The working principle of active inverter and the conditions for its realization

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1. How active inverter works

Let the motor run in the motor state and the back E (E) (+). At this time, the thyristor converter circuit must work in the rectification state, so that the output DC average voltage Ud>0, also (+) down (-), overcomes the action of the back E potential, and the output DC average current Id is supplied to the armature winding. At this time, the thyristor control angle α=0 to π/2, and α is adjusted so that Ud>E. At this time, Id=(Ud-E)/Rd, generally Rd is small, and the limit Id is not too large, and Ud≈E must be controlled. At this time, the electric energy flows from the AC grid to the DC motor side through the converter circuit. From the waveform diagram, the thyristor operates in the rectified state for most of the time in the range of the AC voltage u2>0. When u2<0, the thyristor is still conducting with the forward anode voltage due to the self-inductive potential of the reactor.

In Figure 1b), the motor is operated in the power generation braking state, and the back E potential is reversed. Due to the unidirectional conductivity of the thyristor element, it is determined that the current flow in the circuit cannot be reversed. To change the direction of transmission of the electric energy, only the polarity of the voltage can be changed. In the case where the back EMF polarity is reversed, the polarity of the DC average voltage Ud of the converter circuit must also be reversed. That is, Ud should be (-) down (+), otherwise the back E will be short-circuited with Ud. In order to make the current from the DC side to the AC side, E>Ud must be used. At this time, Id = (E - Ud)/Rd. In order to prevent overcurrent, E≈Ud is also required. At this time, it is possible to flow from the DC motor side to the AC grid through the converter circuit, thereby realizing the conversion of DC power into AC power.

2. Inverter generated conditions

The following two conditions must be met at the same time to achieve active inverter

(1) There is a direct current potential that can reverse the electrical energy. The polarity of the potential is the same as the unidirectional conduction direction of the thyristor element, and the magnitude of the potential is slightly larger than the DC average voltage of the converter circuit;

(2) The DC side of the converter circuit should be able to generate a negative DC average voltage.