US1798668A - Automatic synchronizer - Google Patents

Description

March 31, 1931.

F- H. GULLIKSEN ET AL AUTOMATIC SYNCHRONIZER Filed May 29, 1928 'Aine' Na chin e I INVENTORS Homer C. Nycum and Figyn H. Gu l-i sen ATTdRNEY Patented Mar. 31, 1931 FINN H. GULLIKSEN AND HOMER C. NYCUM, OF

WILKINSBURG, PENNSYLVANIA, AS-

SEGI ORS TO \VEST'INGHOUSE ELECTRIC AND MANUFACTURING COMPANY, A COR- PGRA'IIQH OF PENNSYLVANIA AUTOMATIC SYN CHRONIZER Application filed May 29,

This invention pertains to automatic synchronizers or devi es designed to control the operation of a switch for connecting two alternatingcurrent sources in parallel, when the phase and frequency relations of the voltages of the two sources are suitable.

One of the principal objects of this in vention is the provision of an automatic synchronizer capable of connecting two alternating-current sources in parallel at the precise instant of exact phase coincidence of the voltages of the sources to be paralleled.

Another object of our invention is the provision of an automatic synchronizer which shall be effective to initiate the operation of the paralleling switch before the occurrence of exact phase coincidence, so that allowance may be made for the finite time required for the closing of the switch, whereby paralleling will actually be effected precisely at zerophase difference.

It is a further object of the present invention to provide an automatic synchronizer which shall operate to start the paralleling switch to close an interval in advance of eact phase coincidence, which is proportional to the beat frequency or the difference between the frequencies of the sources to be paralleled. This feature of our invention makes it possible for the paralleling switch to be finally closed exactly at the moment of zero phase din ence, regardless of the beat frequency. Obviously, for higher -beat speeds it will be necessary for the paralleling switch to be started toward the closed position sooner than at relativelylow-beat frequencies, and the paralleling system'of our invention functions to accomplish this pur pose.

when such differences exist than when the sources have equal voltages, by a time proportional to the magnitude of such difference.

A still further object of our invention is to provide a simple and inexpensive syn- 1928. Serial No. 281,552.

chronizer characterized by durability and accuracy in operation.

In accordance with our invention, we employ a system of pivoted levers or walking beams. Two of these levers are actuated solenoids connected in parallel between the sources to be paralleled, so that they are subject to the resultant of the voltages of said sources. he third lever is connected to one of said solenoid-actuated levers through an air dash-pot or other equivalent means. Contacts mounted on said levers cooperate with other fixed and moving contacts to set up a circuit to the paralleling switch in a mannor to be described more in detail hereinafter.

It is a feature of this synchronizer of our invention-that paralleling of the two sources will be positively prevented so long as the beat frequency is above a predetermined value.

Another feature of our invention is the provision of voltage compensation by means of which the synchronizer is caused to operate sooner if the voltage of one of the sources to be paralleled is below normal. where a generator is being connected in parallel to a supply source which is heavily loaded, the paralleling operation will be effected more rapidly than would be the case were the line voltage normal. ously an advantage, since it is desirable, when a supply circuit is overloaded, that additiona generating equipment be connected thereto with as little loss of time as possible.

Another feature of our invention resides in the provision of means whereby the initiation of the closing of the aaralleling switch will be deferred for two cycles of the beat frequency after the first occurrence of phase coincidence at a beat frequency below the value above which paralleling is prevented. This time interval is provided to avoid a premature closing of the paralleling switch, since the synchronizing contacts are normally engaged when the device is deenergized, and it 7 is necessary that paralleling be prevented until after the synchronizer has commenced to function.

Another advantageous feature of our invention is the provision of means whereby Thus,

This is obvithe aforementioned time delay of two cycles of the beat frequency may be obviated in case the beat frequency is very low, in which case, the two-cycle delay would be unnecessarily long.

The above features and advantages, as well as the details of the construction and arrangement of our invention will now be described and the features which we consider novel will be pointed out in the appended claims.

For a full understanding of our invention, reference should be had to the accompanying drawing, the single figure of which illustrates diagrammatically the automatic synchronizer of our invention, together with the circuits necessary for a proper functioning thereof.

Referring in detail to the drawing, the principal features of our invention are the levers 10, 11 and 12 which are pivoted respectively at 13, 1e and 15 and normally occupy the positions in whi h they are shown in the drawing. The lever 10 is maintained in engagement with a stop 16 by a spring 17 The lever 11 is similarly actuated by spring 19. A spring forces the lever 12 into enga ement with a stop 21.

