6.2.3. Armature reaction (rotor reaction)
Mode of operation
The magnetic main field of the direct current machine runs symmetrically from the north to the south pole (Figure 92 (1)) in the stator. The current-saturated rotor winding generates a second magnetic field which runs vertically (transverse) to the main field (Figure 92 (2)).
(1) Main field of the currentless rotor, (2) Armature transverse field, (3) Resultant overall field
Despite rotor rotation the transverse field remains motionless and its value only changes in proportion to the current intensity in the rotor winding.
The armature transverse field superimposes itself with the main field to a resulting field (Figure 92 (3)) whose neutral zone has been displaced with regard to the main field.
The neutral zone is displaced by the armature transverse field. Its rotation ensues in the generator in rotational direction and, in the motor, contrary to the rotational direction of the rotor.
A displacement of the neutral zone leads to brush sparking. Furthermore, the armature reaction weakens the main field and this, in turn, paves the way for a decrease in rotor-induced voltage.
The negative influence of the armature transverse field can be overcome by superimposing a counter-directed magnetic field. So-called interpoles (Cp. Figure 93) are established between the main poles.
1 Generator operation
The interpole windings are so switched that their magnetic field is counter-positioned to the armature transverse field. In this motor, therefore, a principal north pole is followed by a north interpole in rotor rotational sense.
The interpole shall generate a magnetic field ouncterpositioned to the armature transverse field. The interpoles are in the neutral zone. Interpole and rotor windings have been series connected,
The armature transverse field can also be compensated by a magnetic field which is generated by the so-called compensation winding.
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