US2797380A - Electromagnetic switch - Google Patents

Description

E. J. DIEBOLD ELECTROMAGNETIC SWITCH June 25, 1957 12 Sne'et-sheei. 1

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ELECTROMAGNETIC SWITCH Filed Nov. 2s, 1951 12 sheets-smeet 12 n marsan Patented June 25, 'i957 2,797,380 ELECTROMAGNETIC SWITCH Edward John Diebold, Ardmore, Pa., assigner to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Application November 23, l1951, SerialNo. 257,901

27 Claims. (Cl. 321-43) My present invention relates to electromagnetic switches and more particularly to an electromagnetic switch have ing an auxiliary winding in combination with ak commutating reactor.

Heretofore in the prior art various devices were used to provide a uni-directional current from an alternating current. Each of these devices inherently have disadvantages which the present invention proposes to alleviate.

Dry -cell rectifiers or selenium rectifiers have the disadvantage of ohmic losses in the forward direction and relatively low reverse resistance so that there are considerable losses as current flows in the reverse direction. Gas discharge tube rectiiiers and mercury arc rectifiers present a substantial arc voltage drop. Moreover, the gas discharge tube and mercury arc rectifiers require an interval of time to heat up and are additionally sensitive to changes in temperature. A motor generator set used as a rectifier has the disadvantage of large initial cost, large bulk and has losses `due to the transformation from electrical energy to mechanical energy and back again to electrical energy.

Mechanical rectiiiers occupy a small volume and havek high efiiciency but they require a synchronous motor drive which makes them subject to backfire whenever the load, the voltage `or the frequency exceed a relatively narrow limit.

The present invention overcomes the limitations of the prior art by providing an electromagnetic switch khaving a magnetization coil and a commutating reactor in series with the contacts. The commutating reactor essentially restrains the current through the switch to a relatively low value when the switch opens or closes.

It is then an important object of the present invention to provide an electromagnetic switch or valve which has a resistance when conducting in the order of milliohms or less.

The utilization of electromagnetic switches having in the associated circuitry saturable transformers and reactors materially increases the cost of such a switch.

If the transformer is to be easily saturable and have an air gap, almost unsurmountable manufacturing difiiculties arise, as the desirable characteristics are lost when an air gap is cut into such a core or when the core is made in two valves and assembled.

Moreover, electromagnetic switches as utilized in the prior art require heavy expensive windings.

Another important `object of the present invention is then the provision of a novel electromagnetic switch having a minimum of associated coil and winding.

Another important object of the present invention is the provision of a novel electromagnetic switch having a low recovery volt across its opening contacts.

Another important object of the present invention is the provision of a novel electromagnetic switch having relatively few heavy windings.

Another important object of the present invention is the provision of a self-operating rectifier that is instantaneously ready for operation, independent of the ambient temperature and having a high efiiciency and small volunie.

Another important object of the present invention is the provision of a novel electromagnetic valve where the inrush and break currents through the valve contacts are always maintained at a low value independent of the voltage load, number of phases or frequencies.

Another important object of the present invention is the provision of a novel rectifier having a control by meansv of a current which is much smaller than the output of the rectifier.

Another object of the present invention is the provision of a novel saturable transformer without an air gap having the characteristics of a transformer having an air gap. Y Y

Another important object of the present invention is the provision of a rectifier where the transition from one mode of commutation to another occurs without disturbance.

Further objects and advantages will become apparent oni consideration of the following description in connection with the drawings in which Figure 1 is a schematic diagram of a modification of the novel electromagnetic switch of the present invention.

Figure 2 is an enlarged top view of a portion of the novel electromagnetic switch of the present invention.

Figure 3 is an enlarged front View of a portion of the novel electromagnetic switch of the present invention.

Figure 4 is a series of voltage and current v. time curves illustrating the operation of the novel electro* magnetic switch.

Figure 5 is a flux current curve of the commutating reactor of the present invention.

Figure 6 is the flux current curve of the saturable transformer of the present invention.

Figure 7 is a schematic diagram of a modification of the novel electromagnetic switch of the present invention.

Figure 8 is a series of voltage and current versus time curves illustrating the operation of the circuit of Figure 7.

