ARMATUREREACTION MADE BY : DEEPAK (006) RUPESH JAIN (031) SIDHARTH SHARMA (029)
Basic terminology of a DC generator• A Generator is a machine which converts mechanical energy into electrical energy.
Stator: The stator is the fixed part of the generator thatsupplies the magnetic field in which the coils rotate. It mayconsist of two permanent magnets with opposite poles facingand shaped to fit around the rotor.Rotor: In its simplest form, the rotor is a part of generatormade to rotate within a magnetic field. In actual practice, thearmature is usually referred to as rotor.
ARMATURE REACTION:All current-carrying conductors producemagnetic fields. The magnetic field produced bycurrent in the armature of a DC generatoraffects the flux pattern and distorts the mainfield. This distortion causes a shift in the neutralplane, which affects commutation. This changein the neutral plane and the reaction of themagnetic field is called ARMATURE REACTION.
Armature reaction can be better understood with this figure:-
UNDESIRABLE EFFECTS OF ARMATURE REACTION1. Armature reaction causes a net reduction in the field flux per pole. Due to this net flux decrease, induced armature e.m.f. decreases and also the torque decreases.2. Distortion of the main field flux along the air gap i.e. MNA axis shifted. Due to this there is a problem of commutation which results in copper losses, iron losses, sparking etc.
Methods To Reduce Armature Reaction1. By high Reluctance at POLE TIPS:At the time of construction we use chamfered poles. Thesepoles have larger air gap on the tips and smaller air gap at thecentre. These poles provide non-uniform air gap. The effect ofarmature reaction is more near to edge of poles andnegligible near the centre of pole.If air gap is kept non uniform i.e., larger air gap at theedges(Pole Tip) and smaller near the centre of the pole andthen armature flux near the pole tip decreases and armaturereaction decreases.
2. By Laminated Pole Shoe:We insert Laminated objects in the pole. By having Laminatedpole shoe the reluctance in the armature flux pathincreases. Hence the armature flux gap gets reduced.
3. By Reduction in Armature flux:The effect of Armature Reaction is reduced by creatingmore reluctance in the path of Armature flux. This isachieved by using field Pole Laminations having severalRectangular holes punched in them. It gives highReluctance in the path of armature flux. Due to thisarmature cross flux reduces whereas main field remainsalmost unaffected.
4. By having Strong main magnetic field:During the design of DC machine it should be ensured that themain field m.m.f. is sufficiently strong in comparison with fullload armature flux.Greater the main field, lesser will be the distortion.
5) By using Inter Poles:The effect of Armature reaction is reduced by interpolesplaced in between the main poles. The magnetic axis ofinterpoles is in the Direction of q-axis. Interpoleswindings are connected in series with the armaturewinding. So that, interpoles flux is able to neutralize theeffect of armature flux. The interpoles are narrow andtapered with large air gap.
6. By Using Compensating Windings:The Armature Reaction can also be limited bycompensating windings. It is the best and mostexpensive methods. The compensating windings areconnected in series with armature winding.