In order to understand the armature reaction a two pole dc machine is taken for study. When field winding and is supplied with power, magnetic poles starts producing magnetic filed lines (magnetic flux). This magnetic flux originate from north pole and will pass through the armature of the dc machine and reaches the south pole. Under no load, dc machine armature draws only little current to supply I2R losses. This low current produces a flux which is of no significant.
Figure (i) shows the filed lines (blue lines and red arrows shows the flow of flux lines) passing from north pole to south pole of the dc machine through armature. During this period, armature is not energized. Figure (ii) shows the no load operation of the dc machine. During no load motor draws very little current. This armature current produces a magnetic field according to ampere right rule. Direction of magnetic flux lines generated by armature is shown in green arrows). Under no load condition of the dc machine, magnetic neutral axis (MNA) and geometric neutral axis (GNA) coincides with each other. This can be seen in figure (i)
DC machine under load:
When dc machine is loaded, it draws more current from the supply mains. This currents produces a magnetic significant to distort the main magnetic field of the machine. Figure (ii) shows the armature flux
distorting the main filed at trailing pole tip and aiding at leading time. This results in shifting of MNA from GNA.
Cross Magnetizing effect of armature reaction:
The cross-magnetization effect of armature reaction distorts the field of the air gap between armature and field poles. The figure (iii) we can observe that the distortion of air gap in such a way that:
- The magnetic field is created in the iner-polar region where the brushes are placed for commutation
- Weakening of field strength in the air gap under leading pole tips and strengthening of air gap under trailing pole tips
The above two effects causes the distortion of the main field under load, leading to increase in iron losses, poor commutation or even sparking at the commutation surface.
Increase in Iron losses:
The iron losses depends on the maximum value of the flux density (B). The increase in the iron losses due to increase in flux density under one half of the pole arc is much more greater than the decrease in the flux density in another half. This results in increase in iron losses more under loaded condition than during no load operation (usually iron losses remain same during different loading conditions when no armature reaction exist)
For good commutation, the coils short circuited by brushes should have zero emf induced in them. The brushes are usually placed in the inter-polar axis. Since zero crossing of the flux density wave shifted from inter-polar axis, the coils undergoing commutation will have certain value of induced emf. Thus there will be delay in the reversal of current in the short circuited coil which may cause sparking.
Under heavy loads the flux density wave distorts significantly. If the two induced coil sides of the coil are under the points of the maximum flux density, a much greater voltage may be induced in the coil. If the voltage between adjacent commutator segments exceeds 30 to 40 volts, a spark may occur between the adjacent commutator segments. Sometimes, this spark may be too large that it may spread around the commutator in the form of ring fire.
Demagnetizing effect of armature reaction:
The demagnetization effect of armature reaction reduces the total flux per pole from its no load value due to magnetic saturation. Reduction in flux depends on the degree of saturation. The reduction in the main flux due to armature reaction may be about 10% of the total main flux.
The decrease in the flux due to armature reaction reduces the magnitude of the generated emf (results in the reduction of the terminal voltage) in case of generator and decrease in the electro-magnetic torque in case developed in case of dc motor