Stabilized Three-Phase Voltage Source for Supplying Electrolytic Heating Baths for Parts

E. G. Zavatskii, V. P. Obrusnik, M. A. Zhitkov

(Presented by the scientific and technical seminar of the Static Converters Department of the Research Institute of Automation and Electromechanics at TPI)

In recent years, the electrolytic method of heating parts has been finding ever wider application, especially in the automotive-tractor industry. Extensive work on the creation of automated installations for heat treatment of parts using their heating in an electrolyte is being carried out by the Altai Scientific Research Institute of Machine-Building Technology.

Until recently, motor-generator systems have usually been used as the power sources for such installations; these systems have a number of significant shortcomings. These systems require increased expenditures for routine maintenance, have a relatively low efficiency, increased weight and overall dimensions, sometimes require separate rooms for installation and, what is especially important, complex control systems for stabilizing the output voltage and limiting short-circuit currents. In addition, the particularly unstable nature of the load, characteristic of electrolytic-heating installations, leads to the fact that the generators operate under severe conditions, which makes it necessary to overrate their installed power.

At the Research Institute of Automation and Electromechanics under the Tomsk Polytechnic Institute, a static three-phase source of stabilized DC voltage, adjustable over the range of 100–250 volts and with a power of 80 kVA, has been developed for supplying electrolytic-heating baths.

A feature of this source is that, as the power-regulating element, it employs a transformer regulated by magnetization of a magnetic shunt (TRPSh), with a semiconductor power rectifier for the output voltage and a smoothing filter.

In order to increase the speed of response of the system, reduce its size and weight, and increase the power gain factor, a pulsed method of magnetizing the transformer [1, 2] has been adopted. It consists in feeding narrow voltage pulses into the magnetizing winding of the TRPSh, \(W_y\) (Fig. 1), shunted by the inverse valve \(D\), these pulses being located at the minimum of the negative half-wave of the emf of the odd harmonics. In this case, the minimum average current is consumed by the winding \(W_y\) from the magnetization source, and through this same winding positive feedback is provided for the currents of harmonics that are multiples of three. Such a control method provides the transformer with a high power gain factor, and a saving in weight and overall dimensions is achieved—

Fig. 1. Schematic circuit diagram of a stabilized three-phase voltage source for supplying electrolytic-heating baths.

dimensions, since there is no need to make feedback windings, bias windings, etc. At the same time, the regulator has external characteristics \(U_{\text{out}} = f(I_{\text{load}})\) of the excavator type, which solves the problem of protecting the power supply from overloads and short-circuit currents by simply limiting the width of the control pulses, without introducing additional cutoff, disconnection, and similar devices.

The actuating element of the pulsed premagnetization of the TRPSh is a semiconductor rectifier with a thyristor switch. The thyristor switch in Fig. 1 is a system consisting of the power thyristor \(T_1\), connected in series with the premagnetization winding \(W_y\) of the TRPSh, and a forced-commutation unit for this thyristor. An autonomous system for forced commutation of thyristor \(T_1\) is described in detail in [3]. In the circuit of the commutating unit, in order to reduce energy losses and unload the charging circuit while maintaining high turn-off capability, direct recharging of capacitor \(C_3\) is used. Energy from the commutation circuit is not diverted; rather, to eliminate the phenomenon of its accumulation during the capacitor-charging stage, an additional circuit is created, not including the premagnetization source, through which \(C_3\) is recharged. The recharge circuit is formed by inductor \(L_3\) and diode \(D_8\). The switch is controlled by two commands, one of which turns on the power thyristor \(T_1\), while the other is the command for turning it off and is applied to the control electrode of thyristor \(T_2\). When thyristor \(T_2\) is opened, capacitor \(C_3\), having a voltage of the order of \(2E\), discharges in the \(L_2 C_3\) circuit. When the polarity of the voltage on \(C_3\) corresponds to that indicated in Fig. 1, diode \(D_8\) opens and the capacitor is recharged in the \(L_3 D_8 C_3\) circuit. Thyristor \(T_2\) closes when the current in the circuit passes through zero. At the same time, \(C_3\) continues to be recharged through the \(L_3 D_8\) circuit and is topped up from the power source through the circuit \(E — L_4 — D_9 — C_3\). The voltage to which capacitor \(C_3\) is charged before the next commutation depends on the ratio of the inductances \(L_3/L_4\).

The commands for controlling the thyristor switch are issued by the control circuit outlined by the dashed line in Fig. 1. The timing diagrams explaining its operation are shown in Fig. 2.

