What is Armature Reaction?
In a DC Machine, the main field is produced by field coils. In both the generating and motoring modes, the armature carries current and a magnetic field is established, which is called the armature flux. The effect of armature flux on the main field is called the armature reaction.
Effects of Armature Reaction on Field Flux:
The armature reaction produces two effects :
- Demagnetization Effect
- Cross Magnetization Effect
How to overcome Demagnetizing and Cross magnetizing effects?
The demagnetizing effect can be overcome by adding extra amp-turns on the main field winding. The cross magnetizing effect can be reduced by providing inter poles between the gap of main poles. The brushes of a generator must be set in the neutral plane; that is, they must contact segments of the commutator that are connected to armature coils having no induced emf. If the brushes were contacting commutator segments outside the neutral plane, they would short-circuit "live" coils and cause arcing and loss of power. Without armature reaction, the magnetic neutral axis (MNA) would coincide with the geometrical neutral axis (GNA). Armature reaction causes the neutral plane to shift in the direction of rotation, and if the brushes are in the neutral plane at no load, that is, when no armature current is flowing, they will not be in the neutral plane when the armature current is flowing. For this reason, it is desirable to incorporate a corrective system into the generator design.
Two principal methods by which the effect of armature reaction is overcome. The first method is to shift the position of the brushes so that they are in the neutral plane when the generator is producing its normal load current. in the other method, special field poles, called inter-poles, are installed in the generator to counteract the effect of the armature reaction.
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| Armature Reaction in DC Machines |