INDICATION CIRCUIT FOR BACKFIRES, CURRENT OVERLOADS, AND THYRATRON MISFIRES IN THE EXCITATION SYSTEM OF THE ELECTROMAGNET OF THE “SIRIUS” SYNCHROTRON

V. V. Ivashin, L. A. Zolotaya

(Presented by the seminar of the “Sirius” facility of the Nuclear Physics Research Institute)

In the excitation system of the electromagnet of the 1500-MeV synchrotron, 18 thyratrons are connected in inverse-parallel fashion in two groups [1]. Each group has 9 thyratrons operating under severe conditions. The valves are loaded by pulsed currents whose amplitude is twice the rated value, while the duration of current flow through each valve is \(84 \cdot 10^{-3}\) sec. The power circuit of the excitation system has a high \(Q\), and at the instant of blocking a reverse voltage of 10 kV is applied abruptly to the valves.

To improve the reliability of the circuit, speed up troubleshooting, and make possible continuous monitoring of the operation of all thyratrons, an indication circuit was developed and tested. This circuit makes it possible to indicate current overloads, backfires, and valve misfires.

The use for this purpose of indication circuits described in the literature [2, 3, 4] is unacceptable because they are either more complicated or do not make it possible to identify the faulty valve when the number of valves is odd, etc.

The indication circuits used in the synchrotron power-supply circuit cover all types of emergency operating conditions of the valves, are simple, and consist of elements of the same type. This simplifies maintenance of the circuits and does not require highly qualified specialists for servicing.

The basic element of the indication circuits is an electromagnetic sensor made in the form of a closed magnetic core encircling the anode or cathode bus of the valve. The sensor has two windings: the signal winding \(w_c\) and the magnetizing winding \(w_0\). The number of sensors in the indication system of each type corresponds to the number of valves.

The operating principle of the electromagnetic sensors, as usual, is based on remagnetization of the sensor in an emergency mode. A voltage is induced in the signal winding, indicating in one way or another the emergency operating mode. The only exception is the valve-misfire indication system, in which the magnetic core of the emergency-circuit sensor is not remagnetized and does not participate in producing the valve-misfire signal.

Backfire indication

When indicating backfires, the magnetic core of the sensor is biased to point “a” on the bend of the magnetization curve (Fig. 1).

The magnetizing force from the current in the operating busbar in this case coincides with the direction of premagnetization. During normal operation of the circuit the magnetic core of the sensor operates in the saturated part of the magnetization curve. The increment of the magnetic field is small ($\Delta B_1$), and only a small voltage is induced in the signal winding. In the event of a backfire of the valve, the magnetizing forces from the backfire current

Fig. 1. Reversal magnetization of the sensor of the backfire indicator

($i_{\mathrm{rev}} w$) and from the premagnetizing current ($a w_0$) are directed not in agreement, but oppositely. In this case the magnetic core of the sensor is remagnetized, the change in induction $\Delta B_2$ is considerably greater than $\Delta B_1$, and, accordingly, a considerably larger voltage is induced in the winding $w_c$, which is used for indicating the backfire.

In the signal winding $w_c$ (Fig. 2) two voltage pulses are induced. The first, positive one, is induced when the backfire current rises. The induction in the magnetic core then changes from $-B_a$ to $+B_c$. The second, negative pulse is induced when the current falls; in this case the induction changes from $+B_c$ to $-B_a$. Relay $P$ operates on the first positive pulse and becomes self-holding.

Fig. 2. Schematic diagram of indication of backfires and current overloads

At the output of the relay power supply an electrolytic capacitor $C_\phi$ is connected (Fig. 2). When the contacts $k_1$ close, the relay winding is shunted by this capacitor; therefore the negative current pulse passes through the capacitor, and no voltage is developed across the relay winding. If there is no appreciable capacitance at the output of the rectifier, under the action of the negative voltage on $w_c$ the current through the relay winding may become less than the drop-out current, the relay will release, and its contacts $k_1$ will open.

When the relay operates, a signal indicating an emergency operating mode can be sent to the control console by means of its second contacts. In the implemented indication system, this signal is the extinguishing of a lamp on the control console.

Instead of a relay, any other relay device may be used, for example, a monostable multivibrator, a contactless relay, etc.

In the excitation system of the electromagnet of the “Sirius” synchrotron, the parameters of the elements of the reverse-ignition indication circuit were chosen as follows: the sensor is assembled on Sh-32 iron with the central lamination cut out, \(w_o = 280\) turns, \(I_o = 0.42\) A, \(w_c = 1700\) turns, relay RSM-2, \(C_\phi = 500\) µF.

Fig. 3. Schematic circuit diagram of indication with a short sensor remagnetization time

The circuit described above will not operate if the time of variation of the current in the working winding \(w_p\) is significantly less than the relay operating time. In view of this, the indication of emergency modes of the TR1-85/15 thyratrons in the power rectifier, assembled according to the Larionov circuit and feeding the synchrotron excitation circuit, has been somewhat modified. To broaden the voltage pulse on the relay winding, an integration method is used.

