1.Brief of a variable frequency drive
A Variable Frequency Drive (VFD), also known as an AC drive or frequency inverter, is an electronic device that controls the speed and torque of an AC electric motor by adjusting the frequency and voltage of its power supply. By converting a fixed-frequency AC power to a variable-frequency AC power, VFDs allow motors to operate at different speeds, leading to significant energy savings, improved process control, reduced mechanical stress, and a longer motor lifespan.
2.Operation steps of variable frequency drive
1.Rectification:The VFD takes the incoming fixed-frequency AC power from the mains supply and converts it into DC power.
2.Filtration/DC Bus:The DC power then passes through a DC bus, which smooths out any ripples using components like capacitors.
3.Inversion:An inverter converts the DC power back into AC power, but this time with a variable frequency and voltage. The VFD adjusts the switching of electronic components, like IGBTs (insulated gate bipolar transistors), to create this variable output.
3.Control methods of variable frequency drive
1.Speed control: This is the most basic control method, which changes the speed of the motor by adjusting the output frequency of the inverter. Speed control can be divided into open-loop control and closed-loop control. The former is simple and easy to use but not very accurate, while the latter requires additional speed sensors for feedback to achieve more accurate speed regulation.
2.Torque control: Torque control aims to control the output torque of the motor, which is particularly suitable for occasions requiring constant torque. By adjusting the output voltage and frequency of the inverter, it can be ensured that the motor can output the required torque at different speeds.
3.PID control: PID control is a control method based on proportional, integral, and differential algorithms, which is suitable for precise control of process variables. In the application of inverters, PID control is often used to achieve closed-loop control of physical quantities such as pressure and temperature.
4.Multi-speed control: Multi-speed control allows users to preset multiple speed sections and switch them at any time as needed. This control method is suitable for occasions that require frequent speed changes, such as different processes on a production line.
5.Communication network control: With the development of intelligent technology, inverters are increasingly supporting remote control through industrial networks. This control method can achieve collaborative work between multiple inverters and improve the automation level of the system.
4.Selection principles of variable frequency drive
1.Load type: The load type (such as constant voltage control, constant current control, etc.) must be clearly defined. Different loads have different control methods and performance requirements for the inverter. For example, it is recommended to use a dedicated inverter for torque-reducing loads such as fans and water pumps, while a inverter with closed-loop control function should be selected for scenes that require precise control.
2.Power and voltage matching: The rated power and voltage of the inverter must match the motor. Ordinary centrifugal pumps and other loads are mainly based on current matching, while special loads such as deep-water pumps need to refer to the maximum current of the motor and appropriately enlarge the inverter capacity. Voltage matching requires ensuring that the input/output voltage of the inverter is consistent with the rated voltage of the power supply and the motor.
3.Control mode: Select the control mode (such as V/F control, vector control, servo control) according to application requirements. For example, machine tool equipment requires constant power characteristics, and a vector control inverter can be selected; fans, water pumps, etc. can use general inverters.
4.Current and torque matching: The inverter capacity needs to be matched according to the rated current and load characteristics of the motor. High-speed motors may increase output current due to low reactance, so the inverter capacity needs to be appropriately increased. Heavy loads or large inertia loads (such as cranes and centrifuges) need to be enlarged by 1-2 gears to select inverters and configure brake units.
5.Environmental adaptability: Environmental factors such as temperature, humidity, and dust need to be considered, and appropriate protection levels (such as IP54, IP65) and operating temperature ranges need to be selected to ensure inverter stability.
6.Function and efficiency: Prioritize inverters with high energy efficiency levels, and consider whether they support network communication, fault diagnosis and other functions. Some inverters have built-in energy-saving functions to reduce operating costs.
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