The working principle of the frequency converter

The working principle of the frequency converter

The main circuit is the power conversion part that provides the voltage and frequency power supply for the asynchronous motor. The main circuit of the frequency converter can be roughly divided into two categories: the voltage type is the frequency converter that converts the DC of the voltage source to the AC, and the filtering of the DC loop is the capacitor. . The current type is a frequency converter that converts the DC of the current source into AC, and the DC loop filter is an inductor. It consists of three parts, the "rectifier" that converts the industrial frequency power supply into DC power, the "flat wave loop" that absorbs the voltage ripples generated in the converter and inverter, and the "reverse circuit" that converts DC power to AC power. Changer".

　　 rectifier

　　 Recently, a large number of diode converters are used, which convert industrial frequency power to DC power. Two groups of transistor converters can also be used to form a reversible converter, which can be regenerated because of its reversible power direction.

　　 flat wave loop

　　The DC voltage rectified by the rectifier contains a pulsating voltage of 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 fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). If the capacity of the device is small, if the power supply and the main circuit components have a margin, the inductor can be omitted and a simple smoothing loop can be used.

　　Inverter

　　 Contrary to the rectifier, the inverter converts the DC power into the AC power of the required frequency, and the 6 switching devices are turned on and off for a determined time to obtain a 3-phase AC output. Take the voltage-type pwm inverter as an example to show the switching time and voltage waveform.

The control circuit is a circuit that provides control signals to the main circuit of the asynchronous motor (voltage and frequency adjustable). It has the frequency and voltage "calculation circuit", the main circuit "voltage and current detection circuit", and the motor "speed detection" Circuit", the "drive circuit" that amplifies the control signal of the arithmetic circuit, and the "protection circuit" of the inverter and the motor.

　　 (1) Calculation circuit: compare the external speed, torque and other commands 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: It is isolated from the main circuit to detect voltage and current.

　　 (3) Drive circuit: The circuit that drives the main circuit device. It is isolated from the control circuit so that the main circuit device is turned on and off.

　　 (4) Speed ​​detection circuit: Take the signal of the speed detector (tg, plg, etc.) installed on the asynchronous motor shaft machine as the speed signal, send it to the calculation loop, and make the motor run at the command speed according to the command and calculation.

　　 (5) Protection circuit: Detect the voltage and current of the main circuit. When an abnormality such as overload or overvoltage occurs, in order to prevent damage to the inverter and asynchronous motor, stop the inverter or suppress the voltage and current value.