Standalone current inverters
The power circuit diagram of a three-phase bridge inverter is shown in Fig. 5. 28.
The notation used in Fig. 5.28:
SUV - rectifier control circuit;
PT - current controller;
SUP - inverter control circuit;
FP is a functional converter.
Fig. 5.28. Power circuit diagram of a three-phase bridge inverter
Two-stage converter devices are performed on the basis of a three-phase AC voltage rectifier and an autonomous inverter that converts the rectified voltage into a three-phase AC with adjustable frequency and amplitude. In spite of the double conversion of energy and the consequent slight decrease in efficiency, frequency converters with an intermediate DC link are most widely distributed in a regulated electric drive.
In the autonomous current inverter, the controlled rectifier, performed on the thyristors KS1 ... KS6, operates in the current regulator mode, and the inverter on the thyristors VS7 ... VS12 generates the required frequency of the output current. A filter with inductance L0 provides smoothing of rectified current pulsations
The thyristors of the current inverter VS7 ... VS12, switched on in a three-phase bridge circuit, flow current for 120 e. deg. Switching is performed with a frequency of 60 ° in a sequence corresponding to the numbering of the thyristors. The current diagrams for each phase of the inverter are shown in Fig. 5.29.
Current switching and reactive power compensation are carried out by capacitors C1 ... C3 on the AC side. With an active-inductive load at the output of the inverter and on thyristors VS7..VS12 , significant overvoltages can occur at the commutation moments, caused by the action of the EMF of the self-inductive load. Their limitation is achieved by installing the appropriate containers. When the frequency decreases and at a constant torque on the motor shaft, the capacity of the capacitors increases inversely proportional to the square of the frequency.
Fig. 5.29. Current diagrams for each phase of the inverter
When the motor is powered from an autonomous inverter, the direction of the inverter's counter-emf changes, which switches to the rectifier operating mode, which could cause an increase in the current in the DC link. However, due to the strong negative feedback on the current that the rectifier covers, the current in the DC link remains at the same level, and the rectifier is put into the mode of the inverter driven by the network. As a consequence, energy is recuperated into the supply network without changing the current direction in the DC link.
Thus, in autonomous current inverters, the braking modes of the engine with energy recovery into the network are easily realized, which makes it preferable to use it in reversible electric drives.
A more perfect scheme of this class is the circuit of an autonomous current inverter with clipping diodes in Fig. 5.30.
In the circuit of the autonomous current inverter (Figure 5.30), the capacitors C1 ... C6 are separated from the load by means of diodes VDl ... VD6 , so that the capacitors participate in the inverter only in a relatively short time switching, and the rest of the time the current through them does not flow. This allows a significant reduction in the capacitance of capacitors.
The main advantages of frequency converters with autonomous current inverters:
• the possibility of energy recovery to the network;
• output voltage close to sine wave; failure-free short circuit in the load.
Fig. 5.30. Power circuit circuit of an autonomous current inverter with clipping diodes
• Limiting the output frequency at the level Hz;
• switching overvoltages on thyristors, which makes it more difficult to force the circuit;
• The inability to work on group load;
• Significant weight and dimensions of the inductive filter.
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