In this circuit the relay winding \(P\) is shunted by the capacitance \(C\), and an active resistance \(r\) is connected in series with the signal winding (Fig. 3). It is not difficult to show that, in such a circuit, for certain parameters \(r\) and \(C\), the voltage on the capacitance will depend only weakly on the rate of remagnetization of the sensor.

\[ U_C = U_r + U_{cu}. \tag{1} \]

Hence

\[ U_{cu}=\frac{WS\cdot 2B_s}{rC+\Delta t}. \tag{2} \]

It follows from relation (2) that, for \(rC \gg \Delta t\), the voltage on the capacitance changes little. The choice of the time constant \(rC\) is determined by the condition of the relay operating time.

\[ t_{\mathrm{op}}=\frac{rC+\Delta t}{1.2 \div 1.5}. \]

Indication of Current Overloads

The electrical schematic diagram of the overload sensor differs almost not at all from the backfire-indication circuit. The only difference is that the direction of bias magnetization of the sensor and the direction of the magnetizing force from the current in the working bus are opposite. In this case the bias magnetization is so strong that, under normal valve loading, the sensor does not leave the saturated state. The magnetic core of the sensor can be remagnetized only during overloads. By the value of the bias current one can set the critical value of the load current at which the sensor is remagnetized and the circuit indicates an abnormal operating mode.

Indication of Valve Misfires

In the circuit for indicating valve misfires, all signal windings \(w_c\), relays \(P\), and diodes \(D\) are connected as shown in Fig. 4.

The bias current of the sensor is equal in magnitude to the bias current of the backfire sensor, but opposite in direction. Therefore, with each pulse in the winding \(w_p\), the misfire sensors are remagnetized.

Fig. 4. Schematic diagram of a circuit for indicating valve misfires

During normal operation all magnetic cores are remagnetized simultaneously, and equal voltages \(U_{c1}\), \(U_{c2}\), \(U_{c3}\) .... \(U_{cn}\) are likewise induced simultaneously in the signal windings. With equal voltages the circuit is in a state of electrical equilibrium; no current flows through the winding, and no voltages are developed across the diodes \(D_1, D_2, D_3, \ldots, D_n\).

If one of the valves connected in parallel fails to operate, the magnetic core of the sensor of this valve will not be remagnetized, and no voltage will be induced in the signal winding of the sensor. The voltage from the remaining windings \(w_c\) will then be applied to the diode of the sensor that has failed to operate. This voltage is applied to the diode in the reverse direction, appears across it, and can be taken as a signal indicating a valve misfire.

By connecting electromagnetic relays or any other signaling devices to the diodes \(D_1, D_2, D_3, \ldots, D_n\), each valve misfire can be recorded.

Biasing the sensor with direct current makes it possible to reduce the cross section of the sensor magnetic core by more than a factor of two, owing to the increase in the induction used. In the absence of bias magnetization, the magnetic core of the sensor is remagnetized along partial hysteresis loops, changing—

the change in induction is insignificant, i.e., the magnetic core of the sensor is underutilized. The second biasing circuit increases the voltage (Fig. 4) across the diode by reducing the voltage drop across the inductive reactance of the signal winding when the sensor is saturated.

It follows from Fig. 4 that, if one of the parallel circuits fails to operate, voltage is applied to the actuating mechanism through the circuit containing the winding \(w_c\) of the sensor of that same circuit. Obviously, in order to reduce the voltage drop across the winding \(w_c\), the resistance of the winding must be minimal. This is achieved by saturating the sensor magnetic core with the bias current. The magnetomotive forces from the current of winding \(w_c\) and from the bias current add together, and, when the valve fails to operate, the sensor magnetic core operates in the saturation region. The inductive reactance of the sensor is then small, and only an insignificant voltage is developed across it.

The parameters of the elements of the misfire-indication circuit are: \(w_c = 1700\) turns, \(w_0 = 280\) turns, bias current \(0.42\ \mathrm{A}\), D7Zh diodes, RSM-2 relay.

The indication circuit proved sufficiently reliable in operation and substantially improved the operation of the accelerator power-supply circuit.

REFERENCES

1. V. M. Kuznetsov. Excitation of the synchrotron electromagnet with accelerated-electron energy of 1500 MeV. Tomsk, Research Institute of Nuclear Physics, Candidate’s dissertation, 1963.

2. G. B. Chernikov. Monitoring extinction of ion-converter valves by anode current. Elektrotekhnika, No. 1, 1965.

3. Yu. A. Shmain, S. Kh. Kharlamov. Monitoring misfires in the ignition of ion-converter valves. Vestnik elektropromyshlennosti, No. 12, 1962.

4. E. G. Bobrov. Device for protecting a rectifier against incomplete-phase operating mode. Author’s Certificate No. 160217, Bulletin No. 3, 1964.