US2166785A - Synchronizing apparatus - Google Patents

Description

V. AIGNER SYNCHRONIZING APPARATUS July 18, 1939.

Filed .May 12, 1937 4 Sheets-Sheet 2 Inventor: Viktor Aigner",

by 3 & is Attorney Fig. 6.

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July 18, 1939. v. AIGNER SYNCHRON-IZiNG APPARATUS Filed May 12, 1937 4 Sheets-Sheet 3 Fig. 8.

Inventor \/i kto r- Aiggner, by Hi Attorney Pig. 9. ,llllllhh. ,llllllllh.

y 8, 1939. v. AIGNER SYNCHRONIZING APPARATUS Filed May 12, 1937 4 Sheets-Sheet 4 new 4 d, d TIME Irwventor: ViktorAign T, y c5. 4,

His Attorney.

Patented July 18, 1939 UNITED STATES PATENT OFFICE SYNCHRONIZIN G APPARATUS Viktor Aigner, Berlin-Charlottenburg, Germany,

assignor to General Electric Company, a corporation of New York Application May 12, 1937, Serial No. 142,273 In Germany May 25, 1936 11 Claims. (Cl. 171-118) My invention relates to improvements in syn- In the drawings, Fig. 1 illustrates diagramchronizing apparatus and more particularly to matically a synchronizing arrangement embodyimprovements in synchronism indicators and also ing my invention; Figs. 2 and 3 are curve diasynchronizing devices for automatically controlgrams explanatory of the embodiment of my ling the closing operation of means for connectinvention shown in Fig. 1; Fig. 4 illustrates diati ing two alternating current circuits or systems grammatically a modification of a part of the whereby to start the closing operation at a perembodiment of my invention shown in Fig. 1; missible slip frequency sufllciently prior to syn- Fig. 5 is a curve diagram explanatory of the chronism to insure a completion of the connecembodiment of my invention shown in Fig. 4;

tion when their circuit voltages are substantially Fig. 6 illustrates diagrammatically another mod- 1 alike in phase without material shock to the sysification of my invention; Fig. 7 is a curve diatems. gram explanatory of the embodiment of my in- It has been proposed to control the paralleling vention shown in Fig. 6; Fig. 8 illustrates diaor synchronizing connection of two alternating grammatically a further embodiment of my incurrent circuits or systems by a device to which vention; Figs. 9 and 10 are curve diagrams ex- 1 a slip voltage, that is, a voltage dependent on planatory of the embodiment of my invention the phase difference between the electro-motive shown in Fig. 8, and Fig. 11 illustrates diagramforces of the circuits, is supplied through storage matically a modification of a part of the embodicircuits which permit the current effective in the ment of my invention shown in Fig. 8.

o device to attain its minimum value only when In Fig. 1 two polyphase alternating current i the slip frequency, that is, the difference becircuits or systems, represented by phase contween the frequencies of the electro-motive forces ductors l5 and i6 respectively, are arranged to of the circuits, is sufilciently small. One advanbe interconnected by suitable switching means tageous arrangement for accomplishing this opillustrated as a latched closed circuit breaker I1.

