Suppliers of frequency converter supporting equipment remind you that frequency converters have been widely used in industrial production today. Equipment controlled by frequency converters can significantly save energy to a certain extent, thus gaining the favor of many industrial manufacturers.
To achieve features such as soft parking, soft starting, stepless speed regulation, or special requirements for increasing or decreasing speed, a speed regulation device called a frequency converter is needed in modern asynchronous motors. The main circuit of the device uses AC-DC-AC circuits with a working frequency of 0-400Hz. The output voltage of the low-voltage universal frequency converter is 380-460V, and the output power is 0.37-400kW.
Choose a reasonable frequency converter
The problems that arise during the use of frequency converters, such as abnormal operation, equipment failure, etc., leading to production stoppage and unnecessary economic losses, are often caused by improper selection and installation of frequency converters. Therefore, it is necessary to choose an economical and practical frequency converter that can better meet the basic conditions and requirements of production and process.
As the main driving object of the frequency converter, the motor should be selected to match the working parameters of the motor when choosing the type of frequency converter.
(1) Voltage matching: The rated voltage of the frequency converter matches the load voltage of the motor.
(2) Current matching: The capacity of the frequency converter depends on the rated current continuously output by the frequency converter. When selecting a frequency converter for motors that require speed regulation, it is necessary to choose a frequency converter with a continuous rated current greater than the rated current of the motor when operating at rated parameters, and with a quantitative margin; For general frequency converters with more than 4 poles, the selection cannot be based on the capacity of the motor, but on the current seat verification standard of the motor; Even if the load on the motor is relatively light and the current is less than the rated current of the frequency converter, the selected frequency converter cannot be too small in capacity compared to the motor.
(3) Capacity matching: Depending on the different load characteristics of the motor, there are different requirements for selecting the capacity of the frequency converter.
Control method of frequency converter
The main control methods of frequency converters currently include the following.
(1) The first generation used U/f=C control, also known as sine pulse width modulation (SPWM) control method. Its characteristics include a simple control circuit structure, low cost, good mechanical properties and hardness, which can meet the smooth speed regulation requirements of general transmission. However, this control method reduces the maximum output torque at low frequencies due to the lower output voltage, resulting in decreased stability at low speeds. Its characteristic is that without feedback device, the speed ratio ni is less than 1/40, and with feedback, ni=1/60. Suitable for general fans and pumps.
(2) The second generation adopts voltage space vector control (magnetic flux trajectory method), also known as SVPWM control method. It is based on the overall generation effect of three-phase waveforms, generating three-phase modulation waveforms at once and controlling them by cutting polygons to approximate circles. To eliminate the influence of stator resistance at low speeds, the output voltage and current are closed loop to improve dynamic accuracy and stability. Its characteristics: no feedback device, speed ratio ni=1/100, suitable for speed regulation in general industry.
(3) The third generation adopts vector control (VC) method. The practice of vector control variable frequency speed regulation essentially equates an AC motor to a DC motor, and independently controls the speed and magnetic field components. By controlling the rotor magnetic flux and decomposing the stator current to obtain two components, torque and magnetic field, orthogonal or decoupled control can be achieved through coordinate transformation. Its characteristics: speed ratio ni=1/100 without feedback, ni=1/1000 with feedback, and starting torque of 150% at zero speed. It can be seen that this method is applicable to all speed control, and when equipped with feedback, it is suitable for high-precision transmission control.
(4) Direct Torque Control (DTC) method. Direct torque control (DTC) is another high-performance variable frequency speed control mode that differs from vector control (VC). Obtain magnetic flux and torque data using magnetic flux simulation models and electromagnetic torque models, compare them with given values to generate hysteresis comparison state signals, and then switch the switch state through logic control to achieve constant magnetic flux control and electromagnetic torque control. It does not require imitation of DC motor control, and this technology has been successfully applied to the AC drive of traction electric locomotives. Its characteristics: without feedback device, the speed ratio ni=1/100, with feedback ni=1/1000, and the starting torque can reach 150% to 200% at zero speed. Suitable for heavy-duty starting and large loads with constant torque fluctuations.
Installation environment requirements
(1) Environmental temperature: The environmental temperature of the frequency converter refers to the temperature near the cross-section of the frequency converter. Due to the fact that frequency converters are mainly composed of high-power power electronic devices that are highly susceptible to temperature, the lifespan and reliability of frequency converters largely depend on temperature, generally ranging from -10 ℃ to+40 ℃. In addition, it is necessary to consider the heat dissipation of the frequency converter itself and the extreme situations that may occur in the surrounding environment, and a certain margin is generally required for temperature.
(2) Environmental humidity: The frequency converter requires a relative humidity of no more than 90% in its surrounding environment (with no condensation on the surface).
(3) Vibration and shock: During the installation and operation of the frequency converter, attention should be paid to avoiding vibration and shock. To avoid the connection solder joints and loose parts of internal components of the frequency converter, which may cause poor electrical contact or even serious faults such as short circuits. Therefore, it is usually required that the vibration acceleration of the installation site be limited to below 0.6g, and seismic resistant measures such as shock-absorbing rubber can be added in special places.
(4) Installation location: The maximum allowable output current and voltage of the frequency converter are affected by its heat dissipation capacity. When the altitude exceeds 1000m, the heat dissipation capacity of the frequency converter will decrease, so the frequency converter is generally required to be installed below 1000m altitude.
(5) The general requirements for the installation site of the frequency converter are: no corrosion, no flammable or explosive gases or liquids; Dust free, floating fibers and metal particles; Avoid direct sunlight; No electromagnetic interference.
The research on variable frequency speed regulation is currently the most active and practically valuable work in electrical transmission research. The potential of the frequency converter industry is enormous, as it is widely used in industries such as air conditioning, elevators, metallurgy, and machinery. Variable frequency speed regulating motors and their corresponding frequency converters will develop rapidly.