Principle and application of frequency converter
Working principle of frequency converter
The main circuit is the power conversion part that provides voltage regulation and frequency modulation power supply for asynchronous motor. The main circuit of frequency converter can be roughly divided into two types [1]: the voltage type is a frequency converter that converts DC of voltage source into AC, and the filtering of DC loop is a capacitor. Current mode is a frequency converter that converts DC of current source into AC, and its DC loop filter is inductance. It consists of three parts, namely, a rectifier that converts the power frequency power supply into DC power, a smoothing circuit that absorbs the voltage pulsation generated by the converter and inverter, and an inverter that converts DC power into AC power.
(1) Rectifier: Recently, diode converters are widely used, which convert power frequency power supply into DC power supply. Two groups of transistor converters can also be used to form a reversible converter, which can be regenerated because of its reversible power direction.
(2) Smoothing circuit: The DC voltage rectified by the rectifier contains a pulsating voltage with 6 times the frequency of the power supply, and the pulsating current generated by the inverter also changes the DC voltage. In order to suppress voltage fluctuation, inductance and capacitance are used to absorb pulsating voltage (current). When the capacity of the device is small, if the power supply and the main circuit have a surplus, the inductance can be omitted and a simple smoothing circuit can be used.
(3) Inverter: Contrary to rectifier, inverter converts DC power into AC power with required frequency, and three-phase AC output can be obtained by turning on and off six switching devices in a determined time. Taking voltage source pwm inverter as an example, the switching time and voltage waveform are shown.
The control circuit is a circuit that provides control signals for the main circuit that supplies power to the asynchronous motor (with adjustable voltage and frequency). It consists of an arithmetic circuit of frequency and voltage, a voltage and current detection circuit of the main circuit, a speed detection circuit of the motor, a driving circuit that amplifies the control signals of the arithmetic circuit, and a protection circuit of the inverter and the motor.
(1) operation circuit: compares the external instructions such as speed and torque with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter.
(2) Voltage and current detection circuit: isolated from the main loop potential to detect voltage and current.
(3) Driving circuit: the circuit that drives the main circuit device. It is isolated from the control circuit to turn on and off the main circuit device. ? (4) Speed detection circuit: The signals of speed detectors (tg, plg, etc.) installed on the shaft of asynchronous motor are used as speed signals, which are sent to the operation circuit, and the motor can be operated at the instructed speed according to the instructions and operations.
(5) Protection circuit: detect the voltage and current of the main circuit, and stop the inverter or suppress the voltage and current values in order to prevent the inverter and asynchronous motor from being damaged when there are abnormalities such as overload or overvoltage.
Function of frequency converter
Inverter integrates high voltage and high power transistor technology and electronic control technology, and is widely used. The function of frequency converter is to change the frequency and amplitude of AC motor power supply, thus changing the period of its moving magnetic field and achieving the purpose of smoothly controlling the motor speed. The appearance of frequency converter simplifies the complex speed control. The combination of frequency converter and AC squirrel-cage induction motor replaces most of the work that can only be done by DC motor, which reduces the volume and maintenance rate and makes the transmission technology develop to a new stage.
Pump applications:
Replace the slip motor speed regulation system with high failure rate caused by the environment and various valve control methods with poor control accuracy. Save energy by 20~40%, reduce equipment wear and tear, reduce investment in spare parts, improve control accuracy and prolong equipment service life.
Application of fans:
Replacing the wind deflector with serious energy waste to adjust the air volume, it can reduce the vibration of the fan, eliminate the current impact of starting the large motor, avoid mechanical vibration, greatly reduce the failure rate of equipment, reduce the workload of equipment maintenance, and easily realize closed-loop automatic adjustment of air volume and pressure by microcomputer.
Application in machine tools;
Reduce the logarithm of variable speed transmission gear, reduce noise, improve spindle accuracy, and have strong characteristics of adapting to spindle speed distribution required for processing of various products and different materials. It is convenient to realize numerical control, and its cost is much lower than that of similar numerical control systems composed of DC speed regulation. The drag system of audio quenching machine body in Kunming Milling Machine Factory, the numerical control milling machine in Power Company, the main drag system of 450 vertical rolling mill in Liangting Rolling Mill of Kunming Iron and Steel Company, the grinder in Yunnan Bearing Factory and the missile shell grinder in Beijing Aerospace Department Factory have achieved good results. At present, VLT inverter has been used for a long time in the products of professional manufacturers such as engraving machine, extruder, wire drawing machine and CNC lathe.
Cigarette machine application:
Eliminate the phenomenon of high failure rate caused by the original old cigarette machine, such as (MK8R) step speed regulation, improve the equipment utilization rate, prolong the service life of the equipment, save 20% electricity, and realize the localization and standardization of the main motor. Saving foreign exchange, expanding the source of spare parts and improving the working environment.
Through the introduction of variable-frequency drive in Xiaobian, do you have a deeper understanding of variable-frequency drive and its role? If you master the working principle of frequency converter, it will be a great progress and breakthrough for high-voltage and high-power transistor technology and cushion control technology. The working principle of frequency converter is divided into two parts, the main circuit and the auxiliary circuit, and it uses the current of alternating current and direct current, which is very high-tech. Well, the introduction of Xiaobian is over here.
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