Excitation of the Electromagnet of a Compact Betatron from an Autonomous Power Source of Limited Power
Abstract Generated abstract
This paper examines the feasibility of exciting the electromagnet of a compact PMB-6 betatron from autonomous gasoline-electric power units of limited capacity. Experimental studies compare direct supply from an AB-4 generator with supply through a rectifier, inverter, and stabilizing frequency-conversion system, including transient starting behavior and steady-state operation. Direct supply is shown to be possible but sensitive to generator frequency and requiring adjustment, with a reported 10 to 15 percent reduction in average radiation dose. Use of an intermediate converter with soft starting stabilizes voltage and frequency, permits operation near 100 Hz, and in tests doubles the bremsstrahlung dose rate relative to standard 50 Hz excitation, while suggesting mixed excitation circuits as a promising way to reduce reactive and consumed power.
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EXCITATION OF THE ELECTROMAGNET OF A COMPACT BETATRON FROM AN AUTONOMOUS POWER SOURCE OF LIMITED CAPACITY
P. G. Gordeev, P. P. Rumyantsev, Yu. P. Yarushkin
(Submitted by the Research Institute of Nuclear Physics, Electronics, and Automation)
For a number of practical applications of a compact betatron, it becomes necessary to supply it with power in places where centralized electric power supply is absent. In this case, normal operation of the betatron is provided by autonomous units with generators of alternating single-phase and three-phase alternating current of industrial frequency (50 Hz) and increased frequency (400 Hz).
In autonomous units, synchronous and induction alternating-current generators are used for voltages of 115, 230, or 380 V. The powers of the generators of autonomous units are relatively small (from a few kilowatts to several tens of kilowatts).
Autonomous power-supply units are equipped with special devices that make it possible automatically to maintain the frequency and voltage at the generator output with an accuracy on the order of ±2–3%.
The most suitable autonomous units for supplying portable compact betatrons of the PMB-6 type, from the standpoint of minimum weight, are gasoline-electric units of the AB-4 type. These units consist of a 7 hp internal-combustion engine and a 4 kW induction-type alternating-current generator with a three-phase output of 127/220 V at a frequency of 50 Hz, or with a single-phase output of 230 or 115 V and a frequency of 425 Hz. In the first type of generator there is no special voltage-stabilization device, which is present in the generator with a frequency of 425 Hz. To stabilize the frequency in AB-4 units, a centrifugal regulator is used, maintaining constant speed of the internal-combustion engine.
To determine the possibility of powering the PMB-6 betatron from gasoline-electric units of the AB-4 type, studies were carried out on supplying a production-model PMB-6 betatron directly from an AB-4-T/230-M unit with a three-phase output of 127/230 V at a frequency of 50 Hz, and on supplying a modernized PMB-6 betatron through a frequency converter from an AB-4-0/230/ч-425 unit with a single-phase output of 230 V at a frequency of 425 Hz.
1. Experimental studies of steady-state and transient conditions under direct supply from a gasoline-electric unit.
The output characteristic of the AB-4-T/230-M unit (the dependence of the voltage at the generator terminals on frequency), obtained under no-load conditions, is shown in Fig. 1. A large change in the voltage at the terminals
of the generator within the range of 48–52 Hz was alarming when starting the betatron electromagnet. Under load, however, the output characteristic becomes considerably more uniform, and the resonance characteristic of the electromagnet (the dependence of the primary current on frequency) ensures stable operation of the betatron and the gasoline-electric generator on the right-hand branch, i.e., when the generator frequency is set before the load is switched on to more than 49 Hz. In this case the internal-combustion engine tends to increase its speed and the voltage at the generator output should rise; however, the increasing load of the circuit reduces this voltage and, consequently, the engine speed as well. A decrease in frequency also reduces the load current, causing an increase in the generator voltage and frequency.
Setting the resonance frequency (49 Hz) or lower (left-hand branch) in the no-load mode, when the load is switched on, leads to generator pull-out, since as the frequency decreases the load current increases, producing an even greater decrease in frequency and an increase in the load current.
Since a parametric (ferroresonant) voltage-stabilization system, sensitive to changes in frequency, is used in the power-supply circuit of the betatron electromagnet, a decrease in the average dose of bremsstrahlung radiation was to be expected. A quantitative estimate of the average radiation dose over equal time intervals, carried out when the betatron was powered from an industrial mains supply and from a gasoline-electric unit, showed that in the latter case the average dose decreases by 10–15%.
The experiments performed show that, in principle, autonomous power sources of limited capacity can be used to supply the standard compact betatron PMB-6.
Fig. 1. \(U_{\mathrm{xx}}=f(I)\) and \(U_{\mathrm{H}}=f(I)\)—output characteristics of the AB-4-T/230-M unit (dependence of the voltage at the generator terminals on frequency) at no load and under load. \(I=f(f)\)—resonance characteristic of the electromagnet (dependence of the primary current on frequency).
The disadvantages of directly supplying a betatron from autonomous power units include, first, a decrease in the average radiation dose due to the instability of the frequency at the generator output and, second, the need for preliminary adjustment of the autonomous unit for stable operation with the betatron installation.
To eliminate these disadvantages, the betatron must be supplied from the autonomous unit through an additional device that stabilizes voltage and frequency.
- Experimental studies of steady-state and transient modes when supplying the betatron from a gasoline-electric unit through an intermediate stabilizing device.