Figure`9 is a flux current curve of the saturable transformer in Figure 7.

Figure 10 is a flux current curve of the commutating reactor 12S in Figure 7.

Figure 11 is a series of circuit diagrams of a portion of the present invention.

Figure 12 is a series of voltage and current versus time curves of the circuits of Figure 11.

`Figure 13 is a series of circuit diagrams of a portion of the present invention.

Figure 14 is a circuit diagram of a modification of the present invention.

Figure 15 is a series of voltage and -current versus time curves for the circuit as shown in Figure 14.

Figure 16 is a circuit diagram of another modification of the present invention.

Figure 17 is a single phase rectifier utilizing the electromagnetic switch of the present invention.

Figure 18 is a three phase rectifier utilizing the electromagnetic switches of the present invention.

Figure 19 is a modification of the electromagnetic switch of the present invention.

Referring now to Figure l the novel electromagnetic valve circuitry `of the present invention is connected between the points 21 and 22. The source of alternating voltage 23 provides the power to the load 24 which isV connected in series with the electromagnetic valve circuitry' connected between 21 and 22 and the source 23. The' load 24 may be inductive, capacitive or resistive,y or any combination thereof.

The electromagnetic switch or valve 25 has two iron cores 26 andk 27 whichl are connected electrically, as

hereinafter described, to the two terminals of the switch 25. The magnetic poles 26 and 27 are connected by a small bridge contact 28 which makes an electrical and a magnetic connection therebetween. In order to achieve a good electric and magnetic connection the movable bridge or armature 28 is made of a magnetically soft material, as iron, and coated with a layer of high conducting material, as silver or gold. The stationary poles 26 and 27, as shown in Figures 2 and 3 are surrounded by copper conductor 29 and 30 which serve the double purpose of carrying the current and conducting the heat generated in the small armature 28 away from its source. For high current carrying capacity the conductors 29 and 36 may be equipped with cooling tins, now shown.

Referring again to Figure 1, the electromagnetic switch 25 has a main winding 31 connected in series with the switch poles 26 and 27 and the armature 28. The winding 31 essentially magnetizes the magnetic poles 26 and 27 when a current flows therethrough. rihe current owing through the coil 31 causes the armature 28 to be attracted and held tightly against the magnetic poles 26 and 27.

As shown in Figures 2 :and 3, the current from the copper conductor 29 is carried to the armature 28 across a small steel silver plated plate 32 which lies across the pole face of the magnetic pole 26. An identical plate 33 carries the current to the conductor 30 across the pole face of the magnet 21. The plates 32 and 33 are each held in position by two screws 38. The armature 28 is suspended by spring 34 held between a clamped plate 35 and two insulators 36. The spring 34 resilient- 1y resn'ains the armature 28 against the bumper 37 which is essentially a stack of thin laminations. The laminations absorb the shock of the armature 28 preventing rebound.

Referring again to Figure l, the electromagnetic switch 25 carries a premagnetization winding, polarization winding, or auxiliary winding 40 on the pole 26. The winding 40 carries a direct current supplied by a battery 41 which also provides through a coil 42 the preexcitation for the core 43 of the commutating reactor 44. The commutating reactor has its main coil 45 connected in series with the switch 25 and also with a winding 46 of the transformer 47. Connected then between the points 1 and 2 in series is the winding 46, the winding 45, the pole 26, the armature 28, the pole 27 and the coil 31, all described above. The transformer 47 consists of a core 50 having an air gap 51 upon which are wound the three coils 46, 52 and 53. The coil 53 is connected through an impedance 58 across the alternating voltage source 23. The coil 52 is connected on one side to the junction between the coils 46 and 45 and on the other side through a selenium rectifier 55.

The rise of current through the switch 25 is initiated through the selenium rectifier S and kept away from the armature 28 for the time interval necessary to close the switch plus an additional interval necessary to increase the current through the main winding 31 to a relatively high value suiiicient to assure a good contact, as is hereinafter described in reference to the curves shown in Figure 4.

The curves drawn in Figure 4 have the same time axis and are hereinafter described before proceeding with the analysis of what occurs during the make or break intervals of the cycle during the times tt) to t8.