eration embodies a connection in which the rec- This is shown as having a closing coil l8 whose .2 titled slip voltage and also a voltage proportional circuit may include a circuit breaker auxiliary to its rate of change are supplied to the indiswitch H! which is closed when the circuit breaker eating or control devices in such a way that, is open and vice versa. In order to obtain the when these quantities reach their minimum voltages desired for the synchronism indication values, the synchronizing operation is started. and control of the switch l1, there may be pro- 30 In order to determine the rate of change of vided potential transformers 20 and 21 which the rectified slip voltage, a circuit comprising a are respectively connected to be energized from rectifier and a condenser is used. In this concorresponding phases of the systems l5 and I6. nection, the function of the condenser is to pro- The secondaries of these transformers are con- 5 vide the first derivative with respect to time of nected in opposition so as to obtain the differthe rectified slip voltage or in other words, the ence between the electromotive forces of the ra e o c e o t e ed S p Voltage systems or the slip voltage which is dependent W this rr m the l p v l e is pon the phase angle between the electromotive plied to a rectifier and condenser and the voltf rces The secondary circuit may i d an age to be impressed on the device taken from the indicating lamp '4 which will be dark when the 40 rectifier. This, however, has the disadvantage Systems are in synchronism that the Influence of the normal power system Any suitable synchronism indicating or control frequency 15 present m the final result" means 22, illustrated as having two energizing In order to avoid such a disadvantage, I provide in accordance with my invention an arg g gi g g gif gi gg g g gg g 45 ran ement for eliminatin the normal ower s sg g p y torque on the same shaft. As shown, the detem or supply frequency effect from the slip voltage before obtaining its first derivative. vlce n has movable mdwatmg or Contact con- Thi and other obj ct of my invention W111 trolling member 25 which maybe biased to cirpear in more detail hereinafter cuit closing position by a spring 26. In some My invention will be better understood from cases it m y be advantageous to p ovi a ui the following description when considered in conably adjustable stop 21 for the contact member nection with the accompanying four sheets of 25 so that it can move from its in phase" podrawings and its scope will be pointed out in the sition only a small amount. In this way any appended claims. stored kinetic energy which might tend to cause 55 an undesired switching operation is so limited as practically to eliminate such a possibility.

In order to control the device 22 in accordance with my invention, the winding 23 is connected through resistances 23, 29 and 3E! and a condenser 3! to a rectifier 32 which is shown fullwave and which is connected in the secondary circuit of the potential transformers 29 and 2! so as to be energized in accordance with the slip voltage E. Consequently the winding 23 is energized in dependence on the rectified slip voltage Er. As shown in the upper portion of Fig. 2 schematically by section lines the rectified voltage Er as measured across the resistance 28 contains impulses at power system frequency. These in turn would affect the winding 23 except for the filtering and smoothing action of the condenser 3|. The current Ir in th winding 23, therefore, assumes a form such as shown in the lower portion of Fig. 2 opposite a point below the condenser 3|. The current Ir in the winding 23 accordingly corresponds directly to the slip voltage.

In. accordance with my invention, the winding 24 of the device 22 isconnected to be energized in dependence on the rate of change or the first derivative with-respect to time-of the rectified and filtered slip voltage Erf. winding 24 may be connected in series with a condenser 33 which is energized from the se'c ondary circuit of the potential transformers 29 and 2!- through a rectifier 34, shownfull-wave, a filtering network comprising condensers .35 and inductances 36, and a loading resistance 31. In this way impulses at power system frequency are eliminated from the rectified slip voltage Er, shown. in the upper portion of Fig. 3 beforethe rectified and filtered voltage Erf in the form shown in themiddle portion of Fig. 3 as measured across the loading resistance 31, for example, is

efiective on the condenser 33. Consequently, in the current circuit of the condenser 33 and the winding 24,. a current Irf flows which is dependent on the firstderivative with respect to time of the rectified and filtered slip voltage.

Whenthe. sum. of the effects exerted. by Ir and Irrthrough the windings'23 and 24'falls below a predetermined value, the control member 25, under the influence of spring 26,;closes its contacts 38 in the circuit 39 of the. closing coil l-8'- to energize the'coil from a suitable source 40. This ef fects the closing of the circuit breaker l1 whereby to interconnect the systems l and I6 when they are in synchronism. The closing coil circuit 39 may be provided with a control switch 4|, which maybe normally open, but closable at will when an automatic synchronizing operation is desired. Similarly the secondary alternating currentvoltage difierence circuit of the transformers 20 and 2| may be provided with a control switch 4| which may be normally open but closable at will when an automatic synchronizing operation is desired.