Power to the control circuit is supplied from transformer \(TR_1\) through rectifier \(D_{24} \div D_{29}\). In the initial state transistor \(PT_1\) is in the closed state, since the current in its base circuit is zero; \(PT_2\) is saturated. When the negative half-wave of the sinusoidal voltage of Fig. 2, a, arrives from the winding of the control transformer at the base of transistor \(PT_1\), the triode opens. At the output of \(PT_1\), rectangular voltage pulses are then formed (Fig. 2, b), the duration of which is determined by the period of the control voltage. When \(PT_1\) is fully open, the base current of \(PT_2\) is equal to zero and it is closed. Thus, at the collector of transistor \(PT_2\) rectangular pulses are obtained, equal in duration to half the period of the control voltage, Fig. 2, b. Through the differentiating circuit \(C_7 R_{13}\), pulses from the leading edge of the signal from \(PT_2\) are applied to the input of a triggered blocking oscillator made on triode \(PT_3\). From the output of pulse transformer \(Tr. 2\) a control signal is taken, which is applied to the control electrode of the commutating thyristor \(T_2\) and opens it. At the same time, the pulse from transformer \(Tr. 2\) is applied to the left arm of trigger \(PT_5\) through diode \(D_{44}\) and flips it into the first stable state. From the moment triode \(PT_1\) closes, capacitor \(C_{11}\) of the sawtooth-voltage generator begins to charge. By appropriate selection of the charging time constant, it is possible to obtain the desired slope of the “sawtooth” and its linearity.

The capacitance \(C_{11}\) in Fig. 2, ж, charges until the moment of comparison of the pi-

Fig. 2. Timing diagrams

of the sawtooth voltage \(U_{\text{saw}}\) with the reference voltage \(U_{\mathrm{e}}\) in the comparator implemented on transistor \(PT_4\). The reference voltage, taken from the divider \(R_{19}R_{20}\), is applied through diode \(D_{40}\) to the winding of transformer \(Tr.\,3\), which performs the function of negative feedback; and the sawtooth voltage \(U_{\text{saw}}\), taken from \(C_{11}\) through diode \(D_{41}\), is applied to the winding of \(Tr.\,3\), which performs the function of positive feedback of the comparator. Until the moment when \(U_{\text{saw}}\) is compared with \(U_{\mathrm{e}}\), diode \(D_{40}\) is in the conducting state, \(D_{41}\) is cut off, and the comparator does not generate. At the moment when \(U_{\mathrm{e}} = U_{\text{saw}}\), diode \(D_{41}\) becomes conducting; a current flows through the positive-feedback winding; the comparator is excited, and a voltage pulse appears on the output windings. Through diode \(D_{45}\), this pulse is applied to the base of triode \(PT_6\) and switches the trigger into its second stable state. In this process, transistor \(PT_5\) opens and, through diode \(D_{43}\), shunts capacitor \(C_{11}\). The capacitor discharges rapidly. The output pulse from the trigger is amplified by triodes \(PT_7\) and \(PT_8\) and, from the winding of pulse transformer \(Tr.\,4\), is fed to the control electrode of thyristor \(T_1\), opening it. Thus, by changing the voltage \(U_{\mathrm{e}}\), we change the turn-on instant of the power thyristor and thereby regulate the current flowing in the bias winding \(W_{\mathrm{u}}\) of the control transformer. When \(U_{\mathrm{e}}\) is limited at a certain level, the limiting value of the short-circuit current of the TRPSh is also limited.

Regulation of the source output voltage is carried out by changing the position of potentiometers \(R_4\) or \(R_5\). In this case, the difference between the voltage \(U_{\text{set}}\), taken from the Zener diode \(D_{16}\) or \(D_{17}\), and the voltage proportional to the source output voltage is compared with \(U_{\mathrm{e}}\) and determines the bias current of the transformer.

When the output voltage of the source deviates from the specified value, the difference between \(U_z\) and the feedback voltage changes,

\[ U_{\mathrm{fb}} = U_{\mathrm{set}} - aU_{\mathrm{source}}, \]

where \(aU_{\mathrm{source}}\) is the voltage taken from \(R_4\) or \(R_5\).

Consequently, the duration of the open state of the power thyristor \(T_1\) changes, which leads to a change in the bias current of the TRPSh. The transformer switches to operation with another output characteristic, which corresponds to the previously established output voltage; that is, stabilization of the source output voltage is effected.

In the described power source for electrolytic-heating baths, with a power of 80 kVA, when the output voltage is regulated from 100 to 250 V, a stabilization accuracy of \(\pm 2\% U_{\mathrm{out}}\) was obtained over a load-current variation range of 100–300 A. The short-circuit current at nominal bias does not exceed 800 A. The overall dimensions of the source in its housing are \(750 \times 1650 \times 1420\) mm; weight, 920 kg.

The described power source for electrolytic-heating baths has successfully passed production tests and may be recommended for wide application. With changes in the winding data of the TRPSh and in the feedback unit, the system may be used in many electrochemical installations.

Appendix

Technical data and parameters of the system elements
(for manufacturing)

Data of the transformer regulated by biasing:

supply-line voltage: 380 volts;
transformation ratio \(K_t = 1.53\);
nominal bias current: 20 A;
dc voltage: 250–100 volts;
nominal load current: 300 A.

