Mar 18, 2021
Some thoughts on inverter braking
1. Introduction: In the traditional variable frequency speed regulation system composed of general frequency converter, asynchronous motor and mechanical load, when the potential energy load driven by the motor is lowered, the motor may be in a regenerative braking state; or when the motor is from high speed When decelerating at a low speed (including parking), the frequency can be suddenly reduced, but due to the mechanical inertia of the motor, the motor may also be in a state of regenerative power generation. The mechanical energy stored in the transmission system is converted into electrical energy by the motor, and the inverter is connected to the six continuations. The flow diode is sent back to the DC circuit of the inverter. At this time, the inverter is in a rectifying state. At this time, if energy consumption measures are not taken in the inverter, this part of the energy will cause the voltage of the energy storage capacitor of the intermediate circuit to rise. If the braking is too fast or the mechanical load is a hoist, this part of the energy may cause damage to the inverter, so we should seriously consider this part of the energy. In general-purpose inverters, there are two most common ways to deal with regenerative energy: 1. Dissipate into the "brake resistance" artificially set in the DC circuit in parallel with the capacitor, which is called the dynamic braking state; 2. To feed it back to the grid is called the feedback braking state (also known as the regenerative braking state). There is also a braking method, that is, DC braking, which is used for situations that require accurate stopping or the irregular rotation of the braking motor due to external factors before starting. In books and publications, many experts have talked about the design and application of inverter braking, especially in recent times there have been many articles on "energy feedback braking". Today, the author provides a new type of braking method, which has the advantages of "feedback braking" four-quadrant operation and high operating efficiency. It also has the advantages of "dynamic braking" without pollution to the power grid and high reliability. 2. Dynamic braking: The method of using the braking resistor set in the DC circuit to absorb the regenerative electric energy of the motor is called dynamic braking. The advantages of the dynamic braking principle diagram are simple structure; no pollution to the power grid (compared with feedback braking action) and low cost; the disadvantage is low operating efficiency, especially when frequent braking will consume a lot of energy and the capacity of the braking resistor Will increase. Generally, in general-purpose inverters, low-power inverters (below 22KW) have built-in brake units, and only need to add brake resistors. High-power inverters (above 22KW) require external braking units and braking resistors. 3. Regenerative braking: the realization of energy regenerative braking requires the same frequency and phase voltage control, feedback current control and other conditions. It adopts active inversion technology to invert the regenerative energy into AC power with the same frequency and phase as the grid and return it to the grid, so as to realize the braking four-quadrant operation diagram. The advantage of feedback braking is that the four-quadrant operation power feedback improves the system’s performance. effectiveness. The disadvantages are: 1. This feedback braking method can be used only under the stable grid voltage that is not prone to failure (the grid voltage fluctuation is not more than 10%). Because the grid voltage failure time is greater than 2ms during gen-brake operation, commutation failure may occur and the device may be damaged. 2. In the feedback, there is harmonic pollution to the power grid. 3. The control is complicated and the cost is high. 4. New type of braking method (I call myself: "capacitor feedback braking") 4.1 Reversible chopper: Reversible chopper The circuit composition of the reversible chopper is as follows. The energy flow direction of this chopper can adopt two directions, and it has the characteristic of feeding the energy generated on the load to the power source. When energy is supplied to the load by the power supply E, the device Q2 is disconnected, and the device Q1 and the diode D2 function as a step-down chopper; and when the energy is fed from the load side, the device Q1 is disconnected, and the device Q2 and the diode D1 It functions as a boost chopper. The role of the reversible chopper in energy feedback will be described in detail below. Capacitor feedback brake main circuit principle diagram 4.2 Main circuit principle diagram 4.3 System brief description: The rectifier part adopts ordinary uncontrollable rectifier bridge for rectification, the filter circuit adopts general electrolytic capacitors, and the delay circuit adopts contactors or silicon controlled rectifiers. . The charging and feedback loop adopts the principle of a reversible chopper, which is composed of power module IGBT charging, feedback reactor L and large electrolytic capacitor C (capacity is Farad level, which can be determined according to the operating system of the inverter). The inverter part is composed of power module IGBT. The protection circuit of the large electrolytic capacitor is completed by the chip, IGBT, and power resistor. 