Figure 4:: shows the distribution of the Voltage on the electromagnetic switch 25, indicated as c25, and on the load 24, indicated as e24. The curve of e24 shows that the voltage across the load is mainly positive and the curve of e25 shows that this voltage is mainly negative. Except for a short interval between the times t1 and t2 at the beginning of the positive half cycle Iand between the times t5 and t8 at the beginning of the negative half cycle, the two voltages e24 and e25 in the positive and negative portion thereof are respectively equal to the generator voltage e23, which is shown as a thin solid line in Figure 4b.

Figure 4d shows the current :'24 through the load 24, through the alternating voltage source 23 and through the coil 31. The current :'24 is shown lagging the voltage e24 as the load 24 is considered for the present discussion to be resistive and inductive.

Figure 4b in addition to showing the voltage e23 also shows the voltage across the winding 46 of the saturable transformer 47 as a heavy solid line.

Figure 4c shows the voltage across the winding 45 of the commutating reactor 44, and

Figure 4e shows the current :'55 through the selenium rectifier 55.

Figure 4f shows the current :'28 through the armature 28, and through the winding 45 .as is hereinafter described.

Figure 4g shows the pre-excitation current :'53 owing through the winding 53 of the saturable transformer 47.

The current :'53 is limited by the reactor 58 and, therefore, lags the voltage e23 by 90 degrees. The winding 53 is wound in opposition to the winding 46 of the saturable transformer 47 and, therefore, the current :'53 appears to be reversed, i. e., leading the voltage e23 instead of lagging it. The sole purpose of the current :'53 is to saturate the core 50 of the saturable transformer 47 at the beginning of the positive half cycle of the generator voltage @23 and to permit an early saturation of the core 50 at the end of the positive half cycle of the generator voltage e23 as is hereinafter described.

The times t0 to t8 indicated in Figures 5 and 6 are the same instants of time as those of Figure 4. A preexcitation current :'42 in Figure 5 is applied by the battery 41 in Figure l to displace the curve into the iield of positive currents. 42 is a constant current which is very small compared to the main current i25 described above.

The curve as shown in Figure 5 is the dynamic iiux current curve of the commutating reactor 44. Figure 6 shows a similar dynamic flux current curve of the saturable transformer 47. Again the times t0 to t8 are the same times t0 to t8 as in Figures 4 and 5. The preexcitation current :'53 is an alternate current and varies between the limits of 53 and :'53 In the following explanation of the operation of the electromagnetic valve it is assumed that 1'53 is equal to :'53 during the time interval t0 to t3 and equal to :'53 during the time interval t4 to t8. The zero line of :'24 is therefore at two different places according to the time as shown in Figure 6. The appreciable slope of the magnetization curve of the saturabie transformer 47 is due to the gap 51 in the core 50.

Consider the time t9 at which the source voltage e23 becomes positive. The current :'24 through the main line is 0, at this time, and the armature 28 'is pulled away from the stationary contacts by the spring 34. The core 50 of the saturable transformer 47 issaturated by the current :'53 and a free passage for the current exists then only through the selenium rectifier 55 which commences to carry the current :'55 in phase with the generator voltage e23 as shown in Figure 4e. The current :'55 is equal to the current :'24 during the time interval t0 to t2 since all of the current that iiows through the load 24 iiows through the series connection 55. When the current :'24 owing through the winding 31 reaches a value high enough to produce a magnetic ilux suflicient to attract the armature 28 to the fixed poles 26 and 27, the switch 25 closes. The armature 28 being very small and the attractive force being relatively high, the movement requires a time about 'lmgoth of a second which is negligible for vpulse control current or other outside action. therefore, also work under extraordinary conditions and for :'24 is shifted to the negative side.

through the switch 25. The current :'25, however, is limited by the commutating reactor 44 which has the winding 45 in series with the contacts of the armature 28.

Figure shows this action having the iiux of the core 43 plotted against the current :'25. After the time t1, the current :'25 cannot riseto a value higher than :'M unless the flux is fully reversed from practically its negative minimum to its positive maximum. The current limit :'M can be made extremely small and hence protects the armature 28 from an excess of inrush current.

