First, the main function of the braking unit
In some applications, the need for rapid speed reduction, according to the principle of asynchronous motor, if the slip is larger, the greater the torque, the same thing is that the braking torque will increase with the increase of the speed reduction rate, so that the system speed reduction time is greatly shortened, the energy feedback is greatly accelerated, and the DC bus voltage rises rapidly, so it is necessary to quickly consume the feedback energy, and insist that the DC bus voltage is 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). It effectively makes up for the shortcomings of slow braking speed and small braking torque (≤ 20% additional torque) of general inverter, and is very suitable for some occasions that require fast braking but low frequency.
Second, the advantages of the braking unit
Because the working condition of the braking unit is attributed to short-term work, that is, the power-on moment is very short each time, and the temperature rise is far from reaching the stable temperature rise in the power-on time; The intermittent moment after each power on is longer, and the temperature is enough to drop to the same temperature as the ambient temperature during the intermittent time, so the additional power of the braking resistor will be greatly reduced, and the price will also be reduced; In addition, because there is only one IGBT, the braking time is ms level, and the transient performance indicators for power tube registration and shutdown are low, and even the shutdown time is required to be as short as possible to reduce the shutdown impulse voltage and protect the power tube; The control mechanism is also relatively simple, and it is relatively easy to implement. Because of the above advantages, it is widely used in potential energy loads such as cranes and occasions that require rapid braking but are short-time working systems.
Third, the action process of the braking unit
1. When the motor decelerates under the effect of external force, the motor runs in the condition of power generation, and regenerative energy occurs. The three-phase AC electromotive force generated by it is rectified by the three-phase full-control bridge composed of six freewheeling diodes of the inverter inverter, so that the DC bus voltage in the inverter continues to rise.
2. When the DC voltage reaches a certain voltage (the opening voltage of the braking unit), the power switch of the braking unit is registered, 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 stop voltage of the braking unit), the power tube of the braking unit is turned off. At this moment, no braking current flows through the resistor, and the braking resistor naturally dissipates heat and lowers its own temperature.
5. When the voltage of the DC bus rises again to make the brake unit move, the brake unit will repeat the above process to balance the bus voltage and make the system operate normally.
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