3 pair of solenoids 22 and 23 cooperate with their respective armatures which are connected to the ends of levers 11 and 12, respectively. These solenoids are adapted to be subjected to the resultant of the voltages of the two sources to be paralleled.

The lever 10 is connected to the lever 12 by means of a linkage mechanism, including a dash-pot 24, which is so designed that, when the lever 12 is actuated by the spring 20 against the pull of the solenoid 23, the lever 10 will be actuated thereby. The pull exerted on the lever 10 by the dash-pot linkage is proportional to the angular velocity of the lever 12. When the solenoid 23 actuates the lever 12 against the tension of the spring 20, the dashpot permits this lever to move without affecting the lever 10. A needle valve (not shown) may be provided for varying the efiectiveness of the dash-pot in downwardly actuating the left-hand end of the lever 10.

In addition to the spring 19, other springs 19 and 19 are provided. The additional springs do not become effective until the lever 11 has moved slightly in the counter clockwise direction. Similarly, a spring 20 is provided to assist the spring 20. These extra springs serve to cause the relation between total spring pull and the angular movement of the lever to be a straight line or stated in a different manner, these extra springs, when properly adjusted, serve to cause the relation between the travel of the armatures of the solenoids 22 and 23 and the phase displacement of the voltages to be a straight line.

Mounted on the lever 10 is a synchronizing contact 25 and a lookout contact 26. A second synchronizing contact 27 is mounted operate with contact 26.

on the lever 11 for cooperation with contact 25, and a fixed contact 28 is positioned to co- Contacts 26 and 28 are preferably adjustable in the same manner as contacts 25 and 27. The springs 17 and 19 are so adjusted that contacts 25 and 27 are lightly engaged when lever 10' engages stop 16. The lever 12 is provided with a plurality of contacts 29, 30, 31 and 32 which are arranged to cooperate, respectively, with the fixed contacts33, 3a, 35 and 36. These contacts serve, in a manner to be described hereafter, to complete certain circuits which result in the closing of the paralleling switch.

In order to make the showing of the drawing complete, the two circuits to be paralleled are indicated at 37 and 38. It is assumed, for the purposes of this description, that the conductors 37 and 38 and 39 and 40 correspond to certain phases of polyphase distribution circuits, the other phases of which are not shown. The switch 41, having a closing coil as, is provided to effect the connection of the phases 37 and 38 when proper conditions obtain. Obviously, the same or similar switches may be employed to connect in parallel the remaining phases of the two systems which are not shown. One pair of corresponding phases, 39 and 40, for example, may be permanently connected together.

The other elements necessary to the functioning of our invention will be described in the course of the following explanation of the method of operation thereof.

As is apparent from the drawing, the solenoids 22 and 23 are connected in parallel across the phases 37 and 38 of the sources to be paralleled. A switch i3 is provided to control the energization of the solenoids. From the circuit illustrated, it will be apparent that the solenoids will be subject to the resultant of the voltages of the phases 37 and 38. This resultant has a value varying from zero, if the voltages of the two sources are equal, to twice the voltage of either source, and the frequency of this resultant voltage applied to the solenoids is equal to the difference between the frequencies of the circuits37 and 38.

Assuming that the conductor 37 is connected to one phase of a generator which it is desired to connect to a supply circuit, such as 38,.if the switch L3 is closed when the machine speed is so low that its frequency is considerably below that of the line, it will be obvious that the voltage impressed upon the solenoids 22 and 23 will have a considerable frequency. The armatures of the solenoids 22 and 23 are slightly damped, for example, by the dash-pots't land 4:5 and tend to respond to the variation in the current through the solenoids. When the current through the solenoids 22 and 23 is of a consicerable frequency, it will be obvious that the armatures, because of their inertia, and the damping effect of the dash-pots 44 and 45, will be unable to follow the variations of the current in the solenoids closely. Thus, when the machine frequency is low and the line frequency normal, it has been observed that'the armatures of the solenoids 22 and 23 will merely quiver, without any well defined movement. As the machine speed increases, the beat frequency decreases and the movement of the armatures of the solenoids becomes slower and of greater amplitude.

