INVERSION OF DC VOLTAGE IN AN ELECTROMAGNETIC PARAMETRIC FREQUENCY DIVIDER

R. A. VAINSTEIN, A. G. CHIKUNOV

(Submitted by the Department of Electric Power Stations)

Electromagnetic parametric generators are widely used in automation devices, computer technology, and also as power frequency dividers (PFD) [1, 2, 3].

Frequency conversion in a PFD is performed when it is supplied from a sinusoidal or rectangular-wave source and is most effective when the primary frequency is divided by two. The unipolar magnetization of the ferromagnetic material of the divider cores, necessary for the occurrence of oscillations at half frequency, is then accomplished either by including a rectifier in the supply circuit or through an additional bias winding. It is quite obvious that the necessary excitation conditions for a PFD can be provided if it is supplied from a direct-current source through a controlled interrupter, for example, a transistor or thyristor.

If the commutation frequency of the controlled rectifier and the value of the supply voltage are in a definite relation with the natural frequency of the oscillatory circuit of the divider, then undamped oscillations arise in the latter, close in form to a sinusoid. Part of the power of these oscillations can be transmitted to the load through a load winding or by direct connection of the load in parallel with the circuit capacitor. Figure 1, a, b, c shows the basic circuit diagrams of inverters.

The control circuit (CC) ensures the conducting or nonconducting state of the inverter power switches and may have various circuit implementations. In particular, it may be an autogenerator and be supplied from the same direct-current source as the inverter.

In the circuit of Fig. 1, a, diode D and resistor R prevent overvoltages across the switch. Resistor R is selected on the basis of the permissible voltage across the transistor at the maximum interrupted current. In it, the power stored in the ferromagnetic material of the cores during the conducting state of the switch is dissipated. It should be noted that the use of only one diode to reduce commutation voltages across the switch is inadmissible, owing to the small modulation of the inductance under continuous current in the pump winding. To reduce the loss of power stored in the core material, an additional winding ($w_{\mathrm{d}}$, Fig. 1, b) and a diode may be used; through them this power is returned to the source during the moments when the switch is in the nonconducting state. Fig. 1, c shows the basic circuit diagram of an inverter with commutation

Fig. 1. Inverter circuits. \(w_{\mathrm{n}}\)—drive winding: 100 turns, \(\varnothing\) 0.69; \(w_{\mathrm{d}}\)—additional winding: 100 turns, \(\varnothing\) 0.51; \(w_{\mathrm{k}}\)—tank winding: 300 turns, \(\varnothing\) 0.96

through the thyristor. Such a circuit can be recommended only for cases of high inverted voltages or high powers.

Figures 2 and 3 show the test results for the inverter according to the circuit of Fig. 1b and Fig. 2, made on two ferrite cores of grade NN600, with a cross section of \(3.84\ \mathrm{cm}^2\) and a magnetic path length of \(18\ \mathrm{cm}\). The commutation frequency of the transistor switch is \(800\ \mathrm{Hz}\), and that of the output voltage is \(400\ \mathrm{Hz}\).

In the plots, the solid lines correspond to modes of stable operation of the inverter, by which it is meant that the loaded inverter is reliably excited when power is applied. The dashed lines indicate operating modes obtained with a smooth increase of the load, but in which, after removal and repeated application of the supply, oscillations in the tank do not arise.

The experimental data show that such an inverter, in the region of stable operation, has a stiff external characteristic from no-load operation to nominal load and output-voltage stability within \(+3.2\%\) to \(-1.5\%\) when the supply voltage deviates by \(\pm 20\%\). The efficiency at nominal load reaches \(66\%\), i.e., it is only slightly inferior to conventional inverters of low and medium power. The harmonic coefficient of the output voltage did not exceed \(15\%\) at nominal load. Changing the capacitance of the capacito-

Fig. 2. Inverter characteristics. 1 — efficiency; 2 — external; 3 — output power; 4 — harmonic coefficient of the output voltage; 5 — power consumed by the inverter.

Fig. 3. Dependences of the output voltage on changes in the supply voltage. 1 — at \(C=2\ \mu\mathrm{F}\); 2 — at \(C=2.2\ \mu\mathrm{F}\); 3 — at \(C=2.6\ \mu\mathrm{F}\).

… has little effect on the magnitude of the output voltage and on the region of stable operation.

If one calculates the apparent power of the cores used for the transformer mode of their operation at a frequency of 400 Hz, then the useful power obtained from them in the parametric frequency divider mode amounted to 40% of the apparent power.

It should be noted that, in the range of loads close to complete disruption of oscillations, intermittent generation of the parametric frequency divider is possible. This can be avoided by correctly choosing the range of load variation, without going beyond the limits of stable operation of the inverter. Figure 4 shows oscillograms of the voltages and currents of the inverter under the nominal-load condition.

Such an inverter does not require special measures to protect the power switch against short circuits and overloads, since in such cases the parametric oscillations are disrupted; they are automatically restored after the short circuit is eliminated or after the load is reduced to a value corresponding to stable operation of the inverter.

Fig. 4. Oscillograms of voltages and currents at nominal load. 1—voltage on the pumping winding; 2—source current; 3—voltage on the circuit

The reduction in the number of power switching elements, as compared with conventional inverters, may also be counted among the advantages of the proposed method of inversion.

An inverter with a three-phase output, consisting of two parametric frequency dividers (PFDs), was tested experimentally. A symmetrical three-phase voltage, independent of the load power factor, is obtained if the commutation of each PFD is controlled with a shift of \(180^\circ\). Owing to frequency division, the output voltages of the PFDs are shifted by \(90^\circ\), which makes it possible to obtain a three-phase system according to any of the circuits proposed in [1], and also to provide contactless control of the phase-sequence order.

Conclusions

1. Sufficiently efficient conversion of direct current into single-phase and three-phase alternating current of nearly sinusoidal form is possible on the basis of an electromagnetic frequency divider.

References

1. A. M. Bamdas, S. V. Shapiro, L. N. Davydova. Ferromagnetic Frequency Dividers. Energiya, 1967.
2. Parametrons (translated from Japanese and English). IL, 1962.
3. M. A. Rakov, Yu. M. Shumkov. Stationary processes in a frequency divider with a valve under rectangular excitation voltage. Elektrichestvo, 1969, No. 2.
4. V. A. Labuntsov, G. A. Rivkin, G. I. Shevchenko. Electric Drives with Semiconductor Control. Autonomous Inverters. Energiya, 1967.