Constant torque control method for permanent magnet synchronous motor based on position sensor-less vector control

The invention discloses a constant torque control method for a permanent magnet synchronous motor based on position sensor-less vector control. The method includes the following steps: the motor is started and operated, a target current value iq_A of a q-axis is set to be equal to a current given value iq_limit, and constant torque control is achieved by utilizing a current PI control ring of theq-axis; when a torque command value is small or a load becomes heavy, a rotating speed decreases until a real-time detected rotating speed V is less than a preset reference rotating speed Vref1, a constant torque control mode is exited to enter constant rotating speed control, and the rotating speed is intervened through addition of extra current increment delta_iq based on the current given valueiq_limit of the q-axis to make the motor perform constant rotating speed control; and when the real-time detected rotating speed V rises and reaches a stable state, when a rotating speed difference Err is greater than a certa