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

When using an inverter, different control methods must be selected based on one's control requirements. Below, let's introduce several common control methods for inverters:
I. V/f Constant Control:
V/f control changes the voltage along with the frequency of the motor's power supply to maintain a constant magnetic flux, ensuring that the motor's efficiency and power factor do not decrease over a wide range of speeds. This method controls the ratio between Voltage (V) and Frequency (F), hence it's called V/f control. A primary issue with constant V/f control is its poor performance at low speeds; extremely low speeds cannot overcome significant static friction forces, making it difficult to properly adjust the motor's torque compensation and adapt to load torque changes. Furthermore, it cannot accurately control the motor's actual speed since it uses open-loop speed control, leading to stability errors.
II. Slip Frequency Control:
Slip frequency refers to the difference between the frequency applied to the motor and the motor's speed. According to the stable mathematical model of asynchronous motors, when the frequency is constant, the electromagnetic torque is proportional to the slip rate, resulting in linear mechanical characteristics.
Slip frequency control regulates torque and current by controlling the slip frequency. It requires detecting the motor's speed to form a speed closed-loop, where the output of the speed regulator is the slip frequency. Then, the sum of the motor speed and slip frequency serves as the set frequency for the inverter. Compared with V/f control, this method improves acceleration/deceleration characteristics and current limiting capabilities. However, achieving good dynamic performance remains challenging.
III. Vector Control, also known as Field-Oriented Control
First proposed in the early 1970s by F. Blasschke et al. from West Germany, vector control compares DC and AC motors to explain this principle, pioneering the equivalence between AC motors and DC motors. In vector control variable frequency speed regulation, the three-phase stator AC currents Ia, Ib, and Ic of an asynchronous motor are transformed into two-phase stationary coordinate system currents Ia1 and Ib1 through a three-to-two phase transformation. Then, these are converted into synchronous rotating coordinate system currents Im1 and It1 (Im1 corresponds to the excitation current of a DC motor; It1 corresponds to the armature current). By mimicking DC motor control methods, the control quantities of a DC motor are obtained and then transformed back to control the asynchronous motor. Vector control has made variable frequency speed regulation of asynchronous motors dominant in the field of motor speed control. However, accurate estimation of motor parameters is required for vector control technology, which remains a topic of ongoing research.
IV. Direct Torque Control
In 1985, Professor DePenbrock from Ruhr University in Germany first introduced direct torque control theory, significantly addressing the shortcomings of vector control. Instead of indirectly controlling torque through current or flux linkage, it directly controls torque. The superiority of torque control lies in its ability to control stator flux without needing speed information, offering robustness against all motor parameter variations except for stator resistance. The introduced stator flux observer can easily estimate synchronous speed information, enabling sensorless speed control, 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.”
Address: Sunye Electric Industrial Park, No. 669 Xinsheng Road, High-Tech Zone, Changsha City
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