In order that the operation of the device 22 may be dependent on a current of less pulsating value than III, the embodiment of my invention, shown in Fig.1, may be modified as shown in Fig. 4. In this modification, the connection and arrangement of parts above the points 42, 43, 44 and 45 may be just as shown above the correspondingly numbered points in Fig. 1. In circuit with the loading resistance 37, there are connected in series the condenser 33 and a loading resistance 46 whereby to obtain across the resistance 46 a voltage e which is shown in'the top and 5|.

For this purpose the portion of Fig. 5 and which corresponds to the current Irf, shown in the bottom portion of Fig. 3. Across the resistance 46, or a portion thereof, I connect a rectifier 41 which is accordingly energized in dependence on the voltage e or Ke, K representing a constant. From this rectifier, which is shown full wave, the winding 24 of the device 22 is supplied through a loading resistance 48 in a circuit which includes a condenser 49 and may also include series limiting resistances 50 With this arrangement, the voltage across the resistance 48 corresponds to the voltage er, shown in the middle portion of Fig. 5. The condenser 49 is thuscharged through the resistance 59. Consequently, the winding 24 is energized' by a current 2', shown in the bottom portion-of'Fig. 5. This current is obtained indirectly from the voltage 6 which is proportional to In by rectifying Irf which corresponds to Erf, the first derivative of the rectified slip voltage after filtering. It will. be apparent that the variation in magnitude of this current i for a given slip frequency can be kept as small as desired by suitably proportioning the resistances 48, 50 and 5| and the condenser 49.

When thersumof the effects exerted by Ir and-z through-the windings 23 and 24 falls belowa predetermined value, the condition is then satisfied that the slip frequency does not exceed an amount safe for synchronizing and that the phase angle between. the voltages of the systems is not too large for safe closing. Under these conditions, the control member 25'closes its contacts 38 whereby to effect the closing of the synchronizing switch l1.

Instead of having a single device responsive to both. the slip voltage and the rate of change thereof, my invention may be embodied in separate indicating or synchronizing devices in such a way that the synchronizing operation is dependent on thesimultaneous operation of these devices, which operate to permit circuit interconnection, one when the rate of change of, the rectified slip voltage-is less than a predetermined mean value and another when the rectified slip voltage is less than a predetermined value. In accordance with my invention, the rectifier and filter circuits are so disposed that, atv least, in the circuit which supplies thevoltage proportional to the: rate of change of the rectified slip voltage, the pulsations at power system frequency are eliminated. Thus, as shown in Fig. 6, the device 226, which operates in response to the rate of change of the rectified slip voltage Er, is supplied through the rectifier 34 which is energized bythe slip voltage'E. On the direct current side of this rectifier, the voltage indicated as Er at the top of Fig. '7 is freed of the power system frequency pulsations by the filter network comprising the condensers 35 and the inductances 36 so that the envelope of the rectified slip voltage, indicated as Erf in Fig. '7, appears across the loading resistance 31. This voltage establishes in the condenser-33 a current Irf shown in Fig. 7. This current produces a voltage drop, e, also shown in Fig. 7. across the resistance 46. With the rectifier 4"! connected to be energized by this voltage, or a fixed portion Ke thereof, there results the rectified voltage er which is shown in Fig. 7 and which appears across the loading resistance 48. This voltage er is proportional to the rate of change of the slip voltage. Accordingly, the condenser 49 is charged through the resistance 50. The winding 24!] of the device 220 is, therefore, energized from the condenser 49 by a currenti which, with a condenser of suflicient size, canbe made practically constant for any given rate of change of slip voltage and which will have a magnitude dependent on this rate of change.

The device 220, which may be of ammetric type, accordingly operates to open its contacts 22| whenever the slip frequency decreases to a value at which synchronizing is permissible. For control purposes, the device 220 may be arranged to control an auxiliary relay 52 whose contacts 53 are closed when the relay is deenergized. The relay 52 is, in turn, arranged to cooperate in the control of a circuit breaker closing relay 54 which cannot operate to close its contacts 55 in the circuit 39 of the closing coil while the relay 52 is energized.

