What is the control principle of variable frequency motor

Common variable frequency motors include: three-phase asynchronous motors, DC brushless motors, AC brushless motors and switched reluctance motors. What is the control principle of variable frequency motor?
The control principle of frequency conversion motor
Generally speaking, the control strategy of the variable frequency motor is: constant torque control at the basic speed, constant power control above the basic speed, and field weakening control in the ultra-high-speed range.
Basic speed: Since the inverter motor will generate back electromotive force when running, the magnitude of the back electromotive force is usually proportional to the rotational speed. Therefore, when the variable frequency motor runs to a certain speed, the speed at this time is called the basic speed, because the back EMF is the same as the applied voltage.
What is the control principle of variable frequency motor?
Constant torque control: The variable frequency motor performs constant torque control at the basic speed. At this time, the back EMF of the variable frequency motor is proportional to the speed of the variable frequency motor. In addition, the output power of the frequency conversion motor is proportional to the product of the torque and the rotational speed of the frequency conversion motor, so the power of the frequency conversion motor is proportional to the rotational speed.
Constant power control: When the frequency conversion motor exceeds the basic speed, by adjusting the excitation current of the frequency conversion motor, the back electromotive force of the frequency conversion motor is basically kept constant, thereby increasing the speed of the frequency conversion motor. At this time, the output power of the variable frequency motor remains basically unchanged, but the torque of the variable frequency motor decreases inversely proportional to the rotational speed.
Field weakening control: When the speed of the variable frequency motor exceeds a certain value, the excitation current is already very small, and it is basically impossible to adjust. At this time, it enters the weakening control stage.
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