Analysis of Circuit Design Solutions for Cathode-Spot Excitation Circuits in Excitrons

V. A. Paramonov, V. A. Sevastyanov, A. P. Ineshin

(Recommended by the scientific seminar of the Department of “Electric Drive and Automation of Industrial Installations,” UPI)

Modern automated electric drives for metal-cutting machines with a wide speed-control range are based mainly on systems consisting of an electric-machine amplifier and a DC motor (EMA–D), with intermediate semiconductor amplifiers (SAs). At present, a number of design organizations and research institutes are considering the possibility of using, in the main drives of heavy machines, static current converters: magnetic amplifiers with self-saturation (MAs) and ion converters. ENIMS, SibNIZTI, and others are working on this problem.

Despite the known advantages of electric drives with ion converters—high speed of response, high efficiency, absence of rotating parts, the possibility of using a transformerless circuit, ease of switching on, etc.—they have not found wide application in machine-tool building and require further investigation and improvement. Thus, for example, thyratrons, along with their existing advantages, are characterized by the presence of filament heating, require preparation before switching on, and have low overload capacity. Ignitrons do not have these disadvantages, but they are characterized by the absence of grid control, a short service life of the igniter, and the possibility of ignition failures and current pulses.

At present, domestic industry is mastering the production of single-phase mercury rectifiers with a control grid and a pilot arc—excitrons—which combine the advantages of thyratrons and ignitrons without having the indicated shortcomings. The excitron has a metal casing, from which the anode and cathode are insulated, which eliminates the possibility of arc transfer to the casing and increases its operational reliability. The presence of a continuously burning pilot arc eliminates omissions of anode-current pulses. The excitron is distinguished by high efficiency and low power of grid control (2,000 times less than that of an ignitron). Its preparation for operation is determined by the excitation time of the cathode spot and amounts to fractions of a second (Fig. 4). The igniter circuit is normally unloaded with respect to current; the igniter ...

is located outside the cathode-spot zone. The excitron has a tenfold overload capacity with respect to current. Its small dimensions make it possible to mount excitirons in compact cabinets. The only substantial disadvantage of the excitron is water cooling, which somewhat reduces its operating characteristics. At present, however, excitirons with air cooling are being developed, which will facilitate their use in industrial installations.

The question of investigating the possibility of using these devices in main-drive systems of heavy machine tools of the UZTS type, with a drive-motor power of not less than 20 kW, deserves attention. Despite the attractive possibilities of excitirons for their use in drives with a wide range of speed regulation of the actuating electric motor, the circuit designs of both the power and auxiliary circuits have not been sufficiently covered in the modern literature and require further development, investigation, and analysis.

Figure 1 shows a known basic excitation circuit with a thyratron in the igniter circuit, Author’s Certificate No. 158957, 1962, the investigation and analysis of which in the sector laboratory of UPI showed the following results:

a) thyratrons additionally consume considerable filament energy, which is uneconomical. There is a state of unreadiness of the circuit for switching on, owing to the necessity of preheating the cathode, which requires time;

Fig. 1. Thyratron circuit for excitation of the cathode spot of an excitron.

b) a multiphase excitation circuit with thyratrons will be complex and bulky;

c) in terms of dimensions, thyratrons are commensurate with excitirons, i.e., they will require a separate cabinet.

It follows from the above that this excitation circuit for an ionic electric drive using excitirons in heavy machine-tool construction is of little acceptability.

Fig. 2. Electromagnetic circuit for excitation of the cathode spot of an excitron.

Figure 2 gives another circuit for excitation of the cathode spot of an excitron—an electromagnetic one, developed by VEI. Here: Э is the excitron, Tr is the power-supply transformer, MU is the magnetic amplifier in the ignition circuit, Dr is the smoothing choke.

The magnetic amplifier MU is made on toroidal iron, has a small no-load current and sharply pronounced saturation. The MU and valve B1 form the circuit of a single-half-period, fast-acting magnetic amplifier controlled by the current of the pilot arc. When the supply voltage is applied to the circuit, the cathode spot of the excitron is not excited, the pilot-arc current is absent, the magnetic amplifier is opened as a result of feedback action, and current pulses with a steep leading edge flow through the igniter circuit at a frequency of 50 cps. The average value of this current is 1.5 A. If the igniter has a comparatively large nonlinear internal resistance, then, when current pulses pass through it, an “ignitron effect” arises. The electromagnetic excitation circuit is simple in design, contactless, and consists of reliable elements: a choke, a selenium rectifier, and an MU. However, investigations of the electromagnetic circuit for excitation of the cathode spot, carried out at the Department of Electric Drive of UPI, showed that, out of 10 devices, only 6 are reliably excited, since, with the existing manufacturing technology, excitron igniters have a large scatter in the cold resistance of the igniter—from several ohms to kilohms.

