The supplier of the frequency converter braking unit reminds you that in the frequency conversion speed control system, the basic method of speed reduction is to gradually reduce the given frequency. When the inertia of the drag system is large, the decrease in motor speed will not keep up with the decrease in synchronous motor speed, that is, the actual speed of the motor is higher than its synchronous speed. At this time, the direction of the magnetic field lines cut by the rotor winding of the motor is exactly opposite to that of the constant speed operation of the motor. The direction of the induced electromotive force and current of the rotor winding is also opposite to that of the motor rotation direction, and the motor will produce negative torque. At this time, the motor is actually a generator, and the system is in a regenerative braking state. The kinetic energy of the drag system is fed back to the DC bus of the frequency converter, causing the DC bus voltage to continuously rise and even reach a dangerous level (such as damage to the frequency converter).
Working principle of braking unit
The braking unit consists of a high-power transistor GTR and its driving circuit. Its function is to add an external braking component to accelerate the consumption of regenerated electrical energy when the discharge current link capacitor cannot store within the specified voltage range or the internal braking resistor cannot consume it in time, resulting in overvoltage in the DC part.
In certain applications, rapid deceleration is required. According to the principle of asynchronous motors, the greater the slip, the greater the torque. Similarly, the braking torque will increase with the increase of deceleration rate, greatly shortening the deceleration time of the system, accelerating energy feedback, and causing the DC bus voltage to rise rapidly. Therefore, the feedback energy must be quickly consumed to maintain the DC bus voltage below a certain safe range. The main function of the braking unit system is to quickly dissipate the energy (which is converted into thermal energy by the braking resistor). It effectively compensates for the disadvantages of slow braking speed and small braking torque (≤ 20% rated torque) of ordinary frequency converters, and is very suitable for situations where fast braking is required but the frequency is low.
Due to the short-term operation of the braking unit, which means that the power on time is very short each time, the temperature rise during the power on time is far from stable; The interval time after each power on is longer, during which the temperature is sufficient to drop to the same level as the ambient temperature. Therefore, the rated power of the braking resistor will be greatly reduced, and the price will also decrease accordingly; In addition, due to the fact that there is only one IGBT with a braking time of ms level, the transient performance indicators for power transistor turn-on and turn off are 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 transistor; The control mechanism is relatively simple and easy to implement. Due to the above advantages, it is widely used in potential energy loads such as cranes and in situations where rapid braking is required but for short-term work.
The function of the braking unit
1. When the electric motor decelerates under external force, it operates in a generating state, producing regenerative energy. The three-phase AC electromotive force generated by it is rectified by a three-phase fully controlled bridge composed of six inverter specific energy feedback units and freewheeling diodes in the inverter section of the inverter, which continuously increases the DC bus voltage inside the inverter.
2. When the DC voltage reaches a certain voltage (the starting voltage of the braking unit), the power switch tube of the braking unit opens and current flows through the braking resistor.
3. The braking resistor releases heat, absorbs regenerative energy, reduces motor speed, and lowers the DC bus voltage of the frequency converter.
4. When the DC bus voltage drops to a certain voltage (braking unit stop voltage), the power transistor of the braking unit is turned off. At this time, no braking current flows through the resistor, and the braking resistor naturally dissipates heat, reducing its own temperature.
5. When the voltage of the DC bus rises again to activate the braking unit, the braking unit will repeat the above process to balance the bus voltage and ensure the normal operation of the system.