US2193421A - Conversion of currents or voltages with the aid of periodically actuated break contacts - Google Patents

Description

Mam}! 1940- r-:. JANETSCHKE 193,421 CONVERSION OF CURRENTS 0R VOLTAGES WITH THE AID OF PERIODICALLY ACTUATED BREAK CONTACTS Filed Feb. 27, 1957 6 Sheets-Sheet l March 12, 1940. E JANETSCHKE 2,193,421

CONVERSION OF cunnnu'rs 0R VOLTAGES WITH THE AID 0F PERIODICALLY ACTUATED BREAK CONTACTS Filed Feb. 27, 193'! 6 s hgets-Sheet 2 2,193,421 CONVERSION OF CURRENTS on VOLTAGES WITH THE AID 6 Sheets-Sheet 5 E. JANETSCHKE Filed Feb. 27, 1937 OF PERIODICALLY ACTUATED BREAK CONTACTS March 12,

Mam}! 1940- E. JANETSCHKE 9 CONVERSION OF CURRENTS' 0R VOLTAGES WITH THE AID 0F PERIODICALLY ACTUATED BREAK CONTACTS Filed Feb. 27, 1957 6 Sheets-Sheet 4 March 12, 1940. E, JANETSCQKE 2,193,421

CONVERSION OF CURRENTS 9R VOLTAGES WITH THE AID 0F PERIODICALLY ACTUATED BREAK CONTACTS Filed Feb. 2'7, 1937 6 Sheets-Sheet 5 March 12, 1940. E. JANETSCHKE ,193,421 CONVERSION OF CURRENTS 0R VOLTAGES WITH THE AID 0F PERIODICALLY ACTUATED BREAK CONTACTS 6 Sheets-Sheet 6 Filed Feb. 27, 1937 Patented Mar. 12, 1940 CONVERSION OF CURRENTS R VOLTAGES WITH THE AID 0F PERIODICALLY ACTU- ATED BREAK CONTACTS Erwin Janetscbke, Berlin-Charlottenbnrg, Germany, assignor to Siemens-Schuckertwerke Aktiengesellschaft, Berlin-Siemensstadt, Germany, a corporation of Germany Application February 27, 1937, Serial No. 128,185 In Germany March 2, 1936 24 Claims.

My invention" relates to the conversion of currents or voltages with the aid of periodically actuated break contacts.

It has already been proposed to employ in converters variable resistances for improving the commutation i. e., the flow of current from the break contact in one phase to the following contact in another phase, which resistances are connected in series with the break contacts and which considerably reduce the current in the contact to be opened at the moment of break. To this end, saturated inductances may, for instance, be employed which are desaturatedif the current in the contact to be broken drops below a given value.

Further, it has already been proposed, with relation to the above-mentioned converters, to shift the moment at which the contacts are closed within the period of the current cycle in order to control the voltage.

The object of my invention is to ensure always a proper commutation in converters oi the abovedescribed character when the converter is used for exchanging energy between two current supply circuits with timely difierent' current curves, i. e., between a direct-current supply circuit and an alternating-current supply circuit or between two alternating-current supply circuits with different frequency.

According to the invention the contact arrangement is designed in such a manner that upon a change of the direction of energy, a change in the synchronous position of the contact movement is effected, referred to the variations of the alternating voltage with respect to time.

Further details of my invention will be apparent from the following description taken in connectionwith the drawings and graphic repr sentations of the relation between current and voltage.

Fig. 1 shows an embodiment of my invention in atic form- Fig. 1a shows alateral viewoi a constructional detail of this embodiment. Fig. 1b illustrates a modification of a portion of the network shown in Fig. 1. Figs. 2,-3, 4 and 5 show graphic representations of the relation between current and voltage'for different operating conditions of an arrangement according to the invention. 6 shows a modified mechanical design of a circuit breaker suitable for the invention. Figs.,7 and 8 show two other embodiments of arrangements according to the in.- vention. Figs. 9 to 12 refer to converters which are employed for converting alternating current of one frequency to alternating current of another frequency, and more precisely Figs. 9 and 10 illustrate current and voltage curves, while Figs. 11 and 12 show two different embodiments of the total converter system.

The invention will be first described in connection with arrangements which serve to exchange energy between an alternating-current supply circuit and a direct-current supply circuit. -Inthis case, the shifting of the moment at which the contact closes or opens with respect to the cycle of the alternating voltage is eflected advantageously so that when passing from rectifier operation to inverter operation the flow of current from one contact to the following contact is shifted with respect to time from a moment which, depending upon the degree of control, more or less lags behind the moment oi the voltage coincidence of the phases which are connected in succession, to a moment which leads the abovementioned moment of the voltage coincidence. If during the reversal of current the voltage supplied by the converter should not vary, the time leading of the commutation process with respectv to the moment of the voltage coincidence in the case of inverter operation must be approximately equal to the lagging of the commutation process in the case of rectifier operation.

The arrangement shown in Fig. 1 is designed in the manner just described.

AC is the alternating-current supply circuit and DC the direct-current supply circuit between which an exchange of energy is to be effected. The primary windings ll, 2|, 3| of a main transformer are connected to the alternating-current supply circuit AC. The secondary windings I2, 22, 32 of this transformer are connected to each of the stationary contacts I5, 25, 35 of an interrupting device through each winding I3, 23, 33 of each reactor. The magnetic bodies It, 24, 34 of the reactors are so designed that they are desaturated when the intensity of current decreases below a given value which lies below the average intensity of the alternating current. The magnetization curve 0! the material employed has a s'har'p saturation bend so that the inductance of the reactor greatly increases if the intensity of current drops below a given low value. Owing to this effect, the current curve is so distorted in the neighborhood of the time point at which the current passes the zero value that the intensity oi. current remains low during a given interval. Within this interval the contacts are broken. Consequently, the occurrence of an interrupting arc is avoided.

Each reactor is provided with an exciting winding I8, 28, 88. The windings are connected in series to a voltage source 88 through a reaotance coil 80 and a resistance 88. By regulating the resistance 88, the magnetic bias of the reactors may be varied and, therefore, the interval within which the intensity of current is reduced may be shifted with respect to the cycle of the alternating curren Each contact device of the switching gear is provided with a second stationary contact I8, 28, 38 as well as a movable contact I1, 21, 81 which in the switching-in position conductively connects the two stationary contacts, for instance I! and I8, with each other. The stationary contacts I8, 28, 88 are electrically connected with one another and connected to a pole of the direct-current supply circuit DC through a reactance coil 85 and a resistance 84. The second pole of-this direct-current supply circuit is connected to the neutral point of the secondary side of the main transformer.