As the beat frequency decreases still farther, the armatures move up and down in accordance with the rise and fall of the current in the solenoids, the armatures both moving upward when the voltage across the solenoids decreases as the voltages of the two sources approach synchronism and moving downwardly as the resultant voltage increases when the voltages of the sources approach the positions in which they are 180 out of phase.

It will thus be obvious that, when the beat frequency has been reduced to a relatively low value, for example, to or of a cycle per second, the armatures of the solenoids 22 and 23 will move up and down simultaneously at the beat frequency. The amplitude of the movement of the armatures is, as hereinbefore stated, inversely proportional to the beat frequency.

As the armatures of the solenoids 22 and 23 move up and down they communicate a corresponding movement to the levers 11 and 12 which are normally maintained in the position shown by the springs 19 and 20, respectively. As the lever 11 oscillates, no result is produced but oscillation of the lever 12 causes the successive engagement of the contacts 2933, 30-34, 31-35 and 3236..

At the same time, the movement of the lever 12 is communicated, through the dashpot 24, to the lever 10 which, because of the opposing torques exerted by the springs 17 and the dash-pot, assumes a mean position dependent upon the frequency of the oscillations of the lever 12 which depends, of course, upon the beat frequency. In other words, the higher the beat frequency, the greater the angular velocity of the lever 12, the pull of dash-pot 24 on lever 10 and the are through which the lever 10 is actuated thereby, and the lower is the position taken by the contact 25, so that the synchronizing con tacts 2527 are engaged at a time in advance of zero phase which is proportional to the beat frequency.

Since the armatures of the solenoids 22 and 23 oscillate simultaneously, it will be seen that the contact 27 will engage the contact 25 each time the former moves upward as the voltages of the sources approach synchronism.

It will also be obvious that the contacts 26 and 28 will be engaged before the contacts 25 and 27 engage until the beat frequency decreases to a definite value at which contacts 25 and 27 are engaged previous to the engagement of contacts 26 and 28. It will thus be apparent that the engagement of the contacts 26 and 28 may be employed to control a circuit to prevent the closing of the paralleling switch 41, and that the engagement of the contacts 25 and 27 may be employed to contion of the paralleling switch before the automatic synchronizer has been brought into operation In order to allow the synchronizer sufiicient time to begin to operate, we int-roduce a time delay of two cycles of the beat frequency between the first occurrence of phase coincidence at a heat frequency at which paralleling is feasible, and thefinal initiation of the operation of theparalleling switch.

This end is accomplished by the use of a plurality of relays which are successively energized by the engagement of the contacts operated by the lever 12, the energization of each relay being dependent upon the operation of the previous relay in the series.

Let it now be assumed that the beat frequency has decreased to such value that the contacts 25 and 27 are engaged prior to the engagement of contacts 26 and 28. The operation of the latter contacts to prevent paralleling, will be described hereafter. As the contact 27 on the lever 11 moves upwardly to engage the contact 25, the armature of solenoid 23 likewise moves upwardly. At this stage of the operation, the engagement of the contacts 3l-35 and 3236, carriedby the lever 12, is ineffective except to energize the timedelay relay 52 for an interval which is insuflicient to permit it to operate.

As already stated, the armatures of the solenoids 22 and 23 are released and drawn upwardly by'the springs19 and 20 when the voltages of the sources 37 and 38approach the zero phase relation. The spring 19 and the cooperating springs 19' and 19 are so arranged and adjusted that the movement of. the lever 11 has a substantially straight-line relation to the phase angle between thevoltages of the sources. Unlike previous synchronizers, the performance of which followed the upper flat part of the sine curve, the movement of the synchronizing contact 27 is quite large near synchronism, which has the effect of magnifying the accuracy of the system in' the synchronizing range.

I After the first occurrence of phase coincidence, following a reduction in heat speed to the value at which the contacts and 27 are engaged prior to the engagement of the lockout contacts 26 and 28, the voltages of the sources move away from the position of phase coincidence, and the armatures of the solenoids 22 and 23 are again attracted by the solenoids. When the voltages of the sources 37 and 38 have reached a position 180 out of phase, both armatures will be completely withdrawn from their normal positions, and the contacts 25 and 27 will be disengaged, while the contacts 29 and 33 will be engaged.

The engagement of the contacts 29 and 33 sets up a circuit from a control source, such as the battery 46, to the lever 12, through contacts 29 and 33, the coil of an auxiliary relay 47 and a resistor 48, to the negative pole of the battery. The completion of this circuit causes the operation of the relay 47 to close its switches. The closure of the up per switch of the relay 47 completes a looking circuit therefor in parallel with the contacts 29 and 33. The energization of the operating coil of relay 47 is thereafter independent of the contacts 29 and 33. The closure of the lower switch of relay 47 sets up a circuit for a relay 49 which is next in the series.

