First, the inverter converts the three-phase AC power supplied by the grid into DC via a rectifier circuit.

　　First, the inverter converts the three-phase AC power supplied by the grid into DC via a rectifier circuit. In this process, the rectifier typically consists of a six-diode uncontrolled full-wave bridge, which transforms the AC into a pulsating DC waveform.

　　The DC output obtained after rectification is pulsating and contains a significant ripple component. To achieve a smooth DC voltage, it must be filtered using a filtering circuit. Such circuits typically employ large capacitors to reduce voltage fluctuations, resulting in a more steady and nearly constant DC waveform.

　　The filtered DC is then converted by an inverter circuit into AC with adjustable frequency and voltage, which is supplied to the motor. The inverter circuit is the core component of the variable-frequency drive; by controlling the on–off states of power semiconductor devices (such as IGBTs), it transforms DC into AC at the desired frequency and voltage. During the inversion process, by adjusting the switching frequency and duty cycle of the semiconductor devices, precise control of the output voltage and frequency can be achieved.

　　The key to the operation of a variable-frequency drive lies in the on/off control of its internal IGBTs (insulated-gate bipolar transistors). An IGBT is a high-speed switch that can rapidly turn on or off in response to control signals, thereby regulating the current in the circuit. In a VFD, the rectifier unit first converts mains‑frequency AC into DC, with IGBTs playing a crucial role in this process. Subsequently, high‑capacity capacitors store and filter the DC voltage, stabilizing the current and preparing it for the subsequent inversion stage.

　　The inverter unit is the core component of the variable-frequency drive, consisting of electronic switches made up of multiple IGBTs. These switches are toggled according to a specific control strategy, converting DC power into square waves of varying frequency and amplitude. Finally, the controller regulates the amplitude and pulse width of the output square wave according to the programmed sequence, superimposing these pulses to generate an AC waveform that approximates a sine wave, thereby driving the AC motor.

　　It is worth noting that the inverter’s braking unit and drive unit are equally indispensable. The braking unit ensures that, during motor deceleration or shutdown, the electrical energy generated by the motor is either fed back into the power grid or dissipated in a braking resistor, thereby preventing overvoltage that could damage other electrical components. Meanwhile, the drive unit directly controls the motor’s start-up, operation, and stopping, guaranteeing smooth performance under a wide range of operating conditions.

　　The protection functions of a variable frequency drive are also among its key features. Functions such as overcurrent, overvoltage, and overload protection ensure the safety of both the motor and the drive itself under abnormal operating conditions. For example, when the motor is subjected to excessive load, the drive can promptly adjust voltage and frequency to prevent damage caused by overload.

　　The control section of a variable-frequency drive comprises the control circuit and the protection circuit. The control circuit, based on the set speed or frequency command, uses a microprocessor or a dedicated control chip to generate the corresponding control signals, thereby switching the semiconductor devices in the inverter circuit on and off. Meanwhile, the protection circuit continuously monitors the operating conditions of both the motor and the drive; upon detecting abnormalities such as overcurrent, overvoltage, or overload, it promptly implements protective measures to prevent damage to the motor and the drive.

　　When using a variable-frequency drive for speed control, the motor speed can be adjusted according to actual requirements. Speed-control methods include manual adjustment, automatic regulation, and remote operation. Manual speed control is performed via the VFD’s operator panel or buttons on the control cabinet; automatic speed control is achieved through PLCs or other controllers; and remote control enables speed adjustments via a remote controller or a host computer.