Voltage Stabilizer for Powering a Betatron Electromagnet

L. A. Volynskaya, V. M. Razin

The sensitivity of instrumental methods of betatron flaw detection of materials and products can be brought to 0.3–0.1%; however, when scintillation and spectrometric methods are used, good results can be obtained only with high stability of the excitation voltage of the betatron electromagnet [1].

Since the active power consumed by the supply circuits of a betatron installation can reach 20–30 kW, parametric stabilizers using the nonlinear properties of the electromagnet [2] and compensating single-cycle thyristor stabilizers [3], providing a stabilization factor no higher than 15–20, may find application as voltage stabilizers for powering betatrons. In addition, electromechanical stabilized sources of alternating voltage [4] are used; their use is associated with considerable technical difficulties.

A feature of the electrical circuits for supplying the windings of a betatron electromagnet is the presence of a matching autotransformer [5]; therefore, when choosing the type of stabilizer, preference should be given to a compensating pulse stabilizer, in which the regulating element, in the form of two thyristors connected in antiparallel, is connected in series with the main winding of the step-up autotransformer [6].

The electrical circuit of an AC voltage stabilizer for powering a betatron electromagnet is shown in Fig. 1, where 1 is the autotransformer, 2 the electromagnet, 3 the control and synchronization circuit, and 4 the regulating element.

The operating principle of the stabilizer is as follows. If the thyristors T1–T2 do not conduct current during the entire period of the supply voltage, then the winding of electromagnet 2 will be connected to the AC voltage source \(U_c\) through a saturated choke formed by the series winding of autotransformer 1 with the number of turns \(W_1\). If, however, thyristors T1–T2 conduct current for the entire period, then the excitation voltage of the electromagnet will be equal to

\[ U_{\text{em}} = \frac{W_1 + W_2}{W_2}\cdot U_c . \tag{1} \]

Regulation of the firing angle of the thyristors ensures stabilization of the voltage across the load when the magnitude of the line voltage \(U_c\) changes over sufficiently wide limits.

For stabilizing the supply voltage of the electromagnet of the B-15 betatron for an energy of 15 MeV, a semiconductor control and synchronization circuit was used; its block diagram is shown in Fig. 2.

The control and synchronization circuit operates as follows. The line voltage \(U_c\) is applied to the input of synchronization circuit 1, which generates rectangular pulses at the instant when the voltage \(U_{\mathrm{st}}\sin \omega t\) passes through zero values. Synchronization circuit 1 starts sawtooth-voltage generator 2, whose output is connected to one of the inputs of comparator 3. A constant control voltage is applied to the other input of comparator 3. To form the control voltage, a circuit is used consisting of comparison circuit 4 and dc amplifier 5. The input of the comparison circuit receives the rectified feedback voltage taken from the primary winding of the betatron electromagnet.

Fig. 1. Voltage stabilizer circuit

Fig. 2. Control and synchronization circuit of the voltage stabilizer

At the instant when the control voltage and the voltage at the output of generator 2 are equal, a rectangular pulse appears at the output of comparator 3 and is amplified by amplifier 6. The control pulses arriving at the control electrodes of thyristors \(T_1\)—\(T_2\) are formed by means of expanders 7, 9 and power amplifiers 8, 11. Delay line 10 is intended to delay the control pulse applied to the electrode of thyristor \(T_2\) by 10 μsec. Since the duration of the pulses at the output of generator 3 is 10 μsec, the circuit considered provides a range of variation of the thyristor firing angle of \(140\)—\(160^\circ\). Standard elements of the “Logika” series, type G-403, are used as power amplifiers 8, 11 in the control and synchronization circuit. The regulating element is made in the form of two type PTL-50-8 thyristors connected in antiparallel.

Laboratory tests showed that, when the input voltage varied within 150—240 V and the output voltage was 255 V, the stabilization coefficient of the stabilizer was 120 at a load current of 17.5 A. The coefficient of nonlinear distortion of the output voltage was no more than 2.5%. The power factor of the stabilizer when operating into a resonant load was not less than 0.85. Smooth adjustment of the output-voltage magnitude over the range 200—300 V was provided. The transient time in the load circuit when the stabilizer was switched on was 0.9 sec. The transient time for a stepwise change of the input voltage from 160 to 200 V was 0.25 sec.

References

1. V. V. Evstigneev, K. A. Dergobuzov, V. M. Zykov, V. N. Rudenko, Yu. M. Stepanov. Experimental assessment of the possibilities of electron defectoscopy. Defektoskopiya, 1971, 2, 97.

2. L. Sipek, Stabilizace napajeni magnetu betatronu. Elektrotechnicky obzor. 1962, 51, 5, str. 233.

3. L. M. Ananiev, M. M. Shtein. Stabilization of the supply voltage of a betatron electromagnet. Abstracts of papers of the VI Interuniversity Conference on Electron Accelerators. Tomsk, 1966, 48.

4. S. Haltrich, M. Janovici. Die Stabilisierung der Energieversorgung beim Betatron des Instituts für Atomphysik Bukarest. Wissenschaftliche Zeitschrift der Friedrich-Schiller-Universität Jena. 1964, H. 4, S. 485.

5. L. M. Ananiev, A. A. Vorobev, V. I. Gorbunov. An induction electron accelerator—the betatron. Moscow, Atomizdat, 1961.

6. A. Ya. Yakushev. The use of thyristors for stabilizing the voltage of auxiliary circuits of ER-9 alternating-current electric trains. Proceedings of LIIZhT, issue 277, 1968, 45.