As the voltages of the sources 37 and 38 again approach synchronism, the armatures of the solenoids are released and move upward under the influence of the springs 19 and 20. The contact 27 engages the contact 25, but is ineffective to cause the closing of the paralleling switch since the paralleling circuit is open at another point. At the same time, contacts 31 and 35 are engaged, and a circuit is completed from the positive pole of the battery, through the lever 12, contacts 31 and 35, the lower switch of relay 47, the operating coil of the auxiliary relay 49 and resistor 48, to the negative pole of the bat tery. The relay 49, being energized, closes its switches. The upper switch completes a locking circuit similar to that already described for the relay 47, and the lower switch sets up a circuit to the operating coil of the relay which is the next relay in the series.

As the voltages of the two sources 37 and 38 again depart from synchronism and approach the out-of-phase condition, the armatures of the solenoids are again attracted. The downward movement of the contact 30 brings it into engagement with the contact 34. The engagement of the contacts 30 and 34 completes a circuit from the positive pole of the battery 46, through the lever 12, contacts 30 and 34, the lower switch of relay 49, the operating coil of the third auxiliary relay 50, andthence, to the negative pole of the battery. The relay 50 thereupon closes its switches, the upper switch completing a lock ing circuit for the operating coil and the low er switch setting up a circuit to the operating coil of an additional auxiliary relay 51. This circuit however is open at the contact 27.

When the voltages of the sources again approach synchronism the armatures of the solenoids are released and the contact 27 and 25 become engaged to complete a circuit from the positive pole of the battery, through the lever 10, contacts 25 and 27, the lower switch of relay 50 and the operating coil of relay 51, to the negative pole of the battery. Simultaneously, contacts 32 and 36 become engaged to complete a circuit from the positive pole of the battery, through the lever 12, contacts 32 and 36 and the operating coil of a relay 52, to the negative pole of the battery. The relay 52 is a time-delay relay having some means, such as a dash-pot 53, for delaying the closing of its switch. The switch of the relay 52 is in parallel with the lower switch of the relay 50.

' The energization of the relay 51 results in the closing of its switch which completes a circuit from the positive pole of the battery, through the closing coil 42 of the'paralleling switch 41 and the switch of the relay 51, to the negative pole of the battery through the resistor 48. Switch 41 is thereupon closed, and the circuits 37 and 38 are connected in parallel.

It is believed to be apparent from the above discussion that, at zero beat frequency the lever 10 will occupy the position in which it is shown. The lever 11 will assume a position depending upon the phase angle of the circuits 37 and 38, and the contact 27 will engage the contact 25 when the phase angle between the circuit voltages becomes zero. Thus, for a zero beat frequency, the synchronizer of our invention causes the paralleling switch to start to close precisely at the instant of phase coincidence. hen the beat frequency has a value greater than zero, however, the oscillation of the lever 12 will affect the position of contact 25 so that it is engaged by the contact 27 before the occurrence of exact phase coincidence, as the voltages of the sources approach synchronism. It will be apparent, furthermore, that after the first engagement of the contacts 25 and 27 following the occurrence of the beat frequency at which paralleling is permissible, the passage of two complete cycles of the beat frequency is necessary to effect the closing of the paralleling switch.

If the beat frequency is of a higher value, however, such that the contacts 26 and 28 are engaged prior to the engagement of contacts 25 and 27, it will be seen that the operating coils of all the relays 47, 49 and 50 will be short-circuited and the relay switches will be reset. For this reason, it is impossible for the synchronizer of our invention to effect paralleling until the beat frequency has decreased to a predetermined value.

If it is desired to employ the device of our invention to parallel portions of the same system, it is obvious that the provision of means for securing an advance initiation of the operation of the paralleling switch is unnecessary, since, in the case mentioned, the beat frequency is always zero, the two portions of the systems being energized from the same source.

In connection with installations of this kind, the delay introduced by the relays 47, 49, and 50 is unnecessary and they may, therefore, be omitted, the relay 52 being relied upon to furnish the time delay necessary to prevent paralleling before the synchronizer has been started. In such installations, however, the voltage difference between the portions of the system becomes important, and it is advantageous to provide means for compensating for difference in the voltages of the portions of the circuit so that the operation of the synchronizer will not be affected thereby.

