Armature Reaction

 Armature reaction

• Due to relative motion between armature conductors and field mmf there is an emf induced in armature conductors. This emf causes flow of current in armature conductors which cause armature flux. This flux is produced as a reaction to field flux and hence is called as Armature Reaction.

• Geometric Neutral Axis ( GNA ) is defined as the axis that is perpendicular to the field axis of the stator. Magnetic Neutral Axis ( MNA ) is defined as the axis perpendicular to the net flux that is flux due to field as well as armature mmf. The perpendicularity is taken in terms of electrical angle and not mechanical angle.

• It is well known that the brushes are placed on the MNA to collect maximum emf and also to ensure that the undergoing commutation have zero rotational voltage to prevent serious commutation problems.

• On no-load, the MNA coincides with the GNA because on no-load there is no armature current and armature reaction can be neglected so net flux is same as field flux.

• However, when the machine is loaded then the armature MMF that is triangular in wave shape and along the brush axis causes a cross magnetizing effect on the main field resulting in to concentration of flux on the trailing pole tips in generator action and leading pole tips in motor action.

• Leading Pole Tip is the tip of the pole encountered first by the armature in the direction of rotation and trailing pole tip is the tip which is encountered after the leading pole tip.

• Flux is strengthened where armature and field flux lie along the same direction and weakened where armature and field flux lie in opposite directions.

• So, magnetic field is strengthened at trailing pole tips and weakened at leading pole tips.

• If strengthened effect = weakening effect, the average flux under the pole remains same and therefore no demagnetizing effect of Armature Reaction. But due to saturation, strengthening effect < weakening effect so average flux under each pole reduces and hence Armature Reaction is demagnetizing.

• The Phasor Diagram for Armature Reaction is shown below :

• Consequently, the MNA is no longer on the GNA but shifts in the direction of rotation for generator action and is a direction opposite to rotation for motor action.

• Since, Armature Flux is perpendicular to Field Flux so the nature of Armature Reaction is cross magnetizing.

Effect of Cross Magnetizing Armature Reaction :-

• MNA shifts in direction of rotation in case of generator and opposite to direction of rotation in case of motor. The amount of shift ∝ Ia

• The iron losses are increased because Bmax is increased.

• Due to presence of air gap flux along the brush axis, an emf is induced in coil which is undergoing commutation, so commutation process id delayed, so sparking at brushes occurs.

• The ⲫ is maximum under trailing pole edge, so induced emf is maximum for conductors lying under this pole edge and if this induced emf increases beyond 30-40 V, the mica insulation fails, hence sparking takes place between adjacent commutator segments.