The inverter controls the motor by controlling the voltage, current and frequency provided by the motor to meet the load requirements according to the characteristic parameters of the motor and the motor operation requirements. So just like the main circuit of VFD, the inverter components are the same and the number of microcontroller bits are the same, only the control method is different and its control effect is also different. So the control method is very important. It represents the level of the inverter. The current control method of inverter for motor can be roughly divided into U/f constant control, differential frequency control, vector control, direct torque control and non-linear control.

U/f constant control

U/f control is to change the voltage of the motor power supply while changing the frequency of the motor power supply, so that the flux of the motor remains constant and the efficiency and power factor of the motor does not drop in a wider speed range. Because it controls the ratio of voltage to frequency, it is called U/f control. The main problem of constant U/f control is poor performance at low speeds. When the speed is very low, the electromagnetic torque cannot overcome the large static friction force, and the torque compensation of the motor cannot be properly adjusted to adapt to the change of load torque. Second, the actual speed of the motor cannot be precisely controlled. Since constant U/f VFD is an open-loop speed control, it can be seen from the mechanical characteristics diagram of the asynchronous motor that the setting value is the stator frequency, i.e. the ideal no-load speed, while the actual speed of the motor is determined by the turndown rate, so it is impossible to control the stability error existing in the U/f constant control method, and therefore the actual speed of the motor cannot be accurately controlled. Differential frequency control

Differential frequency is the difference frequency between the AC power frequency applied to the motor and the motor speed. According to the mathematical model of asynchronous motor stability, when the frequency is certain, the electromagnetic torque of the asynchronous motor is proportional to the difference rate, and the mechanical characteristics are linear.

Differential frequency control is to control the torque and current by controlling the differential frequency. Differential frequency control needs to detect the motor speed to form a closed loop speed. The output of the governor is the differential frequency, and then the sum of the motor speed and the differential frequency is used as the given frequency of the VFD. Compared with U/f control, it has improved acceleration and deceleration characteristics and the ability to limit overcurrent. In addition it has speed regulator and uses speed feedback to form closed-loop control with small speed static error. However, good dynamic performance is not possible to achieve steady-state control of an automatic control system.

Vector Control

Vector control, also known as magnetic field oriented control. It was first proposed by F. Blasschke et al. in West Germany in the early 1970s and elaborated the principle by comparing DC motors with AC motors. Thus, it pioneered the AC motor and the equivalent DC motor. The method of vector frequency regulation is to compare the stator AC currents Ia, Ib, and Ic of asynchronous motors in a three-phase coordinate system. equivalent to AC currents Ia1 and Ib1 in a two-phase stationary coordinate system through a three-phase to two-phase transformation, and then equivalent to DC currents Im1 and It1 in a synchronous rotating coordinate system based on the rotor magnetic field through a directional rotational transformation (Im1 is equivalent to the excitation current of the DC motor, and It1 is equivalent to the armature current of the DC motor). Then, the control quantities of the DC motor are obtained by imitating the control method of the DC motor, and the asynchronous motor is controlled by the corresponding coordinate inversion. The advent of vector control methods has put asynchronous motor variable speed regulation in a fully dominant position in the field of motor speed regulation. However, vector control techniques require correct estimation of motor parameters, and how to improve the accuracy of the parameters has been the subject of research.

Direct Torque Control

The theory of direct torque control was first introduced by Professor DePenbrock of Ruhr University in Germany in 1985. This technique largely solves the shortcomings of vector control. Instead of controlling the torque indirectly by controlling the current and magnetic chain, the torque is directly controlled as the control quantity. The advantage of torque control is that torque control is controlling the stator magnetic chain, which essentially does not require speed information and is robust to all motor parameters except the stator resistance. The introduced stator magnetic chain observer allows easy estimation of the synchronous speed information, thus allowing easy implementation of direct torque control without speed sensors.