A current path including an apparent resistance' is arranged in parallel relation toeach con tact device. This apparent resistance is denoted by 9|, 82, 88 and will be hereinafter more fully described.

For the drive of the movable contacts the following mechanical device is employed.

Eachcontact is arranged on a ta'ppet i8, 28, 38 which is slidably mounted on the frame of the switching gear, A tension spring I8, 28, 88 has the tendency to bring the contacts I1, 21, 81 into the oil-position. Further two shafts 48 and 18 are mounted on the frame of the interrupting device. On the shaft 40 is mounted for each contact device an eccentric 4|, 42, 48 which is firmly secured to the shaft 48. The three eccentrics 4|, 42, 48 are angularly spaced 120. On the shaft 48 are mounted the eccentrics SI, 82, 88 immediately adjacent to each of the eccentrics 4!, 42, 48. However, the eccentrics ii, 82, 88 may be rotated with respect to the shaft 48. Each of the eccentrics Si, 82, 83 is firmly connected to the gears 8|, 82 and 88 respectively. Three gears 1|, 12, 18 are firmly secured to the shaft 18. These gears mesh with the gears 8|, 82, 88. The shaft 48 is connected to a synchronous motor 48 through a coupling and the shaft 18 to a synchronous motor 16 through a coupling 14.

If the cams 4|, 8i and 42, 82 and 48, 83 are so adjusted as to be in registry with each other the interval during which the movable contact is raised is shortest. If the corresponding eccentrics are, however, displaced with respect to one another as shown in Fig. 1a, the time at which the contacts close is increased. If both synchronous motors 48 and 15 rotate at the'same speed, the relative position of both-eccentrics remains the same during the rotation thereof. However, if the phase angles of both synchronous motors 48 and 18 are adjusted with respect to each other, the time at which the contacts close is thereby varied. I

For regulating the synchronous motors, the

following means are employed:- Both motors are connected to the secondary windings I2, 22, 82 of the main transformer through a regulating transformer 48. With the aid of the transformer 48 both synchronous-motors may be so'controlled that the breaks take place at the time at which the intensity of current is reduced by the reactors I8, 28 88. One of the synchronous motors is further so designed that its phase may be displaced with respect to that of the other motor. To this end, a second regulating transformer 18 is employed in the embodiment shown. By means of this transformer the interval may, consequent- 1y, be varied within which the contacts are closed.

Theabove-described controls are made conditional upon the direction of the energy flowing through the switching gear"by the following means. Both regulating transformers 48 and 16 are provided each with a device 41 and 11 respectively for automatically adjusting the transformers. Both adjusting devices are each con.- nected through a switch41' and 11' to a relay which is actuated in response to the changes of direction of the energy in a conductor extending between both supply circuits AC and DC. The relay in the embodiment shown consists of a horizontal pendulum 88 which, depending upon the position thereof, brings about a contact between a power source 88' and a stationary contact 18 or 18.

The horizontal pendulum 80 is under the action of a tension spring 8| which tends to hold the contact 18 in a closed position and, therefore, to bring either regulating transformer or also both in a certain regulating state depending upon the operating position of the switches 41' and 11 A fleld winding in series with a tube 88 is shunted to a resistance 84 lying between both supply circuits AC and DC. The field winding 82 workson the control member 88 of the relay. The field windsistance 84 in a direction in which the fleld wind- .ing 82 is not energized the contact between the power source 88' and the contact 18 is closed. Thereby at least one of the transformers 48 or 18 is adJusted in a definite manner which corresponds either to a certain position of phase of the cams of the interrupting device or to a certain duration of the contact making. L'If now the current changes its direction in the resistance 84, the winding 82 is energized, the relay 80 changed over, and at least one of the regulating transformers 48 or 18 is brought thereby into another position of phase. In this manner either the position of phase of the cams of the switching gear or the duration of the switching-in period is varied.

For a better understanding of the operation of the above-described arrangement. reference may be made to the graphs shown in Figs. 2 to 5. The curves l. 2 and 8 in these graphs represent the three phases. The currents corresponding to the single phases are indicated by the curves 4, 8 and 8. The portions of the curves in full line represent the variations of the voltage across the direct-current terminals of the inverter with respect to time. Fig. 2 shows the conditions prev'ailing during rectifier operation for a full degree of control, 1. e., for such a degree of control whereby the greatest possible value of the directa current voltage is supplied. During the flow of current, the intervals of closure of the contacts which are closed and opened in succession (for instance i1 and 21 in Fig. 1) overlap one another by a certain time. During this time both contacts with the corresponding transformer phases (l2 and 22)' form-a closed path of the short-circuit current. The current in the contact l1 to'be opened is, therefore, composed of the load current and the current produced by the voltage effective in the'path of the shortcircuit current (I2, 22, 28, 28, 21, 28, l8, l1, I8,

l8, l2). In order to attain a proper 'commutation it is preferable to choose the moment at which the path of the short-circuit current closes, in such a manner that the short-circuit current in the contact I! to be opened is in opposition to the current flowing into the current consuming device so that the resultant current in this contact is reduced. In rectifier operation, this condition is fulfilled if the voltage of the phase belonging to the following contact 21 is greater at the moment at which the contact closes than the phase voltage of the contact I! to be broken or if the voltage at the closing contacts increases at least during the duration of the short-circuit to a greater amount than the phase voltage of the contact to be broken. The difierential voltage between both phase voltages then acts in opposition to the loadcurrent in the contact to be opened. If, for instance, in rectifier operation shown graphically in Fig. 2 the following contact is closed at the moment t1, 3, s etc, that is at the moment of the voltage coincidence of both contacts to be broken and opened in succession the voltage of the following contact 21 increases during the period of the shortcircuit, whereas the voltage of the contact I! to be broken decreases. If, for instance, the passage from phase 1 to phase 2 is considered, the voltage during the short-circuit varies in the direct-current circuit according to a middle curve C-D between the phase voltage 2 and the phase voltage I The difference between the phase voltage 2 and the phase voltage I is, however, effective in the path of the short-circuit current. The current increases in the following is equal to the current flowing in the current consuming device. In this case, it is assumed that the current flowing in the current consuming device is completely smooth, i. e., remains always constant.

