A variable-frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of the motor's power supply.
A frequency converter mainly consists of a rectification unit (AC to DC), a filter, an inverter (DC to AC), a braking unit, a drive unit, a detection unit, and a microprocessor unit. As one of the drive and control devices for three-phase motors, the frequency converter plays a crucial and indispensable role in industrial automation.
Domestic frequency converter technology has been developing for about 30 years. Currently, domestically produced frequency converters occupy half of the domestic market and are exported abroad. Well-known brands such as Puchuan Technology, Huichuan, and Blue Ocean Huateng have emerged. Their products have evolved from simple soft starters to vector frequency converters and servo control systems. Products such as Puchuan Technology's PI500 series high-performance vector frequency converters not only occupy a large market share in China but also enjoy a good reputation and market share internationally.
Frequency converters mainly have the following functions:
I. Soft start energy saving
Hard starting of motors causes severe impacts on the power grid and places excessive demands on grid capacity. The large current and vibration generated during startup cause significant damage to baffles and valves, severely shortening the lifespan of equipment and pipelines. However, using a variable frequency drive (VFD) energy-saving device utilizes the VFD's soft-start function to ensure the starting current starts from zero, with a maximum value not exceeding the rated current. This reduces the impact on the power grid and the demand on power supply capacity, extending the lifespan of equipment and valves and saving on equipment maintenance costs.
Variable frequency drives (VFDs) were initially designed for the most commonly used three-phase asynchronous squirrel-cage motors. The speed formula for a three-phase asynchronous motor is n=60f(1-s)/p. Changing the motor's speed by altering its input frequency is the most thorough, convenient, and reliable method. The instantaneous current for direct starting of a motor is 6-8 times that of normal stable operation, while reduced-voltage starting is 4-5 times. Variable frequency speed control, however, will not exceed 1.5 times the VFD's rated current. The advantages in terms of both grid impact and equipment adaptability are obvious.
II. Stepless speed regulation
Stepless speed regulation (CVT) means speed control without steps or increments. It allows for a seamless transition from zero speed to a given speed and back to zero, resulting in a continuous, smooth, and stable speed curve. This greatly adapts to load inertia, improves the interoperability of multiple mechanical devices, increases production efficiency, significantly reduces mechanical friction, vibration, and noise, extends equipment life, and lowers maintenance costs. Ordinary frequency converters typically output frequencies between 0.00Hz and 400.00Hz, with a frequency resolution of 0.01Hz. Some reach 650.00Hz, while high-frequency converters can output up to 5000Hz, completely covering the entire range of industrial speed regulation.
Before the advent of frequency converters, we already had many mechanical speed control methods, ubiquitous in our daily lives, such as gears, belts, and reducers. However, these methods could only output a fixed speed ratio at a time, making frequent speed changes difficult. There were also slip-ring motors, but their application was limited, and maintenance and costs were high. The emergence of core components such as transistors and microprocessors spurred a major breakthrough in continuously variable speed (CVT) technology. The development of frequency converters was a result of meeting the needs of industrial automation and the development of several related industries.
III. Variable Frequency Energy Saving
To ensure production reliability, all production machinery is designed with a certain margin of safety in its power drive system. When the motor cannot operate at full load, the excess torque beyond meeting the power drive requirements increases active power consumption, resulting in energy waste. Traditional speed control methods for equipment such as fans and pumps adjust the airflow and water flow by regulating the opening of inlet or outlet baffles and valves. This method has high input power, and a significant amount of energy is consumed during the flow throttling process of the baffles and valves. When using variable frequency speed control, if the flow requirement decreases, the requirement can be met by reducing the speed of the pump or fan. For example, consider energy saving in fan-pump type loads: a centrifugal pump motor with a power of 55 kW consumes 28.16 kW of electricity when the speed is reduced to 4/5 of its original speed, saving 48.8%; when the speed is reduced to 1/2 of its original speed, the electricity consumption is 6.875 kW, saving 87.5%.
The energy-saving function of frequency converters has been widely used in various industrial and civil production and daily life, especially when used for square torque loads, such as fans, blowers, pumps, and propellers. This is determined by the torque curve of such loads. At low speeds, the load torque is very small because the fluid velocity is low. As the motor speed increases, the flow velocity increases, and the load torque and power also increase.
IV. Equipment and Self-Protection Functions
As we all know, in the actual operation of equipment, various factors often lead to various faults and equipment damage. It would be very necessary to have intelligent detection, automatic alarm and protection. For example, undervoltage, overvoltage, lightning strike, overload, load stall, overheating, communication failure, electromagnetic interference, power supply phase loss, motor phase loss, load short circuit, ground short circuit, etc. Some faults are fatal, and once they occur, the consequences are unimaginable.
During the research and development of frequency converters, multiple detection and protection mechanisms are incorporated primarily for the safety of the internal components. For example, the maximum allowable current of the inverter module generally does not exceed twice the rated current, the allowable temperature of the electrolytic capacitor is 85℃, and electromagnetic compatibility design is implemented. In addition, load-side detection and feedback (such as motor temperature detection and protection, and uninterrupted operation during instantaneous power outages) make today's frequency converters highly intelligent, not only protecting their own safe and normal operation but also greatly protecting the safe operation of upstream and downstream equipment.
V. Power Factor Compensation Energy Saving
Reactive power not only increases line losses and equipment heating, but more importantly, the reduction in power factor leads to a decrease in the active power of the power grid. A large amount of reactive power is consumed in the lines, resulting in low equipment efficiency and serious waste.
After using a variable frequency speed control device, the reactive power loss is reduced and the active power of the power grid is increased due to the effect of the internal filter capacitor of the frequency converter.
VI. Automated Control Functions
Today's frequency converters have comprehensive control functions, allowing them to work seamlessly with other control equipment and instruments to achieve centralized real-time monitoring and control through systematic networking. This includes integration with DCS, PLC, sensors, HMI, CNC, host computers, servo drives, etc., significantly improving production line automation, saving manpower and resources, increasing efficiency, and reducing costs. For example, frequency converters offer various frequency setting methods, such as digital setting, potentiometer, external analog input (0~10V, 4~20mA), multi-speed, PID, etc. They can be connected to various external controllers and feedback signals, enabling frequency commands and remote/local control switching, facilitating diverse user control needs. Furthermore, some companies have launched specialized industry-specific models, offering customized development and integrated solutions for specific industry users, saving users the hassle of selection and system compatibility issues, and reducing costs.
Frequency converters are an important component of industrial automation. With the rapid development of China's automation industry, the performance and stability of Chinese frequency converter brands have surpassed many foreign brands in some aspects, and the market share of domestic brands is gradually increasing.
