Frequency inverters with direct coupling (PCNS)
Controlled rectifiers serve as the basis for constructing a number of frequency converters for controlling AC motors. The simplest and most natural arrangement of such a device, which makes it possible to obtain a low frequency from a high frequency and received the name of a frequency converter with direct coupling (sometimes the term "cyclo-converter" is used) is shown in Fig. 3.12, a.
Each phase of the engine (shown as a three-phase version) is equipped with two sets of back-up rectifiers (three-phase ones, executed in the zero scheme are shown, the fragment related to one phase of the engine is shown in Figure 3.12, b). By controlling the rectifiers, you can ensure that at each phase of the motor in the positive half-wave of the output voltage ( + in Figure 3.12, c) runs one set of thyristors - A, and in the negative (-) - another,/12.
From Fig. 3.12, in it follows that the output voltage period T and its frequency/= 1 /T depend on the switching time of the thyristor kits and can vary over a wide range. True, the upper frequency is limited: as the Tc T is approached, its output voltage is so distorted that it can not be used; in practice, it is often assumed/& lt; fjl.
The amplitude of the output voltage can be changed by changing the angle a. In addition, with the appropriate control, the harmonic composition of the output voltage, the power characteristics of the drive can be significantly improved.
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Fig. 3.12. Frequency converter with direct communication
Frequency converters with an explicit DC link
The principal feature of the devices considered was the connection with an alternating current network: it was this network that created the conditions and determined the moments of closing the thyristors (the negative potential of the anode). At the same time, it was the network that limited the possibilities of regulating the converter, which is especially evident in the case of PCHP.
The main class of frequency converters are converters with an explicit DC link (Figure 3.13, a), in which the alternating voltage of the supply network is first rectified by a conventional controlled rectifier, and then converted to an alternating voltage with the desired number of phases, with adjustable frequency by means of an autonomous inverter AI.
Fig. 3.13. Frequency converter with an explicit DC link
The principle of the autonomous voltage inverter (ANN) is as follows: if you switch in pairs T/2 keys K1-KZ and K2-K4, the voltage on the load will be variable, rectangular with amplitude U and the frequency/= /T (Figure 3.13, c); under the load in the form of active resistance, the current curve will be repeat the voltage curve. It is not difficult to increase the number of phases by adding pairs of keys and connecting the load to the points between them (the dashed lines in Figure 3.13, b). In the three-phase version, the scheme is very similar to bridge rectifier (see figure 3.9, b); the only difference is that the power source and load are reversed.
The shape of the output voltage and, consequently, its harmonic composition can be changed by changing the duration of the switched-on state I and the switching-on time/'of the key pair (dashed line in figure 3.13, c).
In a multiphase (three-phase) version of the ability to control the shape of the voltage, changing the duration of the key pair inclusion is even richer.
External characteristic = Д/") of the simplest inverter under consideration with ideal keys - a horizontal line.
Processes become more complicated if the load is a chain R-L. Strictly speaking, the circuit (Figure 3.13, b) will be unworkable, because when the inductive circuit breaks, the voltage on the keys becomes infinitely large. To ensure normal operation, the keys are shunted by diodes, and the power source, if its internal resistance is large, is a capacitor (Figure 3.14, a). The current diagrams consist of sections of exponentials (Figure 3.14, b), the voltage and current have a different shape and harmonic composition, the first harmonics are shifted relative to each other by some angle.
The considered autonomous inverters are called voltage inverters: it is formed forcibly, and the current essentially depends on the load. When used as thyristor switches, the circuit becomes more complicated; In addition, circuits that quench thyristors are included.
Fig. 3.14. Stand-alone voltage inverter with R-L load
Autonomous current inverters (AIT) are similar in appearance to voltage inverters, they have the same structure (Figure 3.15, a), however, the processes in them differ significantly.
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Fig. 3.15. Offline current inverter
The main difference in the feeding method: the reactor A L d is connected to the AIT input, the inductance of which is sufficient to maintain the load current practically unchanged during the half-cycle of the output frequency. Thus, the instantaneous current value is set in the AIT, it is powered by a current source. Voltage is a dependent variable. Usually, the load is shunted by the capacitor Ck in order to create the conditions for switching the keys (thyristors) and ensuring normal operation with an active-inductive load; the voltage across the load varies exponentially (Figure 3.15, b).
External characteristic of AIT is soft, steeply increasing at low currents, as the capacitor is charged to high voltages; The idle speed is not permissible due to excessive overvoltages.
The principle of operation of impulse converters is explained in the simplest scheme (Figure 3.16, a). Using the K, load - let it be the motor's anchor, i.e., the R-L-E chain, - alternately connects to the power source & pound;., and disconnects from it, so that the voltage to the load is applied in the form of pulses and or, as is sometimes said, modulated (Figure 3.16, 6).
The average value of the voltage on the load is determined as
where - pulse ratio.
Fig. 3.16. Diagram (a) and diagram of the basic values of the impulse converter (b)
The load is shunted by the diode V, so that when the switch is opened, the current in the load continues to flow due to the energy stored in the inductance L. The current in the load varies along segments of exponentials with time constant r = L/R (Figure 3.16, b); the average current that determines the electromagnetic moment is expressed as
the root-mean-square current that determines the loss can be found from the expression:
So, a simple pulse converter modulates the supply voltage in such a way that the average voltage on the load can vary from zero (the load is always off) to the source voltage (the load is always connected).
There are two types of modulation: frequency, when the time of the included (or disabled) state of the key is invariable, and the period T or the frequency/= 1 /T changes and latitude when T and/are constant, and the on time (pulse width) changes from tJT = 0 to 1JT = 1, The last type of modulation is Pulse Width Modulation (PWM), and the devices implementing it are not widely used only in DC drives, but also in AC drives .
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