If now the direct-current voltage is to be reduced, the moment at which the contact closesmay be retarded, whereby a state is finally attained in which the direct-current voltage supplied is equal to zero. This is attained, if the moment at which the contact closes lies approximately at B. Consequently, in rectifier operation, the interval AB is available for the closure of the contact.

Fig. 3 also applies to rectifier operation and more precisely to a degree of control which is already extremely small. Also in this case, the curve of the direct-current voltage supplied is drawn in full line. As will be-seen, the closure of the contacts is efl'ectedat a moment at which the phase voltage 01. the contact to be broken has already become negative. That the contact to be broken still carries current in the negative portion of its phase voltage curve is rendered possible by the smoothening means (85 in Fig. 1) provided in the direct-current circuit. If the flow of current from the contact I! to the contact 21 is considered, it will be seen that in this case the direct-current voltage also varies from the moment at which the contact closes according to a middle curve E-F between the curves 2 and 1. Upon the opening of the contact to be broken the curve or the rectifier voltage is suddenly shifted to the corresponding point 01' the voltage of the following contact.

The inverter operation is characterized by the fact that the current is in opposition to the phase voltages. It, therefore, the differential voltage efiective in the path of the short-circuit current is to be in opposition to the current during the short-circuit, the voltage of the following contact 21 must be greater than the voltage of the contact I 'l to be broken. To fulfill this condition the closure of the contacts must be effected before the moment of the voltage coincidence.

Fig. 4 shows a graph for the inverter operation with full degree of control. The closure of the contacts takes place at the moments i1, is, is, etc., and the break of the contacts takes place each time at the moment of the voltage coincidence,

i. e., at the moments t2, i4, is, etc. In the same manner as in the above-mentioned instances the direct-current voltage varies during the short circuit according to a middle curve between the corresponding phase voltages. If now the directcurrent voltage is to be reduced, the time between the moment at which the closure of the contacts takes place and the moment of the voltage coincidence must be further increased. In this case, a point is finally obtained at which the direct-current voltage is again equal to zero.

The moment ofclosing the contacts at which this is' attained corresponds approximately to the point H; consequently, an interval of (3-H is available for the control range in the case of inverter operation.

Fig. 5 is a graphic representation of the relation between voltages and currents in the case of an inverter operation and a partial degree of control.

According to the above explanation it is possible to operate the converter arrangement shown in Fig. 1 always under favorable commutating conditions both for rectifier and inverter operation if the synchronous position of the contact movement referred to the time variation of the alternating-current voltage is varied according to the invention upon the reversal of the current. This variation of the synchronous position is attained in the arrangement according to Fig. 1 by adjusting the synchronous position of the driving motor 15 by the device 1'! and the relay 80 in accordance with the direction of current in the shunted resistance 84, or by simultaneously adjusting the synchronous position of both motors 45 and 15 and, therefore, of the driving cams 4|, 5| and 42, 52 and 43, 53. However, the variation of the synchronous position of the contact movement may also be effected in another manner. The means employed for the particular purpose in question depend as a rule upon the type of the contact circuit breakers and on the type of their drive. If, for instance, a contact arrangement is employed in which a rotating contact and a plurality of stationary contacts are provided, the stationary contacts may, for instance, be'rotated in the case of an unchanged synchronous position of the rotating contact or the rotating contact may be rotated with respect to the driving shaft. In a contact arrangement with reciprocating circuit breakers as shown in Fig. 1. also different cams for rectifier and inverter operation may be provided for each contact on the same shaft and may be displaced upon reversal of the current, for instance, by displacing the entire shaft. However, also an individual cam shaft for inverter operation and a second cam shaft for rectifier operation may be provided and the drive may be so designed as shown, for instance, in Fig. 6.

In Fig. 6, I8 represents the same tappetwhich is denoted by I8 in Fig. 1. Instead of the driving arrangement shown in Fig. 1 the following means as shown in Fig. 6 are employed for actuating the tappet I8. The tappet has a member IIO which may alternately cooperate with one of the cam shafts I and I42. Both cam shafts I and I42 are arranged on an oscillating lever I43 pivotally mounted as indicated at I40. Stops I and I serve to limit the movements of lever I43. Lever I43 has an actuating arm I44 which is connected with the armatures I46 and I41 of two magnet coils I46 and I40. If coil I46 is energized, lever I43 assumes the position illustrated. Upon demagnetizationof coil I46 and energization of coil I48, lever I43 is rotated against stop I50 so that now cam shaft I42 comes into operation. A relay I8, I3, 80, 8| which corresponds to the relay designated in Fig. 1 by the same reference numerals, is connected with an auxiliary voltage source .80 and with the coils I46 and I48. Assoon as the transfer of energy between the direct current supply circuit and the alterhating current circuit changes, its direction, the

relay is actuated, coil I46 is deenergized while.

coil I48 now is connected to the voltage source 80'. Accordingly, lever I43 changes its position, thus bringing the cams of shaft 2 into operation. Shaft I42 and shaft I are continuously driven and so adjusted that the cams .of one shaft show a given phase difference with respect to the cams of the other shaft. The change of the operating position of lever I43 thus results in the desired variation of the phase position with which tappet I8 is actuated.

Further, the synchronous position of the contact movement may be changed by changing in the case of the driving motor the synchronous position of the rotor with respect to the stator revolving three-phase field. To this end, for instance, the rotor is provided with two exciting windings each being capable of being regulated separately and lying indifferent axes. An extremely rapid adjustment may be, however, also attained if the stator of the .motor is also so mounted as to be capable of being rotated a given angle. Embodiments of these two ways of varying the phase position of the contact movement will be described hereinafter with reference to Fig. 8. Another means for changing the synchronous position of the contact movement with respect to the feeding phase voltages consists in shifting the position of phase of the alternating-current voltage supplied to the contacts, for instance, by interchanging the connec-' tions of the contacts at the phases of the feeding three-phase current supply circuit. An example of such means is illustrated in Fig. 117. II, 2| and 3| designate the primary windings of the main transformer, as indicated in Fig. l by the same numerals. In departure from the arrangement of Fig. 1, a switch I63 is connected between the transformer and the A. C. circuit. The switch allows the interchanging of the connections oi the three transformer phases in such a way that the voltages in the three phase windings of the transformer are phase shifted by 120. The switch I53 is actuated by means of a magnet armature I50 and two magnet coils I6! and IE2 which are connected to an auxiliary voltage source 80' and a relay [0, I0, 80, 3| which corresponds to the relay designated-in Fig. 1 by the same numerals. It is to be understood that the relay circuit shown in Fig. 1b replaces the corresponding part circuit in Fig. 1. Accordingly, the

cordance with each other.

