McPherson, George, Jr. Department of Electrical Engineering, School of Engineering, University of Missouri, Columbia, Missouri.
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The effects of the magnetomotive force (mmf) caused by the current flow in the armature windings on the air-gap field of direct-current (dc) and synchronous alternating-current (ac) machines. Since armature current depends on the electrical or mechanical load on the machine, armature-reaction effects are therefore load-dependent. In dc machines, the armature reaction causes a distorted flux-density distribution in the air gap. Figure 1 shows the magnetic field produced in the air gap of a two-pole dc motor or generator by (a) the field mmf, (b) the armature mmf, and (c) both armature and field mmf acting together. Figure 2 shows the distorted flux distribution in the air gap, curves a, b, and c corresponding to the above three cases. Due to the saturation of the armature teeth, the flux density is decreased by a greater amount under the pole tip e than it is increased under pole tip f, and therefore the armature reaction produces a demagnetizing effect, and the generated voltage or countervoltage will be reduced when the armature is loaded. In a generator this degrades the voltage regulation. In a motor it tends to increase the speed and may cause instability.
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