In order that the closing operation of the circuit breaker I! may be dependent on the slip voltage, I provide a device 222 which may be of an alternating current voltmetric type and whose winding 230 may be connected directly to the secondary circuit of the potential transformers 20 and 2| for energization in accordance with the slip voltage without rectification thereof. The movement of the circuit controlling member 25! of the device 222 may, accordingly, be dependent on the momentary value of the slip voltage or more correctly the effective value thereof. It thus shows how the effective value of the power system frequency varies, and this variation corresponds with the slip voltage. Through its contacts 22l, the device 222 controls an auxiliary relay 56, which is deenergized when the contacts 22l are open, to close its contacts 51 and thereby to control in cooperation with the device 220, the energization of the closing control relay 54 and the closing of the circuit breaker l1. Instead of having the device 222 of an alternating current type energized in accordance with the slip voltage before rectification, it may be a direct current device with its winding 230 connected across the resistance 31 to be energized in dependence on the rectified and filtered slip voltage Erf, as shown in Fig. 8. Thus, when both the devices 220 and 222 operate simultaneously to open their contacts, under the influence of suitable biasing means 26 the closing control relay 54 is energized because of the deenergization of both of the auxiliary relays 52 and 56 and the consequent closing of their contacts and the switch closing operation is started.

This means that for the first time since the slip frequency s has decreased sufficiently to permit the opening of the contacts 22!, (which had been previously closed under the influence of a higher slip frequency) the difference voltage has dropped, indicating an approach to synchronism at a low enough slip frequency to permit closing the connecting means I1. Moreover for the given slip frequency which determines the operation of the contact controlling member 250 there is a corresponding slip voltage at which the operation of the contact controlling member 25! should occur. This relation establishes an ad-- vance angle at which, with a fixed slip, the switching may be started to insure switch closing when the electromotive forces of the systems are in synchronism.

In order not to be limited to one slip frequency at which the switch closing operation may be effected, the position of the contacts 22! may be adjusted in dependence on any existing slip frequency at which it is permissible to connect the systems. In this way it is possible to have the same time interval between the simultaneous operation of the contacts 221 and 221 and the passage through zero of the rectified slip voltage, which means, of course, for every permissible value of the slip frequency an in-phase condition of the voltages at the moment of switch closing. The movement of the contacts 22l' may be effected by an ammetric device 220' whose movable control member 250 is arranged to actuate the contacts 22 I Since the greater the slip frequency is, the earlier the synchronizing operation must start, the greater the movement of the contact controlling member 250 should be. Accordingly, the device 220 may be of an ammetric type and have its winding 240 energized by the current i so as to respond to the rate of change of the slip voltage like the device 220. In order to control the sensitivity of response of the devices 220 and 220, independently adjustable resistances 58 and 59 may be connected in parallel with the windings 240 and 240' of these devices.

The contacts 22| are thus opened as the slip frequency decreases below the value permissible for synchronizing. The contacts 22 I are opened when the slip voltage E decreases to the value Es which is so proportioned to the particular slip frequency as to match the specific closing time of the connecting switch ll. If the contacts 22 I open before the permissible slip frequency value is reached, then the contacts 22! will not yet have opened. Consequently, the contacts 51 are closed but the contacts 53 are open, and no switch closing operation can occur. If, however, the contacts 221 are open; that is, if the slip frequency is sufilciently small, then the switch closing operation occurs on the first opening of the contacts 22I' at an instant which always insures the same elapsed time before the connection of the systems. This follows by virtue of the relative positioning and movement of the contacts MI in dependence on the instantaneous value of the slip frequency and the contact-controlling member 25! in dependence on the slip voltage whereby at any slip frequency below a predetermined value the opening of the contacts 22 I is effected at the same time displacement relatively to the in-phase position of the electromotive forces of the two circuits although the actual phase displacement may differ.

