The basic principle of frequency converters:
Control method
1: VVVF is an abbreviation for Variable Voltage and Variable Frequency, which means changing voltage and changing frequency, also known as voltage and frequency conversion.
2: CVCF is an abbreviation for Constant Voltage and Constant Frequency, which means constant voltage and constant frequency, also known as constant voltage and constant frequency.
VVC 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.
Furthermore, the synchronous 60° PWM method integrated into the ASIC circuitry determines the optimal switching time of the inverter semiconductor device (IGBTS).
The following principles should be followed when determining the switching time:
The phase with the largest numerical value maintains its positive or negative potential for 1/6 of a cycle (60°).
3. Unlike sinusoidal PWM control, VVC operates based on the digital value of the required output voltage. This ensures that the inverter output reaches the rated voltage, the motor current is sinusoidal, and the motor operates as if it were directly connected to AC power.
4: Since the motor constants (stator resistance and inductance) are considered when calculating the optimal output voltage of the frequency converter, the optimal motor excitation can be obtained.
Because the frequency converter continuously monitors the load current, it can adjust the output voltage to match the load, so the motor voltage can adapt to the type of motor and follow the changes in the load.
The control principle of VVC+ 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 very important for the control system, controlling the angle of the voltage vector can significantly improve the dynamic performance in the 0-12Hz range, while the 0-10Hz range generally has problems in standard PWMU/F drives.
Calculating the inverter's switching mode using the SFAVM or 60°AVM principle can minimize air gap torque ripple (compared to inverters using synchronous PWM).
Because the motor constants (stator resistance and inductance) are taken into account when calculating the optimal output voltage of the frequency converter, the optimal motor excitation can be obtained.
Because the frequency converter continuously monitors the load current, it can adjust the output voltage to match the load, so the motor voltage can adapt to the type of motor and follow the changes in the load.
Principles of each component:
1. Rectifier. It is connected to a single-phase or three-phase AC power supply to generate a pulsating DC voltage. There are two basic types of rectifiers—controlled and uncontrolled.
2. Intermediate circuit. It has the following three types:
a) Convert the rectified voltage into direct current.
b) To stabilize or smooth the pulsating DC voltage for use by the inverter.
c) Convert the fixed DC voltage after rectification into a variable DC voltage.
3. Inverter. It generates the frequency of the motor voltage. In addition, some inverters can also convert a fixed DC voltage into a variable AC voltage.
4. Control circuit. It transmits signals to the rectifier, intermediate circuit, and inverter, and also receives signals from these components.
For more information, please visit the Medium and Low Voltage Frequency Inverter channel.
