Commonly used frequency converters are selected based on the load characteristics, speed range, static speed accuracy, and starting torque requirements of the production machinery. They typically employ V/F control or vector control methods. The variable frequency motors they drive differ significantly from ordinary motors, and these motors have a wider frequency range. Therefore, prolonged low-frequency operation of the frequency converter can be harmful. However, for long-term low-frequency operation, the installation environment and ventilation performance must be carefully considered. (See the diagram below.)
Here, we'll use V/F control to explain. The frequency converter changes the motor power supply voltage while changing the motor power supply frequency, keeping the motor magnetic flux constant. Within a wide speed range, the motor's efficiency and power factor do not decrease.
Because it controls the ratio of voltage (V) to frequency (F), it is called V/F control. The characteristics of V/F control are a simple control circuit structure, low cost, and good mechanical stiffness, which can meet the smooth speed regulation requirements of general transmissions.
During the process of the inverter's output frequency rising from 0Hz to the fundamental frequency, the output voltage rises proportionally from 0V to the V/F line of the maximum output voltage, which is called the fundamental V/F line, as shown in Figure 1.
Figure 1 shows the definition of the basic V/F characteristic parameters. It is the most widely used type of frequency converter. When the output frequency of the frequency converter increases from 0Hz to 50Hz, the output voltage increases proportionally from 0V to 380V.
As shown in the V/F characteristic parameter definition diagram above, there is an FL (which represents the lower limit frequency), FH (which represents the upper limit frequency), FB (which represents the rated frequency), and Fmax (which represents the maximum frequency); for example, this type of V/F frequency converter has a maximum frequency range of 50 to 500 Hz, a rated frequency of 50 Hz, a basic operating frequency of 1 to 500 Hz, and a basic maximum output voltage of 1 to 480 V;
In addition, special attention must be paid to the V/F curve setting of the frequency converter, and corresponding settings should be adopted for different load characteristics. This is because there are multiple control settings for the V/F curve, and the multiple voltage points also need to be adjusted according to your own load conditions (0.00~100%). The default values set by the frequency converter at the factory may not be suitable for you.
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