Saturday, January 9, 2016
Armature Reaction In DC Machines [Motor & Generator]
What is Armature Reaction?
The effect of magnetic field set up by the armature current on the distribution of flux under the main poles of a DC generator or a DC motor is known as armature reaction.
The armature m.m.f produces two undesirable effects on the main field flux. They are,
1. Distortion of the main field flux wave along the air gap periphery.
2. Net reduction in the main field flux per pole.
Reduction in main field flux per pole reduces the generated voltage and torque, whereas distortion of main field flux gives three harmful effects. They are increase in iron losses, poor commutation and sparking.
Consider a two-pole machine as shown in figure [in graph] at no-load i.e., having no armature currents. The main field flux is shown on a horizontal phasor OA which is produced by field m.m.f (IfNf).
MNA (Magnetic Neutral Axis) :
MNA (Magnetic Neutral Axis) may be defined as the axis along which no emf is generated in the armature conductors as they move parallel to the flux lines. Brushes are always placed along MNA because reversal of current in the armature conductors takes place along this axis.MNP refers to magnetic neutral point which is a point on MNA
GNA (Geometrical Neutral Axis):
GNA (Geometrical Neutral Axis) may be defined as the axis which is perpendicular to the stator field axis.GNP refers to geometrical neutral point which is a point on GNA
The geometrical neutral plane and the magnetic neutral plane are coincident at no-load, i.e., magnetic lines of force intersect the MNA at right angles.
If D.C machine is loaded then the armature winding receives the current. These currents are shown in figure by dots under south pole and by crosses under north pole. These currents setup armature flux. Armature flux (φa)is shown by a vertical phasor OB.φa is produced by armature m.m.f IaNa. If the D.C machine is working as generator, then its armature must be driven clockwise by prime mover and anti-clockwise for motoring operation.
From above figure it is seen that φa is perpendicular to φf i.e., the path of armature flux crosses the path of main field flux.This effect is known as cross magnetizing effect.
If the current is flowing in both the windings, the resultant flux distribution is obtained by super imposing the two fluxes as shown in below figure.
From above, it is clear that armature flux aids the main field flux at upper end of north pole and at lower end of south pole. The DC machine practically gets saturated and the strengthening effect is very low as compared with weakening effect and the resultant flux get decreased from its no-load value.
This effect on armature flux is called demagnetizing effect. This effect reduces the total flux/pole and found to be 1 to 5% from Its no-load to full—load.
How To Reduce Armature Reaction?
Usually, no methods employed for small machines (up to few kilowatts) to reduce the armature reaction. But for large DC machines, interpoles and compensating winding are used to reduce armature reaction.
Methods to Reduce Armature Reaction
(a) High Reluctance Pole Tips
By flattening the pole faces slightly so that the air gap is longer at the pole tips rather than at the center of the pole results in increase in reluctance of the pole tips and the magnitude of armature cross flux is reduced and the distortion of the resultant flux density wave is minimized. This can be achieved by using chamfered or eccentric pole face.
(b) Reduction in Armature Flux
To reduce armature cross flux without reducing the a main field flux, it is required to create more reluctance in the path of armature flux. This is done by using field pole laminations.
The reluctance offered to armature flux is more pronounced due to four air gap openings introduced in the path of cross flux.
(c) Strong Main Field Flux
During the design of DC machine, it should be ensured that main field m.m.f should be strong when compared to full load armature m.m.f. The distortion produced by armature cross flux can be minimized by increasing the ratio of main field m.m.f to the full load armature m.m.f.
(d) Interpoles
Interpoles are small poles placed in between the main poles. These are connected in series with armature,so that they carry armature current. The e.m.f induced by the interpoles neutralizes the effect of armature m.m.f in the intelpolar region,. thus making commutation sparkless.
(e) By Using Compensating Winding
A compensating winding is an auxiliary winding embedded in slots located in the faces of main poles. This winding is connected in series with armature in such a manner so that the direction of current flowing in this winding should be quite opposite to the direction of current flowing in the armature conductor the m.m.f produced by the compensating winding should be equal to the m.m.f produced by the armature conductors. To maintain a uniform distribution of flux in the main poles and to neutralize the effect of armature reaction. compensating windings are provided. This winding adds cost of the machine and doubles the armature copper loss. but it makes the machine to withstand the most violent fluctuations of load that is applied to it.