The reversal of the ilux is due to the generator voltage e23-appearing substantially across the winding 45. The voltage c45 is shown as described above in Figure 4c. The time interval r1 to t2 is the time required to reverse the ilux, after which the voltage c25 disappears and the current :'25 is free to rise. Figure 4f shows how the current :'25 is kept at the low value :'M during the interval t1 t0 t2 and rises suddenly at the time t2.

The area of the voltage e45 in the time interval t1 to t2 in Figure 4c is called the make-step. The make-step is a constant and is determined by the size and material of the core 43. Comparing Figures 4c and 4b, it is evident that the source voltage e225 is shifted from the switch voltage c25 to the load voltage e24 with an intermediate stage at the commutating reactor voltage c45.

The `rise of current, then, through the electromagnetic valve connected between points 21 and 22 is initiated by the selenium rectifier 55 and kept away from the armature 28 for a time interval t0 to t1 required to close a switch and for another time interval Il to t2 required to increase the current through main winding 31 to a value suciently high to assure goed contact pressure. This action takes yplace in a fraction of a millisecond and yet assures that the contact never closes incompletely or ever rebounds while a high inrush current flows. The current ilowing through the selenium rectifier 55 closes the electromagnetic switch 25 without any current flowing through the switch 25 itself during the closing time. The operation is completely self-controlled and does not require any im- It will,

cease to operate instantaneously whenever the conditions of operation cease to exist.

After the time t2 the current :'25 through the armature 28 of the electromagnetic switch 25 rises to a value deter- -mined by the source voltage 23 and the load impedance 24 only, as shown in Figures 4f and 4d. The current :'55

through the dry cell rectier 55 decreases rapidly to zero since the rectifier 55 is short circuited by the armature 28.

This condition is shown in Figure 4e where the current :'55 reaches zero at the time t3.

As long as the current through the switch 25 and therefore through the winding 31 is maintained at an appreciable value, the switch 25 remains :closed by the magnetic force due to the magnetic ux induced by the current :'24 itself. Thus, there is no danger of the switch ever opening when current is ilowing therethrough.

Figure 6 shows that the flux in the time interval t0 to t3 remains almost unchanged and so the saturable transformer 47 is inoperative during the whole closing cycle and has thus been disregarded in the discussion thereof.

The opening cycle of the electromagnetic switch 25 occurs in the interval between t4 and t8 when the preexcitation of the saturable transformer 47 is reversed, as shown in Figures 6 and 4g. In Figure 6 the preexcitation is assumed to be :'53 and hence the zero line As :'24 decreases towards zero, it passes through a point at which the core 50 of the saturable transformer 47 unsaturates. The current :'24 at this unsaturating value is shown as 24 (5) in Figures 4d and 6. This value of current :'24 (5) occurs at the instant of time t5. After the time t5 the core 50 of rthe saturable transformer 47 is essentially equivalent to the core of a conventional transformer. In a conventional transformer the sum of ampere turns of all the VHwindings determines its magnetization and the electromotiveforces in its windings are directionally proportional to thenumber of turns. As the corey 50 of the saturable transformer 47 becomes unsaturated, and its reactance rises to a high value, the voltage e46 across the winding 46 as shown in Figure 4b suddenly rises to the value of the source voltage e23. The current 24 through the winding 46 decreases at a lower rate after the time t5. The voltage e46 on the Winding 46 is transformed to the winding 52, and hence a current :'55, as shown in Figure 4e, commences to flow. As in any normal transformer, a decrease of the primary current corresponds to an increase in the secondary current and vice versa. Therefore, as the `.current :'24 decreases, the current :'55 increases. Both the line current :'24 and the bypass current :'55 flow through the armature 28 and the commutating reactor coil 45. The by-pass current :'55 Hows in the opposite direction tothe current :'24 and hence the resulting current through the armature 28 and the coil 45 is the difference of the two. The current 28 is shown in Figure 4f as decreasing at a much higher rate than 24 during the time interval t5 to t6.

