For normal field strength and constant torque capability, the ratio of the output voltage to output frequency (V/Hz) is nominally constant. At output frequencies above the field weakening point, the V/Hz and (thus the field strength) is reduced as the frequency increases. Usually, that is the point at which the nominal output voltage is equal to the nominal input voltage and the VFD doesn't have the capacity to keep increasing voltage with further increase in frequency.
With field weakened operation, the torque capability is reduced as the frequency (speed) increases. In comparison to speed, the torque capability drops more rapidly than the drop described by a constant horsepower torque vs. speed curve. However, constant horsepower operation can be maintained up to some point like 150% of base speed or so.
