Braking unit, the full name of "inverter-specific energy consumption braking unit", or "inverter-specific energy feedback unit", is mainly used to control the occasions where the mechanical load is relatively heavy and the braking speed is required to be very fast. The regenerative electric energy generated by the motor is consumed by the braking resistor, or the regenerative electric energy is fed back to the power supply. The main functions, advantages and action process of the braking unit are briefly introduced below.
First, the main function of the brake unit
In some applications, rapid deceleration is required. According to the principle of asynchronous motor, the greater the slip, the greater the torque. Similarly, the braking torque will increase with the increase of the deceleration rate, making the system decelerate. The speed time is greatly shortened, the energy feedback is greatly accelerated, and the DC bus voltage rises rapidly, so the feedback energy must be quickly consumed to keep the DC bus voltage below a certain safety range. The main function of the braking unit system is to quickly dissipate this energy (the energy is converted into heat energy by the braking resistor and dissipated). It effectively makes up for the shortcomings of slow braking speed and small braking torque (≤20% rated torque) of ordinary inverters, and is very suitable for some occasions where rapid braking is required but the frequency is low.
Second, the advantages of the braking unit
Because the working conditions of the braking unit are short-term work, that is, the power-on time is very short each time, and the temperature rise is far from the stable temperature rise during the power-on time; and the intermittent time after each power-on is longer, In the intermittent time, its temperature is enough to drop to the same as the ambient temperature, so the rated power of the braking resistor will be greatly reduced, and the price will also drop; in addition, because there is only one IGBT, the braking time is ms level, and the power tube is turned on and The turn-off transient performance index is required to be low, and even the turn-off time is required to be as short as possible to reduce the turn-off pulse voltage and protect the power tube; the control mechanism is also relatively simple, and the implementation is relatively easy. Due to the above advantages, it is widely used in cranes and other potential energy loads and occasions requiring rapid braking but short-time duty.
3. The action process of the braking unit
1. When the motor decelerates under the action of external force, the motor runs in a power generation state to generate regenerative energy. The three-phase AC electromotive force generated by the inverter is rectified by a three-phase fully-controlled bridge composed of six inverter-specific energy feedback unit freewheeling diodes in the inverter part, so that the DC bus voltage in the inverter continues to rise.
2. When the DC voltage reaches a certain voltage (the turn-on voltage of the braking unit), the power switch tube of the braking unit is turned on, and the current flows through the braking resistor.
3. The braking resistor releases heat, absorbs regenerative energy, the motor speed decreases, and the DC bus voltage of the inverter decreases.
4. When the DC bus voltage drops to a certain voltage (the braking unit stop voltage), the power tube of the braking unit is turned off. At this time, no braking current flows through the resistor, and the braking resistor naturally dissipates heat and reduces its own temperature.
5. When the voltage of the DC bus increases again to make the braking unit act, the braking unit will repeat the above process to balance the bus voltage and make the system run normally.
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