current id is caused to flow closely to the region preceding to the saturation it can be considered that the operating point lies on the straight line portion of the characteristic curve representing the linear relation between the field current if and the flux linkage u03c8 see the point m u00b7 if shown in fig2 to say in another way with the field weakening control based on the d axis armature current id the field current if is effectively or virtually lowered whereby the operating point is changed from the flux linkage u03c8o to u03c81 see fig2 which means that the magnetic saturation is virtually or effectively nullified in the foregoing description it has been presumed that the q axis armature current iq assumes a fixed value in this conjunction it is to be noted that by changing the phase difference angle u00f8 see fig1 with the q axis current value being held constant the flux linkage u03c8 and hence the torque te as produced can overall be increased although the q axis armature current iq decreases more or less because the influence of the magnetic saturation is mitigated by the d axis armature current id whereby the flux linkage u03c8 can correspondingly be increased next reference is made to fig3 which is a characteristic diagram graphically illustrating a characteristic relation between the phase difference angle u00f8 and the output power of the claw pole synchronous machine when it is operated as the generator i e in the generator operation mode more specifically there is shown in this figure the relation between the phase difference angle u00f8 and the output power pe as measured when only the phase difference angle u00f8 is changed in the state where the field weakening armature current id is being fed.