Finally, there is another possibility of changing the synchronous position of the contact movement which consists in providing two separate contact arrangements, one of which is always controlled as a rectifier and the other as an inverter. Upon the reversal of current the directcurrent circuit is then changed over from one contact arrangement to the other. Both contact arrangements are preferably controlled in ac- This may be accomplished according to the embodiment shown in Fig. 7.

According to Fig. 7, two control arrangements connected with each other are employed for the exchange of energy between the alternating-( current supply circuit AC and the direct-current supply circuit DC. The arrangement shown on the left-hand side is designed in a manner substantially similar to that shown in Fig. 1 as will be seen by' the corresponding reference numerals indicating similar parts. The circuit breakers I1,

21, 31, however, may be driven by any other; suitable drive which actuates the three contacts in succession and in synchronism with the voltage of the alternating-current supply circuit. The

drive is indicated by the shaft 40, the synchronous Both'lnterrupting devices are driven simul-.

taneously. and are, therefore, always ready for operation. Both arrangements are connected to the direct-current supply circuit DC through a changeover switch M0, by means of which the supply circuit is' connected to the rectifier or to the inverter depending upon the direction of current. The change-over is effected with the aid of a magnetic device represented in the drawin: by the following parts. The switch is equipped with an armature I0l and a magnetic winding I02. A spring I03 tends to bring the switch into a given switching position. However, if the field winding I02 is energized, the switch is brought into the other operating position. The magnetic winding I02 is connected to the power source I06 through a relay contact I04. The relay winding I0! is connected to a resistance I00 through a tube I01. The tube causes an energization of the relay winding I06 and, therefore, also of the field winding I02 if the current inthe resistance I00 has a given direction. Accordingly, an automatic change from the rectifier to the inverter is effected depending upon whether energy is transmitted from the alternating-current supply circuit to the direct-current supply circuit or in latter may be provided with a biasing winding and the magnetic bias of the magnetic cores of the inductors may be varied by varying simultaneously the synchronous position of the contacts. An arrangement of this kind is described above with reference to Fig. l where the magnetic bias of the inductances i 3. 23 and 33 may be varied by adjusting the resistor 50. If resistor 50 is mechanically coupled with the phase shifting device for varying the phase adjustment of the interrupters, the adjustment is effected automatically. A similar connection between the magnetic bias and the phase shifting means is illustrated in Fig. 8.

Fig. 8 shows a wiring diagram in which a contact arrangement'consisting of a rotating contact I09 and three stationary contacts In, I21, I 31 is employed for coupling the three-phase current supply circuit A. C. to the direct-current supply circuit D. C. The highly saturated reactors l3, 23, 33 which serve to limit the current during the commutation are arranged in series with the stationary contacts. A synchronous motor whose stator 200 is rotatably mounted and whose rotor is provided with two exciting windings 20! and 202 displaced 90 with respect to.

each other. The rotation of the stator 200 is effected by a particular device 203, for instance, an electromagnet, which is fed by an auxiliary direct-current supply circuit 209. For-checking the occurrence of a reversal of current any suitable relays may be employed which operate in response to a change of direction of current or of power, or also an arrangement may be employed which compares the conditions of voltage of both supply circuits. In the embodiment of Fig.8, the same relay 18, 18, 80, 8|, 02 as in Fig.1, is employed, the pendulum 80 of which oscillates as soon as the current in the resistance 84 changes its direction, so that the rightor the left-hand contact is closed, thus causing the electromagnet 203 to rotate the stator 200 of the motor a given angle limited by stops in either direction. Upon the rotation of the stator 200 a change of the magnetic bias of the reactors I3, 23, 33 is at the same time eilected. The windings I0, 20 and 30 are connected to the direct-current supply circuit 209 through a contact pair 208 and a resistance arranged in parallel relation thereto. If the stator of the motor assumes the position shown in the embodiment, the contact pair 200 is closed and a comparatively large biasing currentfiows in the windings I0, 20, 30. If the motor stator is rotated to the right the contacts 208 are interrupted, the resistance 20! is connected in series with the windings I0, 20, 30 and, therefore, a smaller biasing current is adjusted.

To regulate the current in the exciting winding 20| of the synchronous motor an automatic regulator 201, for instance, a so-called Thoma regulator, is provided which in the embodiment shown .serves to control the contact arrangement in accordance with the load current. Ito-this end, the voltage present across the resistance 208 inserted in thedirect-current conductor is sup-.

plied to the regulator. Under certain circumstances, the regulator may also be so connected that upon a reversal of current the regulating principle is changed, for instance, in such a manner that in the case of a rectifier operation the voltage and in the case of an inverter operation the current is regulated to a constant value. The latter case is, as a rule, desired if the inverter operation is utilized for the regenerative braking. The means for efiecting the last-men tioned change may consist of a contact device 203' and a relay 2l0 with change-over contacts 210' as illustrated in Fig. 8.

In many cases it is preferable to combine various means for changing the synchronous position of the switching gear in one and the same system. For instance, one means may be then employed for the .coarse adjustment and the other in addition thereto for the fine adjustment. If, for instance, the stator of the synchronous motor is rotated, this rotation is in general, as in the case of the arrangement of Fig. 8, effected within a given angle. This angle of rotation will not always be, however, convenient for every operating case, particularly if the zone of commutation is only narrow. The rotation of the stator need then be employed only for the coarse adjustment,

while for the fine adjustment, 1. e. for the adaptation to the corresponding degree of control, the rotor may additionally be rotated relatively to the rotating stator field.

Arrangements according to the invention may also be designed for converting alternating current of one frequency into alternating current of another frequency. For instance, with the aid of arrangements of the above-described type an alternating current may be first rectified and the direct current produced may be again converted to alternating current of the desired frequency. To this end, two separate contact arrangements may be employed, each of which comprises all auxiliary devices necessary therefor.

However, a considerably simple arrangement may be provided according to the invention by using one and the same group of contact devices for transmitting'both the positive and the negative half waves of the converted alternating current.

