1. Introduction Many production machines require rapid deceleration or stopping during operation. Some equipment requires maintaining a certain speed difference or stretching ratio between several machines during production. This leads to the problem of regenerative braking, causing the motor to operate in the second or fourth quadrant. However, in practical applications, most general-purpose frequency converters use voltage source control, with a large capacitor clamping the voltage in the intermediate DC link, preventing rapid reversal. Furthermore, the AC/DC circuit usually uses an uncontrolled rectifier bridge, which cannot reverse the current. Therefore, achieving regenerative braking and four-quadrant operation is quite difficult. [align=center]Figure 1: Two operating states of a frequency converter speed control system[/align] In a frequency converter speed control system, motor deceleration and stopping are achieved by gradually reducing the frequency. At the instant the frequency decreases, the synchronous speed of the motor decreases accordingly, while the rotor speed remains unchanged due to mechanical inertia. When the synchronous speed w1 is less than the rotor speed w, the phase of the rotor current changes by almost 180 degrees, and the motor changes from motoring to generating. Simultaneously, the torque on the motor shaft becomes braking torque Te, causing the motor speed to drop rapidly, and the motor enters a regenerative braking state. The regenerated electrical energy P is fed back to the DC circuit after full-wave rectification by the freewheeling diode. Since the electrical energy in the DC circuit cannot be fed back to the grid through the rectifier bridge, it is absorbed only by the inverter's own capacitor. Although other parts can consume electrical energy, the capacitor still accumulates charge for a short time, forming a "pump-up voltage," causing the DC voltage Ud to rise. Excessively high DC voltage will damage various components. Therefore, necessary measures must be taken to handle this regenerative energy when the load is in a regenerative braking state. This article describes the methods for handling regenerative energy: regenerative braking and regenerative braking. 2. Working Mode of Regenerative Braking The regenerative braking method uses a discharge resistor unit component added to the DC side of the inverter to dissipate the regenerative electrical energy in the power resistor to achieve braking (as shown in Figure 2). This is a direct method for handling regenerative energy. It dissipates the regenerative energy through a dedicated energy-dissipating braking circuit, converting it into heat energy. Therefore, it is also called "resistive braking." It consists of two parts: a braking unit and a braking resistor. 2.1 Braking Unit The function of the braking unit is to connect the energy-dissipating circuit when the DC circuit voltage Ud exceeds a specified limit (such as 660V or 710V), allowing the DC circuit to release energy as heat after passing through the braking resistor. Braking units can be divided into two types: built-in and external. The former is suitable for low-power general-purpose frequency converters, while the latter is suitable for high-power frequency converters or operating conditions with special braking requirements. In principle, there is no difference between the two; both act as a "switch" to connect the braking resistor. It includes a power transistor, a voltage sampling and comparison circuit, and a drive circuit. 2.2 Braking Resistor The braking resistor is the carrier used to dissipate the motor's regenerative energy as heat energy. It includes two important parameters: resistance value and power capacity. In engineering applications, corrugated resistors and aluminum alloy resistors are commonly used. The former features a vertical corrugated surface for better heat dissipation and reduced parasitic inductance, and uses a highly flame-retardant inorganic coating to effectively protect the resistance wire from aging and extend its service life. The latter has superior weather resistance and vibration resistance compared to traditional ceramic frame resistors, making it widely used in demanding and harsh industrial control environments. It is easy to install tightly, easy to add heat sinks, and has an aesthetically pleasing appearance. 2.3 The braking process involves the following steps: A. When the motor decelerates or reverses under external force (including being driven), the motor operates in a generator state, feeding energy back to the DC circuit, increasing the bus voltage. B. When the DC voltage reaches the state where the braking unit is on, the power transistor of the braking unit conducts, and current flows through the braking resistor. C. The braking resistor consumes electrical energy as heat, reducing the motor speed and lowering the bus voltage. D. When the bus voltage drops to the value at which the braking unit should be turned off, the power transistor of the braking unit is cut off, and no current flows through the braking resistor. E. The bus voltage value is sampled, and the braking unit repeats the ON/OFF process to balance the bus voltage and ensure normal system operation. 