1. SVMIT ELECTRICAL Prepared By : - Rohan Kapadia (016) Bhargav Khambhata (017) Pradeep Malaviya (018) Hemik Patel (019) Snehal Mistry (020)

2. Presentation on ARMATURE REACTION

3.  Armature Reaction  The effect of magnetic field set up by armature current on the distribution of flux under main poles of a generator. The armature magnetic field has two effects: (i) It demagnetises or weakens the main flux and (ii) It cross-magnetises or distorts it.  This results in reduced generated voltage and the sparking at the brushes.

4. Terms used in armature reaction :-  Geometrical Neutral Axis It is the axis which is perpendicular to the centre lines of poles.  Magnetic Neutral Axis The axis along which no emf is produced in the armature conductors.  Leading Pole Tip (LPT) It is the end of the pole which comes first in contact with armature in the direction of rotation by the armature conductors.  Trailing Pole Tip (TPT) It is the end of the pole which comes later with the armature in the direction of rotation.

5. Process  Consider bipolar dc generator running in the clockwise direction. When there is no load connected to the generator, only the flux due to field pole is established by mmf produced by the field current and there is no flux due to the armature. This flux is uniform in the air gap and the GNA coincides with the MNA.  The flux at the leading pole tip(LPT) is equal to the flux at the trailing pole tip.  The main pole flux distribution on no-load is shown in the fig. NO-LOAD CONDITION :-

6.  The Fig. shows the flux due to the armature current and there is no flux due to the field poles.  The current flowing in the armature conductors creates armature mmf, which acts at 90 o main field flux axis and is known as cross magnetizing mmf.  Armature Flux Distribution :-

7. ON LOAD CONDITION :-

8.  It is clear fro m the Fig. that he flux at the LPT has decreased while at the TPT has increased.However, as the field poles work near their saturation point and the decrease of the flux at the leading pole tip(LPT) is much more than the increase of flux at the TPT, there is a net decrease in the total flux and the emf induced also decreases.  Due to the strengthening of the TPT and weakening of the LPT, the MNA is advanced from the GNA in the direction of rotation. This effect goes on increasing as the armature current increases with increase of load.  If the brushes are not shifted to this new position of the MNA, heavy sparking can takes place at the brushes.

9.  Effects  It can be concluded that the armature flux due to armature current under load conditions produces the following effects…… It decreases the efficiency of the machine. It produces sparking at the brushes. It produces a demagnetizing effect on the main flux. It produces the cross magnetizing effect and hence distorts the uniformity of the main flux. It reduces the emf induced. Self excited generators some times fail to build up emf.  The demagnetizing and distorting effects of armature reaction will increase with increase in load or armature current.

10.  REMEDIES  Brushes must be shifted to the new position of the MNA  Extra turns in the field winding  Slots are made on the tips to increase the reluctance  The laminated cores of the shoe are staggered  In big machines the compensating winding at pole shoes produces a flux which just opposes the armature mmf flux automatically.

11.  Methods To Reduce Armature Reaction

12. 1. By high Reluctance at POLE TIPS: At the time of construction we use chamfered poles. These poles have larger air gap on the tips and smaller air gap at the centre. These poles provide non-uniform air gap. The effect of armature reaction is more near to edge of poles and negligible near the centre of pole. If air gap is kept non uniform i.e., larger air gap at the edges(Pole Tip) and smaller near the centre of the pole and then armature flux near the pole tip decreases and armature reaction decreases.

13. 2. By Laminated Pole Shoe: We insert Laminated objects in the pole. By having Laminated pole shoe the reluctance in the armature flux path increases. Hence the armature flux gap gets reduced.

14. 3. By Reduction in Armature flux: The effect of Armature Reaction is reduced by creating more reluctance in the path of Armature flux. This is achieved by using field Pole Laminations having several Rectangular holes punched in them. It gives high Reluctance in the path of armature flux. Due to this armature cross flux reduces whereas main field remains almost unaffected.

15. 4. By having Strong main magnetic field: During the design of DC machine it should be ensured that the main field m.m.f. is sufficiently strong in comparison with full load armature flux. Greater the main field, lesser will be the distortion.

16. 5) By using Inter Poles: The effect of Armature reaction is reduced by interpoles placed in between the main poles. Interpoles windings are connected in series with the armature winding. So that, interpoles flux is able to neutralize the effect of armature flux. The interpoles are narrow and tapered with large air gap. Interpoles are added [NN & SS]

17.  Demagnetising Ampere Turns/pole: 

18. To neutralize the demagnetizing effect of armature reaction, an extra amp-turns (AT) are added to he main field winding. Hence it is essential to calculate the number of extra ampere turns(AT) required. The exact number of conductors which produce demagnetizing effect are shown in the fig. where the brush axis is given a forward lead of Ɵ m. So as to lie along the new axis of MNA. The flux produced by the current carrying conductors laying in between the angles POR & QOS is such that, it opposes the main flux and hence they are called as demagnetizing armature conductors. Let, Z = total number of conductors T = number of turns = ( because each turn consists of two armature conductors ) I = Current in each armature conductor for simplex lap winding for simplex wave winding

19. Ɵ m = forward lead in mechanical or angular degrees. Total number of armature conductors within the angles POR & QOS = Since each turn consists of two conductors, the total number of turns between these angles = Demagnetizing AT per pair of poles = Demagnetizing ST per pole (At d per pole ) =

20.  Cross magnetizing Ampere Turns/pole:

21. As shown in fig. the armature conductors laying in between the angles AOC & BOC are carrying the current in such a away that the direction of the flux is downwards i.e. at right angles to the main flux. This results in the distortion in the main flux. Hence these conductors are called cross-magnetizing or distorting ampere conductors. Total armature conductors/pole = Demagnetizing conductors/pole = > Cross magnetizing conductors/pole = > Cross magnetizing ampere conductors per pole =

22. > Cross magnetizing AT per pole (AT c /pole) = ( because one turn consists of two conductors) No. of Extra turns/pole to neutralize the demagnetizing effect = for shunt wound generator = for series wound generator

23.  Compensating Windings The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature in such a way that the current in them flows in opposite direction to that flowing in armature conductors directly below the pole shoes.

24. The series-connected compensating windings produce a magnetic field, which varies directly with armature current. As the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field.

25. Thank You…..