If, for instance, a voltage of a polyphase system is to be converted to a monophase alternatingcurrent voltage of lower frequency, the control of the contact arrangement must be eflected in the following manner:

During the duration of a half cycle of the monophase voltage the contacts are closed or' opened at such moments with respect to the polyphase voltage that the voltage across the output terminals has always the same sign.- A change in the control of the contacts is then effected in such a manner that the sign of the voltage across the output terminals is reversed and the new direction is maintained during the duration of the next half wave.

v For a better understanding of such a device reference may be first made to Figs.'9 and 10. It is assumed that the voltage of a six-phase system is to be converted to the voltage of a monophase system, for instance, with a frequency ratio of 60 to 16% cycles. The figures show in which manner the curve of the converted voltage is formed of the voltage curves of the feeding alterhating current system, the period being represe'nted within which the passage of the converted voltage current from the positive to the negative half wave is eflected. This passage from the positive to the negative half wave is effected in of higher frequency within the positive and within the negative half wave of the converted alternating-current voltage of lower frequency. Thus,- for instance, in Fig. 9 the passage from the voltage curve of one phase to the voltage curve of the following phase takes place at the moment of the voltage coincidence a or a. The curve of the resulting voltage of lower frequency has the form of a trapezoid.

However, a passage from a phase voltage curve to the other phase voltage curve which takes place exactly at the moment of the voltage coincidence, as was assumed for the sake of simplicity in Fig. 9, is not always possible, since the commutation' frequently entails that the moments at which the contacts are closed overlap each other by a certain amount, 1. e., that the contact to be A broken is opened a certain time after the following contact has been closed.

In Figs. 9 and 10 are,'furthermore, plotted the curves for the alternating current J of the lower frequency, ohmic load being assumed in Fig. 9, so that the passages of the current and of the conversion voltage through the zero value take place simultaneously. Since the current has always the same direction as the voltage both in the positive and in the negative half wave, the

converter operates continuously as a rectifier.

' In order to attain in the case of a rectifier operation favorable commutation conditions, the contacts must be closed at such a moment that upon a breaking of the contacts the voltage in the phase of the following contact is greater than the voltage in the phase of the contact to be opened. If, therefore, as assumed in Fig. 9 for the sake of simplicity, the maximum voltage is'to be supplied-4nd this is the case if the passage takes place substantially at the moment of the voltage coincidence-the following contact must always be closed at the moment of the voltage coincidence and the contact to be broken must be opened a given interval later. If the full voltage should not be supplied but a smaller voltage the moment at which the commutation begins must be retarded so that a certain retardation of the closure of the following contact results, thereby obtaining a partial degree of control of the system. This applies also to the negative half wave. In this case the voltage of the following contact must have during the commutation a greater negative value than the voltage offlthe contact to be broken.

The magnetic bias of the inductors I3, 23, and

33 arranged in series with the break contacts is preferably so influenced that the moment at which the inductance increases to its maximumvalue, is varied during the passage from the negative to the positive half wave. To this end, the windings of the reactors are biased by a current component which when passing from the positive to the negative voltage half wave changes the state of magnetization of the inductors in such a manner that the moments at which the inductance attains the maximum values, are approximately shifted the same amount as the mo-' ments at which the corresponding contacts close. This may be attained in a "simple manner by supplying an exciting current to the inductors ,which changes its sign approximately at the same time as the alternating-current voltage of low frequency produced. a

By changing the control of the contacts, it is v of the original voltage.

- cycle.

possible to change the alternating-current voltage curve produced in such a manner that the middle portion thereof. approaches the sine shape. The individual break contacts should not havethe same degree of control, but the moment at which the contacts are closed must be correspondingly shifted from phase to phase. Also in this conversion process the moments at which the maximum value .of the inductance is attained in the individual inductors may be easily adapted to the voltage curve. This may, for instance, be accomplished by energizing the reactors by a sinusoidal current of the frequency The moment at which the maximum value of the inductance is attained is then shifted continuously in the successive phases.

The above considerations are based on ohmic load. With converters according to the invention also pure wattless loads or a mixed load must, however, be transmitted. In this case, it is immaterial whether the flow of energy is directed from the supply circuit of high frequency to the supply circuit of low frequency or vice versa, that is to say, whether a reversal of current occurs. As soon as a wattless load is transmitted, the current curve is out of phase with I respect to the curve of the converted voltage so that the passages of both curves through the zero value donot take place any longer at the same moment. This load' condition is represented in Fig. 10. With this 'load both in the positive .and in the negative half wave, there are not only periods during which the current flowing through the break contact has the same direction as the voltage (rectifier operation) but also periods during which current and voltage of the transformer phase in operation are in opposition (inverter operation). With a purely inductive load, the rectifier operation is changed over to the inverter operation after every quarter With a greater power factor, the times of the inverter operation become shorter, whereas the periods of the rectifier operation become correspondingly longer. In the case of the current'flowing from the supply circuit of low frequency to the supply circuit of high frequency, the periods of the inverter operation will be longer than those of the rectifier operation. If this flowing back of the current is effected with a power factor equal to unity all tubes are operated as inverters during the entire period of .low frequency.

In the caseof an inverter operation the commutation process is preferably effected at such a moment at which the following contact has a lower voltage than the contact which is to supply the current. In the path of the short-circuit current whichis formed by the overlapping contacts of different phases (1. e., by contact pairs which are simultaneously closed) a compensation current will then flow in such a direction that it weakens the current in the contact to be broken and supports the increase of the current in the following contact. If, therefore, a passage from one phase to the other phase is to be attained at the moment of the voltage coincidence (full degree of control), the commutation process must be initiated'by closing the following contact a certain time before the moment atthe current J is in opposition to the converted voltage during the positive half wave; that is to say, inverter operation prevails. Accordingly,

the commutation is initiated with the full degree of control as above assumed by the amount Icv the case of the inverter operation equal to the corresponding lagging of the point behind the I point a in the case of the rectifier operation, the

mean value of the voltage for rectifier and inverter operation will be the same and the trapezoid curve produced will be a straight line in the horizontal portion thereof, provided a corresponding smoothening is effected. The commutation point is thenshifted from b to c to the right by the value 2k.

The moment at which the maximum value of inductance of the inductors ll, 23 and 33 is attained need not under circumstances be shifted when passing from inverter operation to rectifier operation, i. e., when the commutation point passes from b to 0, since the time within which the inductors attain the maximum value of inductance may be chosen longer than the total shifting of the commutation period. If a shifting of the moment at which the maximum value of inductance is obtained is nevertheless desirable, this may be easily accomplished by influencing the excitation of the inductors by an additional component which changes its sign when the current passes through the zero value. In the negative voltage half wave the conditions are similar to those in the positive half wave.

