On Current Switching from a Shock Generator

V. V. Ivashin

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

Known circuits for switching energy from a shock generator usually have at least two valve devices, one of which conducts current when the contacts are closed, and the other when they are opened [1, 2]. In [3] a device is described for switching current from a shock generator in which, when the contacts are closed and opened, the current passes through a valve (or a group of valves) connected in parallel. The implementation of such a device requires special manufacture of the shock generator, with a separate tap from part of the turns of its winding. The need for such a tap is undoubtedly a disadvantage. In addition, the pulse currents flowing through the auxiliary valve when the apparatus is switched on constitute a considerable part of the amplitude of the shock-generator current, which requires the use of valves with high pulse overload capacity. Figure 1 shows a device for switching current from a shock generator in which the power of the auxiliary valves is substantially reduced.

Fig. 1.

The main elements of the switching device are, as in the known devices, a high-speed switching apparatus 3, a saturation reactor 4, and an auxiliary valve 5. These elements are connected into a single circuit and included in the current circuit of the shock generator as shown in Fig. 1.

The circuit operates as follows. At the instant \(t_0\), valve 5 is opened by the control circuit, and the current of shock generator 1 begins to flow through inductive load 2 and valve 5, which shunts the circuit with contacts 3 and reactor 4. At the same time, a signal is applied to actuate the drive of contacts 3, and they begin the process of closing.

The valve must be opened at such an angle \(\omega t_1\) of the generator emf that the time \(t_0—t_2\), during which the current passes through the valve, is sufficient for the contacts to close. It follows from Fig. 2 that the larger the angle \(\omega t_1\), the longer the time \(t_0—t_2\). Taking into account that the operating time of the contacts of modern high-speed switches can be taken as \((4 \div 5)\cdot 10^{-3}\) sec at a generator voltage of 10–15 kV, the angle \(\omega t_1\) may be taken equal to \(30 \div 35^\circ\) at a generator-emf frequency of 50 Hz. When the frequency is reduced, the angle \(\omega t_1\) decreases correspondingly.

Fig. 2.

At the instant \(t_2\), when the current passes through zero, the valve is locked, and the generator current for \(t > t_2\) passes through saturation choke 4 and the contacts. The entire positive current wave passes through the contact apparatus. When the current passes through zero at the instant \(t_3\), choke 4, made of magnetically soft steel with a large squareness coefficient, begins to be remagnetized, and a voltage \(e_2\) is induced in it, applied to the valve in the forward direction. At the instant \(t_3\), an igniting pulse is applied to valve 5; the valve opens, and subsequently, during the time \(t_3—t_4\), the current passes through valve 5, while the contacts are de-energized. Simultaneously with the application of the igniting pulse to valve 5 at the instant \(t_3\), a signal is applied to actuate the contact drive, which opens them.

By the instant \(t_4\) the contacts must be opened to the required gap, capable of withstanding the recovery voltage of the circuit when the valve is locked at the instant \(t_4\).

Saturation choke 4 is necessary when closing the contacts in order to create a current step of small magnitude, equal to the remagnetization current of the choke core, which prevents sparking when the contacts touch, their sticking, and electrical wear. When the circuit is opened, the choke creates the anode voltage necessary for normal operation of the valve and prevents the current in the circuit with the contacts from increasing for \(t > t_3\) during the remagnetization time. In practice, the contacts must open before the choke is completely remagnetized under the action of the voltage drop across the conducting valve.

The advantages of the switching method considered, in comparison with known ones, are as follows:

1. The process of arc-free closing of the contacts is ensured by means of the same valve that ensures arc-free opening of the contacts. In comparison, for example, with the device described in [1, 2], this makes it possible to reduce the number of valves by at least a factor of two and simpli-

This is valid in those cases where the number of valves is determined not by the average value but by the amplitude value of the current passed through the valve.

In practice, when switching current from a shock generator, the number of valves is determined precisely by the amplitude values of the current and by the selected switching circuit.

In comparison with the switching device described in [3], the method considered has the advantage that it imposes no additional requirements on the design of the shock generator (such as, for example, the need to tap off part of the turns of the shock-generator winding). With the switching method considered, the load on the turn-on valve is considerably smaller both in terms of the average and the amplitude values of the current. Fig. 3 shows the currents during turn-on according to the known circuits (a) and according to the method considered (b), for the same amplitude value of the current taken from the shock generator.

Fig. 3.

It follows from the figure that the amplitude value of the current \(I_m\) in the valve during switching according to the known circuits is approximately 2.5 times greater than the amplitude value of the current \(I_m^*\) in the valve during switching according to the method considered. The average current value, for the same operating time \(t_{\mathrm{cp}}\) of the contact apparatus, is also substantially reduced, by a factor of \(1.5 \div 2\), as follows from comparison of the areas with vertical and oblique hatching in Fig. 3.

REFERENCES

1. V. V. Ivashin, G. A. Sipailov. Author’s certificate No. 158959.
2. V. V. Ivashin, G. A. Sipailov. Author’s certificate No. 166354.
3. V. V. Ivashin, G. A. Sipailov. Author’s certificate No. 156200.