At the time t6 the armature current :'28 has reached la very small value :'b, which is practically Zero and the Alay-pass current 55 is now equal to the main current :'24. In other words, the saturable transformer 47 displaces the line current 24 from the switch 25 to the dry cell rectifier 55 as soon as it becomes unsaturated. At the time t6 the core 43 as shown in Figure 5 of the commutating reactor 44 unsaturates and the current :'55 is henceforth xed at the small value :'B. After the time t6 the line current i24 flows through the dry cell rectifier 55 and the generator voltage e23 appears partly on the winding 46 of the saturable transformer 47 as shown in Figure 4b and partly on the winding 45 of the commutating reactor 44 as shown in Figure 4c. The line current :'24 decreases slowly according to the slope of the left-hand curve in Figure 6 shown also as the lines in Figures 4d and 4e.

At a time t7 the current :'24 through remaining winding 31 of the electromagnetic switch 25 becomes too low to hold the armature 28 closed against the poles 26 and 27. The spring 34 pulls the armature 28 back against the bumper 37. This opening process is similiar to the closing process in that it is a very rapid oper ation taking approximately j,/10,000th tof a second. The armature 28 will interrupt the current :'28 which is at that time t7 equal to :'B, a current so small as to be practically negligible. Line current :'24 which is now equal to the by-pass current :'55 will decrease until it reaches Zero at the time t8. The current :'24 cannot reverse due to the non-symmetric characteristics of the dry cell rectifier 55 and, therefore, remains at the zero value.

The decrease then of the line current :'24 demagnetizes the saturable transformer 47 which displaces the current :'24 from the electromagnetic switch 25 to the dry cell rectier 55. The current in the switch 25 is maintained at a very low value while it opens due to the suppression by the unsaturation of the commutating reactor. The remaining current :'24 flows through the dry cell rectifier 55 until it creases to flow naturally as it would in a normal dry cell rectifier. The opening of the switch 25 is completely self-controlled without any additional pulses or signals necessary.

The shape of the step due to the commutating reactor 44 follows essentially the shape of the hysteresis loop as shown in vFigure 5 and may be corrected by straightener circuits as disclosed in application Serial No. 212,017 filed February 2l, 1951.

The armature 28 of the electromagnetic switch 25 is constructed so as to have a natural frequency in the order of one kilocycle to be definitely mismatched with the frequency of the alternating voltage source 23. This mismatch is provided so that if the frequency of the 'source 23 changes suddenly say-from 60 cycles tto-65 cycles, the armature 28 will commutate at 65 cycles and not tend to remain at its natural frequency. Another property of such a switch is the possibility of sudden start (zero to 60 cycles) or sudden stop without affecting the commutation.

The circuit of Figure 1 is only a modification of the present invention and may be improved upon in a variety of ways as shown for example in Figure 7. Figure 7 shows a single phase half-wave rectifier similar to that of Figures l and 2 with the exception that it provides for regulation of the direct voltage output as is hereinafter described. 1

Referring now to Figure 7, the alternating voltage source 123 is connected to the load 124 through the points 121 and 121A across the source 123 to the points 122 and 122a connected across the load 124. Connected in series between points 121 and 122 is the saturable transformer 147, the commutating reactor 144 and the electromagnetic switch 125. The saturable transformer contains two windings 146 and 152 which are wound on the core 150 having an air gap 151. 'Ihe coil 146 is connected between point 121 and the coil 145 of the commutating reactor 144.

The commutating reactor 144 has a pre-excitation coil 142 which is biased by the battery 141. Both coils 142 and 145 are wound on the core 143. The other end of the coil 145 is connected to the conductor 160 of the electromagnetic switch 125. The other conductor 162 of the electromagnetic switch 125 is connected to the main coil 131 and thence to the point 122. The electromagnetic switch 125 has two poles 126 and 127 and an armature 128 mounted on a spring 134 and backed by laminations 137. The electromagnetic switch 125 also has a permanent magnet 163. The permanent mag net 163 is insulated from the remainder of the switch 125 by the insulator 164 and 165.

The contacts of the armature 128 are paralleled by a rectifier 166 in series with a parallel circuit containing a resistor 170 and a capacitor 171. The essential differences between the circuit of Figure 7 and that of Figure 1 resides in the absence of the pre-excitation winding S3 of Figure l and the stabilizing reactor 58 of Figure 1 from Figure 7. The iiux current curve of the core 150 of the saturable transformer follows the curve must have approximately twice the crosssectional area of the core 50. The coils 53 and 58 in Figure l, however, can be omitted.

