Suppliers of energy feedback devices for frequency converters remind you that in traditional frequency control systems composed of frequency converters, asynchronous motors, and mechanical loads, when the potential load transmitted by the motor is lowered, the motor may be in a regenerative braking state; Or when the motor decelerates from high speed to low speed (including parking), the frequency may suddenly decrease, but due to the mechanical inertia of the motor, it may be in a regenerative power generation state. There are two methods to handle the regenerative energy of the frequency converter: one is the resistance energy discharge method; Another method is the inverse feedback method. The inverse feedback method is a "dual PWM" structure composed of fully controlled switching elements, but its high cost limits its widespread use. Below is an introduction to a new feedback method for regenerating energy in a frequency converter.
Working principle of energy feedback
The feedback of regenerative energy is to feed back the accumulated electrical energy at both ends of the filtering capacitor generated by the motor in regenerative braking state to the power grid. As a feedback circuit, two conditions should be met:
(1) When the frequency converter is working normally, the feedback device does not work. The feedback device only works when the DC bus voltage is higher than a certain value. When the DC bus voltage drops back to normal, the feedback device should be turned off in a timely manner, otherwise it will increase the burden on the rectifier circuit.
(2) The feedback current of the inverter should be controllable.
Inverter section
V1-V6 thyristors form a three-phase bridge inverter circuit. Thyristors have the advantages of low cost, simple control, reliable operation, and mature technology. But thyristors are semi controlled components, and the inverter circuit composed of thyristors must ensure that the minimum inverter angle is greater than 30 °, otherwise it is easy to cause inverter failure, but this makes the normal voltage of the DC bus higher than the inverter voltage. The inverter circuit composed of thyristors can start the inverter by emitting a trigger pulse, but cannot stop the inverter by canceling the trigger pulse. If the trigger pulse is cancelled during inversion, it will result in serious consequences of inversion failure. Therefore, it is necessary to use the method of cutting off the DC circuit to stop the inverter.
The function of VT is twofold: one is to control the start or stop of the inverter circuit. When VT is turned on, the DC voltage is applied to the inverter bridge to start the inverter; When VT is turned off, the DC circuit is cut off and the inverter stops (at this time, the trigger pulse is optional). The normal voltage of the DC bus is approximately DC600V (considering a fluctuation of ± 10% in the grid voltage). The start stop of the inverter depends on the magnitude of the DC bus voltage and adopts hysteresis control. When the DC bus voltage is higher than 1.2 × 600V, the inverter is started, and when it is lower than 1.1 × 600V, the inverter is turned off. Another function of VT is to control the magnitude of the inverter current.
Control of inverter current
When reversing, the DC bus voltage and the inverter voltage are connected in parallel with the same polarity, and the bus voltage is higher than the inverter voltage. Inductance L is used to balance the voltage difference. The control of VT can adopt PWM current hysteresis control method, and the current hysteresis method is used here.
When iL<I Α L-IL, VT conducts; The direct current voltage is applied to the inductor L and the inverter bridge, forming a current in path ①, and the current iL begins to rise; When iL rises above I3 L+IL, VT is turned off and the inductor continues to flow through diode D. The current iL begins to decrease. When iL drops to I3 L-IL, VT conducts again and iL begins to rise again. By the on/off changes of VT, the inverter current iL is maintained at a set value I3, and regardless of how the peak value of the inverter voltage changes, due to the use of high-frequency switch control, the inductance L can be kept very small.
In summary, the conduction of VT should meet two conditions simultaneously: (1) the DC voltage Uc is higher than the set voltage upper limit; (2) When the inverter current iL is less than the set lower limit of current.
The shutdown of VT should meet one of the following two conditions: (1) the DC voltage Uc is lower than the set voltage lower limit; (2) When the inverter current iL exceeds the set upper limit.
In order to avoid frequent VT switching, hysteresis control is used for voltage Uc and current iL, and the loop width is the difference between the set upper and lower limits.
Calculation of inductance
To simplify the calculation and ignore the instantaneous variation of the inverter voltage Vd Β, which is considered a constant quantity, the following equation can be obtained: L diL dt=Uc Ud Β Solving the equation yields t1=2ILL Uc Ud Β, where IL - current hysteresis width;
Uc - DC voltage; Ud Β - average value of inverter voltage.
In the t2 interval, VT is turned off and the voltage continues to flow through D.
Existe a seguinte equação: L diL dt=- Ud Β Solução: t2=2ILL Ud Β Período de chaveamento: T=t1+t2=2ILLUc Ud Β (Uc Ud Β) Frequência de chaveamento: f=Ud Β (Uc Ud Β) IILLUc Indutância: L=Ud Β (Uc Ud Β) 2ILUCf. A equação acima indica que, quando f é muito alto, L é muito pequeno. Isso é diferente dos circuitos inversores de tiristores típicos. A fórmula acima pode ser usada como base para selecionar a indutância.
Cálculo da corrente de descarga do capacitor
Somente quando VT está conduzindo, pode haver uma corrente de descarga fluindo para fora do capacitor. Portanto, o valor médio da corrente de descarga é: Ic = t1 TI 3 L. Substituindo a fórmula acima na fórmula do ciclo de chaveamento, o resultado é: Ic = Ud Β Uc I 3 L