Parameters of the forced-commutation unit:

source voltage: 120 volts.

\(D_1\) — PVKL 100-10 diode
\(T_1\) — PTL-50-8 thyristor
\(T_2\) — VKU-10-5 thyristor
\(D_8\) — D216B diode
\(D_9\) — D246B diode
\(L_2\) — inductance \(L = 0.167\) mH
\(L_3\) — inductance \(L = 3\) mH

\(L_4\) — inductance \(L = 17\) mH
\(C_1\) — MB20-10.0—400 V capacitor
\(C_2\) — K50-3N capacitor — 500 µF — 450 V
\(C_3\) — MBGO-10.0—400 V capacitor
\(R_1\) — 100 W, 40 Ω resistor
\(R_2\) — OMLT-2-470 Ω resistor.

Parameters of the control circuit:

supply voltage: 24 volts.

\(PT_1\) — MP25 transistor
\(PT_2\) — MP25 transistor
\(PT_3\) — MP25 transistor
\(PT_4\) — MP25 transistor
\(PT_5\) — MP25 transistor
\(PT_6\) — MP25 transistor
\(PT_7\) — MP25 transistor
\(PT_8\) — MP25 transistor
\(C_6\) — ETs—1000-30 capacitor
\(C_7\) — BM2-0.0125—200 capacitor
\(C_9\) — BM2-0.125—200 capacitor
\(C_{10}\) — ETO \(15 \times 20\) V capacitor

\(R_8\) — OMLT-0.125—1.3 kΩ resistor
\(R_9\) — OMLT-0.5—240 Ω resistor
\(R_{10}\) — OMLT-0.5—7.5 kΩ resistor
\(R_{11}\) — OMLT-0.5—1.3 kΩ resistor
\(R_{12}\) — OMLT-0.5—12 kΩ resistor
\(R_{13}\) — OMLT-0.5—1.3 kΩ resistor
\(R_{17}\) — OMLT-0.5—3.9 kΩ resistor
\(R_{18}\) — OMLT-0.5—11 kΩ resistor
\(R_{19}\) — PPZ-3 kΩ variable resistor,

$C_{11}$ — OMBG-20—400 capacitor,
$C_{13}$ — BM2-0.1—160 V capacitor,
$C_{14}$ — MBM-1.0—160 V capacitor,
$C_{15}$ — BM-3300—200 V capacitor,
$C_{16}$ — BM2-0.05—160 V capacitor,
$C_{17}$ — BM-3300—200 V capacitor,
$C_{18}$ — MBM-0.025—150 capacitor,
$C_{19}$ — K50-3—200×50 V capacitor,
$D_{24}$—$D_{29}$ — D7Zh diodes,
$D_{32}$ — D7Zh diode,
$D_{34}$—$D_{36}$ — D7Zh diodes,
$D_{37}$—$D_{38}$ — D211 diodes,
$D_{39}$—$D_{46}$ — D7Zh diodes.
$R_{20}$ — OMLT-1—910 ohm resistor,
$R_{21}$ — OMLT-2—5.1 k resistor,
$R_{22}$ — OMLT-0.5—4.3 k resistor,
$R_{23}$ — OMLT-0.5—47 k resistor,
$R_{24}$ — OMLT-0.5—9.1 k resistor,
$R_{25}$ — OMLT-0.5—110 ohm resistor,
$R_{26}$ — OMLT-0.5—4.3 k resistor,
$R_{27}$ — OMLT-0.5—4.3 k resistor,
$R_{28}$ — OMLT-0.5—12 k resistor,
$R_{29}$ — OMLT-0.5—12 k resistor,
$R_{30}$ — OMLT-0.5—4.3 k resistor,
$R_{31}$ — OMLT-0.5—4.3 k resistor,
$R_{32}$ — OMLT-0.5—7.5 k resistor,
$R_{33}$ — OMLT-1—910 ohm resistor,
$R_{34}$ — OMLT-0.5—240 ohm resistor,
$R_{35}$ — OMLT-0.5—240 ohm resistor.

REFERENCES

1. V. P. Obrusnik, A. V. Kobzev. Method of pulse biasing. Inventor’s certificate, No. 245842, class 21a² 18/08, Bulletin of Inventions, No. 20, 1969.

2. V. P. Obrusnik, A. V. Kobzev. Operating modes and characteristics of single-phase TPSh under pulse biasing. Proceedings of the VI Scientific and Technical Conference on Problems of Production Automation, vol. 2. TSU Publishing House, Tomsk, 1969.

3. A. I. Zaitsev, M. A. Zhitkov. Device for forced commutation of thyristors with direct recharging of the commutating capacitor. Proceedings of the VI Scientific and Technical Conference on Problems of Production Automation, vol. 5, TSU Publishing House, Tomsk, 1969.