2.4 Installation Requirements: The wiring distance between the braking unit and the frequency converter, and between the braking unit and the resistor, should be as short as possible (wire length less than 2m), and the wires should be thick enough. During operation, the resistor will generate a significant amount of heat; therefore, adequate heat dissipation is crucial, and heat-resistant wires should be used. The wires should not touch the resistor. The discharge power resistor should be securely fixed with insulating baffles, and the installation location should ensure good heat dissipation. It is recommended that the resistor be installed on the top of the electrical control cabinet. 2.5 Selection of Braking Unit and Braking Resistor: A. First, estimate the braking torque. Generally, during motor braking, there is a certain amount of internal loss in the motor, approximately 18-22% of the rated torque. Therefore, if the calculated result is less than this range, a braking device is unnecessary. B. Next, calculate the resistance value of the braking resistor. During the operation of the braking unit, the rise and fall of the DC bus voltage depends on the constant RC, where R is the resistance value of the braking resistor, and C is the capacitance of the electrolytic capacitor inside the frequency converter. Here, the braking unit's operating voltage is generally 710V. C. Then, select the braking unit. The maximum operating current of the braking unit is the sole criterion for selection. The calculation formula is as follows: D. Finally, calculate the nominal power of the braking resistor. Since the braking resistor operates on a short-time basis, based on the resistor's characteristics and technical specifications, we know that the nominal power of the resistor will be less than the power consumed when energized. This can generally be calculated using the following formula: Nominal power of braking resistor = Derating factor of braking resistor × Average power consumed during braking × Braking utilization rate. 2.6 Braking Characteristics The advantage of energy-consuming braking (resistive braking) is its simple construction. The disadvantage is reduced operating efficiency, especially during frequent braking, which consumes a large amount of energy, and the capacity of the braking resistor will increase. 3. Regenerative Braking with Shared DC Bus For motors that frequently start and brake, or operate in four quadrants, how to handle the braking process not only affects the system's dynamic response but also raises economic concerns. Therefore, regenerative braking has become a focus of discussion. However, given that most general-purpose frequency converters cannot achieve regenerative energy through a single frequency converter, how can regenerative braking be achieved in the simplest way? To address the above issues, a regenerative energy feedback system using a shared DC bus is introduced here. This method fully utilizes the regenerative energy generated during braking, achieving both energy savings and regenerative energy processing. 3.1 Working Principle We know that a typical asynchronous motor multi-drive system includes a rectifier bridge, a DC bus power supply circuit, and several inverters. The energy required by the motor is output in DC mode through the PWM inverter. In a multi-drive system, the induced energy during braking is fed back to the DC circuit. Through the DC circuit, this feedback energy can be consumed by other motors in a motoring state. When braking requirements are particularly high, only a shared braking unit needs to be connected in parallel on the shared bus. In practical applications, multi-drive systems are expensive and have few brands, often used in high-end markets such as steel and paper manufacturing. Referring to numerous small braking system applications, this is also a highly efficient and energy-saving braking method. Figure 3 shows a typical braking method using a shared DC bus. M1 is in motoring mode, while M2 is frequently in generating mode. A three-phase AC power supply of 380V is connected to the inverter VF1 terminal on motor M1 in motoring mode, while VF2 is connected to the bus of VF1 via a shared DC bus. In this mode, VF2 is used only as an inverter. When M2 is in motoring mode, the required energy is obtained from the AC grid through the rectifier bridge of VF1; when M2 is generating mode, the feedback energy is consumed by M2 in motoring mode via the DC bus. 3.2 Application Scope The braking method using a shared DC bus can be typically applied to papermaking machinery, printing machinery, centrifuges, and system drives. These applications share a common characteristic: the capacity of M2 in generating mode is much smaller than the capacity