ON PARALLEL OPERATION OF IGNITRONS IN A PULSE REGIME

I. P. Chuchalin, V. M. Razin

(Presented by the scientific seminar of the Faculty of Physics and Engineering)

In some cases it is necessary to obtain, at definite moments of time, large current pulses. Such current pulses can, as is known, be obtained by discharging a bank of capacitors. In order for these pulses to occur at definite time intervals, a controlled switching device must be placed in the discharge circuit of the capacitor bank. The controlled switching device capable of passing the greatest current is the ignitron.

The purpose of the present work is:

1) to determine the possibility of parallel operation of ignitrons in a pulse regime, and

2) to determine the time of nonsimultaneity in switching ignitrons into parallel operation, i.e., the time interval between the beginnings of operation of the ignitrons.

In the book by B. M. Shlyaposhnikov [1], the parallel operation of ignitrons in rectifier circuits is considered with the use of a so-called equalizing reactor (Fig. 1).

On each core there are placed two windings belonging to different branches. The connection of these branches is made in a zigzag with opposing connection. With this method of winding arrangement, the magnetic fluxes associated with the operating current have opposite directions in the branches placed on one and the same core, as is shown by the arrows in Fig. 1. As a result, these fluxes will be compensated if the branch currents are equal.

Fig. 1

Any disturbance of equality of the currents in the branches of the equalizing reactor leads to a disturbance of the equilibrium of the magnetomotive forces and, consequently, to the appearance of a certain additional emf in the reactor winding, which tends to preserve the equilibrium. This circumstance promotes equalization of the currents through the ignitrons.

If ignitron 1 begins to conduct earlier than ignitron 2, the entire current \(I\) will flow through branch 1. In the iron of the equalizing reactor a magnetic flux is created, which will induce in the turns of branch 2 a certain emf, promoting ignition of ignitron 2 and the passage of current through it.

In rectifier circuits, the spread in the ignition instants of ignitrons is not of essential importance. When operating with short current pulses of large magnitude, simultaneous ignition of the ignitrons is especially important, since the spread of ignition instants of the ignitrons in time may be comparable with the width of the current pulse.

For parallel operation of ignitrons in the pulsed mode, special circuits must be used to ensure simultaneous firing.

Fig. 2

Figure 2 shows the circuit by means of which the parallel operation of two I-100/1000 ignitrons was investigated.

The circuit operates as follows. A capacitor bank of capacitance \(C_1 = 50\ \mu\mathrm{F}\) is charged through the gasotron VG-163 and the limiting resistance \(r_1 = 50\ \Omega\). Negative voltages that block the thyratrons are applied to the grids of the thyratrons from the rectifier. Positive voltage pulses from the pickup unblock the thyratrons. At the moment when they compensate the negative biases, the capacitors \(C_2\) and \(C_3\), which are charged through the gasotron VG-129, discharge through the thyratrons and the igniters of the ignitrons. The ignitrons are fired, and the capacitor bank \(C_1\) discharges, creating current pulses in the load. Coils of inductance \(10\ \mu\mathrm{H}\) and \(7.3\ \mathrm{mH}\) were used as the load.

The amplitude and width of the current pulse through the load are determined by the voltage to which the capacitance \(C_1\) is charged and by the parameters of the circuit \(C_1L_1\). The sharpness of ignition of the ignitrons depends substantially on the sharpness of ignition of the thyratrons. When identical blocking potentials are applied to the grids of the thyratrons, the thyratrons, owing to differences in their ignition regions and to the spread of ignition instants within the region, ignite non-simultaneously. Even with simultaneous ignition of the thyratrons, the ignitrons, owing to the spread of their ignition instants, begin to conduct current non-simultaneously.

If a winding is fastened around the iron core of the equalizing reactor and its ends are connected to the input of an oscillograph, then during the time interval between the beginnings of operation of the ignitrons, when an equalizing magnetic flux exists in the iron of the reactor, the emf pattern induced in this winding can be observed on the oscillograph screen.

By adjusting the blocking potentials on the grids of the thyratrons by means of resistances \(r_4\) and \(r_5\) (Fig. 2), it is possible to cause first one to ignite,

one or the other ignitron. It is possible to arrange that the ignitrons will be fired approximately in succession. Figure 3 presents oscillograms of the emf in the turn around the reactor core for different sequences of ignitron firing. The first peak corresponds to the non-simultaneity of the start of operation of the ignitrons, the second peak to the non-simultaneity of the end of operation of the ignitrons. As can be seen from Fig. 3, the ignitron that first begins to conduct current is also the first to cease conducting.

Fig. 3

Fig. 4

Figures 4 and 5 show oscillograms of the emf in the turn around the core of the equalizing reactor (the first peak, Fig. 3). The same figures also show the calibration frequencies of generator oscillations with period \(L_2 = 3\ \mu\text{sec}\). From Fig. 4 it is seen that the half-width of the first peak is approximately \(3\ \mu\text{sec}\). With a current-pulse width in the load equal to \(120\ \mu\text{sec}\) (\(L_1 = 10\ \text{mH}\)), the non-simultaneity of firing of the ignitrons, relative to this pulse width, is approximately \(2.5\%\). From Fig. 5 it is seen that the half-width of the first peak is approximately \(15\ \mu\text{sec}\). With a current-pulse width in the load equal to \(1900\ \mu\text{sec}\) (\(L_1 = 7.3\ \text{mH}\)), the non-simultaneity of firing of the ignitrons is approximately \(0.8\%\).

If the ignitrons are connected in parallel without an equalizing reactor, the current through them is distributed nonuniformly. Figure 6 gives an oscillogram of the current through one of the ignitrons without a reactor. The width of the current pulse in the load in this case was \(1900\ \mu\text{sec}\). From the oscillogram shown in Fig. 6 it is seen that the current through the ignitrons without a reactor is distributed nonuniformly. Sometimes there may even be misfires of one of the ignitrons.

For narrow current pulses in the load (tens of \(\mu\text{sec}\)), the distribution of current between the ignitrons, even without an equalizing reactor, occurs

more or less uniformly. In this case, with a small load inductance, the role of the equalizing reactor is evidently performed by the connecting leads, whose inductance proves to be commensurate with the load inductance.

Fig. 5

The calculation of the equalizing reactor for each particular case of parallel operation of ignitrons can be carried out by the method of Razin V. M. [2].

Fig. 6

Conclusions

1. Ignitrons can operate in parallel not only in rectifier circuits, but also in pulsed operation.

2. For parallel operation of ignitrons in pulsed operation, an equalizing reactor and special ignition circuits for the ignitrons are necessary.

1. The scatter of the ignitron ignition instants in time, relative to the width of the current pulse, in our experiments was, for a current pulse of width 120 μsec, 2.5%; for a current pulse of width 1900 μsec, 0.8%.

Fig. 7

1. If necessary, it is possible to make not only two ignitrons operate in parallel, but four or more as well, by using a cascade connection of equalizing reactors (Fig. 7).

REFERENCES

1. Shlyaposhnikov B. M. Ignitron Rectifiers, Transzheldorizdat, 1947.
2. Razin V. M. On the calculation of an equalizing reactor for parallel operation of ignitrons. Proceedings of the Tomsk Polytechnic Institute, vol. 76, 1954.