It is obvious that, if the voltages of the circuits 37 and-38 differ, a resultant voltage will be impressed upon the solenoids 22 and 23, even at phase coincidence. This voltage will prevent the engagement of the contacts 25 and 27 to cause paralleling, and, to avoid this condition, we provide a voltage-compensating solenoid 54 to exert a torque on the lever 11 in opposition to that exerted by the solenoid 22. The solenoid 54 has two opposing windings which are actually superimposed windings energized by direct current obtained from rectifiers 55 and 56 connected to the conductors 37 39 and 38, 40, respectively. These rectifiers may be of the fullwave copper-oxide disc type, such as are now well known. It is believed to be apparent that the solenoid 54 will exert a torque on the lever 11 in proportion to the differences in the voltages of the circuits 37 and 38 and thus counteract any tendency of the solenoid 22 to maintain the contact 27 out of engagement with the contact 25 when the voltages of the circuits 37 and 38 are in phase but unequal. Although the solenoid 54, as shown, works on a much shorter lever arm than solenoid 22, any suitable provisions may be made to equalize the torque exerted on the lever by the solenoids.

The voltage-compensating means above described can be employed only when the beat frequency is zero, such as that between two portions of the same distribution system. For paralleling circuits having different frequencies, however, the compensating means must be applied to the lever 10. Such means are illustrated as the solenoid 54, which is similar in construction and operation to that shown at 54. The solenoid .54 tends .to de-= press the contact 25 and, consequently, to cause paralleling to take place in advance of zero phase by a time proportional to the voltage difference.

The function of the relay 52 is to effect the energization of relay 51 in case the beat frequency should be of such value that the delay introduced by the relays 47, 49 and 50, which require two cycles of the beat frequency for their operation, should prove too long. Thus, where the beat frequency is very low, instead of waiting for tl e completion of two cycles of the beat frequency, the relay 52 is efiective after the lapse of the time for which it is set, to complete the circuit of the relay 51 when contacts 25 and 27 are engaged slightly in advance of the occurrence of phase coincidence.

Among other advantages of our invention may be mentioned the fact that it is practically proof against failure in service. Failure of any of the direct-current circuits would only prevent paralleling, and the only failure that could cause faulty operation would be an opening of the alternating-current circuit, and even this would have no effect unless it occurred in the last half of the second beat cycle just before synchronizing would normally be effected.

Another important feature of our invention is its easy adjustability. The dash-pot needle valve may be adjusted to vary the advance angle at which circuit breaker operation is initiated, so that paralleling will be effected precisely at zero phase whatever may be the operating time of the circuit breaker employed. By adjusting contacts 26 and 28, the beat frequency above which paralleling is prevented may be varied to suit different conditions.

Other advantages of our invention have already been mentioned and the features thereof which we consider novel will now be defined in the following claims.

Although we have illustrated but one modification of our invention, it will be apparent to those skilled in the art that changes and alterations may be made therein to suit given conditions. It is our desire, therefore, not to be limited by the disclosure herein, but that the claims be construed to cover as broad a range of equivalents as the prior art will permit. Y

We claim as our invention:

1. An automatic synchronizer comprising three pivoted lever arms, solenoids for actuating two of said arms, an, air dash-pot linking the third arm to one of saidsolenoidactuated arms, synchronizing contacts on said third arm and one of the solenoid-actuated arms, three relays, contacts on the other solenoid-actuated arm for energizing said relays successively when the lastqn'elntioned arm is acti'iated, and a contact closed by the third relay to be energized and cooperating with said synchronizing contacts to set up a paralleling circuit.

2. An automatic synchronizer comprising three pivoted lever arms, solenoids for actuating two of said arms, an air dash-pot linking the third arm to one of said solenoidactuated arms, synchronizing contacts on said third arm and one of the solenoid-actuated arms, three relays, contacts on the other solenoid-actuated arm for energizing said relays successively when the last-mentioned arm is actuated, a contact closed by the third relay to be energized and cooperating with said synchronizing contacts to set up a synchronizing circuit, and a lookout contact on said third arm for short-circuiting all of said relays.

