1. Introduction In the late 1980s, AC variable frequency speed control gradually emerged as a key method for industrial drive speed regulation. Variable frequency speed control offers significant advantages in terms of speed range, accuracy, flexibility, efficiency, and ease of use, making it one of the most promising AC speed regulation methods. Most ordinary frequency converters use a diode rectifier bridge to convert AC to DC, and then use IGBT inverter technology to convert the DC back to AC with adjustable voltage and frequency to control the AC motor. This type of frequency converter can only operate in motoring mode, hence the name "two-quadrant frequency converter." Because two-quadrant frequency converters use a diode rectifier bridge, they cannot achieve bidirectional energy flow, so there is no way to feed the energy from the motor feedback system back to the power grid. In applications where the motor needs to feed energy back, such as elevators, hoists, and centrifuge systems, a resistive braking unit must be added to the two-quadrant frequency converter to dissipate the energy fed back from the motor. Furthermore, in some high-power applications, the diode rectifier bridge generates severe harmonic pollution to the power grid. IGBT power modules can achieve bidirectional energy flow. If IGBTs are used as the rectifier bridge, and a high-speed, high-computing-power DSP generates PWM control pulses, on the one hand, the input power factor can be adjusted to eliminate harmonic pollution to the power grid, making the frequency converter a truly "green product." On the other hand, the energy generated by the motor can be fed back to the power grid, achieving a thorough energy-saving effect. Jia Neng Company has been developing and researching four-quadrant frequency converters since 2003. Currently, it has developed mature products and technologies in two series (380V and 660V) with various power levels, which are widely used in hoisting, coal mining, and oilfield fields. 2. Working Principle of Four-Quadrant Frequency Converters 2.1 Circuit diagram of a four-quadrant frequency converter is shown in Figure 1. Figure 1 Circuit diagram of a four-quadrant frequency converter 2.2 Working Principle When the motor is operating in motor mode, the DSP of the rectifier control unit generates six high-frequency PWM pulses to control the switching on and off of the six IGBTs on the rectifier side. The switching on and off of the IGBT, together with the input reactor, generates a sinusoidal current waveform that is in phase with the input voltage, thus eliminating the 6K±1 harmonic generated by the diode rectifier bridge. The power factor reaches 99%, eliminating harmonic pollution to the power grid. At this time, energy flows from the power grid to the motor through the rectifier and inverter circuits, and the frequency converter operates in the first and third quadrants. The waveforms of the input voltage and input current are shown in Figure 2. When the motor operates in generator mode, the energy generated by the motor is fed back to the DC bus through the diodes on the inverter side. When the DC bus voltage exceeds a certain value, the energy feedback control section on the rectifier side is activated, converting DC to AC. By controlling the phase and amplitude of the inverter voltage, energy is fed back to the power grid, achieving energy saving. At this time, energy flows from the motor to the power grid through the inverter and rectifier sides. The frequency converter operates in the second and fourth quadrants. The main function of the input reactor is current filtering. The feedback current and grid voltage waveforms are shown in Figure 3: Figure 3 Feedback current and grid voltage waveforms 2.3 System Composition of the Four-Quadrant Inverter The main circuit consists of: a pre-charge circuit, input reactor, intelligent power module, electrolytic capacitor, and output reactor. The functions of each part are listed below: Pre-charge circuit: Composed of AC contactor, power resistor, and corresponding control circuit. Its main function is to pre-charge the DC bus capacitor when the system is powered on, preventing the powerful inrush current from burning out the power module. Input reactor: In motoring mode, it stores energy and forms a sinusoidal current waveform. In feedback mode, it filters out the high-frequency components of the current waveform. Intelligent power module (SkiiP): IGBTs on the rectifier and inverter sides, isolation drive, current detection, and various protection and monitoring functions. Electrolytic capacitor: Energy storage and filtering. Output reactor: Reduces output dv/dt, providing some protection for the motor. Control section composition: System auxiliary power supply module, pre-charge control, power interface board, DSP control board, and human-machine interface board. The system auxiliary power supply generates the 5V, 15V, and 24V power required for system control; the pre-charge control is used to control the operation of the pre-charge AC contactor; the power interface board feeds back the current, voltage, and temperature signals required for system control and transmits the PWM control waveform to the drive board. The interface board filters the signals; the DSP control board completes the rectification and inverter PWM control algorithm, acting as the system's brain. The human-machine interface board displays various operating statuses of the inverter and user parameter inputs. 3. Rectification Section The system control block diagram of the rectification section is shown in Figure 4. The system's setpoint is the DC bus voltage command. The error between this command and the DC bus voltage feedback is sent to the PI regulator in the voltage loop. The product of the voltage loop PI regulator and the three-phase input sine wave becomes the three-phase current command. The three-phase current command is compared with its respective current feedback, and the error is sent to the PI regulator in the current loop. The output of the current loop PI regulator can generate PWM control signals for each phase IGBT through carrier modulation, or it can generate PWM signals to control the IGBTs using space vector modulation. The above calculations are all performed using a DSP. 4. Typical Applications Typical applications of four-quadrant frequency converters are in situations with potential load characteristics, such as hoists, locomotive traction, oilfield pumping units, centrifuges, etc. In some high-power applications, four-quadrant frequency converters are also needed to reduce harmonic pollution to the power grid. Taking a hoist as an example, when lifting heavy objects, the four-quadrant frequency converter drives the motor to overcome gravity, and the motor is in an electric state. When lowering the heavy object, the inverter side generates an excitation current, and gravity pulls the motor to generate electricity, and the motor is in a generating state. Potential energy is converted into electrical energy and fed back to the power grid through the rectifier side. 5. Conclusion Focusing on a frequency converter with a PWM-controlled rectifier, which has four-quadrant operation capabilities, can meet the speed regulation requirements of various potential loads. It can convert the regenerative energy of the motor into electrical energy and send it back to the power grid, achieving maximum energy saving. Furthermore, it can reduce power supply harmonic pollution, and the power factor can approach 1, making it a truly "green" frequency converter.