In drawing the diagram, the reactance drop is
subtracted from the terminal voltage to obtain the
internal voltage and the direction of the
quadrature and direct axes. The internal voltage
sets the level of flux in the motor, which, in turn,
sets the mmf required (ampere-turns or
magnetizing current). The current flowing in the
armature winding produces an mmf called
direct axis, must be added (or subtracted, for
lagging P.F.) to the mmf, producing the internal
voltage. Since the field winding on the rotor poles
is in line with the direct axis, dc in this winding
can supply the required mmf
armature reaction. The portion, in line with the