The Components of the VFD and Its Working Principle

In addition, the frequency inverter also has many protection functions, such as overcurrent, overvoltage, overload protection, etc. With the continuous improvement of industrial automation, frequency inverters have also been widely used.

1. Main circuit

The main circuit is the power conversion part that provides voltage and frequency modulation power to the asynchronous motor. The main circuit of the frequencyinverter can be roughly divided into two categories: the voltage type is a frequencyinverter that converts the DC voltage source into AC, and the filter of the DC circuit is a capacitor. The current type is a frequencyinverter that converts DC from a current source into AC, and its DC loop filter is an inductor.

2. Rectifier

Diode converters are widely used, which convert power frequency power into DC power. Two sets of transistor converters can also be used to form a reversible converter. Since its power direction is reversible, it can perform regenerative operation.

3. Smooth wave circuit

The DC voltage rectified by the rectifier contains a pulsating voltage six times the frequency of the power supply. In addition, 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 pulsating voltage (current). When the device capacity is small, if the power supply and main circuit components have margin, the inductor can be omitted and a simple smoothing circuit can be used.

4. Inverter

Contrary to the rectifier, the inverter converts DC power into AC power at the required frequency. By turning on and off six switching devices at a determined time, a three-phase AC output can be obtained. Taking a voltage type 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 the asynchronous motor (voltage and frequency are adjustable). It has a "computation circuit" for frequency and voltage, a "voltage and current detection circuit" for the main circuit, and a "speed detection circuit" for the motor. Circuit", a "drive circuit" that amplifies the control signal of the arithmetic circuit, and a "protection circuit" for the inverter and motor.