Inverters with sinusoidal form of voltage curve.

For receiving of the high efficiency, the inverters of power transistors operate in the key mode and the waveform of the output voltage in this case is rectangular. The variable voltage of the rectangular waveform can be used for supplying with energy the lighting or heating systems, supplying electric engines, but the line of AC electricity receivers needs the voltage of sinusoidal waveform for its operation. In addition, variable voltage of rectangular shape can create significant obstacles in operation of radioelectronic equipment, and so tend to receive the sinusoidal voltage waveform on the converter's output.

In low-power converters the sinusoidal voltage waveform can be obtained using the technique of setting the inverter's transistors in linear mode and supplying their bases with sinusoidal control voltages. The inverter in this case operates as a voltage amplifier, its efficiency is

,

where U ₌ – constant voltage of invertor’s feeding; U _~ – amplitude of the sinusoidal voltage on its output.

For U _~/ U ₌ = 0, 5 h = 0, 39, For U ~ / U = = 0, 5  = 0, 39, that is, transistor dissipates more than 60% of the capacity consumed by the inverter. This explains that linear mode of operation of inverter's transistors is used only in low-power converters, and in most aiviation converters the transistors are used mainly in the key mode.

The rate of the approximation of the voltage waveform to the sinusoidal one is characterized with the coefficient of non-linear distortions:

,

where U _н – operating value of the sinusoidal waveform of the voltage curve; U ₁ – – operating value of its first harmonics.

For the voltage of rectangular waveform k _н = 0, 484. For making the waveform of the signal more similar to the sinusoidal one and for decreasing of the coefficient of non-linear distortions the switching of the pairs of the invertor’s transistor is provided with the delay t _п (Fig. 4.8, а).

Approximation of the waveform of the curve of the output voltage to the sinusoidal one can be reached also while continuos switching of transistors during a half-period of the basic frequency (Fig. 4.8, b). Higher harmonics approximate to the basic one effectively reduce at width modulation of pulses that fill basic wave according to trapezoidal or sinusoidal law. Thus, when the number of pulses is equal to seven during the half-period, the output voltage curve contains higher harmonics, starting from the fourteenth one. According to this technical solution the mass of the filter reduces, but the circuit of the control system becomes much more complicated. Because of the fact that losses of the transistors' commutation are proportional to the number of commutations, the efficiency of the inverters in the process of growing of numbers of pulses per period decreases.

а б

Fig. 4.8

As filters that distinguish the first harmonic of the signal and absorb higher harmonics LC - filters are used.

Adjustment of the voltage of static convertors. When changing the load of inverters and the voltage feeding network the output voltage of the inverter can vary over a wide range, so to stabilize the output voltage of the converter the inverter is powered with an adjusting converter of DC - with a converter. The value of the output voltage of the converter is formed by the signal of measuring body, which is switched on the output voltage of the converter. The signal at the output of the measuring body is proportional to voltage converter deviation from its specified value. The simplified principal diagramm of the transformer's converter of the type ПТС-250 is represented in Fig. 4.9.

The structure of the control circuit includes the multivibrator on transistors VT1, VT2 and magnetic amplifier AM. Multivibrator is the inverter of self-excitation. The control circuit generates pulses that switch on and off the transistors VT3-VT6 of the inverter. Transformer T1 of the multivibrator is wounded on the toroidal core from Permalloy, which has a rectangular loop of hysteresis (Fig. 4.10). When you switch on DC power supply through various parameters of transistors VT1 and VT2 in one winding from two, w ₁ or w _2, the current is greater than in another. Let VT1 be moreopen at the moment of examining, its collector’s current I _к1 is greater than current of collector VT2, ie I _к1> I _к2.

Fig. 4.9

Then, currents begin to pass through windings w ₁ and w ₂, so under the influence of the resultant magnetic force, the induction in the core changes, in every winding T1, the in-phase EMF occurs

F_н = w ₁ I _к1 - w ₁ I _к2 = w (I _к1 - I _к2),

w ₁ = w ₂ = w, ,

where n is a number of a winding; w_n number of springs in the given winding;
Fig. 4.10 S – view of the core Т 1.

Let us suppose for definiteness that at the moment of switching on of the source the core was demagnetized and its induction equals to B_s (point 1 of the demagnetization curve Fig. 4.10). Because of the fact that the current flows from the beginning of winding w ₁ (marked with a point), it and all other windings T1 have positive potential that also occurs at the beginning of the winding. The voltage through the winding w3 begins to open VT1, and the voltage on the winding w ₄ - to close VT2. This leads to increasing I _к1 and decreasing I _к2, magnetizing power increases, EMFs increase, that means that an avalanche process starts, which will fully open of the VT1 and close the VT2. After the transistor being opened completely, the entire supply voltage is applied to the transformer primary winding w ₁ the induction in the core will vary from- В_s to + В_s (section 4 - 3 in Fig. 4.10). When the induction in the core reaches the value of saturation + B_s, EMF in windings of the transformer will be equal to zero.