In Figs. 11 and 12 are shown as embodiments the connections of two converters according to the invention which are adapted to fulfil the conditions above-described with reference to Figs.

9 and 10. The converters serve to exchange energy between a three-phase current supply circuit AC 3 and a'single phase current supply circuit AC i and vice versa.

In the converter according to Fig. 11, the break contacts I are similar to those shown in Fig. 1. They are operated by the cam shaft 40 which in turn is driven by the synchronous motor 48. The windings 3 of the reactors 2 are series connected to the break contacts which are connected to the six-phase secondary winding 6 of the main transformer. Besides. the series winding 3 each reactor is provided with two exciting windings 4 and 5. to a'potential transformer 41 so that they are traversed by a current which is in phase with the voltage of the single phase current supplycircuit. In this manner the moment at which the maximum value of inductance of the single reactors is attained, is shifted when passing from the positive to the negative half wave. In the case "of an inductive load, in order to-shift the moment at which the maximum value of inductance is attained in inverter operation, with respect to the moment of the maximum inductance in rectifier operation, it may be preferable not to energize the biasing windings 4 in response to the voltage of the single phase supply circuit, but in response to the single phase current. To

If the leading of the commutation point b before the -point of the voltage coincidence a is chosen in The exciting windings 4 are connected this end, the windings must be connected to a current transformer arranged in series with the single phase supply circuit. The additional exciting windings permit an additional influence of the preexcitation of the inductors which may be effected at will or in accordance with any opcrating magnitudes. The shifting of the switching-in and switching-out moments may be obtained in this arrangementby varying the phase position of the synchronous motor 45 in accordance with a control device which is influenced by the reversal of current.

In the arrangement shown in Fig. 12 two inductors, one of which is allotted to the positive whereas the other to the negative half wave of the single phase voltage, are connected in series with each break contact. During the positive half wave, the inductor'relative to the negative half wave is then so highly .preexcited that its inductance practically disappears. The same occurs during the positive half wave with the inductor allotted to the negative half wave. In this manner the inductors need not be alternately biased and may be tuned to the most favorable values for the half wave in question. In order to effect this magnetic bias so as to reduce the inductance, the biasing windings 51 and 58 are connected to the resistances 66 and 61 which are connected to the secondary side of the trans former 9| through the tubes 68 and 69 which transformer is connected on the primary side to the single phase voltage. The primary side of this current transformer is'series connected to the single phase supply circuit. In case no single .phase supply circuit is available, the transformer II is connected to'a pilot time tapper whose frequency is constant with respect to the feeding primary frequency.

In the arrangement shown in Fig. 12, the shifting of the switching-in and switching-out moments in accordance with the reversal of current is effected in the following manner: The shaft 40 may be shifted in the longitudinal direction. The cams arranged on the shaft are so shaped that by displacing the shaft the switching-in and switching-out moments are shifted with respect to the cycle of the alternating-current voltage. Between the motor 45 and the shaft 48 is provided a coupling 48 which permits a shifting of the shaft 40 in the longitudinal direction.

The shifting is effected by means of a magnetic device indicated by a magnet armature 49,

. rheostat I! in accordance with the direction of energy, thereby influencing the excitation of the winding '9. r

I claim as my invention:

1. An arrangement forexchanging energy between two current supply circuits with different current curves, comprising a converter having mechanical contact interrupters, each of said interrupters being disposed in one of the phases connecting said two supply circuits, means for operating said interrupters in synchronfsm with the timevariation of the current to be interrupted, 'a reactor of variable reactance arranged in each of said connecting phases in series with the interrupter of said phase, means associated with said reactor for periodically eilecting a sudden increase of said reactance within an interval including the moment of the opening of said interrupter, and means for varying the synchronous position oi. the operation of said interrupters upon the reversal of the current transmitted.

2. An arrangement for exchanging energy be: tween-two current supply circuitswith diii'erent current curves, comprising a converter having mechanical contact interrupters, each of said interrupters being disposed in one oi. the phases connecting .said two supply circuits, means for operating said interrupters in synchronism with the time variation of the current .to be interrupted, an inductor connected in-series with each or said interrupters, said inductor having a magnetic body designed to be saturated at low current intensities below the effective intensityot the alternating current so as to increase the inductance of said inductor at a time near the zero value of said current, and means for varying the synchronous position oi. the operation of said in-' terrupters upon the reversal of the current transmitted.

3. An arrangement for exchanging energy between an alternating current supply circuit and a direct current supply circuit, comprising a converter having mechanical contact interrupters, each of said interrupters being disposed in one of the phases connecting said two supply circuits,

means for operating said interrupters in synch'ronism with the time variation of the current to be interrupted, an inductor connected in series with'each oi. said interrupters, said inductor.

having a magnetic body designed to be saturated at low current intensities below the eifective intensity of the alternating current so as to increase the inductance of said inductor at a time near the zero value of said current, and means for varying the synchronous position of the operation of said interrupters upon'the, reversal of the current transmitted so as to shift the beginning of the commutations when passing from rectifier to inverter operation from a moment which, depending upon the degree of control, lies behind the moment of the voltage coincident to a moment which lies an equal amount before the moment or the voltage coincidence.

4. An arrangement for exchanging-energy between two current supply circuitswith different current curves, comprising a commutating device having interrupters with reciprocating contacts connected between said two supply circuits, a driving shaft provided with cams for operating said contacts, and means for driving said shaft so as'to actuate said interrupters in synchronism with the current to be interrupted, a variable impedance serieseconnected to each of said interrupters, said impedance being adapted to periodically reduce the current intensity at a.

time in the neighborhood of the zero value oi. the current, and me "for adjusting said came upon the reversal of the current transmittedso as to vary the synchronous position of the operation of said interrupters.

5. An arrangement for exchanging energy between an alternating current supply circuit and a direct current supply circuit, comprising two converting systems, each of said systems having an interrupter allotted to each phase of said alternating current circuit, means for operating said interrupters in synchronism with the alternating current and variable impedances series- -connected with said interrupters, said impeda'nces being designed to periodically increase their impedance magnitude at a time near the versal of the current transmitted.

6. An arrangement for exchanging energy between an alternating current supply circuit and a direct current supply circuit, comprising a converter having mechanical contact interrupters,

each of said interrupters being disposed in oneof the phases connecting said two supply circuits, means for operating said interrupters in synchronism with the time variation of the current to be interrupted, a variable impedance arranged in each of said connecting phases in series with the -interrupter of said phase, said impedance being adapted to periodically increase its impedance magnitude within an interval including the moment ofv the opening of said interrupter, and means disposed in said connecting phases for "interchanging in response to the reversal of the current transmitted the connections of said interrupters to said phases in order to vary the synchronous position of said interrupters. with respect to the alternating current voltage.

