What is a variable frequency drive?

Variable frequency drive (VFD) is the application of frequency conversion technology and microelectronics technology to control the electric drive components of AC motors by changing the frequency and amplitude of the working power supply of the motor. The frequency conversion technology was born under the wide demand of the stepless speed regulation of the AC motor due to the large volume, high failure rate and large application limitations of the traditional DC speed regulation. Other names for VFDs are variable speed drives, adjustable speed drives, adjustable frequency drives, AC drives, micro drives, and inverters.

How the inverter works

The main circuit of the frequency converter is the power conversion part that provides the voltage regulation and frequency regulation power supply for the asynchronous motor. The main circuit of the frequency converter can be roughly divided into two types: voltage type and current type.

The voltage type is an inverter that converts the DC of the voltage source into an AC, and the filter of the DC circuit is a capacitor. The current type is a frequency converter that converts the DC of the current source into an AC, and its DC loop filter is an inductance. It consists of three parts, a ° rectifier that converts power frequency power into DC power. , a "smoothing loop" that absorbs the voltage pulsation generated in the converter and inverter, and an "inverter" that converts DC power to AC power.

① The rectifier is a diode converter that converts the power frequency power supply into a DC power supply. Two sets of transistor converters can also be used to form a reversible converter, which can perform regenerative operation because its power direction is reversible.

② In the DC voltage rectified by the rectifier, the smoothing circuit contains a pulsating voltage six times the frequency of the power supply, and the pulsating current generated by the inverter also fluctuates the DC voltage. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). If the capacity of the device is small, or the power supply and the main circuit components have a margin, the inductor can also be omitted, and a simple smoothing loop can be used.

③ The inverter is the opposite of the rectifier. The inverter converts the DC power into the AC power of the required frequency, and turns on and off the 6 switching devices in a certain period of time, so as to obtain a 3-phase AC output. The control circuit is a circuit that supplies power to the main circuit of the asynchronous motor with adjustable voltage and frequency, and provides a control signal. ", the "drive circuit" that amplifies the control signal of the arithmetic circuit, and the "protection circuit" of the inverter and the motor.