Analysis of braking technology for frequency converter

1. Introduction

In the traditional variable frequency speed regulation system composed of general frequency converter, asynchronous motor and mechanical load, when the potential load driven by the motor is released, the motor may be in a regenerative braking state; or when the motor decelerates from high speed to low speed (including parking), the frequency can drop suddenly, but due to the mechanical inertia of the motor, the motor may be in a regenerative state, and the mechanical energy stored in the transmission system is converted into electrical energy by the motor and sent back to the DC circuit of the frequency converter through the six freewheeling diodes of the inverter. At this time, the inverter is in a rectifier state. At this time, if the frequency converter does not take measures to consume energy, this part of the energy will cause the voltage of the energy storage capacitor in 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 frequency converter, so we should consider this part of the energy.
In general frequency converters, there are two most common ways to deal with regenerative energy: (1) dissipating it into the "brake resistor" artificially set in parallel with the capacitor in the DC circuit, which is called the dynamic braking state; (2) feeding it back to the power grid, which is called the feedback braking state (also called the regenerative braking state). There is another braking method, namely DC braking, which can be used in situations where accurate parking is required or before starting to brake the irregular rotation of the motor caused by external factors.
Many experts have talked about the design and application of frequency converter braking in books and publications, especially in recent years there have been many articles on "energy feedback braking". Today, the author provides a new braking method, which has the advantages of four-quadrant operation and high operating efficiency of "feedback braking", and also has the advantages of "energy consumption braking" that is pollution-free to the power grid and high reliability.

2. Energy consumption braking

The method of absorbing the regenerative electric energy of the motor using the brake resistor set in the DC circuit is called energy consumption braking.
Its advantages are simple structure; no pollution to the power grid (compared with feedback braking) and low cost; disadvantages are low operating efficiency, especially when frequent braking will consume a lot of energy and the capacity of the brake resistor will increase.
Generally, in general-purpose inverters, small-power inverters (below 22kW) have built-in brake units and only require external brake resistors. High-power inverters (above 22kW) require external brake units and brake resistors.

3. Regenerative braking

The realization of energy regenerative braking requires voltage frequency and phase control, feedback current control and other conditions. It uses active inverter technology to invert the regenerated electric energy into AC power with the same frequency and phase as the power grid and send it back to the power grid to achieve braking. The advantage of regenerative braking is that it can operate in four quadrants, as shown in Figure 3. Electric energy feedback improves the efficiency of the system. Its disadvantages are: (1) This regenerative braking method can only be used under stable grid voltage that is not prone to failure (grid voltage fluctuation is not more than 10%). Because during the generation braking operation, if the grid voltage failure time is greater than 2ms, commutation failure may occur, damaging the device. (2) During feedback, there is harmonic pollution to the grid. (3) The control is complex and the cost is high. IV. New braking method (capacitor feedback braking)

1. Main circuit principle
The rectifier part uses an ordinary uncontrolled rectifier bridge for rectification, the filter circuit uses a general electrolytic capacitor, and the delay circuit uses a contactor or a thyristor. The charging and feedback circuits are composed of a power module IGBT, a charging and feedback reactor L, and a large electrolytic capacitor C (the capacity is about a few tenths of a farad, which can be determined according to the working system of the inverter). The inverter part is composed of a power module IGBT. The protection circuit is composed of IGBT and a power resistor.

(1) In the motor generating state,
the CPU monitors the input AC voltage and DC circuit voltage νd in real time to decide whether to send a charging signal to VT1. Once νd is higher than the DC voltage value corresponding to the input AC voltage (such as 380VAC-530VDC) to a certain value, the CPU shuts down VT3 and realizes the charging process of the electrolytic capacitor C by pulse conduction of VT1. At this time, the reactor L and the electrolytic capacitor C divide the voltage to ensure that the electrolytic capacitor C works within a safe range. When the voltage on the electrolytic capacitor C is about to reach a dangerous value (for example, 370V), and the system is still in the generating state, and the electric energy is continuously sent back to the DC circuit through the inverter part, the safety circuit comes into play to realize energy consumption braking (resistance braking), control the shutdown and opening of VT3, so that the resistor R consumes excess energy. Generally, this situation will not occur.
(2) Motor electric running state
When the CPU finds that the system is no longer charging, it pulses VT3, so that an instantaneous left positive and right negative voltage is formed on the reactor L (as shown in the figure). Together with the voltage on the electrolytic capacitor C, the energy feedback process from the capacitor to the DC circuit can be realized. 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 circuit, thereby controlling the feedback current and ensuring that the DC circuit voltage νd does not appear too high.
2. System Difficulties
(1) Selection of Reactors
(a) Considering the particularity of the working conditions, assume that a certain fault occurs in the system, causing the potential load carried by the motor to fall freely at an accelerated rate. At this time, the motor is in a power generation operation state, and the regenerated energy is sent back to the DC circuit through six freewheeling diodes, causing νd to increase, and quickly putting the inverter in a charging state. The current at this time will be very large. Therefore, the selected reactor wire diameter must 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 electric energy as possible before the next charge, it is not possible to achieve the goal by selecting an ordinary iron core (silicon steel sheet). It is best to select an iron core made of ferrite material. Looking at the current value considered above, it is so large that it can be seen how big this iron core is. I don’t know whether there is such a large ferrite core on the market. Even if there is, its price will definitely not be very low. Therefore, the author recommends using an inductor for each charging and feedback loop.
(2) Difficulties in control
(a) In the DC circuit of the inverter, the voltage νd 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 the control method of energy braking (resistance braking). The instantaneous voltage drop generated on the reactor is, and the instantaneous charging voltage of the electrolytic capacitor C is νc=νd-νL. In order to ensure that the electrolytic capacitor works within a safe range (≤400V), the voltage drop νL on the reactor must be effectively controlled, and the voltage drop νL depends on the instantaneous change rate of the inductance and current.
(b) During the feedback process, it is also necessary to prevent the electric energy discharged by the electrolytic capacitor C from passing through the reactor to cause the DC circuit voltage to be too high, so that the system has overvoltage protection.
3. Main application occasions and application examples
It is precisely because of the advantages of this new braking method of the inverter (capacitor feedback braking) that many users have proposed to equip this system in recent years based on the characteristics of their equipment. Due to certain technical difficulties, it is still unknown whether there is such a braking method abroad? At present, only Shandong Fengguang Electric Co., Ltd. in China has changed from the inverter that used to use the feedback braking method (there are still 2 units in normal operation) to the new mining hoist series with this capacitor feedback braking method.
With the expansion of the application field of frequency converter, this application technology will have great development prospects. Specifically, it is mainly used in the mine cage (for manning or loading), inclined mine car (single or double barrel), lifting machinery and other industries. In short, it can be used in any occasion where energy feedback device is needed.