Convert the input AC power source (such as mains electricity) into DC power via a rectifier circuit.

　　Rectification: The input AC power supply (such as mains electricity) is converted into DC by a rectifier circuit. A rectifier circuit typically consists of a bridge rectifier, which rectifies both the negative and positive half-cycles of the AC waveform, producing a pulsating DC voltage.

　　Filtering: The DC voltage obtained after rectification is pulsating and contains significant ripple. To achieve a smooth DC voltage, it must be filtered using a filtering circuit. Such circuits typically employ large‑value electrolytic capacitors, which smooth the DC voltage by charging and discharging, thereby eliminating its pulsating components.

　　Inverter: The filtered DC voltage is further converted by the inverter circuit into an AC voltage whose frequency and amplitude can both be adjusted, supplying power to the motor. The inverter circuit is the core component of the variable-frequency drive and consists of multiple power electronic devices, such as IGBTs. By controlling the on–off states of these devices, it is possible to convert DC into AC with the desired frequency and voltage. During the inversion process, by adjusting the switching frequency and duty cycle of the power electronic devices, precise control over the output voltage and frequency can be achieved.

　　Control: The control section of a variable-frequency drive comprises the control circuit and the protection circuit. Based on the speed or frequency setpoint provided by the user, the control circuit employs a microprocessor or a dedicated control chip to compute the corresponding control signals, which then regulate the switching of power electronic devices in the inverter circuit. Meanwhile, the protection circuit continuously monitors the operating conditions of both the motor and the drive; upon detecting any abnormality—such as overcurrent, overvoltage, or overload—it promptly initiates protective measures to prevent damage to the motor and the drive.

　　The operating principle of a variable-frequency drive (VFD) involves power electronics and control theory, and can be broadly divided into four main stages: rectification, filtering, inversion, and control. First, the rectifier circuit within the VFD converts the incoming AC supply into DC. This is typically accomplished using semiconductor devices such as diodes or thyristors. By controlling their on‑off states, these devices transform the AC into a unidirectional pulsating DC, thereby providing a stable DC power source for the subsequent stages of the circuit.

　　The filter circuit smooths the rectified DC voltage by removing its ripple components. This process ensures the stability of the DC voltage, which is critical for the subsequent inversion stage. Typically, a filter circuit consists of capacitors and inductors; it leverages the energy‑storage capability of capacitors and the current‑limiting effect of inductors to achieve smooth filtering of the DC output.

　　The inverter circuit then converts the filtered DC into an AC output with adjustable frequency, amplitude, and phase, which is supplied to the motor. This process is accomplished using semiconductor devices such as thyristors or IGBTs (insulated-gate bipolar transistors). These devices, driven by signals from the control circuit, rapidly switch between on and off states to modulate the frequency, amplitude, and phase of the output AC, thereby enabling precise control of the motor’s speed and torque.

　　The control circuit is the core component of a variable-frequency drive. It compares and processes user‑set parameters—such as speed setpoints and load variations—with feedback signals—such as the motor’s actual speed and current—and then outputs corresponding control signals to the inverter circuit. This process enables precise regulation of the motor’s speed and torque, meeting the requirements of diverse application scenarios.

　　A variable-frequency drive achieves precise control of motor speed by adjusting the frequency of the power supply to the motor. Based on the direct proportionality between motor speed and input power‑supply frequency—expressed as n = 60f/p, where n is the speed, f is the frequency, and p is the number of pole pairs—the drive can output AC at different frequencies, thereby varying the motor’s speed. Specifically, to increase the motor speed, the drive raises the output frequency; conversely, to reduce the speed, it lowers the output frequency.