The conditions for operation. of the device 222 are, however, present not only when the slip voltage approaches its zero value but also for some time thereafter as the slip voltage increases No switching should, of course, take place at this time. This would be possible if the device 220 were to open its contacts 22 I that is to say, when the current i has decreased sufficiently at a time when the voltage E is increasing but is as yet sufficiently small to allow the contacts 22l' to open,

In order to avoid this, I may provide a circuit including a half-wave rectifier 60 which is connected across the resistance 46, as shown in Fig. 8, so that the half-waves of the voltage e, corresponding to the rising portion of the Erf curve, are passed whereby to energize an auxiliary relay 6|. When energized, this relay closes its contacts 62 so that the relay 56 is kept energized even though the contacts 22I should open. Thus the relay 6| is so connected that the voltage e, which is proportional to the rate of change of the rectified and filtered slip voltage Erf, prevents the release of the relay 56 and the possibility of a synchronizing connection only when the value of e is positive. Obviously, the arrangement could be such as to effect the release of the relay 56 when the voltage 6 is negative. The first arrangement is, however, preferable because beginning at the positive half-wave of e, the full operating current is at once available and, consequently, effects the closing of the contacts of the relay 6| with a powerful impulse and without any delay,

The arrangement of the contacts 22l', which eifects closing of these contacts when equality occurs between a quantity proportional to the rate of change of the slip voltage and a quantity proportional to the momentary value of the slip voltage, can also be obtained electrically instead of mechanically by the embodiment of my invention illustrated in Fig. 11. The part of Fig. 11 above the points 42 and 43 will be understood to be similar to the arrangement shown in Fig. 8 above the points 42 and 43. For this electrical balance I provide, in place of the devices 220 and 222, a device such as a differential relay 63 whose windings 64 and 65 are arranged for opposing efiects.

If, in the connection of the winding 65 for energization in dependence on Erf, the effect thereof in the decreasing range of the: slip cycle, that is, from 180 to 360, as shown in Fig. 10, is overcome by the effect of the winding 64 which, as shown in Fig. 11, is energized in dependence on i, then the contacts 66 close prior to synchronism at an instant which is determined by the slip frequency dependent advance angle to insure a time dependent on the switch closing time. The conditions will be apparent from Fig. 10 in which the releasing conditions; that is, the points of intersection of er curves with the Erf curve are represented for difierent angles can, a1, a2, (13. Thus the contacts 66 close as soon as Erf, which is proportional to the slip voltage, falls below the corresponding values of i which is proportional to er. If the comparison of the two measuring quantities is made with the devices 226' and 222, as in the arrangements illustrated in Figs. 6 and 8, then the device 220 must have a high torque in order that the position of the contacts 22 i may not be altered by engaging the controlling member 25L The arrangement may, however, be so arranged that the control member of the device 222 has a high torque and the control member of the device 220' a small torque. In this way the device 222 carries the contacts 22| into the range in which the contacts are to be closed. The interruption occurs as soon as MEri is less than Ncr, M and N being constants. In order to insure a positive contacting action, the natural frequency of the device 226 should be greater than that of the device 222.

As shown in Fig. 11, the winding 64 of the device 63 is connected to be energized in the same way as the winding 24E! of the device 229 shown in Figs. 6 and 8. This, however, brings in the delayed or retarding action involved in the discharging and charging of the condenser 49. This delayed action might result in the deviation of the movement of the device 220' from the desired movement corresponding to the momentary slip frequency. To avoid this, the winding 24!) of the device 221! and similarly, the winding 64 of the device 53 may be connected directly across the loading resistance 48. In this case, a current proportional to Erf flows in the winding 23!] of the device 222 of Fig. 8 and, similarly, in the winding 65 of the device 63 of Fig. 11, and the winding 240' of the device 229 of Fig. 8 or the winding 64 of the device 63 of Fig, 11 is energ-ized by a current proportional to 61. If the slip frequency is zero, then the relay 56 of Figs. 6 and 8 is deenergized to close its contacts or the contacts of the device 63 close at :0 of Fig. since the rate of change of slip voltage equals zero. With finite slip frequency, the advance angle increases with the slip frequency since, as appears from. Fig. 10, a point of intersection of the 2: curve and the Erf curve will occur earlier when the slip frequency is higher. Moreover the device 6| shown in Fig. 8 prevents a synchronizing connection during the rising half of the slip cycle curve.

