Armature reaction is the effect of a 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 a clockwise direction in the bipolar field.
Distribution of Flux Due to Main poles when generators are 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 the geometrical neutral plane (GNP). The vector OA represents the main mmf both in magnitude as well as in direction, producing the main field flux. The currents flowing in the armature conductors create a magnetizing effect that acts at right angles to the main field flux. Thus, this effect of the 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.
Distribution of Flux Due to Armature current carrying conductors while Field coils carrying No Current
When the field coils are not energized and the generator is supplying loads, the direction of the current in the armature conductors is the same. The direction of the current can be determined by applying Fleming’s right-hand rule. Fleming’s right-hand rule states that current flows inward in all the conductors under the N-pole (shown by X) and outward in all the conductors lying under the s-pole (shown by.). V
ector OB represents the mmf in both magnitude and direction, i.e., the flux produced 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 the armature current is flowing, and the amount of cross-magnetization so produced is proportional to the amount of current flowing through the armature conductors.
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 the 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 in the direction of rotation. The magnitude of the shift depends on the length of the vector OB and also on the magnitude of the armature current.
Each of the lines of force produced 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, as shown by I, and the flux density in the air gap varies when the current is flowing in the armature conductors, as shown by II.
The magnetic neutral axis will be shifted along the magnetic neutral plane, and the brushes will shift along the mnp to have sparkles commutation. If the brushes are not shifted along mnp and left along gnp, the coils being short-circuited by the 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 the field or shifting of the magnetic neutral plane varies. Therefore, the effect of the action of the armature mmf makes shifting of the brushes with a change in load to secure spark commutation. The distribution of current in armature conductors changes 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 a 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 demagnetizing component of armature mmf results in weakened field flux, which lowers the generated emf, and the action is called the armature reaction.
- cross-magnetizing effects of armature reaction distort the field in the air gap.
- The effects caused by the distortion of the main field are:
- creation of a magnetic field in the inter-polar region.
- weakened field strength in the air gap under the leading pole tips and strengthen field under the trailing pole tips.
- The distortion of the main field under load leads to an increase in iron losses and poor commutations.
- The iron losses depend upon the maximum value of flux density B. The iron losses at the load are more than those on no load.
- The demagnetizing effect of the 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 the case of a generator and electromagnetic torque developed in the case of a motor.
- The effect of the armature reaction on the main field flux is of the opposite sign for the 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: Make the trailing horn of the pole piece longer than the advancing horn and cut farther from the surface of the armature.
- To reduce the distortion of the resultant flux density wave: Flattening 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 causes the reluctance to increase and more ampere-turns are required in the field windings.
- By compensating for the windings.
- By reducing the cross-section of the pole pieces.
Let the total number of armature conductors=Z
Number of parallel paths=A
The angle of lead of brush from gnp=θ*in mechanical degrees
Number of poles=P
current per conductors, Ic=Ia/A
Total armature-amperes-turns=Total armature ampere-conductors/2
Total armature amperes-turns/pole=ZIc/2P
Total demagnetizing conductors=conductors lying within angles AOC and BOD
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
cross-magnetizing ampere-turns/pole, ATc