With the development of industrial automation production, the frequency of using frequency converters has been increasing. In order to maximize production efficiency, associated equipments for frequency converters, such asEenergy Consumption Brakingunits and braking resistors, are often added. The following is a brief analysis of the optimal selection methods for theEnergy Consumption Brakingunit and braking resistor of frequency converters based on the characteristics, deficiencies, and composition of the Energy Consumption Braking of frequency converters.

  1. Energy Consumption Braking of Frequency Converters

  The method adopted for energy consumption braking is to install a braking unit assembly on the DC side of the frequency converter and consume the regenerative electric energy on the braking resistor to achieve braking. This is the most straightforward and simple method for dealing with regenerative energy. It consumes the regenerative energy on the resistor through a specialized energy consumption braking circuit and converts it into heat energy. This resistor is called the resistor braking.

  The characteristics of energy consumption braking are that the circuit is simple and the price is relatively low. However, its drawback is that during the braking process, as the rotational speed of the motor decreases, the kinetic energy of the driving system also decreases. Consequently, the regenerative capacity and braking torque of the motor are also reduced. Therefore, in driving systems with large inertia, it often happens that the system cannot stop at low speeds and a "creeping" phenomenon occurs, which thus affects the accuracy of the stopping time or the stopping position. Hence, energy consumption braking is only applicable to the stopping of general loads.

Energy consumption braking consists of two parts: the braking unit and the 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 loop exceeds the specified limit value, enabling the DC loop to release energy in the form of heat after passing through the braking resistor. Braking units can be divided into two types: built-in type and external type. The built-in type is suitable for general-purpose frequency converters with low power, while the external type is applicable to high-power frequency converters or working conditions with special requirements for braking. In principle, there is no difference between the two. The braking unit serves as a "switch" to connect the braking resistor and includes a power transistor, a voltage sampling and 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, and it has two important parameters: resistance value and power capacity. Usually, in engineering, corrugated resistors and aluminum alloy resistors are two types that are widely used. The corrugated resistor adopts a surface vertical corrugation design, which is beneficial for heat dissipation and reducing the parasitic inductance. It also selects a high-flame-retardant inorganic coating to effectively protect the resistance wire from aging and extend its service life. Aluminum alloy resistors have better weather resistance and shock resistance than traditional porcelain frame resistors,they are widely used in harsh environments with high requirements. They are easy to be installed tightly, easy to attach heat sinks, and have an aesthetic appearance.

  The process of energy consumption braking is as follows: When the motor decelerates or rotates in reverse under the action of external forces (including being dragged), the motor operates in a generating state, and the energy is fed back to the DC loop, 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 is turned on, and the current flows through the braking resistor. The braking resistor converts electrical energy into heat energy, reducing the rotational speed of the motor and also lowering the DC bus voltage. When the bus voltage drops to the turn-off value set by the braking unit, the switch power tube of the braking unit is turned off, and no current flows through the braking resistor.

  The wiring distance between the braking unit and the frequency converter, as well as between the braking unit and the braking resistor, should be as short as possible (the wire length should be less than 2m), and the cross-sectional area of the wire 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 the wires should not touch the braking resistor. The braking resistor should be firmly fixed with insulating baffles. 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 frequencyconverter cabinet.

  2. Selection of braking units

  Generally, when braking an electric motor, there are certain losses inside the motor, which are approximately between 18% and 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 working 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 frequency converter.

  If the resistance value of the braking resistor is too large, the braking will not be rapid; if it's too small, the braking switching elements are prone to damage. Generally, when the load inertia is not too large, it's considered that a maximum of 70% of the energy of the motor during braking is dissipated in the braking resistor, and 30% of the energy is dissipated in the motor itself and various losses of the load.

  The dissipative power of the braking resistor for low-frequency braking is generally (1/4-1/5) of the motor power. During frequent braking, the dissipative power should be increased. Some small-capacity frequency converters have built-in braking resistors. However, during high-frequency or heavy-load braking, the built-in braking resistor has insufficient heat dissipation and is prone to damage. At this time, a high-power external braking resistor should be used. All kinds of braking resistors should be resistors with a low-inductance structure; the connecting wires should be short and twisted-pair or parallel wires should be used. The low-inductance measures are taken to prevent and reduce the inductive energy added to the braking switching tube and cause damage to the braking switching tube. If the inductance of the circuit is large and the resistance is small, it will cause damage to the braking switching tube.

  The braking resistor is closely related to the flywheel torque which uses a motor, and the flywheel torque of the motor changes during operation. Therefore, it is relatively 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 frequency converter and UD is the DC bus voltage of the frequency converter.

  Since the braking resistor is for short-term operation, according to the characteristics and technical indicators of the resistor, the nominal power of the braking resistor in the variable-frequency speed- regulation system can generally be obtained by 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 utilization rate.

  In order to reduce the resistance grades of braking resistors, various frequency converter manufacturers often provide braking resistors with the same resistance value for several motors with different capacities. Therefore, there is a relatively large difference in the braking torque obtained during the braking process. For example, The Emerson TD3000 Series drives offer 3kW and 20Ω brake resistors for drives with motor capacities of 22kW, 30kW and 37kW.When the DC voltage is 700 V, the braking unit conducts, and then the braking current is calculated as follows:

  IB = 700 / 20 = 35 A

  The power of the braking resistor is:

  PB0 = (700)2 / 20 = 24.5 kW

  The braking unit and braking resistor used in the variable-frequency speed-control system are essential components for the safe and reliable operation of the variable-frequency speed-control system with regenerative energy and the requirement of accurate stopping. Therefore, when correctly selecting a variable-frequency speed regulation system, the braking units and braking resistors should be optimally chosen. This can not only reduce the chances of failures occurring in the variable-frequency speed regulation system but also enable the designed variable-frequency speed regulation system to have high dynamic performance indicators.