There are four ways for frequency converters to control motors: constant U/f control, slip frequency control, vector control, and direct torque control.
I. Constant U/f control
U/f control changes the motor's power supply voltage while simultaneously altering the motor's power supply frequency, maintaining a constant magnetic flux and ensuring that the motor's efficiency and power factor do not decrease over a wide speed range. Because it controls the ratio of voltage to frequency, it's called U/f control. The main problems with constant U/f control are poor low-speed performance; at extremely low speeds, the electromagnetic torque cannot overcome the significant static friction, failing to properly adjust the motor's torque compensation and adapt to changes in load torque. Secondly, it cannot accurately control the motor's actual speed. Since constant U/f frequency converters use open-loop speed control, as shown in the asynchronous motor's mechanical characteristic diagram, the setpoint is the stator frequency, i.e., the ideal no-load speed. However, the actual motor speed is determined by the slip rate. Therefore, the stability error inherent in constant U/f control cannot be controlled, thus preventing accurate control of the motor's actual speed.
II. Slip Frequency Control
Slip frequency is the frequency difference between the frequency of the AC power supply applied to the motor and the motor speed. According to the mathematical model of asynchronous motor stability, when the frequency is constant, the electromagnetic torque of the asynchronous motor is proportional to the slip rate, and the mechanical characteristic is linear.
Slip frequency control controls torque and current by controlling the slip frequency. It requires detecting the motor speed to form a speed closed loop. The speed regulator's output is the slip frequency, and the sum of the motor speed and the slip frequency is used as the inverter's setpoint frequency. Compared to U/f control, its acceleration/deceleration characteristics and overcurrent limiting ability are improved. Furthermore, it has a speed regulator and utilizes speed feedback to form a closed-loop control, resulting in small static speed errors. However, it does not yet achieve good dynamic performance to reach steady-state control of an automatic control system.
III. Vector Control
Vector control, also known as field-oriented control, was first proposed in the early 1970s by F. Blasschke and others in West Germany. It elucidated the principle by comparing DC and AC motors, thus pioneering the concept of AC motors and equivalent DC motors. Vector control for variable frequency speed regulation involves transforming the stator AC currents Ia, Ib, and Ic of an asynchronous motor in a three-phase coordinate system into equivalent AC currents Ia1 and Ib1 in a two-phase stationary coordinate system through a three-phase to two-phase transformation. Then, through a rotor field-oriented rotational transformation, these are equivalent to DC currents Im1 and It1 in a synchronous rotating coordinate system (Im1 is equivalent to the excitation current of a DC motor; It1 is equivalent to the armature current of a DC motor). Then, mimicking the control method of a DC motor, the control quantities of the DC motor are obtained, and through a corresponding inverse coordinate transformation, the asynchronous motor is controlled. The emergence of vector control has given asynchronous motor variable frequency speed regulation a comprehensive advantage in the field of motor speed control. However, vector control technology requires accurate estimation of motor parameters, and how to improve the accuracy of these parameters remains a topic of ongoing research.
IV. Direct Torque Control
In 1985, Professor DePenbrock of Ruhr University in Germany first proposed the theory of direct torque control. This technique largely solves the shortcomings of vector control. Instead of indirectly controlling torque through controlling current, flux linkage, or other quantities, it directly controls torque as the controlled variable. The advantages of torque control are: torque control controls the stator flux linkage, and essentially does not require speed information; it is robust to changes in all motor parameters except stator resistance; the introduced stator flux linkage observer can easily estimate synchronous speed information, thus facilitating the implementation of sensorless control. This type of control is called sensorless direct torque control.
