Inverters and frequency converters
Inverters serve to convert DC energy into AC power of the required frequency.
Thyristors are used as switching devices in high-current inverters. In circuits with relatively small values of the flowing currents, powerful field or bipolar transistors can be used.
In Fig. 2.34, and is a block diagram of the frequency converter using an inverter for supplying a three-phase load connected by a star, alternating three-phase voltage with the adjustment of its value U and frequencies f . Power switching devices employ powerful bipolar transistors.
Voltage with the frequency of the industrial network U c is first converted by a controlled rectifier (B) with a filter (Ф) to the DC voltage U _ of the desired value. Then this voltage goes to the inverter (AND) (Figure 2.34, b), consisting of six transistors VT1-VT6, numbered in the sequence of their inclusion, shown in Fig. 2.34, in. Each transistor opens for a time τ equal to the duration of one half-cycle of the required AC voltage.
In Fig. 2.34, d is the step voltage generated in the A phase of the load, and in Fig. 2.35 the process of its formation is explained.
On the diagrams of Fig. 2.35 in the form of closed keys, only open transistors are shown for six consecutive states of the inverter corresponding to the time diagrams of Fig. 2.34, c. From the analysis of these schemes, it is obvious that when the A phase is enabled in parallel or C, , one third of the voltage U = is allocated to it, and when the phase A turns on sequentially with the parallel phases B and C, there are two-thirds of the voltage U =.
Fig. 2.34. Inverter-based frequency converter:
and - a block diagram; b - the inverter circuit; in - a time chart; d is the graph of the output voltage
In the first three states, the voltage in the phase A corresponds to the positive, and in the latter two - to the negative half-cycle of the alternating current of stepped form applied to it.
Fig. 2.35 . The process of generating the output voltage of the frequency converterReasoning this way, we can make sure that the phases B and C will be attached the same as to the phase A, the voltage, but shifted by one third and two thirds of the period G, respectively, forming a three-phase system of voltages. By changing the duration of the open state of the transistor by means of the control circuit, it is possible to regulate the frequency of the three-phase voltage being formed within a wide range, therefore such inverters are used for smooth control of the rotation speed of three-phase asynchronous motors .
Thyristor DC Motor Control
In DC drive drives and tool feeders for metal cutting machines, DC motors with independent excitation are widely used, which are capable of controlling the speed of rotation within a wide range. Such an engine (Figure 2.36, a) consists of a stator at the poles of which the winding of excitation (OH) is wound, and a rotor, called an anchor.The excitation current I B, passing through the OB under the influence of voltage UΒ, creates a magnetic flux F. To the anchor through the brushes the armature voltage Ua is applied, which creates the armature current I Me. Flowing through the windings of the armature winding, the current I i, interacting with the flow Ф, creates the torque Mvr
where To is a coefficient that depends on the engine design (dimensions, number of turns of windings, etc.).
Fig. 2.36. The DC motor connection diagram ( a ) and the graph explaining the principles of speed control engine (b)
When the engine rotates in the armature winding, the EMF E is directed, according to the Lenz rule, against the applied voltage U and proportional to the number of engine rotations n:
where with is a coefficient that depends on the engine design.
For an anchor chain with a uniform rotation of the shaft according to the second Kirchhoff law, we can write the equation:
where R I am the active resistance of the armature winding, including the contact resistance of the brush-collector (in motors, a collector is a set of contact pads through which the brushes are energized in the winding of the rotating anchor).
Substituting the expression E and the current value I into this equation, i obtained from the expression for the torque , we obtain:
where the engine speed is:
Two methods (two zones) of controlling the engine rotation speed are obvious from the formula obtained (figure 2.36, c). In the zone I U B at the same value of the magnetic flux Φ, and hence the constant excitation voltage U B. When the voltage reaches U a nominal value, its further increase is impossible, since it can lead to breakdown of insulation. At the same time, to quickly move, for example, a tool at idle speed or accelerated rotation of the spindle, it is necessary to increase the engine rotation speed 3-5 times higher than the nome. To do this, use the zone II, in which, with a constant voltage U.nom, the value of the magnetic flux Φ is reduced by a corresponding decrease U i> a, and hence the excitation current I B. Note, however, that in the zone II it is necessary to put up with the corresponding decrease in the engine torque; To load the engine with a smaller moment of resistance, which it must overcome with its torque. Indeed, as follows from the formula for MWR, as the flux Φ decreases, the torque decreases and it is impossible to compensate for its increase in the current I , as this will cause the engine to overheat.
In industry-produced thyristor converters, the control of the rotation speed in the zone I is accomplished by using two controlled high-power (up to several tens of kilowatts) three-phase rectifiers (in Figure 2.37 they are dotted) .
Fig. 2.37. Thyristor Motor Speed Controller Circuit
One of the three-phase rectifiers provides the right direction of rotation of the motor, and the other - the left one, changing the polarity U to the opposite. Naturally, these rectifiers should work separately to avoid short-circuiting between them, which is provided by the rectifier control circuit, allowing one of them to be turned on only after a few milliseconds after switching off the other. The control circuits of thyristors are made according to the principle considered in paragraph 2.9 and in Fig. 2.33.
To control the rotation speed in the zone II a single-phase bridge circuit of the thyristor rectifier is used, which provides power to the OB. The circuit allows only to decrease the excitation current value I in, keeping its polarity. The circuit for controlling the thyristors of the bridge circuit is also executed according to the principle considered in paragraph 2.9 and in Fig. 2.33.
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