7. An' arrangement for exchanging energy between two current supply circuits with diil'erent current curves, comprising a converter having mechanical contact interrupters, each of said interrupters being disposed in one of the phases connecting said two supply circuits, means for operating said interrupters in synchronism with the time variation of the current to be interrupted, an inductor connected in series with each of said interrupters, said inductor having a magnetic body designed to be saturated at low current intensities below the effective intensity or the alternating current so as to increase the inductance of said inductor at a time near the zero value of said current, biasing windings provided on each or said magnetic bodies, an energizing circuit connected with vsaid windings, controlling means disposed in said energizing circuits for varying the magnetic bias, and means for varying the synchronous position of the opera tion of said interrupters upon the reversal of the current transmitted, said controlling means of said energizing circuit being designed for being operated simultaneously with said means for varying the synchronous position of said inter rupters.

8. An arrangement for exchanging energy between two supply circuits with diifere'nt current curves, comprising a transformer having its primary windings connected with one of said supply circuits and the neutral point of its secondary .windings connected to one phase of said second supplycir'cuit, a-set of contact interrupters, each having one contact series-connected to one of said secondary windings and a second contact pertaining to said first contact connected with another phase of said second supply circuit, means for actuating said interrupters in synchronism with the current of said first supply circuit so as to interrupt the current flowing through each interrupter at a time near the zero value of said current, an impedance series-connectedto each of said interrupters, said impedance being adapted to periodically increase its magnitude at low currents near said zero value,-

a control device disposed between said interrupter set and said second supply circuit, said device including a relay responsive to the direction of the current flowing between said set and said second supply circuit, variable means for adjusting the phase position of said interrupters with respect to the current to be interrupted, and an operative connection between said relay and said variable means for effecting said adjust-. ment in response to the reversal of the current transmitted.

9. An arrangement for exchanging energy between two alternating current supply circuits of different frequencies, comprising a transformer having its primary windings connected with the supply circuit of the higher frequency and the neutral point of its secondary windings connected with one phase of the supply circuit of the lower frequency, a set of contact interrupters, each having one contact series-connected to one of said secondary windings and a second contact pertaining to said first contact connected with another phase of said second supply circuit, a driving device for actuating said interrupters in synchronism with said high frequency, variable means for adjusting the phase position of said interrupters with respect to the cycle of the current having the higher frequency, an impedance connected in series between each of said interrupters and said high frequency circuit, said impedance having a magnetic body designed to become unsaturated at low current intensities near the current zero value, additional biasing windings provided on said magnetic body, a second transformer having its primary windings connected with the supply circuit of lower frequency and its secondary windings coupled with said biasing windings in order to effect a magnetic bias in dependence upon the current in said low frequency supply circuit.

10. An arrangement for exchanging energy between two alternating current supply circuits of different frequencies, comprising a commutating converter disposed between said two supply circuits, said converter having a movable break contact connected to each phase of one of said sup ply circuits and driving means for operating said contacts in synchronism with the voltage of the supply circuit connected to said contacts, said converter being so adjusted that said contacts temporarily transmit both the positive and the negative half wave of the converted alternating current, inductors connected between-said contacts and the supply circuit connected to said contacts, said inductors being designed to be periodically unsaturated at low current intensities and saturated at an intensity below the effective current intensity, and controlling means for varying the synchronous position of the operation of said contacts upon the reversal of the current transmitted.

11. An arrangement for exchanging energy between two alternating current supply circuits of different frequencies, comprising a commutatintensity, and controlling means for varying the synchronous position of the operation of said contacts upon the reversal of the current transmitted, said controlling means being adjusted to cause at the time of said variation of phase position'said contacts to transmit both a negative and a positive'half wave, so that the phase of the voltage of the higher frequency which forms the last portion of a half wave of the voltage curve of the lower frequency also forms the first portion of the following half wave of the lower frequency.

12. An arrangement for exchanging energy between two alternating current supply circuits of different frequencies, comprising a commutating converter disposed between said two supply circuits, said converter having at least one interrupter allotted to each phase of the supply circuit having the high frequency and driving means for operating said interrupters in synchronism with said high frequency, said driving means being adjusted to have a phase position at which said interrupters are opened and closed at moments which, within a half wave of the inverted alternating voltage of'low frequency, have approximately the same phase position with respect to the high frequency voltage, an inductor connected in series with each of said interrupters, said inductor having a magnetic body designed to be saturated at low curren intensities below the effective intensity of t e a1ternating current so as to increase the inductance of said inductor at a time near the zero value of said current, and means for varying the synchronous position of the operation of said interrupters upon-the reversal of the current transmitted.

13. An arrangement for exchanging energy between two alternating current supply circuits of difierent frequencies, comprising a commutating converter disposed between said two supply circuits, said converterhaving at least one interrupter allotted to each phase of the supply circuit having the high frequency and driving means for operating said interrupters in synchronism with said highfrequency, an inductor connected in series with each of said interrupters, said inductor having a magnetic body designed to be saturated at low current intensities below the effective intensity of the alternating current so as to increase the inductance of said inductor at a time near the zero value of said current, biasing windings provided on each of said magnetic bodies, an energizing circuit connected with said windings, and an electric coupling disposed between said energizing circuit and the low frequency supply circuit in order to bias said inductors by an energizing current which changes its sign at about the same time as the low frequency voltage.

14. An arrangement for exchanging energy between two alternating current supply circuits of different frequencies, comprising a commutating converter disposed between said two supply circuits, said converter having at least one interrupter allotted to each phase of the supply circuit having the high frequency and driving means for operating said interrupters in synchronism with said high frequency, at least two groups of inductors, each of said groups being allotted to one phase of the voltage of the low frequency supply circuit and comprising one inductor for each of said interrupters, each of said inductors being connected in series with an inductor of another group and having a magnetic 15. An arrangement for exchanging energy between two alternating current supply circuits of different frequencies, comprising a commutating converter disposed between said two supply circuits, said converter having at least one interrupter allotted to each phase of the supply circuit having the high frequency and driving means for operating said interrupters in synchronism with said high frequency, at least two groups of inductors, each oi said groups being allotted to one phase oi the voltage of the low frequency supply circuit and comprising one inductor for each of said interrupters, each of said inductors being-connected in series with an inductor of another group and, having a magnetic body designed to be desultorily saturated at a low periodical current below the efle'ctive current intensity and a winding provided on said body for preexciting said body, an energizing circuit disposed for each =aid groups and connected to said windings of the inductors of said group, a transiormer having its primary windings connected with said low frequency supply circuit and its secondary windings with said energizing circuits, and an electric valve series connected in each of said energ circuits so as to preexcite said inductor groups alternately in rhythm with said low frequency voltage and each of said groups only half wave oi said voltage and always in the same direction.

