The supplier of frequency converter supporting equipment 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).
1、 Overview of braking unit
The braking unit, also known as the "frequency converter specific energy consumption braking unit" or "frequency converter specific energy feedback unit", is mainly used to control situations with heavy mechanical loads and very fast braking speed requirements. It consumes the regenerated electrical energy generated by the motor through the braking resistor or feeds back the regenerated electrical energy to the power supply.
2、 The function of the braking unit
When the electric motor stops quickly, it will feedback energy to the frequency converter, causing the DC bus voltage to rise and even damaging the IGBT. Therefore, a braking unit is needed to consume this energy to protect the frequency converter.
3、 Braking method of frequency converter
1. Power braking.
Refers to the method of using the braking resistor set in the DC circuit to absorb the regenerative energy of the motor.
2. Feedback braking.
Mainly targeting current type frequency converters or voltage type frequency converters with inverters installed in the rectification section, the regenerative energy of the motor is fed back to the AC power grid.
3. Multi inverter drive with shared DC bus.
The regenerative energy of motor A is fed back to the common DC bus, and then consumed by motor B. The multi inverter drive with shared DC bus can be divided into two types: shared DC balanced bus and shared DC circuit bus. The shared DC balanced bus method is to use connection modules to connect to the DC circuit bus. The connection module includes reactors, fuses, and contactors, which must be designed separately according to specific circumstances. Each frequency converter has relative independence and can be connected or disconnected from the DC bus as needed. The shared DC circuit bus method is to connect only the inverter part to a common DC bus.
4. DC braking.
When the frequency converter applies direct current to the stator of the motor, the asynchronous motor is in a state of energy consumption braking. In this case, the output frequency of the frequency converter is zero, and the stator magnetic field of the motor no longer rotates. The rotating rotor cuts through this static magnetic field and generates braking torque. The kinetic energy stored in the rotating system is converted into electrical energy and consumed in the rotor circuit of the electric motor.
4、 The function of braking resistor
During the process of decreasing operating frequency, the electric motor will be in a regenerative braking state, and the kinetic energy of the driving system will be fed back to the DC circuit, causing the DC voltage UD to continuously rise and even reach a dangerous level. Therefore, it is necessary to consume the energy regenerated into the DC circuit to keep UD within the allowable range. The braking resistor is used to consume this energy.
Each frequency converter has a braking unit (low-power is the braking resistor, high-power is the high-power transistor GTR and its driving circuit), low-power is built-in, and high-power is external.
5、 Braking process of braking unit and braking resistor
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 freewheeling diodes in the inverter section of the frequency converter, which continuously increases the DC bus voltage inside the frequency converter.
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.