Suppliers of energy feedback devices for frequency converters remind you that currently, simple energy consumption braking is widely used in AC frequency conversion speed control systems, which have disadvantages such as wasting electrical energy, severe resistance heating, and poor fast braking performance. When asynchronous motors frequently brake, using feedback braking is a very effective energy-saving method and avoids damage to the environment and equipment during braking. Satisfactory results have been achieved in industries such as electric locomotives and oil extraction. With the continuous emergence of new power electronic devices, increasing cost-effectiveness, and people's awareness of energy conservation and consumption reduction, there is a wide range of application prospects.
Feedback braking principle
In the variable frequency speed regulation system, the deceleration and stopping of the motor are achieved by gradually reducing the frequency. At the moment when the frequency decreases, the synchronous speed of the motor decreases accordingly. However, due to mechanical inertia, the rotor speed of the motor remains unchanged, and its speed change has a certain time lag. At this time, the actual speed will be greater than the given speed, resulting in a situation where the back electromotive force e of the motor is higher than the DC terminal voltage u of the frequency converter, that is, e>u. At this point, the electric motor becomes a generator, which not only does not require power supply from the grid, but can also send electricity to the grid. This not only has a good braking effect, but also converts kinetic energy into electrical energy, which can be sent to the grid to recover energy, killing two birds with one stone. Of course, there must be an energy feedback device unit for automatic control in order to achieve it. In addition, the energy feedback circuit should also include AC and DC reactors, resistance capacitance absorbers, electronic switches, etc.
As is well known, the bridge rectifier circuit of general frequency converters is three-phase uncontrollable, so it cannot achieve bidirectional energy transfer between the DC circuit and the power supply. The most effective way to solve this problem is to use active inverter technology, and the rectifier part adopts reversible rectifier, also known as grid side converter. By controlling the grid side inverter, the regenerated electrical energy is inverted into AC power with the same frequency, phase, and frequency as the grid, and fed back to the grid to achieve braking. Previously, active inverter units mainly used thyristor circuits, which can only safely perform feedback operation under stable grid voltage that is not prone to faults (grid voltage fluctuations not exceeding 10%). This type of circuit can only safely perform feedback operation of the inverter under stable grid voltage that is not prone to faults (with grid voltage fluctuations not exceeding 10%). Because during power generation braking operation, if the grid voltage braking time is greater than 2ms, commutation failure may occur and components may be damaged. In addition, during deep control, this method has low power factor, high harmonic content, and overlapping commutation, which will cause distortion of the power grid voltage waveform. Simultaneously controlling complexity and high cost. With the practical application of fully controlled devices, people have developed chopper controlled reversible converters using PWM control. In this way, the structure of the grid side inverter is completely the same as that of the inverter, both using PWM control.
From the above analysis, it can be seen that to truly achieve energy feedback braking of the inverter, the key is to control the grid side inverter. The following text focuses on the control algorithm of the grid side inverter using fully controlled devices and PWM control method.
Feedback braking characteristics
À proprement parler, l'onduleur côté réseau ne peut être simplement qualifié de « redresseur », car il remplit à la fois les fonctions de redresseur et d'onduleur. Grâce à l'utilisation de dispositifs d'arrêt automatique, l'amplitude et la phase du courant alternatif sont contrôlées par modulation de largeur d'impulsion (PWM) appropriée, ce qui permet d'obtenir un courant d'entrée proche d'une sinusoïde et de garantir un facteur de puissance du système toujours proche de 1. Lorsque la puissance récupérée par l'onduleur lors du freinage du moteur augmente la tension continue, la phase du courant d'entrée alternatif peut être inversée par rapport à celle de la tension d'alimentation pour assurer un fonctionnement régénératif. Cette puissance récupérée est alors réinjectée dans le réseau électrique alternatif, tandis que le système maintient la tension continue à la valeur souhaitée. Dans ce cas, l'onduleur côté réseau fonctionne en mode actif. Ceci facilite la mise en œuvre d'un flux de puissance bidirectionnel et offre une grande réactivité. Par ailleurs, cette topologie permet au système de contrôler précisément les échanges de puissance réactive et active entre les circuits alternatif et continu, avec un rendement pouvant atteindre 97 % et des avantages économiques significatifs. Les pertes thermiques représentent 1 % de la consommation d'énergie au freinage et ne polluent pas le réseau électrique. Le facteur de puissance est proche de 1, ce qui est respectueux de l'environnement. Par conséquent, le freinage par rétroaction peut être largement utilisé pour optimiser la consommation d'énergie dans les systèmes de transmission CA à modulation de largeur d'impulsion (PWM), notamment lorsque des freinages fréquents sont nécessaires. La puissance du moteur électrique étant élevée, l'économie d'énergie est significative. Selon les conditions de fonctionnement, l'économie d'énergie moyenne atteint environ 20 %.