16. An arrangement for exchanging energy be tween two alternating current supply circuits oi difi'erent frequencies, comprising a commutating converter disposed between said two supply cir= cults, said converter having at least one interrupter allotted to each phase oi the supplycircuit having the high frequency and driving means for operating said interrupters in synchronism with said high frequency, on inductor connected in series with each of said interrupters, said inductor having a magnetic body designed to be saturated at low current intensities below the etfective-intensity of the alternating current so as to increase the inductance of said inductor at a time near the zero value 01' said current, controlling. means for varying the synchronous positlon of the operation of said contacts in response to the reversal of the current transmitted, and variable means for adjusting the synchronous position of said contacts with respect to the cycle of said high frequency voltage within the half waves of the low frequency voltage .0 that the middle portion of the curve 01'- said low frequency voltage is approximately sine shaped.

17. An arrangement for exchanging energy between two alternating current supply. circuits of I difierent frequencies, comprising a oommutating converter disposed between said two supply circuits, said converter having at least one interrupter allotted to each phase of the supply circuit having the high .frequency and driving means for operating said interrupters in synchronism with said high frequency, an inductor connected in series with each of said interrupters, said inductor having a magnetic body designed to be saturated at'low current intensities below the eflective intensity of the alternating current so as to increase the inductance of said inductor at a time near the zero value of said current, biasing windings provided on each of said magnetic bodies, an energizing circuit connected with said windings, and a current source connected with said circuit, said source being designed for producing an approximately sinusoidal energizing current having the frequency of the low frequency supply circuit, whereby the phase position of said interrupters is varied in response to the reversal of the current transmitted.

18. An arrangement for exchanging energy between two alternating current supply circuits of difl'erent frequencies, comprising a commutating converter disposed between said two supply circuits, said converter having a movable break contact connected to each phase of one of said supply circuits and driving means for operating said contacts in synchronism with the voltage of the supply circuit connected to said contacts, said converter being so adjusted that said contacts temporarily transmit both the positive and the negative half wave of the converted alternating current, inductors connected between said contacts and the supply circuit connected to said contacts, said inductors being designed to be periodically unsaturated at low current intensities and saturated at an intensity below the effective current intensity, controlling means for varying the synchronous position of the operation of said contacts upon the reversal of the current transmitted, and variable means for additionally adlusting the closing moments of said contacts in response to the wattless current transmitted.

19. An arrangement for exchanging energy between two alternating current supply circuits of diflerent frequencies, comprising a. commutating converter disposed between said two supply circuits, said converter having a movable break contact connected to each phase of one of said supply circuits and driving means for operating said contacts in synchronism with the voltage of the supply circuit connected to said contacts, said converter being so adjusted that said contacts temporarily trammlt both the positive and the negative half wave of the converted alternating current, inductors connected between said contacts and the supply circuit connectedtosaid contacts, said inductors being designed to be periodically unsaturated at low current intensities and saturated at an intensity below the eflective current intensity, controlling means for varying the synchronous position of the operation of said contacts upon the reversal of the current transmitted, and means for varying the duration of'the closing periods of said contacts in accordance with the wattless current transmitted. v

20. An arrangement for exchanging energy between two alternating current supply circuits oi so as to increase the inductance of said inductor at a time near the zero value of said current,

biasing windings provided on each of said magnetic bodies,'an energizing circuit connected with said windings, and a current source connected with said circuit, said source being designed for producing an energizing current synchronously and in phase with the current of said low irequency, whereby the phase position of said interruptcrs is varied in response tothe reversal of the current transmitted.

21. An arrangement for exchanging energy between two current supply circuits with different current curves, comprising a converter having mechanical contact interrupters, each of said interrupters being disposed in one of the phases connecting said two supply circuits, means for operating said interrupters in synchronism with the time variation of the current to be interrupted, reactive means for periodically distorting the curve of the current to be interrupted so as to decrease the current intensities in the neighborhood of the zero value of said current, and means operatively connected between said two supply circuits for varying the synchronous position of the operation of said interrupters responsive to the reversal of the current transmitted.

22. An arrangement for exchanging energybetween two current supply circuits with difierent current curves, comprising a converter having .mechanical contact interrupters, each '0! said interrupters being disposed in one of the phases connecting said two supply circuits, means for operating said interrupters in synchronism with the time variation of the current to be interrupted, an inductor connected in series with each of said interrupters, said inductor having a magnetic body designed to be saturated at low current intensities below the eflective intensity of the alternating current so as to increase the inthe synchronous position of the operation or said interrupters upon the reversal of the current transmitted.

23. An arrangement for exchanging energy between two supply circuits with dfierent current curves, comprising a transformer having its primary windings connected with one. of said supply circuits and the neutral point of its secondary windings connected to one phase of said second supply circuit, a set of contact interrupters, each having one contact series-connected to one of said secondary windings and a second contact pertaining to said first contact connected with another phase of said Second supply circuit, a synchronous motor connected to said set for actuating said interrupters in synchronism with the current to be interrupted, said motor having a rotatably mounted stator, controlling means disposed between said interrupter set and said second supply current, said controlling means being responsive to the reversal of the current flowing between said set and said second supply current, and an operative connection between said controlling means and said stator, whereby the phase position of said interrupters is varied in accordance with the reversal oi. the current transmitted.

24. An arrangement for exchanging energy between two supply circuits with different current curves, comprising a transformer having its primary windings connected with one of said supply circuits and the neutral point of its secondary windings connected to one phase of said second supply circuit, a set of contact interrupters, each having one contact series-connected to one of said secondary windings and a second contact pertaining to said first contact connected with another phase of said second supply circuit, means for actuating said interrupters in synchronism with the current of said first supply circuit, phase adjusting means for varying the phase position of said interrupters with respect to the cycle or the current to be interrupted, a control device being connected between said interrupter set and, said second supply circuit, said control device being responsive to the reversal said arrangement.

IRWIN J.

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