When using Field Oriented Control, we control the torque by controlling the d and q axis currents. Q-axis component is the torque component and D-axis is the flux component. Below the base speed we can use zero reference for the D-axis current, i.e., id*=0. Above the base speed, in order to satisfy the voltage constraint (Vs,max^2>=vd^2+vq^2) we need to start adjusting the d-axis current in the negative direction and we enter the field weakening mode. Therefore, the torque control problem has two parts: generating the current references and controlling the current references. The way we tune the controller will have influence on the duration of the transient response. The torque accuracy will be influenced by the way we generate the current references. A good starting point in learning more about FOC and torque control of PMSMs can be found in Simscape Electrical.
PMSM Current Reference Generator:
PMSM Field-Weakening Controller:
PMSM Field-Oriented Control:
I hope this helps.
