Suppliers of energy feedback devices for frequency converters remind you that with the expansion of frequency converter application fields, the braking methods of frequency converters have also become diversified:
1. Energy consuming type
This method involves paralleling a braking resistor in the DC circuit of a frequency converter, and controlling the on/off of a power transistor by detecting the DC bus voltage. When the DC bus voltage rises to around 700V, the power transistor conducts, passing the regenerated energy into the resistor and consuming it in the form of thermal energy, thereby preventing the rise of DC voltage. Due to the inability to utilize regenerated energy, it belongs to the energy consumption type. As an energy consuming type, its difference from DC braking is that it consumes energy on the braking resistor outside the motor, so the motor will not overheat and can work more frequently.
2. Parallel DC bus absorption type
Suitable for multi motor drive systems (such as stretching machines), in which each motor requires a frequency converter, multiple frequency converters share a grid side converter, and all inverters are connected in parallel to a common DC bus. In this system, there is often one or several motors working normally in the braking state. The motor in the braking state is dragged by other motors to generate regenerative energy, which is then absorbed by the motor in the electric state through a parallel DC bus. If it cannot be fully absorbed, it will be consumed through a shared braking resistor. The regenerated energy here is partially absorbed and utilized, but not fed back into the power grid.
3. Energy feedback type
The energy feedback type inverter grid side converter is reversible. When regenerative energy is generated, the reversible converter feeds back the regenerative energy to the grid, allowing the regenerative energy to be fully utilized. But this method requires high stability of the power supply, and once there is a sudden power outage, inversion and overturning will occur.
Regenerative braking can be used in all electric machinery, and currently electric machinery is mainly rotary, such as electric motors. Therefore, regenerative braking is commonly used in electric drive systems, abbreviated as electric drive systems.
The purpose of regenerative braking
Convert the kinetic energy generated by the useless, unnecessary, or harmful inertia rotation of electric machinery into electrical energy and feed it back to the power grid, while generating braking torque to quickly stop the useless inertia rotation of electric machinery. Electric machinery is a device with moving parts that converts electrical energy into mechanical energy, commonly known as rotary motion, such as an electric motor. And this conversion process is commonly achieved by transferring and converting energy through changes in the energy of the electromagnetic field. From a more intuitive mechanical perspective, it is a change in the size of the magnetic field. The electric motor is powered on, generating current and building a magnetic field. The alternating current generates an alternating magnetic field, and when the windings are arranged at a certain angle in physical space, a circular rotating magnetic field will be generated. Motion is relative, which means that the magnetic field is cut by the conductor within its spatial range. As a result, an induced electromotive force is established at both ends of the conductor, which forms a circuit through the conductor itself and connecting components, generating current and forming a current carrying conductor. This current carrying conductor will be subjected to a force in the rotating magnetic field, which ultimately becomes the force in the torque output of the motor. When the power is cut off, the motor rotates inertia. At this time, through circuit switching, a relatively low power excitation power supply is provided to the rotor, generating a magnetic field. The magnetic field cuts the winding of the stator through the physical rotation of the rotor, and the stator then induces an electromotive force. This electromotive force is connected to the power grid through the power device, which is energy feedback. At the same time, the rotor experiences force deceleration, which is called braking. Collectively known as regenerative braking.
Under what circumstances is a braking resistor required?
The general principle is that if the DC circuit is prone to overvoltage due to regenerative braking, a braking resistor must be installed to release the excess charge on the filtering capacitor.
In specific work, it is necessary to consider the following situations when configuring braking resistors:
(1) Frequent starting and braking situations;
(2) In situations where rapid braking is required;
(3) In situations where there is potential energy load (potential energy load, "position" can be understood as position and height), such as lifting machinery.