Inverter

The English translation of the inverter is called VFD (Variable-frequency Drive), which may be one of the few examples of the reverse translation of modern technology from Chinese into English. (But VFD can also be interpreted as a Vacuum fluorescent display, vacuum fluorescent tube, so this translation is not commonly used). Inverter is the application of frequency conversion technology and microelectronics technology, by changing the frequency and amplitude of the motor power supply to control the AC motor power transmission components. Inverters have been known as VVVF (Variable Voltage Variable Frequency Inverter) due to the influence of Japanese manufacturers in Asian regions such as China and Korea.

Principle of Inverter Operation The main circuit is the power conversion part that provides the voltage regulating and frequency modulation power supply for the asynchronous motor. The main circuit of the frequency converter can be roughly divided into two categories [1]: The voltage type is the conversion of the DC voltage source to AC frequency conversion. The filter of the DC loop is a capacitor. The current type is a frequency converter that converts the current from a direct current source to an alternating current, and the direct current loop filter is an inductor. It consists of three parts: a "rectifier" that converts the power-frequency power source into a direct-current power, a "fluid-wave circuit" that absorbs the voltage ripple generated by the converter and the inverter, and an "inverse" that converts the direct-current power into alternating-current power. Changer."

Rectifier recently used a large number of diode converters, which convert power frequency power to DC power. It is also possible to use two sets of transistor converters to construct a reversible converter, which can be regenerated due to its reversible power direction.

The flat wave circuit contains a pulsating voltage at a frequency six times that of the power supply in the DC voltage after the rectifier is rectified, and the pulsating current generated by the inverter also causes the DC voltage to fluctuate. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb the ripple voltage (current). When the capacity of the device is small, if the power supply and the main circuit configuration device have a margin, a simple flat wave circuit can be omitted from the inductor.

The inverter is the reverse of the rectifier. The inverter converts the DC power into the AC power of the required frequency. By turning the six switching devices on and off for a determined time, a three-phase AC output can be obtained. Take the voltage type pwm inverter as an example to illustrate the switching time and voltage waveforms.

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

(1) Operation circuit: Comparing the external speed and torque 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 potential to detect voltage and current.

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

(4) Speed ​​detection circuit: The signal of the speed detector (tg, plg, etc.) mounted on the asynchronous motor shaft machine is the speed signal, which is sent to the calculation circuit. According to the instruction and operation, the motor can be operated at the command speed.

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

The role of the inverter The inverter integrates high-voltage, high-power transistor technology and electronic control technology and is widely used. The function of the frequency changer is to change the frequency and amplitude of the power supply of the AC motor, thus changing the cycle of its moving magnetic field and achieving the purpose of smoothly controlling the rotation speed of the motor. The appearance of the frequency converter makes the complex speed control simple, and replaces most of the work that can only be done with a DC motor with a combination of an inverter + AC squirrel-cage induction motor, which reduces the size, reduces the maintenance rate, and makes the drive The technology has developed to a new stage.

The composition of the frequency converter is usually divided into four parts: the rectifier unit, high-capacity capacitors, inverters and controllers.

â–¡ The rectifier unit converts AC with fixed operating frequency to DC.

â–¡ High-capacity capacitors store converted electrical energy.

â–¡ The inverter consists of a large power switching transistor array to form an electronic switch that converts direct current into square waves of different frequencies, widths, and amplitudes.

â–¡ The controller works according to the set program to control the amplitude and pulse width of the output square wave to superimpose the alternating current that is approximately sine wave to drive the AC motor.