The winding 64 of the device 63 of Fig. 11 may be connected across the resistance 46 or a part thereof so that during the slip cycle range of Zero to 180, the winding 64 is energized to assist the winding 65 whereby, within this range, no closing of the contacts 66 can occur. Thus even though the slip frequency is below the permissible value and the contacts 22| of the device 220 open to drop out the relay 52, the circuit of the closing relay 54 will be open at the contacts 65. In the slip cycle range 180-360, the contacts 66 close at an angle whose value is proportional to the slip frequency. The winding 240 of the device 220 may be connected for energization as is the device 6| of Fig. 8. When so connected, the device 226 will eiTect the closing of the contacts 22|' in the slip cycle range 0 to 180, that is, the contacts 22l' are caused to move to engage the control member 25L With this connection of the winding 240, the rectifier 60 would cut off the negative half-waves of e from the winding 24!! as the electromotive forces depart from the in-phase position. Then, since the zero, or rest position, of the contacts 22 i does not permit a closing of these contacts unless the control member 25| is also in the zero, or rest position, the device 6| is not necessary.

Depending upon the direction of approach to synchronisrn, that is, as to whether the decreasing slip direction is directed to the right or left in Fig. 10, the winding 64 of the relay 63 or the winding 24B of the device 220' and the rectifier 60 in circuit therewith would have to have their relative polarity reversed by a frequency comparison relay.

The current flowing in the device 220 can be used-as a measure for the required adjustment of the prime mover of the machine to be synchronized. Since the movement of the control member of the device 22!] is limited by the contact 22l, the slip frequency can be determined from a second ammetric device 68, shown in Fig. 8, whose winding 69 is connected in series with the winding of the device 22!] and which has a movable member H3 whose angular movement is not limited. By means of a frequency comparison relay, examples of which are well known to the art, it can readily be determined whether the approach to synchronism is in one direction or the other.

While I have shown and described my invention in considerable detail, I do not desire to be limited to the exact arrangements shown, but seek to cover in the appended claims all those modifications that fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, two alternating current electric systems, means for connecting said systems, and means for controlling the operation of said connecting means including means for deriving a voltage dependent on the slip voltage of the systems, means for obtaining from said slip voltage a practically constant continuous current Whose magnitude is dependent on the slip frequency of the systems, and means connected to be energized responsively to said slip voltage and said current for controlling the operation of said connecting means in dependence on the simultaneous values of said voltage and current.

2. In combination, two alternating current electric systems, means for connecting said systems, and means for controlling the operation of said connecting means including means for deriving a voltage dependent on the slip voltage of the systems, means for obtaining from said slip voltage a practically constant continuous current whose magnitude is dependent on the slip frequency of the systems, and means connected to be energized responsively to the sum of an efiect dependent on the slip voltage and an efiect dependent on said current.

3. In combination, two alternating current electric systems, means for connecting said systems, and means for controlling the operation of said connecting means including means for deriving a voltage dependent on the slip voltage of the system, means for obtaining from said slip voltage a practically constant continuous current whose magnitude is dependent on the slip frequency of the systems, means connected to be energized responsively to said slip voltage and means connected to be energized responsively to said current, said voltage and current energized means jointly controlling the operation of said connecting means.

