Frequency converter is a power control device that uses frequency conversion technology and microelectronics technology to 

control AC motors by changing the frequency of the motor's working power supply. The frequency converter mainly consists 

of rectifier (AC to DC), filter, inverter (DC to AC), braking unit, driving unit, detection unit, microprocessor unit, etc.

Variable frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics 

technology to control AC motors by changing the frequency of the motor's operating power supply.

The frequency converter mainly consists of rectifier (AC to DC), filter, inverter (DC to AC), braking unit, driving unit, detection 

unit, microprocessor unit, etc. The frequency converter adjusts the voltage and frequency of the output power supply by 

breaking the internal IGBT, providing the required power voltage according to the actual needs of the motor, thereby achieving 

energy conservation and speed regulation. In addition, the frequency converter also has many protection functions, such as 

overcurrent, overvoltage, overload protection, and so on. With the continuous improvement of industrial automation, frequency 

converters have also been widely used.

Frequency converter composition

Main circuit

Variable frequency power analyzer

Variable frequency power analyzer (5 sheets)

The main circuit is the power conversion part that provides voltage regulation and frequency regulation power for asynchronous 

motors. The main circuit of frequency converters can be generally divided into two types: voltage type frequency converters

 convert the DC of the voltage source into AC, and the filtering of the DC circuit is capacitors. Current type is a frequency

 converter that converts the DC of the current source into AC, and its DC circuit filtering is inductance. It consists of three 

parts: a "rectifier" that converts power frequency power into DC power, a "smoothing circuit" that absorbs voltage fluctuations 

generated by converters and inverters, and an "inverter" that converts DC power into AC power.

ectifier

A large number of diodes are used as inverters, which convert power frequency power into DC power. Two sets of transistor 

converters can also be used to form a reversible converter, which can undergo regenerative operation due to its reversible 

power direction.

Smoothing circuit

In the rectified DC voltage of the rectifier, there is a pulsating voltage of 6 times the frequency of the power supply, and the 

pulsating current generated by the inverter also causes changes in the DC voltage. To suppress voltage fluctuations, inductors 

and capacitors are used to absorb fluctuating voltage (current). When the device capacity is small, if there is margin in the 

components composed of the power supply and main circuit, a simple flat wave circuit can be used instead of inductance.

inverter

Contrary to rectifiers, inverters convert DC power into AC power at the required frequency, allowing six switching devices to 

turn on and off for a predetermined time to obtain a 3-phase AC output. Taking a 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 to the main circuit that supplies power to an asynchronous 

motor (with adjustable voltage and frequency). It consists of a "calculation circuit" for frequency and voltage, a "voltage and 

current detection circuit" for the main circuit, a "speed detection circuit" for the motor, a "drive circuit" that amplifies the control 

signal of the calculation circuit, and a "protection circuit" for the inverter and motor.

(1) Operational circuit: Compare external speed, torque and other instructions 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 circuit potential to detect voltage, current, etc.

(3) Drive circuit: The circuit that drives the main circuit components. It is isolated from the control circuit to make the main 

circuit components turn on and off.

(4) Speed detection circuit: The signal from the speed detectors (tg, plg, etc.) installed on the asynchronous motor shaft 

machine is used as the speed signal, which is fed into the calculation circuit. According to the instructions and calculations, 

the motor can operate at the commanded speed.

(5) Protection circuit: Detect the voltage, current, etc. of the main circuit. In case of overload or overvoltage, in order to 

prevent damage to the inverter and asynchronous motor.