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With the development of industrial automated production, the frequency of inverter usage has been increasing. To maximize production efficiency, supporting equipment for inverters, such as energy-consuming braking units and braking resistors, are often added. Based on the characteristics, deficiencies, and composition of inverter energy-consuming braking, this article briefly analyzes the optimal selection methods for inverter energy-consuming braking units and braking resistors.

  1. Inverter energy-consuming braking

  The method used in energy-consuming braking is to install a braking unit component on the DC side of the inverter, and the regenerative electric energy is consumed on the braking resistor to achieve braking. This is the most direct and simple method for handling regenerative energy. It consumes the regenerative energy on the resistor through a dedicated energy-consuming braking circuit and converts it into heat. This resistor is called a resistive brake.

  The characteristics of energy-consuming braking are a simple circuit and low cost. The drawback is that during the braking process, as the motor speed decreases, the kinetic energy of the drag system also decreases. As a result, the regenerative capacity and braking torque of the motor also decrease. Therefore, in a drag system with large inertia, the system often cannot stop at low speeds, resulting in a "creeping" phenomenon, which affects the accuracy of the parking time or position. Therefore, energy-consuming braking is only suitable for the parking of general loads. Energy-consuming braking consists of two parts: a braking unit and a braking resistor.

  (1) Braking Unit

  The function of the braking unit is to connect the energy-consuming circuit when the voltage Ud of the DC circuit exceeds the specified limit value, so that the DC circuit releases energy in the form of heat through the braking resistor. Braking units can be divided into two types: built-in and external. The built-in type is suitable for small-power general-purpose inverters, while the external type is suitable for high-power inverters or working conditions with special braking requirements. In principle, there is no difference between the two. The braking unit acts as a "switch" to connect the braking resistor, which includes a power transistor, a voltage sampling comparison circuit, and a drive circuit.

  (2) Braking Resistor

  The braking resistor is a carrier used to consume the regenerative energy of the motor in the form of heat. It includes two important parameters: resistance value and power capacity. Usually, two types of resistors are widely used in engineering: corrugated resistors and aluminum alloy resistors. Corrugated resistors have vertical corrugations on the surface, which is beneficial for heat dissipation and reduces the parasitic inductance. They are coated with a high-flame-retardant inorganic coating, which effectively protects the resistance wire from aging and extends its service life. Aluminum alloy resistors have better weather resistance and vibration resistance than traditional ceramic skeleton resistors. They are widely used in harsh environments with high requirements, are easy to install closely, can be easily equipped with heat sinks, and have an aesthetic appearance.

  The process of energy-consuming braking is as follows: When the motor decelerates or reverses under external force (including being dragged), the motor operates in a power generation state, and the energy is fed back to the DC circuit, causing the bus voltage to rise. The braking unit samples the bus voltage. When the DC voltage reaches the conduction value set by the braking unit, the power switch tube of the braking unit conducts, and the current flows through the braking resistor. The braking resistor converts electrical energy into heat, the motor speed decreases, and the DC bus voltage also decreases. When the bus voltage drops to the cut-off value set by the braking unit, the switching power tube of the braking unit cuts off, and no current flows through the braking resistor.

  The wiring distance between the braking unit and the inverter, as well as between the braking unit and the braking resistor, should be as short as possible (the line length should be less than 2m), and the wire cross-section should meet the requirements of the current discharged by the braking resistor. When the braking unit is working, the braking resistor will generate a large amount of heat. Therefore, the braking resistor should have good heat dissipation conditions. The wires connecting the braking resistor should be heat-resistant wires and should not touch the braking resistor. The braking resistor should be firmly fixed with insulating spacers, and the installation position should ensure good heat dissipation. When the braking resistor is installed in the cabinet, it should be installed on the top of the inverter cabinet.

  2. Selection of the Braking Unit

  Generally, when braking a motor, there are certain internal losses in the motor, approximately 18% - 22% of the rated torque. Therefore, if the calculated required braking torque is less than 18% - 22% of the motor's rated torque, there is no need to connect a braking device.

  When selecting a braking unit, the maximum operating current of the braking unit is the sole basis for selection.

  3. Optimal Selection of Braking Resistors

  During the operation of the braking unit, the rise and fall of the DC bus voltage depend on the constant RC, where R is the resistance value of the braking resistor and C is the capacitance of the internal capacitor of the inverter.

  If the resistance value of the braking resistor is too large, the braking will not be rapid; if it is too small, the switching elements for braking are easily damaged. Generally, when the load inertia is not too large, it is considered that during motor braking, a maximum of 70% of the energy is consumed by the braking resistor, and 30% of the energy is consumed by various losses in the motor itself and the load.

  For low-frequency braking, the dissipation power of the braking resistor is generally (1/4 - 1/5) of the motor power. During frequent braking, the dissipation power needs to be increased. Some small-capacity inverters have built-in braking resistors. However, during high-frequency or heavy-load braking, the heat dissipation of the built-in braking resistor is insufficient and it is prone to damage. In this case, a high-power external braking resistor should be used instead. All braking resistors should be low-inductance resistors. The connecting wires should be short, and twisted or parallel wires should be used. The low-inductance measures are taken to prevent and reduce the inductive energy from being applied to the braking switch tube, which could cause damage to the tube. If the loop has high inductance and low resistance, the braking switch tube will be damaged.

  The braking resistor is closely related to the flywheel torque of the motor in use. Since the flywheel torque of the motor changes during operation, it is difficult to accurately calculate the braking resistor. Usually, an approximate value is obtained using an empirical formula.

  RZ >= (2 × UD) / Ie, where Ie is the rated current of the inverter and UD is the DC bus voltage of the inverter.

  Since the braking resistor operates in a short-time duty cycle, according to the characteristics and technical specifications of the resistor, the nominal power of the braking resistor in a variable-frequency speed control system can generally be obtained using the following formula:

  PB = K × Pav × η%, where PB is the nominal power of the braking resistor, K is the derating coefficient of the braking resistor, Pav is the average power consumption during braking, and η is the braking duty cycle.

  To reduce the number of resistance levels of braking resistors, inverter manufacturers often provide braking resistors with the same resistance value for several motors of different capacities. Therefore, there is a significant difference in the braking torque obtained during the braking process. For example, for the Emerson TD3000 series inverters, the braking resistor specifications provided for inverters with motor capacities of 22kW, 30kW, and 37kW are all 3kW, 20Ω. When the braking unit conducts at a DC voltage of 700V, the braking current is:

  IB = 700 / 20 = 35A

  The power of the braking resistor is:

  PB0 = (700)^2 / 20 = 24.5kW

  The braking unit and braking resistor used in a variable-frequency speed control system are essential for the safe and reliable operation of the system with regenerative energy and accurate parking requirements. Therefore, when correctly selecting a variable-frequency speed control system, the braking unit and braking resistor should be optimally selected. This can not only reduce the chance of system failures but also enable the designed variable-frequency speed control system to have high dynamic performance indicators.