4. In combination, two alternating current electric systems, means for connecting said systems, and means for controlling the operation of said connecting means including means for deriving a voltage dependent on the slip voltage of the systems, means for obtaining from said slip voltage a practically constant continuous current whose magnitude is dependent on the slip frequency of the systems, means connected to be energized responsively to said current, means connected to be energized responsively to said slip voltage and means responsive to said current for controlling the means responsive to said slip voltage, and means jointly responsive to said current energized means and said slip voltage energized means for controlling the operation of said connecting means.

5. In combination, two alternating current electric systems, means for connecting said systems, and means for controlling the operation of said connecting means including means for deriving a voltage dependent on the slip voltage of the systems, means for obtaining from said slip voltage a practically constant continuous current whose magnitude is dependent on the slip frequency of the systems, and means connected to be energized responsively to said current, means connected to be energized responsively to said slip voltage, means for controlling the advance angle of said slip voltage energized means and means jointly controlled by said current energized means, said slip voltage energized means and said advance angle control means for controlling the operation of said connecting means.

6. In combination, two alternating current electric systems, means for connecting said systems, and means for controlling the operation of said connecting means including means for deriving a voltage dependent on the slip voltage of the systems, means for obtaining from said slip voltage a practically constant continuous current whose magnitude is dependent on the slip frequency of the system, means connected to be energized responsively to said current, means to be connected to be energized responsively to said slip voltage, means for controlling the advance angle of said slip voltage energized means, means jointly controlled by said current energized means, said slip voltage energized means and said advance angle control means for controlling the operation of said connecting means and means for eliminating the control effect of said slip voltage energized means during a predetermined portion of the slip cycle.

'7. In combination, two alternating current electric systems, means for connecting said systems, and means for controlling the operation of said connecting means including means for deriving a voltage dependent on the slip voltage of said systems, means for deriving a voltage dependent on the rate of change of said slip voltage, cooperating means connected to be energized respectively in response to said slip voltage and said rate of change voltage, means connected to be energized in dependence on said rate of change voltage, and means jointly controlled by said cooperating slip voltage and rate of change voltage responsive means and said rate of change voltage dependent energized means for controlling said connecting means.

8. In combination, two alternating current electric systems, means for connecting said systems, and means for controlling the operation of said connecting means including means for deriving a voltage dependent on the slip voltage of the systems, means for deriving a voltage dependent on the rate of change of said slip voltage, means connected to be energized responsively to said slip voltage, means connected to be energized in dependence on only said rate of change voltage, control means connected to be energized in dependence on said rate of change voltage for controlling said slip voltage energized means, and means jointly responsive to said rate of change voltage dependent energized means, said slip voltage responsive means and said last mentioned control means.

9. In combination, two alternating current electric systems, means for connecting said systems, and means for controlling the operation of said connecting means including means for deriving a voltage dependent on the slip voltage of said systerns and means for deriving a voltage dependent on the rate of change of said slip voltage, means connected to be energized responsively to said slip voltage, means connected to be energized in dependence on said rate of change voltage, control means connected to be energized in dependence on said rate of change voltage for controlling said slip voltage energized means, means jointly responsive to said rate of change voltage dependent energized means, said slip voltage responsive means and said last mentioned control means, and means for eliminating the control effect of said slip voltage energized means during a predetermined portion of the slip cycle.

10. In combination, two alternating current electric systems and synchronizing means therefor, comprising means for deriving a voltage dependent on the slip voltage of the systems and means for obtaining from said slip voltage a practically constant continuous current whose magnitude is dependent on the slip frequency of the systems.

filtered voltage acii rrent dependent on the rate of-ch'ange of said slip voltage, and a device haw ing two cooperating windings respectively connected to beenergized responsively to said current and to a currentproportional to said slip voltage for controlling the operation of said connecting means in dependence on the simultaneous values of said slip and -rate-o'f-change voltages.

VIKTOR AIGNER.

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