Les fournisseurs de systèmes de freinage rappellent qu'avec le développement de l'automatisation industrielle, l'utilisation des variateurs de fréquence est de plus en plus fréquente. Afin d'optimiser la productivité, il est souvent nécessaire d'accroître les performances des variateurs de fréquence en y intégrant des équipements auxiliaires tels que des systèmes de freinage à consommation d'énergie et des résistances de freinage. Cet article analyse les méthodes de sélection optimale des systèmes de freinage à consommation d'énergie et des résistances de freinage dans les variateurs de fréquence, en s'appuyant sur leurs caractéristiques, leurs limitations et leur composition.
1. Freinage par consommation d'énergie du convertisseur de fréquence
The method used for energy consumption braking is to install a braking unit component on the DC side of the frequency converter, which consumes the regenerated electrical energy on the braking resistor to achieve braking. This is the most direct and simple way to process the regenerated energy. It consumes the regenerated energy on the resistor through a dedicated energy consumption braking circuit and converts it into thermal energy. This resistor is called resistance braking.
The characteristics of energy consumption braking are simple circuit and low price. However, during the braking process, as the motor speed decreases, the kinetic energy of the driving system also decreases, resulting in a decrease in the regenerative capacity and braking torque of the motor. Therefore, in drag systems with high inertia, it is common to encounter the phenomenon of "crawling" at low speeds, which affects the accuracy of parking time or position. Therefore, energy consumption braking is only applicable to parking with general loads. Energy consumption braking includes two parts: the braking unit and the braking resistor.
(1) Braking unit
The function of the braking unit is to connect the energy dissipation circuit when the voltage Ud of the DC circuit exceeds the specified limit, allowing the DC circuit to release energy in the form of thermal energy after passing through the braking resistor. The braking unit can be divided into two types: built-in and external. The built-in type is suitable for low-power general-purpose frequency converters, while the external type is suitable for 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, which includes a power transistor, a voltage sampling comparison circuit, and a driving circuit.
(2) Braking resistor
Braking resistor is a carrier used to consume the regenerative energy of an electric motor in the form of thermal energy, which includes two important parameters: resistance value and power capacity. Two commonly used types of resistors in engineering are corrugated resistors and aluminum alloy resistors: corrugated resistors use surface vertical corrugations to facilitate heat dissipation and reduce parasitic inductance, and high flame retardant inorganic coatings are selected to effectively protect the resistance wires from aging and extend their service life; Aluminum alloy resistors have better weather resistance and vibration resistance than traditional porcelain frame resistors, and are widely used in harsh environments with high requirements. They are easy to install tightly, easy to attach heat sinks, and have a beautiful appearance.
The process of energy consumption braking is as follows: when the electric motor decelerates or reverses under external force (including being dragged), the electric motor runs in a generating 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 current flows through the braking resistor; The braking resistor converts electrical energy into thermal energy, reducing the speed of the motor and lowering the DC bus voltage; When the bus voltage drops to the cut-off value set by the braking unit, the switching power transistor of the braking unit is cut 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 (with a wire length of less than 2m), and the wire cross-section should meet the requirements for the discharge current of the braking resistor. When the braking unit is working, the braking resistor will generate a large amount of heat. The braking resistor should have good heat dissipation conditions, and heat-resistant wires should be used to connect the braking resistor. The wires should not touch the braking resistor. The braking resistor should be firmly fixed with insulation pads, and the installation position should ensure good heat dissipation. When installing the braking resistor in the cabinet, it should be installed on the top of the frequency converter cabinet.
2. Selection of braking unit
In general, when braking an electric motor, there is a certain amount of loss inside the motor, which is about 18% to 22% of the rated torque. Therefore, if the required braking torque is calculated to be less than 18% to 22% of the rated torque of the motor, there is no need to connect the braking device.
When selecting a braking unit, the maximum operating current of the braking unit is the only basis for selection.
3. Optimization selection of 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 capacity of the internal capacitor of the frequency converter.
The resistance value of the braking resistor is too high, causing slow braking. If it is too small, the braking switch components are easily damaged. Generally, when the inertia of the load is not too large, it is believed that up to 70% of the energy consumed by the motor during braking is consumed by the braking resistor, and 30% of the energy is consumed by various losses of the motor itself and the load.
The dissipated power of the braking resistor for low-frequency braking is generally 1/4 to 1/5 of the motor power, and the dissipated power needs to be increased during frequent braking. Some small capacity frequency converters are equipped with braking resistors inside, but when braking at high frequencies or gravity loads, the internal braking resistors have insufficient heat dissipation and are prone to damage. In this case, high-power external braking resistors should be used instead. All types of braking resistors should use resistors with low inductance structures; The connecting wire should be short and twisted pair or parallel wire should be used. Low inductance measures should be taken to prevent and reduce the energy of inductance from being added to the brake switch tube, causing damage to the brake switch tube. If the inductance of the circuit is large and the resistance is small, it will cause damage to the brake switch tube.
The braking resistance is closely related to the flywheel torque of the electric motor, and the flywheel torque of the electric motor varies during operation. Therefore, it is difficult to accurately calculate the braking resistance, and an approximate value is usually obtained using empirical formulas.
RZ>=(2 × UD)/In the formula: Ie rated current of the frequency converter; UD frequency converter DC bus voltage
Due to the short-term working mode of the braking resistor, based on the characteristics and technical specifications of the resistor, the nominal power of the braking resistor in the variable frequency speed regulation system can generally be calculated 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; η is the braking utilization rate.
In order to reduce the resistance level of braking resistors, various frequency converter manufacturers often provide braking resistors with the same resistance value for several different capacities of motors. Therefore, the difference in braking torque obtained during the braking process is significant. For example, the Emerson TD3000 series frequency converter provides a braking resistor specification of 3kW and 20 Ω for frequency converters with motor capacities of 22kW, 30kW, and 37kW. 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
Le système de freinage et la résistance de freinage utilisés dans la régulation de vitesse à fréquence variable sont des éléments essentiels au fonctionnement sûr et fiable de ce système, notamment pour la récupération d'énergie et les manœuvres de stationnement précises. Par conséquent, lors du choix d'un système de régulation de vitesse à fréquence variable adapté, il est impératif d'optimiser le choix du système de freinage et de la résistance de freinage. Ceci permet non seulement de réduire les risques de panne, mais aussi d'optimiser les performances dynamiques du système.