Once the synchronous motor is synchronized,
the field poles on the rotor are in line with the
rotating magnetic poles of the stator. If dc is
applied to the rotor pole windings, the rotor can
supply the necessary ampere turns to generate the
flux which produces the internal motor voltage.
Thus, the field current can replace part or all of the
magnetizing current. In fact, if more dc field
current is supplied, the increased flux will try to
increase the line voltage. To increase the line
voltage, the motor will supply ac magnetizing
current to all “magnets” on the system to increase
their magnetic flux. This is leading power factor.