4.4 Principle analysis: 4.4.1 Motor generating operation status: CPU monitors the input AC voltage and DC loop voltage in real time, and decides whether to send a charging signal to VT1. Once it is compared with the DC reference voltage value corresponding to the input AC voltage (such as 380VAC- When 530VDC) reaches a certain value, the CPU turns off VT3, and realizes the charging process of the electrolytic capacitor C through pulse conduction to VT1. At this time, the reactor L and the electrolytic capacitor C are voltage-divided to ensure that the electrolytic capacitor C works in a safe range. When the voltage on the electrolytic capacitor C reaches a dangerous value (for example, 370V), and the system is still in the state of generating electricity, and the electric energy is continuously fed back to the DC circuit through the inverter part, the safety circuit plays a role to realize energy consumption braking (resistance system). To control the turn-off and turn-on of VT3, so as to realize that the resistor R consumes excess energy. Generally, this situation will not occur. 4.4.2 Electric motor running state: When the CPU finds that the system is no longer charging, it pulses VT3 on, so that an instantaneous left positive and right negative electricity is formed on the upstream of reactor L, plus the electrolytic capacitor C The voltage can realize the energy feedback process from the capacitor to the DC loop. The CPU controls the switching frequency and duty cycle of VT3 by detecting the voltage on the electrolytic capacitor C and the voltage of the DC loop, thereby controlling the feedback current and ensuring that the DC loop voltage does not appear too high. 5 System Difficulties: 5.1 Selection of Reactor: (a) Considering the particularity of working conditions, we assume that some kind of failure occurs in the system, which causes the potential energy load carried by the motor to freely accelerate and fall, and the motor is in a kind of power generation operation. The regenerative energy is sent back to the DC circuit through six freewheeling diodes, which causes an increase, and quickly puts the inverter in a charging state. At this time, the current will be large. Therefore, the selected reactor wire diameter should be large enough to pass the current at this time. (B) In the feedback loop, in order to make the electrolytic capacitor release as much electrical energy as possible before the next charge, it is not possible to select an ordinary iron core (silicon steel sheet), and it is best to use ferrite material. Iron core. 5.2 Difficulties in control: (a) In the DC circuit of the inverter, the voltage is generally higher than 500VDC, while the withstand voltage of the electrolytic capacitor C is only 400VDC. It can be seen that the control of this charging process is not like energy braking (resistance system). Move) the control method. The instantaneous voltage drop generated on the reactor is, the instantaneous charging voltage of the electrolytic capacitor C is, in order to ensure that the electrolytic capacitor works in a safe range (≤400V), the voltage drop on the reactor must be effectively controlled, and the voltage The drop depends on the inductance and the instantaneous rate of change of the current. (B) During the feedback process, it is necessary to prevent the electric energy discharged by the electrolytic capacitor C from passing through the reactor to cause the DC loop voltage to be too high, which may cause overvoltage protection in the system. 6. Main application occasions and application examples: It is precisely because of the superiority of this new type of braking method (capacitor feedback braking) of the frequency converter that in recent years, many users have put forward requirements based on the characteristics of their equipment. Equipped with this system. At present, Shandong Fengguang Electronics Co., Ltd. has changed from the inverter that used the feedback braking method before (there are still 2 units in normal operation) to the new type of mining hoist series with this capacitive feedback braking method. So far, this Inverters with capacitive feedback braking are operating normally in Ningyang Baoan Coal Mine in Shandong and Taiyuan in Shanxi for a long time, filling this gap in China. With the expansion of the frequency converter application field, this application technology will have a bright future. Specifically, it is mainly used in the hoisting cage (carrying or loading) in the mine, the inclined mine car (single or double drum), Lifting machinery and other industries. In short, it can be used where energy feedback device is needed.