(shown by .).Vector OB represents the mmf in both magnitude and direction i.e flux produces due to armature current carrying conductors will be perpendicular to the polar axis or parallel to the neutral axis.cross-magnetization is present only when armature current is flowing and the amount of cross-magentization so produced is proportional to the amount of current flowing through the armature conductors.

Fig:Distribution of Flux Due to Armature current carrying conductors while Field coils carrying No current

Distribution of Resultant Flux Established From simultaneous Action of Field and Armature current

When the generator is supplying the load,the main mmf is downward represented by vector OA and mmf produced due to armature current is from right to left represented by vector OB.The resultant mmf is OC. The magnetic neutral plane which is always perpendicular to the resultant mmf OC will be shifted round in the direction of rotation.The magnitude of shift depends upon the length of the vector OB and also on magnitude of the armature current.

Fig:Distribution of Resultant Flux Established From simultaneous Action of Field and Armature current

Each of the lines of force produces by the armature crosses the air gap twice .The field strength in the gap is weakened under the leading pole tips and strengthened under the trailing pole tips.The flux density in the air gap is practically uniform when there is no current in the armature which is shown by I and the flux density in the air gap varies when the current is flowing in armature conductors shown by II.

The magnetic neutral axis will be shifted along the magnetic neutral plane also the brushes will shift along the mnp to have sparkless commutation.If the brushes is not shifted along mnp and left along gnp the coils beings short circuited by brushes would cut the flux and therefore generate emf causing an arc to form.If the load on the generator changes the amount of distortion of field or shifting of the magnetic neutral plane varies.Therefore the effect of the action of the armature mmf makes a shifting of the brushes with change in load to secure sparkless commutation.The distribution of current in armature conductors is changed due to the shifting of the brushes.

Distribution of current in the armature conductors when placed along the MNP

The total armature mmf lies along the mnp and is no longer at right angle to the main field.The armature mmf is now no longer in cross-magnetization effect but is partly directed against the main field.The demagnetising component of armature mmf results in weakened field flux causes in lowering the generated emf and the action is called armature reaction.

  • cross-magnetising effects of armature reaction distorts the field in the air gap.
  • The effects caused by the distortion of the main field is:
  1. creation of magnetic field in the inter polar region.
  2. weakened field strength in the air gap under leading pole tips and strengthen field under the trailing pole tips.
  • The distortion of main field under load leads to increased in iron losses ,poor commutations.
  • The iron losses depends upon the maximum value of flux density B.The iron losses at the load is more than those on no load.
  • The demagnetizing effect of armature reaction reduces the total flux per pole.The decrease in the flux due to armature reaction on load reduces the magnitude of generated emf in case of a generator and electromagnetic torque developed in case of motor.
  • The effect of armature reaction on the main field flux are of opposite sign for generator and motor.

Remedies for Field Distortion

The shift of the magnetic neutral plane in the direction of rotation affects the commutation.The brushes must be shifted back and forth as the load changes because the effect of the armature reaction depends upon the value of the armature current.

  • To equalize the distribution of magnetic flux: Making the trailing horn of the pole piece longer than the advancing horn and cutting farther from the surface of armature.
  • To reduce the distortion of the resultant flux density wave:Flattening of the pole faces slightly so that the air gap is longer at the pole tips than it is at the center of the pole.
  • The air-gap length must be increased which cause the reluctance to increases and more ampere turns are required in the field windings.
  • By compensating the windings.
  • By reducing the cross section of the pole-pieces.

Let total number of armature conductors=Z

Number of parallel paths=A

Angle of lead of brush from gnp=θ*in mechanical degrees

Number of poles=P

Armature current=Ia

current per conductors,Ic=Ia/A

Total armature-amperes-turns=Total armature ampere-conductors/2

=ZIc/2

Total armature amperes-turns/pole=ZIc/2P

 

Total demagnetising conductors=conductors lying within angles AOC and BOD

=Z/360 [<AOC+<BOD]

=(Z/360)* 4θ

Total demagnetising turns=(1/2)* (Z/360)* 4θ    =(2θ/360)* Z                  [ 1 turns=2 conductors]

Demagnetising ampere-turns/pole,ATd=(1/2)* (2θ/360)* ZIc

=(ZθIc)/360

cross-magnetising ampere-turns/pole,ATc

=AT-ATd

=(ZIc/2p)-(θZIc/360)

=ZIc[(1/2P)-(θ/360)]      ……………………………………………………….(i)

 

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