of M1 in motoring mode, and when M1 stops motoring (i.e., inverter VF1 is in standby mode), M2 immediately switches from generating mode to motoring mode. This prevents the DC bus voltage from rising rapidly, keeping the system in a relatively stable state. A centrifuge is used as an example for this application. The filter-type screw discharge centrifuge continuously feeds and discharges at full speed, automatically completing processes such as feeding, separation, washing, and discharging. The core of the centrifuge is the filter-type rotating drum, which uses the differential speed between the main and auxiliary units to control the discharge speed, achieving unmanned and safe operation. During processing, the main unit is always in electric mode, while the auxiliary unit, due to the speed difference, is essentially in generator mode. The power ratio of the main and auxiliary units is typically 4:1, such as 22KW and 5.5KW, 30KW and 7.5KW, 45KW and 11KW, conforming to the operating mode described in this section. Considering that the auxiliary unit's power supply is also provided by the rectifier bridge of the main unit's frequency converter, the rated current of the rectifier bridge of VF1 must be taken into account (different frequency converter manufacturers have different rectifier bridge specifications) to determine the selection of VF1. The selection of VF2 must consider a frequency converter capable of shielding input phase loss functionality. After applying this braking method, the centrifuge not only improves efficiency but also achieves good energy savings, stable operation, and simple maintenance. 3.3 Braking Characteristics The braking method using a shared DC bus has the following significant characteristics: a. The shared DC bus and shared braking unit greatly reduce the redundant configuration of rectifiers and braking units, resulting in a simple, reasonable, economical, and reliable structure. b. The intermediate DC voltage of the shared DC bus is constant, and the parallel capacitors have a large energy storage capacity. c. Each motor operates under different conditions, with complementary energy feedback, optimizing the dynamic characteristics of the system. d. It improves the system power factor, reduces grid harmonic current, and improves system power efficiency. 4 Braking Method Feedback to the AC Grid In production operations, we often encounter another problem: how to truly achieve energy feedback from the motor to the DC bus, and then from the DC bus to the AC grid? Since general frequency converters typically use uncontrolled rectifier bridges, other control methods must be adopted to achieve this. 4.1 Working Principle To achieve bidirectional energy transfer between the DC circuit and the power supply, one of the most effective methods is to use active inverter technology: that is, to invert regenerated electrical energy into AC power with the same frequency and phase as the grid and feed it back to the grid, thereby achieving braking. Figure 4 shows the principle of the regenerative braking system. It uses a current-tracking PWM rectifier, which easily enables bidirectional power flow and provides a fast dynamic response. Furthermore, this topology allows for complete control over the exchange of reactive and active power between the AC and DC sides. 4.2 Braking Characteristics Widely used in energy regenerative braking applications of PWM AC drives, offering high energy-saving efficiency; It does not generate any abnormal high-order harmonic current components, making it environmentally friendly; Power factor ≈ 1; In multi-motor drive systems, the regenerative energy of each individual motor can be fully utilized; It saves investment and is easy to control grid-side harmonics and reactive power components; 5 Conclusion General-purpose voltage-type frequency converters can only operate in quadrants one and three, i.e., motoring mode. Therefore, in the following applications, users must consider using a matching braking method: Motors driving large inertia loads (such as centrifuges, gantry mills, tunnel trolleys, and overhead cranes) requiring rapid deceleration or stopping; motors driving potential energy loads (such as elevators, cranes, and mine hoists); motors frequently in a driven state (such as centrifuge auxiliary machines, paper machine guide roller motors, and chemical fiber machinery drawing machines). The common characteristics of these types of loads require the motor to operate not only in motoring mode (quadrants one and three) but also in regenerative braking mode (quadrants two and four). To ensure the system functions properly under regenerative braking conditions, appropriate braking methods must be employed. This paper attempts to discuss the working principles, application scope, advantages, and disadvantages of three typical braking methods from an engineering perspective: energy consumption braking, braking via feedback to a shared DC bus, and braking via feedback to the AC grid.