In the power and electrical industry, frequency converters are mainly used to save energy and improve production processes. As a motor energy saving and speed control device, they are widely used in metallurgy, power, water supply, petroleum, chemical, coal and other fields. The essence of the inverter energy feedback unit is an active inverter. The realization method of the universal inverter energy feedback unit is to anti-parallel a three-phase inverter at the front stage of the universal inverter uncontrolled rectifier to feed the regenerative energy to the grid. The main circuit of the energy feedback unit is mainly composed of an inverter bridge composed of thyristors, IGBTs and IPM modules, and some peripheral circuits.

The output end of the inverter bridge is connected to the input terminals R, S, and T of the inverter through three choke reactors, and the input end is connected to the positive end of the DC side of the inverter through an isolation diode to ensure that the energy is One-way flow in the direction of “source inverter bridge-grid”. The function of the choke reactor is to balance the pressure difference, limit current and filter, and play a key role in the feedback of regenerative energy to the grid.

The working process of the system is: when the motor is running, the active inverter device does not work, and the switch tubes of the inverter are all blocked and in the off state; when the motor is in the state of regenerative power generation, the energy is fed back to the grid by the motor. Start the active inverter device to work.

The activation of the active inverter device during energy feedback is controlled by the magnitude of the inverter's DC side voltage Ud. The basis is that when the motor is in an electric state, the inverter's DC side voltage is basically constant. When the motor is in a gen-brake state, The regenerative energy of the AC motor charges the energy storage capacitor in the DC link of the frequency converter, which causes the DC bus voltage to increase. As long as the size of Ud is detected, the state of the motor can be judged, so as to control the active inverter device to realize energy feedback .

When the energy is fed back to the DC side of the motor and the DC bus voltage exceeds the peak value of the grid line voltage, the universal inverter rectifier bridge is shut down due to the back pressure; when the DC bus voltage continues to rise and exceeds the active inverter operating voltage, the inverter The inverter starts to work and feeds energy back to the grid from the DC side; when the DC bus voltage drops to the operating voltage of the inverter, the active inverter is turned off.

After the general frequency converter uses the active inverter method to feed the regenerative energy generated during the deceleration and braking of the motor back to the grid, it can overcome the inefficiency caused by the traditional general frequency converter using the braking resistance method and it is difficult to meet the rapid braking and frequent braking. The lack of forward/reverse rotation makes the general-purpose inverter run in four quadrants.

1) Energy feedback control system

A complete energy feedback control system should meet the three control conditions of phase, voltage, current, etc., that is, the feedback process must be synchronized with the grid phase, and the active inverter device will only be activated when the DC bus voltage exceeds a certain value; the system It should be able to control the magnitude of the feedback current so that the braking torque of the motor can be controlled to achieve precise braking.

2) Two general-purpose inverter energy feedback units

Earlier, the main circuit of the energy feedback unit was mostly composed of thyristors and IGBTs. In recent years, some new energy feedback units also use intelligent modules such as IPM, which simplifies the system structure of the energy feedback unit.

(1) Thyristor energy feedback unit:

The energy feedback main circuit is composed of thyristor devices, which is also an earlier energy feedback unit. It is not only used in inverters, but also in the braking of some DC reversible speed regulation systems.

①General inverter forward working state: When the motor is in the electric state, the rectifier of the inverter is working, but the thyristor device in the energy feedback unit is not triggered and is in the cut-off state, and the rectifier is working in the forward direction. The controllable inverter part of the inverter is triggered to work, the uncontrollable reverse rectifier part is in the cut-off state, and the inverter is in forward operation.

②Reverse working state of general frequency converter: When the motor is generating power, the rectifier of the frequency converter is in the cut-off state, and the thyristor device in the energy feedback unit is triggered to work. The controllable inverter part of the inverter is still triggered to work, the uncontrollable reverse rectifier part is in working state, and the inverter is in reverse working.

(2) IGBT energy feedback unit:

The energy feedback main circuit is composed of IGBT devices. This energy feedback unit is most used in general inverters. The freewheeling diode integrated with the IGBT devices cannot be used as a rectifier device under the limitation of the isolation diode connected to the DC side. Its cost should be higher than the thyristor energy feedback unit.

① General inverter forward working state: When the motor is in the electric state, the rectifier of the inverter is working, but the IGBT device in the energy feedback unit is not triggered and is in the cut-off state, and the rectifier is working in the forward direction. The IGBT device in the inverter is triggered to work, the uncontrollable reverse rectification part is in the cut-off state, and the inverter is in forward operation.

②Reverse working state of general inverter: When the motor is generating power, the rectifier of the inverter is in the cut-off state, and the IGBT device in the energy feedback unit is triggered to work. The IGBT devices in the inverter are still triggered to work, the uncontrollable reverse rectification part is in working state, and the inverter is in reverse working.