3. In an automatic synchronizer, the combination with two solenoids, armatures for said solenoids, and two independent pivoted lever arms actuated by saidarmatures, a synchronizing contact on one of said arms, auxiliary contacts on the other of said arms, of a third pivoted arm coupled to the latter of said two first mentioned arms, a second synchronizing contact on said third arm for cooperating with the synchronizing contact on said first mentioned arm.

4:. In an automatic synchronizer, a pair of pivoted levers, solenoids for actuating said levers, contacts mounted on one of said levers to be closed by the actuation of the latter, re-

lays successively controlled by said contacts, a third lever, an air dash-pot coupling said lever to said contact-bearing solenoid-actuated lever, a synchronizing contact on said third lever and a contact on the other solenoid-actuated lever for cooperation there with, a lockout contact on said third lever for engaging an associated fixed contact to shortcircuit said relays.

V 5. An automatic synchronizer for paralleling alternating-current sources comprising a plurality of levers, solenoids for. actuating two of said levers, said solenoids being adapted to be energized, by the resultant of the voltages of said sources, a synchronizing contact on one of said solenoid-actuated levers, an air dash-pot for connecting the other of said solenoid-actuated leversto a third lever, a second synchronizing contact on said third lever for cooperating with said firstmentioned synchronizing contact to effect the paralleling of said sources.

6. An automatic synchronizer comprising three pivoted lever arms, normally-engaged contact members mounted on said arms, solenoids for operating two of said lever arms, a dash-pot for linking the third arm to one of said solenoid-operated arms, and relays controlled by said contact members forisetting up a' synchronizing circuit.

7. A system for paralleling alternatingcurrent sources at the instant of phase coincidence comprising a paralleling switch, a contact-making mechanism for controlling the energization of the paralleling switch, means responsive to the difference in phase angle between the sources for actuating the contact-making mechanism, means actuated in accordance with the beat frequency of the sources for causing the contact-making mechanism to be closed at a phase displacement in advance of phase coincidence, by an interval proportional to the beat frequency, and means controlled by the means actuated in accordance with the beat frequency of the sources for rendering the contact-making mechanism ineffective to close the paralleling switch so long as the beat frequency is above a predetermined value.

8. Means for effecting the paralleling of alternating-current sources precisely at the instant when the voltages thereof are in phase comprising a plurality of solenoids energized by the resultant of the voltages of said sources, pivoted levers actuated by said solenoids, a linkage including an air dash pot for connecting two of said levers, contacts on said levers, a relay controlled by said contacts for causing the sources to be paralleled, and means whereby said contacts are engaged only when the beat frequency is below a predetermined value and at a time in advance of the occurrence of phase coincidence of said sources proportional to the beat frequency.

9. A system for paralleling alternatingcurrent sources comprising a paralleling switch, contacts for controlling the closing of said switch, means actuated in accordance with the beat frequency of said sources for closing said contacts when the beat frequency is below a predetermined value and at a time in advance of the occurrence of zero phase difference between said sources which is pro portlonal to the beat frequency, and means for deferring the closing of said contacts after the first occurrence of zero phase difference at a beat frequency below said predetemined value for two cycles of the beat frequency.

10. A system for paralleling alternatingcurrent sources comprising a paralleling switch, contacts for controlling the closing of sa1d switch, means actuated in accordance wlth the beat'frequency of said sources for closing said contactswhen the beat frequency is below a predetermined value and at a time in advance of the occurrence of zero phase difference between said sources which is proportional to the beat frequency, means for deferring the closing of said contacts after the first occurrence of zero phase difference at a beat frequency below said pedetermined value for two cycles of the beat frequency, and means for effecting the closing of said contacts after the lapse of a predetermined time, independently of the completion of two cycles of the beat frequency.

11. A system for paralleling alternating current sources including a paralleling switch, contacts for controlling the closing of said'switch and means for effecting the closing of said contacts at a time in advance of the occurrence of exact phase coincidence proportional to the beat frequency when the latter is below a predetermined value, and means for causing the closing of said con tacts to be effected in advance of phase coincidence by a greater time if the voltages of said sources are unequal.

12. A system for paralleling alternatingcurrent sources including a paralleling switch, contacts for controlling the closing of said switch and means for effecting the closing of said contacts at a time in advance of the occurrence of exact phase coincidence proportional to the beat frequency When the latter is below a predetermined value, comprising a variable linkage between said contacts and means responsive to the beat frequency for controlling said linkage.

In testimony whereof, We have hereunto subscribed our names this 28th day of May,

FINN H. GULLIKSEN. HOMER C. NYCUM.

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