Voltage shrinkage at higher speed around 0.3 of rated of speed and i'm computing line to line voltage from PWM which i captured using logic analyzer.

Hey I'm controlling pmsm motor using FOC , i observed at that higher speed the duty cycle as well as line to line voltage is shriniking what can be the possible reason behind that ?

the pink region is sinousodul waveforms of line to line voltage as you can see that at higher speed it shrinked.