I. Introduction
In a traditional variable frequency speed control system consisting of a general-purpose frequency converter, an asynchronous motor, and a 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 decelerates from high speed to low speed (including stopping), the frequency may decrease abruptly, but due to the motor's mechanical inertia, the motor may be in a regenerative braking state. The mechanical energy stored in the transmission system is converted into electrical energy by the motor and fed back to the DC circuit of the frequency converter through the six freewheeling diodes of the inverter. At this time, the inverter is in rectification mode. If no energy dissipation measures are taken in the frequency converter, this 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 energy may damage the frequency converter, so this energy should be considered.
In general-purpose frequency converters, there are two common ways to handle regenerative energy: (1) dissipating it into a "braking resistor" artificially set in parallel with the capacitor in the DC circuit, which is called dynamic braking; (2) feeding it back to the power grid, which is called regenerative braking (also known as regenerative braking). There is another braking method, namely DC braking, which can be used for situations requiring accurate stopping or for braking the motor before starting due to irregular rotation caused by external factors.
II. Energy Consumption Braking
The method of absorbing the regenerative electrical energy of a motor by using a braking resistor set in a DC circuit is called regenerative braking.
Its advantages are simple construction, no pollution to the power grid (compared to regenerative braking), and low cost; its disadvantages are low operating efficiency, especially when frequent braking will consume a lot of energy and the capacity of the braking resistor will increase.
In general, low-power frequency converters (below 22kW) have a built-in braking unit, requiring only an external braking resistor. High-power frequency converters (above 22kW) require an external braking unit and braking resistor.
III. Regenerative Braking
Achieving regenerative braking requires conditions such as voltage synchronization and phase control, and regenerative current control. It employs active inverter technology to convert regenerated electrical energy into AC power with the same frequency and phase as the grid, which is then fed back to the grid, thus achieving braking. The advantage of regenerative braking is its ability to operate in four quadrants, as shown in Figure 3; energy feedback improves system efficiency.
Its disadvantages are: (1) This regenerative braking method can only be used under stable grid voltage conditions where faults are unlikely to occur (grid voltage fluctuations are no more than 10%). Because during generator braking operation, if the grid voltage fault time is greater than 2ms, commutation failure may occur, damaging the devices. (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 section uses a standard uncontrolled rectifier bridge for rectification, the filter circuit uses general-purpose electrolytic capacitors, and the delay circuit can use either a contactor or a thyristor. The charging and feedback circuit consists of power IGBT modules, charging and feedback reactors L, and a large electrolytic capacitor C (capacity approximately a few tenths of a farad, determined based on the operating conditions of the inverter system). The inverter section consists of power IGBT modules. The protection circuit consists of IGBTs and power resistors.
(1) Electric motor power generation operation status
The CPU monitors the input AC voltage and DC circuit voltage νd in real time to determine whether to send a charging signal to VT1. Once νd is higher than the DC voltage value corresponding to the input AC voltage (e.g., 380VAC-530VDC) by a certain value, the CPU shuts off VT3 and charges the electrolytic capacitor C by pulse-turning VT1. At this time, the reactor L and the electrolytic capacitor C divide the voltage, thus ensuring that the electrolytic capacitor C operates within a safe range. When the voltage on the electrolytic capacitor C is close to a dangerous value (e.g., 370V), but the system is still in the generating state and electrical energy is continuously fed back to the DC circuit through the inverter, the safety circuit comes into play, realizing energy consumption braking (resistive braking), controlling the switching on and off of VT3, thereby allowing the resistor R to consume excess energy. This situation generally does not occur.
(2) Motor operation status
When the CPU detects that the system is no longer charging, it pulses VT3, creating a momentary voltage across reactor L that is positive on the left and negative on the right (as shown in the diagram). This, combined with the voltage across electrolytic capacitor C, enables the energy feedback process from the capacitor to the DC circuit. By detecting the voltage across electrolytic capacitor C and the voltage in the DC circuit, the CPU controls the switching frequency and duty cycle of VT3, thereby controlling the feedback current and ensuring that the DC circuit voltage νd does not become excessively high.
2. System Challenges
(1) Selection of reactor
(a) Considering the special nature of the operating conditions, we assume that a certain fault occurs in the system, causing the potential energy load on the motor to accelerate freely downwards. At this time, the motor is in a generator operation state, and the regenerated energy is fed back to the DC circuit through six freewheeling diodes, causing νd to rise and quickly putting the frequency converter into a charging state. The current at this time will be very large. Therefore, the selected reactor wire diameter must be large enough to carry the current at this time.
(b) In the feedback loop, to ensure the electrolytic capacitor releases as much energy as possible before the next charge, a standard iron core (silicon steel sheet) is insufficient. A ferrite core is preferable. Considering the exceptionally large current value mentioned above, the required core size is considerable. It's unclear whether such a large ferrite core is commercially available, and even if it is, its price would certainly be high. Therefore, I recommend using one reactor for both the charging and feedback loops.
(2) Control difficulties
(a) In the DC circuit of the frequency converter, the voltage νd is generally higher than 500VDC, while the withstand voltage of the electrolytic capacitor C is only 400VDC. Therefore, the control of this charging process is unlike that of energy braking (resistive braking). The instantaneous voltage drop generated on the reactor is νc = νd - νL. To ensure that the electrolytic capacitor operates within a safe range (≤400V), the voltage drop νL on the reactor must be effectively controlled. The voltage drop νL depends on the inductance and the instantaneous rate of change of current.
(b) During the feedback process, it is also necessary to prevent the electrical energy discharged by the electrolytic capacitor C from causing the DC circuit voltage to be too high through the reactor, so as to cause the system to have overvoltage protection.
3. Main application scenarios and application examples
Due to the superiority of this new braking method (capacitor feedback braking) for frequency converters, many users have recently requested to equip their equipment with this system, taking into account its characteristics. Because of the technical difficulties, it is unknown whether this braking method exists abroad. Currently, only Shandong Wind Power Company has converted its previously regenerative braking frequency converters (two of which are still in normal operation) to this new series of mining hoists using capacitor feedback braking.
With the expansion of frequency converter applications, this technology has a promising future. Specifically, it is mainly used in industries such as mine hoists (for personnel or materials), inclined shaft mine cars (single or double cylinders), and hoisting machinery. In short, it can be used in any application requiring energy feedback devices.
