The Control Method of the Frequency Inverter and its Principle

The power supply we use is divided into AC power supply and DC power supply. The general DC power supply is mostly obtained by transforming, rectifying and filtering by the AC power supply. AC power accounts for about 95% of the total power used by people.

Whether used in homes or factories, the voltage and frequency of single-phase AC power and three-phase AC power have certain standards according to the regulations of various countries. For example, in mainland China, the direct user single-phase AC voltage is 220V, and the three-phase AC voltage is 220V. The wire voltage is 380V and the frequency is 50Hz. The power supply voltage and frequency in other countries may be different from those in China, such as single-phase 100V/60Hz, three-phase 200V/60Hz, etc..

Standard voltage and frequency AC power supply ispower frequency alternating current. Generally, a device that converts power frequency alternating current with fixed voltage and frequency into alternating current with variable voltage or frequency is called a "frequency inverter".

In order to produce variable voltage and frequency, the device first converts the alternating current of the power supply into direct current (DC), that is rectification. Generally, an inverter converts DC power into an inverter power supply with a certain frequency and voltage. The inverter with adjustable frequency and voltage of the inverter power supply is called a frequency inverter. The waveform output by the frequencyinverter is a simulated sine wave, which is mainly used for speed regulation of three-phase asynchronous motors, also called a variable frequency speed regulator. For variable frequency inverters that are mainly used in instrumentation and testing equipment and have higher waveform requirements, the waveforms need to be sorted and can output standard sine waves, which are called variable frequency power supplies. Generally, variable frequency power supply is 15-20 times more expensive than thefrequencyinverter. Frequencyinverters can also be used in home appliances, which include not only motors (such as air conditioners, etc.), but also products such as fluorescent lamps.

The frequencyinverter mainly adopts AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). It first converts the industrial frequency AC power into DC power through a rectifier, and then converts the DC power into AC power with controllable frequency and voltage for supply. electric motor.

The control method of the frequency converter and its principle are as follows:

1: VVVF (Variable Voltage and Variable Frequency)means changing voltage and changing frequency, which is what people call variable voltage and frequency.

Control principle:

In VVC, the control circuit uses a mathematical model to calculate the optimal motor excitation when the motor load changes and compensates for the load. In addition, the synchronous 60° PWM method integrated on the ASIC circuit determines the optimal switching time of the inverter semiconductor device (IGBTS). Determine the switching time to follow the following principles:

The numerically largest phase maintains its positive or negative potential within 1/6 period (60°). The other two phases change proportionally so that the output line voltage remains sinusoidal and reaches the required amplitude.

Unlike sinusoidal control PWM, VVC works based on the digital quantity of the required output voltage. This ensures that the output of the frequencyinverter reaches the voltage rating, the motor current is a sine wave, and the motor operates the same as when the motor is directly connected to the mains.

Since the motor's constants (stator resistance and inductance) are taken into account when the frequency converter calculates the optimal output voltage, optimal motor excitation can be obtained.

Because the frequencyinverter continuously detects the load current, the frequency converter can adjust the output voltage to match the load, so the motor voltage can adapt to the type of motor and follow changes in the load.

2: CVCF (Constant Voltage and Constant Frequency)means constant voltage and constant frequency, which is what people call constant voltage and constant frequency.

Control principle:

The principle is to apply the principle of vector modulation to a fixed voltage source PWM inverter. This control is based on an improved motor model that better compensates for load and slip. Because both active and reactive current components are important to the control system, controlling the angle of the voltage vector can significantly improve the dynamic performance in the 0-12HZ range, while in the standard PWM U/F drive, the 0-10HZ range There are generally problems.

Using the SFAVM or 60°AVM principle to calculate the switching mode of the inverter can make the ripple of the air gap torque very small (compared to an inverter using synchronous PWM). Users can choose their favorite operating principle, or the inverter automatically selects the control principle based on the radiator temperature. If the temperature is lower than 75°C, the SFAVM principle is used to control, and when the temperature is higher than 75°C, the 60°AVM principle is applied.

Above are the bried explanations about the control methods and the principles of the frequency inverter.