In a DC motor, the spatial position of the main magnetic flux and armature current distribution is determined and can be controlled independently. The magnetic flux of an AC asynchronous motor is generated by the combination of the stator and rotor currents, and its spatial position is relative to the stator and rotor. Moving, in addition to this, in a squirrel induction motor, the rotor current is still unmeasurable and uncontrollable.
Therefore, the dynamic mathematical model of the asynchronous motor is much more complicated than that of the DC motor, and for a long time, people have not been able to understand its precise expression.
Fortunately, many mechanical loads, such as fans and pumps, do not require high dynamic performance, as long as high-efficiency speed regulation can be achieved within a certain range, so the control system can be designed only based on the steady-state model of the motor.
The steady-state mathematical model of asynchronous motor is described in the first lecture. In order to realize the coordinated control of voltage and frequency, a control scheme of open-loop constant voltage-frequency ratio with low-frequency voltage compensation can be adopted, which is the commonly used general-purpose inverter control system.