Armature reaction in d.c machines –
The armature m.m.f. produces two undesirable effects on the main field flux and these are :
(1) net reduction in the main field flux
(2) distortion of the main field flux
Effects of armature reaction in d.c machine
(1) iron losses – These losses depend on the maximum value of flux density in teeth and in the pole shoes. The armature reaction, by distorting the main field flux waverorm, increases the flux density considerably over its corresponding no-load value. As a result ; iron losses, particularly in teeth, are much greater on load than on no-load. In addition, high degree of saturation in teeth forces the flux to stray into the core-end plates, end covers etc. This all leads to more eddy-current and hysteresis losses. Roughly, iron losses at full-load is taken to be 1.5 times its value at no-load.
(2) Commutation – At no-load, zero-crossing of the flux density wave is along the GNA such as point C, C’. Under loaded conditions of the D.C. machine, zero-crossing of the flux
density wave is shifted by an angle theta, which depends on the magnitude of armature current.
For good commutation, the coils short-cireuited by the brushes should have zero e.m.f. induced In them. The brushes are usually placed along the GNA. Since zero-crossing of the flux density wave is shifted from GNA or q-axis, the coils undergoing commutation do not have zero e.m.f. induced in them. The induced e.m.f. in the commutated coils delays the l
reversal ot armature current in the short-circuited coils ; this may result in detrimental sparking, or poor commutation, at the brushes.
(3) Sparking – Under heavy load, the flux density waveform is distorted considerably. If the two coil-sides of a coil are under the maximum flux density points, a much greater voltage (infinity Blv) may be generated in this coil. If this rotational voltage between adjacent commutator segments exceeds 30 or 40 V, a spark may occur between these adjacent segments.
Sometimes, this spark may spread around the commutator in the form of a ring fire.
(4) Cost of field winding – Demagnetizing effect of cross-magnetizing armature m.m.f.
is to reduce the total flux per pole from its no-load value due to magnetic saturation. In a generator, the magnitude of the e.m.f. generated in the armature decreases with increase in load. In a motor, electromagnetic torque is decreased as the flux per pole is reduced under
load. In order to compensate for this reduction in total flux, the field m.m.f. must be augmented. This is possible (i) by increasing the number of turns in the field winding or (ii) by using a thick wire for field winding. Either of these schemes entails more copper and, therefore, more cost of the field winding.
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