Poor excitation is explained by the fact that an MU with internal feedback has a large internal resistance, i.e., it is a current generator. Therefore, irrespective of the value of the internal resistance of the igniter–mercury contact, the MU provides approximately the same current in the igniter circuit, whose average value is approximately 1.5 A. On the other hand, it is known that the current gain of an MU is determined by the expression:

\[ K=\frac{W_y}{W_{\sim}}\cdot \frac{1}{1-K_{\mathrm{oc}}}, \tag{1} \]

where \(W_y = W_{\sim}\) is the number of turns of the control winding (which is also the working winding);

\[ K_{\mathrm{oc}} \]
is the feedback coefficient taking into account the real valve.

It is also evident from this that the magnetic amplifier is a current stabilizer; therefore, on an igniter with small internal resistance, only a small amount of power is released, proportional to \(I_3^2R_3\), and insufficient for the occurrence of the “ignitron effect.” It can be increased by means of the igniter resistance \(R_3\) or the power \(P_3\) of the excitation-current source, i.e., at the cost of increasing the overall dimensions of the MU, transformer Tr1, valve B1, etc. For compactness, the power supply of the igniter and the pilot arc in the circuit under consideration is taken from one transformer, Tr1. During operation of the igniter, owing to magnetization of the iron of Tr1 by the ignition current, formation of the pilot arc occurs at a reduced voltage, which makes excitation of the excitron more difficult. To smooth the pulsations of the pilot-arc supply voltage, a choke is installed, which maintains its current when the voltage passes through “zero”; however, this leads to a prolongation of the transient process in the pilot-arc circuit and increases the probability of circuit failures.

Figure 3 presents a three-phase system with a contact excitation circuit investigated by the authors. In it, logical switching of the ignition circuits is employed. In this case, voltage pulses with a large steepness of the leading edge are supplied from a step-up transformer to the igniting electrode of the first excitron through the circuit: single-half-period valve B1, fast-saturating choke D4, from

Fig. 3. Three-phase excitron system with a contact excitation circuit.

contact of relay P1. When a standby arc occurs in it, current will flow through the winding of relay P1, which leads to its operation and to switching the ignition circuit from the first excitron to the second. When a standby arc occurs in the second excitron, relay P2 operates and switches the ignition circuit to the third excitron, and so on. When the standby arcs of all the excitrons are burning, relays P1, P2, P3 are picked up, the ignition circuit is open and is under voltage. If the standby arc in any excitron is interrupted, the corresponding relay drops out and the igniter circuit is automatically restored.

Fig. 4. Oscillogram of the transient process of the currents of the standby arcs and the igniter.

Figure 4 gives an oscillogram of the transient process of the currents, explaining the operation of the logic device of the ignition circuit. Here \(I_{\text{ign}}\) is the current of the igniters; \(I_1\), \(I_2\), \(I_3\) are, respectively, the currents of the standby arcs. It follows from it that arc ignition occurs after no more than 2–3 pulses of the igniter current, and the duration of the transient process is approximately 0.2 sec.

The free contacts P1, P2, D3 are used for signaling and interlocking when the standby arc is interrupted.

The circuit is simple and reliable, although it has the drawbacks inherent in contact-type automation, which are not significant for a single switching-on—starting.

Conclusions

An analysis of thyratron and electromagnetic circuits for exciting the cathode spot of an excitron shows:

1. The circuits shown in Figs. 1 and 2 are characterized by insufficient operational reliability and by the relative complexity of the circuit solutions.

2. The single-phase electromagnetic circuit does not ensure reliable excitation of the excitron when the igniter resistance is less than 70 ohms.

3. The absence of interlocking and signaling when the standby-arc circuit is interrupted in the circuits considered may lead to “failure.”

4. The proposed three-phase circuit may be recommended for investigating the possibility of its application in machine-tool drives.

LITERATURE

1. M. A. Rozenblat, Magnetic Amplifiers. Gosenergoizdat, 1955.

2. I. L. Kaganov. Electron and Ion Converters. Gosenergoizdat, 1956.