The device stabilizing frequency and voltage must contain a dc link (to eliminate the influence of frequency instability of the power unit), a voltage stabilizer, and an inverter. Experimental studies of the PMB-6 betatron electromagnet showed that, without substantial thermal overload, it is possible to excite it at a frequency of about 100 Hz, and this, as is known, gives a twofold increase in radiation intensity [1]. Thus, it proved advisable to make the inverter for a frequency of 100 Hz. Additional
... an additional frequency converter for the betatron does not noticeably reduce the weight and overall-size characteristics, since an autonomous power source is necessarily present and the converter can be structurally incorporated directly into it.
For supplying the betatron through a frequency converter, it is most expedient to use gasoline-electric units with generators of increased frequency (400 Hz). The use of increased frequency makes it possible to reduce substantially the weight and overall dimensions of the transformers and smoothing filters of the rectifier devices.
The developed excitation circuit for the betatron electromagnet at increased frequency is shown in Fig. 2. The excitation circuit includes the gasoline-electric unit AB-4-0/230/g-425, a rectifier \((AT; D_1-D_4; L_1; C_1)\), a single-cycle inverter with artificial commutation \((T_1; T_2; D_5; D_6; L_2; L_3; C_2)\), and an oscillatory circuit formed by compensating capacitors and the windings of the electromagnet.
Fig. 2. Excitation circuit of the betatron electromagnet at increased frequency
The introduction of the rectifier and inverter eliminates the influence of changes in the generator frequency on betatron operation, since the frequency stability is determined by the stability of the self-oscillator of the inverter control system, whose stability is \(\pm 0.5\%\). Parametric stabilization of the voltage on the electromagnet is provided by the limiting choke \(L_3\) and the circuit \(C_6 L_6\), with a stabilization factor of about 7.
An experimental study of the transient mode during excitation of the electromagnet from an autonomous power-supply unit showed that, at the instant the betatron electromagnet was switched on, the alternating-current generator dropped out, because during the transient process the power source had to develop a large power caused by the accumulation of reactive energy in the resonant circuit of the betatron. Obtaining a large instantaneous power from the AB-4-0/230/ch-425 unit is difficult because the generator voltage-stabilization system is built on the basis of a magnetic amplifier, which, as is known, has inertia. Since the magnetic amplifier does not have time to respond to the connection of a large load, the voltage at the generator output decreases, and with it the current of the excitation winding also decreases, which causes the alternating-current generator to drop out.
To ensure stable starting of the betatron electromagnet, it is necessary to increase the duration of the transient process, i.e., during the transient process to reduce the amount of energy introduced into the betatron resonant circuit per oscillation period. This makes it possible for the stabilization system to keep up with changes in the load current. In the experimental installation, a softer transient process is provided by reducing the inverter frequency during the transient process, which lasts \(1 \div 2\) s.
Experimental tests of the betatron power-supply system from a gasoline-electric unit through a frequency converter with soft starting showed its stable operability both in transient modes and in steady-state modes when the generator voltage and frequency were varied over a fairly wide range: ±10% in voltage and 2–3% in frequency. At the same time, the dose rate of bremsstrahlung radiation doubled in comparison with the standard PMB-6 unit powered by industrial-frequency voltage (50 Hz).
The experiments carried out show that excitation of the electromagnet of the PMB-6 betatron from an autonomous unit of limited power is in principle possible; moreover, the introduction of an additional frequency converter not only stabilizes the frequency and voltage but also makes it possible to design the most rational betatron power-supply circuits.
From this point of view, it would be very advantageous to use mixed power supply of the betatron electromagnet, making it possible substantially to reduce the reactive power and, consequently, the consumed power, which is especially important when operating from an autonomous source. Moreover, with mixed excitation of the betatron electromagnet it is possible to construct the power-supply circuit in such a way that the function of the protective element and of the frequency converter is performed by a single element. This reduces the weight and dimensions of the entire power-supply system.
One variant of such a combined power-supply system with a parallel current inverter is shown in Fig. 3. The role of the large inductance at the input of the current inverter is performed by the betatron electromagnet, which is simultaneously connected to the output circuit of the inverter.
Fig. 3. Circuit of mixed power supply of a betatron at increased frequency using a current inverter
Magnetization is carried out by means of the dc component of the inverter current. In this case the degree of magnetization and the quality factor determine the transformation ratio of the inverter output transformer. The approximate value of the transformation ratio, if the active voltage drop in the dc circuit is neglected, is determined by the formula
\[ k_{\mathrm{t}}=\sqrt{2\,C_{\mathrm{p}}Q}, \]
where:
\[ k_{\mathrm{t}}=\frac{U_2}{U_1}, \]
\(C_{\mathrm{p}}\) is the degree of magnetization,
\[ C_{\mathrm{p}}=\frac{I_0}{I_{\sim m}}, \]
\(Q\) is the quality factor of the parallel oscillatory circuit formed by the betatron electromagnet, the protective element, and the compensating capacitors;
\(U_2\) is the rms value of the voltage on the betatron electromagnet;
\(U_1\) is the voltage of the dc source.
The experimental studies carried out on such a power-supply circuit show the feasibility of using it not only for supply from autonomous units of limited power, but also for supplying the betatron electromagnet at elevated frequencies from an industrial power grid.
REFERENCES
- Yu. P. Yarushkin. Some questions concerning the study of mixed excitation of the electromagnet of a compact betatron. Candidate’s dissertation. Tomsk, 1965.