An inverter is a device that converts industrial frequency power (50Hz or 60Hz) into AC power of various frequencies to achieve variable-speed operation of motors. The control circuit completes the control of the main circuit, the rectifier circuit converts AC power into DC power, the DC intermediate circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit converts DC power back into AC power.

When using inverters, it is necessary to select different control methods according to your own control requirements. Below, we will introduce several common control methods of inverters:
1. V/f Constant Control:
V/f control involves changing both the voltage and frequency of the motor power supply simultaneously as the frequency changes, maintaining a constant magnetic flux in the motor. This ensures that the motor's efficiency and power factor do not drop significantly over a wide speed range. Since this method controls the ratio of Voltage to Frequency, it is called V/f control. However, the main issue with constant V/f control is poor low-speed performance. At very low speeds, the electromagnetic torque may not overcome static friction, making it difficult to properly adjust the motor's torque compensation and adapt to load variations. Additionally, it cannot accurately control the actual motor speed. As V/f control is an open-loop speed control, from the mechanical characteristics of induction motors, we know that the set value is the stator frequency (i.e., the ideal no-load speed), while the actual motor speed is determined by the slip rate. Therefore, the stable error inherent in V/f constant control cannot be controlled, making it impossible to precisely regulate the motor’s actual speed.
2. Slip Frequency Control:
Slip frequency refers to the difference between the frequency of the AC power applied to the motor and the motor's actual speed. According to the stable mathematical model of induction motors, when the frequency is fixed, the motor's electromagnetic torque is proportional to the slip, and the mechanical characteristics are linear.
Slip frequency control manages torque and current by controlling the slip frequency. This method requires detecting the motor's speed to form a closed-loop speed control system. The output of the speed regulator becomes the slip frequency, which, together with the motor speed, determines the inverter's set frequency. Compared to V/f control, slip frequency control improves acceleration/deceleration characteristics and overcurrent limiting capability. In addition, it includes a speed regulator and uses speed feedback for closed-loop control, resulting in smaller static speed errors. However, although it achieves steady-state control in automatic systems, it still falls short in terms of dynamic performance.
3. Vector Control (also known as Field-Oriented Control):
Vector control was first proposed in the early 1970s by F. Blaschke and others from West Germany. They explained the principle by comparing DC and AC motors, thereby initiating the era of treating AC motors similarly to DC motors. The approach of vector-controlled variable frequency speed regulation involves converting the three-phase stator AC currents Ia, Ib, and Ic of an induction motor into two-phase stationary frame AC currents Ia1 and Ib1 through a three-phase to two-phase transformation. Then, through a rotor field-oriented rotating transformation, these are further converted into DC currents Im1 and It1 in the synchronous rotating frame (Im1 corresponds to the excitation current of a DC motor; It1 corresponds to the armature current). After this, DC motor control techniques are mimicked to obtain the required control variables, which are then transformed back through inverse coordinate transformations to control the induction motor. The emergence of vector control made variable frequency speed regulation of induction motors superior across all aspects of motor speed control. However, vector control requires accurate estimation of motor parameters, and improving parameter accuracy remains an active area of research.
4. Direct Torque Control:
In 1985, Professor Depenbrock from Ruhr University in Germany first proposed the theory of direct torque control. This technology largely addresses the shortcomings of vector control. Unlike vector control, which indirectly controls torque through current and flux, direct torque control directly treats torque as the controlled variable. Its advantage lies in the fact that torque control focuses on stator flux linkage and fundamentally does not require speed information. It is robust to changes in all motor parameters except stator resistance. The introduced stator flux observer can easily estimate the synchronous speed information, enabling sensorless operation, a method known as Sensorless Direct Torque Control.
Changsha Sunye Electric Co., Ltd. (established in 2010) and Shenzhen Sunye Electric Co., Ltd. (established in 2002) are national high-tech enterprises integrating R&D, manufacturing, and sales of inverters, industry-specific all-in-one machines, servo drives, and new energy products.
Sunye Electric owns independent intellectual property rights, with products certified by national authoritative institutions and holding multiple software copyrights and IP certificates. Each year, significant investment is made in research and development of new technologies and products, resulting in numerous approved patents and software copyrights. The company possesses core platform technologies such as construction hoist drivers, high-performance vector inverters, servos, and permanent magnet synchronous motor controls. Medium- and low-voltage inverters and servo drives are widely used in industries such as lifting machinery, stone processing, HVAC, machine tools, metal products, wire and cable, plastics, printing and packaging, textile and chemical fibers, building materials, metallurgy, coal mining, municipal services, and automotive manufacturing. Guided by the philosophy of “Innovation, Technology, Strength,” driven by the corporate spirit of “Unity, Progress, Pragmatism, Innovation,” and adhering to the business principle of “Sincerity and Excellence, Mutual Benefit and Coexistence,” Sunye Electric is committed to providing premium products to customers across various industries through its quality policy of “Implementing total quality management, continuous improvement, and pursuing zero defects.”
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