In a DC machine, two kinds of magnetic fluxes are present; Armature reaction. In a DC machine, two kinds of magnetic fluxes are present; 'armature flux' and 'main field flux'. The effect of armature flux on the main field flux is called as armature reaction.

Armature reaction.

Armature reaction. Consider there is no current in armature conductors, and only field winding is supplied In this case, the magnetic flux lines due to field poles are uniform and symmetrical to polar axis. And the 'Magnetic Neutral Axis' (M.N.A.) coincides with the 'Geometric Neutral Axis' (G.N.A.). Magnetic neutral axis may be defined as the axis along which no emf is produced, as the armature conductors moves parallel to the field flux lines. In case the machine is running, both the fluxes (flux due to armature conductors and flux due to field winding) are present. The armature flux superimposes with the main field flux, and hence disturbing the flux lines of main field flux This effect is called as armature reaction in DC machines.

The adverse effects of armature reaction: Armature reaction weakens (demagnetizes) the main flux. In case of generator, weakening of the main flux reduces the generated voltage. Armature reaction distorts the main flux (cross magnetization effect) , hence the position of M.N.A. gets shifted (M.N.A. is perpendicular to the flux lines of main field flux). Brushes should be placed on M.N.A., otherwise it will lead to sparking at the surface of brushes. So, due to armature reaction, its hard to determine the exact position of M.N.A.

In a DC machine, the carbon brushes are always placed at the magnetic neutral axis. In no load condition, the magnetic neutral axis coincides with the geometrical neutral axis. When the machine is loaded, the armature flux is directed along the inter polar axis (the axis in between the magnetic poles)and is triangular in wave shape. This results an armature current flux directed along the brush axis and causes cross magnetization of the main field. This cross magnetization effect results in the concentration of flux at the trailing pole tip in generator action and at the leading pole tip in motor action.

1. Brush Shift Methods to neutralize armature reaction Brush shift has serious limitations, Brushes have to be shifted to a new position every time the load changes or the direction of rotation changes or the mode of operation changes. Brush shift is limited only to very small machines. This method is generally not preferred.

2. Inter poles The limitation of brush shift has led to the use of inter poles in almost all the medium and large sized DC machines. Inter poles are long but narrow poles placed in the inter polar axis. They have the polarity of succeeding pole(coming next in sequence of rotation) in generator action and proceeding (which has passed behind in rotation sequence) pole in motor action. The inter pole is designed to neutralize the armature reaction mmf in the inter polar axis. This is because the direction of armature reaction m.m.f is in the inter polar axis. It also provides commutation voltage for the coil undergoing commutation such that the commutation voltage completely neutralizes the reactance voltage (L di/dt). Thus, no sparking takes place. Inter polar windings are always kept in series with armature, So inter polar winding carries the armature current ; therefore works satisfactorily irrespective of load, the direction of rotation or the mode of operation. Inter poles are made narrower to ensure that they influence only the coil undergoing commutation and its effect does not spread to the other coils. The base of the inter poles is made wider to avoid saturation and to improve response.

3. Compensating winding The effect of armature reaction can be neutralized by use of compensating winding. It is always placed in series with armature winding. The armature ampere conductors under pole shoe must be equal to compensating winding ampere conductors which will compensate armature m.m.f. perfectly

Compensating winding The compensating windings are used to neutralize the armature flux in the pole arc region which will otherwise cause sever distortion of main field flux. These windings are of concentric type and are placed in axial slots in the pole faces as sown in the Fig.

Compensating winding The current in this windings flow in opposite direction to that in armature conductors below the pole shoes. This will counterbalance the cross magnetizing effect of armature reaction which may cause flashover between the segments. To have perfect neutralization of armature m.m.f. under the pole shoe, the ampere conductors of compensating winding must be equal to total armature ampere conductors under the pole shoe.

Compensating winding