Frequency inverters with direct coupling
The basis of the power circuit of a frequency converter with direct coupling is a reversible thyristor converter. The multiphase output of a frequency converter with direct coupling is achieved by using several reversible converters with a single-phase output (Figure 5.25).
The output single-phase voltage of low frequency is provided by a cyclic change of the angles of opening of thyristors. The commutation of the current by thyristors of each group of a direct frequency converter of one direction is carried out by the mains voltage (see Figure 5.26).
Most direct frequency converters provide for co-consistent control of reversible thyristor groups. In this case, the limiting reactors L1 ... L6 serve to limit the equalizing currents, which increases the mass and overall dimensions of the converter, and also reduces its energy parameters.
Fig. 5.25. System and "direct frequency converter - asynchronous motor"
On the voltage diagrams (see Figure 5.26), control occurs at a variable control angle a. The law of changing the control voltage in the direct frequency converter is determined by a special setpoint. In the case considered, the control voltage varies sinusoidally. In some cases, a control voltage of rectangular shape is used.
Fig. 5.26. Output single-phase voltage of a direct frequency converter
Three-phase system of control voltages of rectangular shape, shifted for each phase by 120 e. deg., is shown in Fig. 5.27.
If each of the controlled rectifiers of a direct frequency converter is covered by deep negative current feedback, the converter acquires the properties of a current source. Thus, direct frequency converters can operate in the voltage source mode or in the current source mode.
Fig. 5.27. Three-phase system of control voltages of rectangular shape
The advantage of direct frequency converters are:
• One-time energy conversion, which determines the high efficiency of the converter;
• the ability to pass reactive power both from the network to the load, and vice versa.
• The complexity of control devices. A large number of thyristors require a large number of systems of pulse-phase control;
• the converter power factor is significantly less than one ();
• the shape of the mains voltage is significantly distorted;
• Difficulties in obtaining frequencies close to the frequency of the supply network.
For the zero circuit, the maximum frequency of the output voltage is usually limited to Hz. The transition to the bridge circuit extends the operating range to Hz.
Therefore, asynchronous electric drives with direct frequency converters are used for gearless electric drives of medium and high power.
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