Transistor VT1 begins to close (because its base potential» 0) and current I _к1 begins to decrease. This changes the sign of the derivative dB/dt, and therefore changes the polarity of EMF, which is provided in the windings of the transformer T1. Transistor VT1 is closed, and transistor VT2 is opened. The voltage U _п is applied to the transformer's primary winding w ₂, MPD of which causes a change in the induction from + B_s to - B_s (section 2 - 1 Fig. 4.10).

When the induction of saturation in the transformer's core occurs the switching of transistors takes place. The duration of transistors' switching is fully determined by the time of the cores' remagnetization:

.

Waveshape of voltage on the winding of the transformer UT1 is represented in Fig. 4.11. The frequency of the multivibrator can be synchronized from external oscillator too. For this purpose, closing pulses with a frequency higher than the nominal frequency of the multivibrator can be supplied to the bases VT1 and VT2. In this case, switching occurs with a frequency of closing pulses, and induction does not reach the saturation induction. The converter of the transformer consists of two power transistors VT3, VT4, a transformer T2, two auxiliary transistors VT5 and VT6, which are designed for closing of power transistors, auxiliary transformers T3, T4.

Power transistors operate in order to alternately open themselves during the half-period qT /2 (T - period of the control pulses). When one of the transistors is opened, for example VT3, a current passes through the winding w ₂ of the transformer T2. The voltage at the output of the inverter is:

,

w ₁= w ₂= w ₃= w ₄, that is why U _вих= 3 U _вх. EMF is added to the voltage U _вх, EMF is set in the windings w ₃ and w ₁. In the last part of the half-period (1- q) T /2both transistors are closed, the output voltage is equal to the input voltage U _вх.

The mean value of the voltage per period:

.

By varying of the filling factor q, you can adjust the output voltage of the converter starting from U _вх and up to 3 U _вх. Capacitors on input and output make the current ripples smoother; this current is consumed by a converter. Transistors VT3 and VT4 are opened with positive impulses shich are fed through resistors on clamps of transformers T3 and T4. For closing of power transformers we use transistors VT5 and VT6. When a positive pulse is present at the base of a transistor it opens and shunts a winding of a transformer. As a result the voltage on the secondary winding of this transformer is reducing to zero, and the power transistor closes. Impulses for opening of power transistors come from windings w5 and w6 of the transformer T1, and pulses for closing - from load resistors R1 and R2 of a magnetic amplifier.

Three-phase transformer can be made of three single-phase inverters that have the common circuit of feeding and atr connected on the output in a star or triangle. The power section of the three-phase converter (Fig. 4.12) consists of three identical inverter cells made according to the push-pull circuit with zero output. Invertor cell includes two transistors VT1 and VT2, which switch on the primary winding of the transformer Т_А to the power supply.

Fig. 4.12

Transistors VT1 and VT2 are switched on alternately in every half-period of an output signal. And voltage U _п of a power supply is applied alternately to the one, then another half of the primary winding of the transformer, creating the variable magnetic flux in its core, this flux creates the secondary (output) winding AC voltage of the rectangular waveform.

In others inverters' cells analogical processes with the shift in 120^о take place. The capacity of each cell is equal to one third of the output capacity of the converter. The pulses that control the operation of transistors VT1-VT6, are formed in the control unit БК, which is a digital distributor of pulses and provides synchronized operation of cells with mutual phase shift of 120 ^о. Distributor of pulses is made on the basis of the JK flip-flops (Fig. 4.13) and forms three mutually shifted in phase for 120 ^о control signals A, B and C.

As it is seen from the given diagrams, the frequency of the reference generator of the converter for the given control scheme should be in 6 times higer than the output frequency. The reference generator is performed on the elements LC. In cases when a very high stability of frequency is required, the quartz stabilization is used. Three-phase converter can be made of two inverter cells output transformers of which are connected according to the scheme of Scott (Fig. 4.14) - voltages on the primary windings of the transformer T1 and T2 are shifted at 90^о relatively to each other, and the number of springs of secondary windings of the transformer is chosen so that (Fig. 4.14).

Fig. 4.13 Fig. 4.14

The phase shift on 90^о is performed using magnetic amplifier or digital phase-shifting devices.

The advantage of Scott scheme is the same loading of two inverter cells as for the full and as for active capacity at any соsj of load. Stabilization of output voltages in this circuit is provided by means of two voltage regulators.

The first stabilizes linear voltage U_AC, influencing the inverter I1, the second stabilizes U_BO, influencing the inverter I2.