The common DC bus technology is to use a separate rectifier/feedback device to provide a certain power DC power supply for the system in a multi-motor AC speed control system, and the inverter for speed control is directly connected to the DC bus. When the system is working in electric state, the inverter obtains power from the bus; when the system is working in power generation state, the energy is directly fed back to the grid through the bus and the feed-back device, so as to achieve the purposes of energy saving, improving the reliability of equipment operation, reducing the amount of maintenance of the equipment and the equipment floor space, and so on.
I. Origin of common DC bus system
For frequent starting, braking, or four-quadrant operation of the motor, how to deal with the braking process not only affects the dynamic response of the system, but also the economic efficiency of the problem. Thus, feed-back braking has become the focus of discussion. However, in the case that most of the current general-purpose inverters are not yet able to realize regenerative energy through a separate inverter, how can feed-back braking be realized in the simplest way?
In order to solve the above problems, a regenerative energy feedback system with a common DC bus is introduced here, through which the regenerative energy generated by braking can be fully utilized, thus achieving the efficacy of both saving power and handling regenerative power.
II. Composition of the Common DC Bus System
Common DC bus control system usually consists of rectifier/feedback unit, common DC bus, inverter unit and so on. Feedback unit can be divided into two ways through the autotransformer energy feedback and not through the autotransformer energy feedback. Through the autotransformer energy feedback can improve the feedback branch circuit in the power supply voltage, the purpose is in the energy feedback process does not have to reduce the intermediate circuit voltage, so that the inverter can obtain a more stable DC power supply; not through the autotransformer energy feedback is actually to keep the system has been in the feedback state, in the rectification process rely on continuous reduction of the intermediate circuit with the phase angle control of the voltage to achieve.
III. Principle of Common DC Bus System
We know that the usual sense of asynchronous motor multidrive includes rectifier bridge, DC bus power supply circuit, and several inverters, in which the energy needed by the motor is output in DC mode through PWM inverter. In the multidrive mode, the induced energy is then fed back to the DC circuit during braking. Through the DC circuit, this part of the feedback energy can be consumed in other motors in the electric state. When the braking requirement is particularly high, it is only necessary to connect a common braking unit to the common bus.
Figure 1 shows the wiring is a typical common DC bus braking mode, M1 is in the electric state, M2 is often in the power generation state, three-phase AC power supply 380V connected to VF1.
Fig. 1 Feedback braking method with common DC busbar
The inverters VF1 and VF2 on the motor M1 in the electric state are connected to the bus of VF1 by means of a common DC bus. In this way, VF2 is only used as an inverter, when M2 is in electric state, the required energy is obtained from the AC grid through the rectifier bridge of VF1; when M2 is in power generation state, the feedback energy is consumed by the electric state of M2 through the DC bus.
IV. Advantages of Common DC Bus System
- The common DC bus system is the optimal solution to the multi-motor drive technology, a good solution to the contradiction between the electric state and the power generation state of the multi-motor. In the same system, at the same time, different devices can work in different states, rectifier feedback unit to ensure a stable supply of public DC bus voltage, and the excess energy back to the grid, realizing the rational use of renewable energy.
- Common DC bus system equipment is compact and stable. In the multi-motor drive system eliminates a large number of braking units, braking resistors and other peripheral equipment, saves the equipment footprint and equipment maintenance, reduces the equipment failure points, and improves the overall control level of the equipment.
- In the roller conveyor and other multi-motor drive applications common DC bus technology is a development direction of roller conveyor speed control, it can achieve higher dynamic and static performance, speed control accuracy at the same time the system regenerative energy to be reasonably utilized and recycled.
V. Some considerations for the design of a common DC bus system
- The inverter needs to share the rectifier, this rectifier is a special device for common DC bus.
- Inverters should be installed together as far as possible to avoid long-distance wiring, preferably in the same electrical room;
- Each inverter must be isolated and protected separately.
- Can not use the general inverter for public DC bus use, otherwise there will be a risk of blowing up the machine.
- The capacity of the motor M1 ~ M4 power can not have to be the same, but must consider whether the energy feedback can be used up during the shutdown.
- The general operating units in 4 ~ 12 (motor power can be different) a set of public DC bus is good.
- The inverter can drive permanent magnet synchronous motor, to solve the problem of starting the impact.
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