2000-Ampere DC Motor-Generator Welding Converter with Automatic Transition to Compensator Mode

G. N. Kok, A. F. Knyazkov

Electric arc welding is widely used in all branches of the national economy as one of the most advanced methods for joining various metals and alloys. At present, arc welding with alternating current is predominant, owing to its high productivity and savings in electrical energy. But it is not applicable everywhere.

Thus, for example, for welding with electrodes having a basic coating of the UONI type, etc., direct current is required, since when welding with such electrodes on alternating current, because of effective deionization of the interelectrode gap, it is impossible, without special equipment, to maintain stable burning of the arc. Therefore, there is still a need for DC sources for welding. Moreover, as practice shows, there is a need for more powerful power sources for DC welding than those produced in series by domestic industry, rated up to 1000 amperes.

The DC welding converter described here is a multi-station source with a rated current of 2000 amperes at a voltage of 60 volts. Regulation of the welding current at the stations is carried out by means of individual ballast rheostats.

The converter consists of a DC generator driven in rotation by a synchronous motor.

The welding generator is a six-pole machine with compound excitation. Owing to the presence of a light series winding, its external characteristic is stiff.

The use of a synchronous drive ensures constancy of the generator’s rotational speed and, consequently, of the voltage, which eliminates mutual influence of the individual stations during operation.

The generator and motor are mounted on a common foundation frame. For excitation of the synchronous motor, an automatic thyratron ionic-excitation system is used. The purpose of this system is to maintain the nominal value of the stator current, i.e., to make maximum use of the free capacity of the synchronous motor to improve the power-system $\cos \varphi$ when the active load on the generator is shed.

At nominal load on the converter, the magnitude of the excitation current of the synchronous motor, and consequently also of the stator, is established by means of the current in the bias winding of the saturation reactor \(DN\) of the phase shifter, and then, depending on the change in the stator current, will vary in such a way as to restore the stator current to its nominal value.

Fig. 1. Excitation circuit of the synchronous motor

Control of the phase shift of the grid voltage relative to the anode voltage of the thyratrons, and consequently also of the magnitude of the motor excitation current, is carried out by means of the phase-shifting bridge \(FS\), connected to the grid circuit \(RC\) of the thyratrons. The change in the phase of the output voltage in the bridge phase shifter is achieved by changing the inductance of the saturation reactor \(DN\), which is included as a variable resistance in one arm of the bridge. In the absence of magnetizing current in the reactor \(DN\), the phase shift of the grid voltage relative to the anode voltage is maximum; in this case the motor excitation current is minimum. When the reactor is saturated, the shift of the grid voltage relative to the anode voltage is minimum, and the excitation current is maximum.

Regulation of the control ampere-turns of the saturation reactor \(DN\) is carried out by a magnetic comparison circuit, which monitors the deviation of the stator current from the nominal value. This circuit consists of two control windings of the saturation reactor \(DN\), connected in opposition. The ampere-turns of one winding are directly proportional to the magnitude of the stator current. The other winding is shunted by a nonlinear resistance—a barretter—and its ampere-turns are directly proportional to the voltage drop across this element. The numbers of turns are calculated so that, at nominal stator current, the resultant ampere-turns of these windings are equal to zero.

When the stator current deviates from the nominal value, a difference of ampere-turns appears, changing its sign together with the change in the sign of the deviation. These resultant ampere-turns, acting in opposition to or in agreement with the ampere-turns of the bias winding, will change the phase of the output voltage of the phase shifter, and consequently also the magnitude

of the motor excitation current so as to maintain the stator current at the specified level.

The motor-generator welding converter was designed and manufactured by the institute for the Altaidieselmash plant under contract conditions.