Figure 8 shows the various vol-tage and current time curves of the circuit as shown in Figure 7. Figure 8h n shows the voltage across the winding 146. As compared to Figure 4b, an additional voltage time area appears between the time l1 and t2. This additional area causes an additional delay in the load voltage @24 so that its average voltage is lowered by 8 to 10%.

The by-pass or arc suppressor circuit containing the elements 179, 171 and 165 affords a passage for the residual current iB shown in Figure 8g which flows through the armature 128 of the electromagnetic switch 125 at -the opening time t7.

rl`his small current ."B will charge the capacitor formed by the opening halves of the contact as this capacitor has an exceedingly small capacity. The voltage across this capacitor will rise to a very high value even with a very small current iB. The capacitor 171 connected across the fixed contacts 16% and 152 of the switch 125 has a much higher capacity and, therefore, a much lower recovery voltage for the same current iB. After the time t8, when the voltage e128 across the open contact rises suddenly to a high negative value, the selenium rectifier 166 prevents a reversal of current. When the voltage e128 rises again in the positive direction at the time l0, the capacitor 171 is charged to this voltage. After the closing or make of the contact 128, at time t1, the capacitor 171 cannot discharge itself immediately as the selenium rectifier 166 is in opposition to the discharge current. In the time interval t1 to t7, the capacitor discharges itself through the resistor 170.

The arc suppressor circuit acts as a capacitor with high capacity connected in parallel to the opening armature 128 and thus absorbs the residual current. On the other hand, it does not discharge itself through the contacts when it is closed and thus prevents contact damage due to an inrush current. When the direct current voltageV is reduced by delaying the makerpoint, the arc suppressor circuit does notvcarry a forward current or damage the contact with a capacitor discharge.

The direct voltage regulation is providedlby the circuitry associated with a saturable reactor having a core 131 and two windings 182 anrd 183. Y A

The regulator coil 182 aords a delay for the make point of the contact 128 and thus a means to reduce the average direct current voltage output of the rectifier. The voltage e182 appearing across the coil 182 is required to magnetize the core 1.81. During this magnetization period the-current i155 through the coil 182 and the selenium rectifier 155 is equal to the magnetizing current iM181 of the core 181 as shown in Figure 8f which is too small to operate the electromagnetic switch 25 through the coil Y131. Only whenthe core 181 is fully saturated, the' current i155 can rise freely and close the electromagnetic switch 125. The voltage e182, shown in Figure 8c, reduces the load voltage e124 shown in Figure 8e by delaying the beginning of the conduction period from the time ttt to the time'tl.

'The shaded area of Figure 8c is equalV to the flux increase ofthe core 181 also shown as A@ 181 in Figure 10. The size of Afl: 181 determines the amount of voltage lost to the direct current load by the make delay. The iiux I 181 does not reverse itself after the time t8 unless a current i183 in Figure l() is applied tothe coil 183. When no current flows through the coil 183, the linx change A@ 181 is very small and the make point is not delayed resulting in the output voltage of the rectifier being high. If a current i183 fiows during the interval t8 through the winding 183, in the direction given by the rectifier 184, connected from the coil 183 to a potentiometer 185, then the flux 181 is reversed to an extent depending' on the amount of current. If 11 181 is reversed completely, the delay of the make point t01 from the highest possible point t1 is the longest possible and the rectifier works at the lowest possible direct current voltage. The flux reversal by means of coil 183 might be effected by several means, the example in Figure 7 shows a very simple one, a voltage proportional to the generator voltage e128 is taken from the voltage divider and pushes a current through the coil 183 according to the position of potentiometer185.

The current flowing through the winding 182 of the voltageregulator is the small current i155.and not the large current i129V flowing through the load; the regulator nevertheless controls the voltage and current through the load.y Moreover, thecontrol current i183 of the Vregulator is `again much smaller than the current i155,

thus the ratio of control power to controlled power can be made to be one to thousand or better. The response .time of the regulation, as described above, is approximately one-half cycle.

Another possibility for regulation is to apply the output current of a small magnetic amplifier, not shown, to the control windings 183. This modification would Ybe important for high power rectifiers with multi-anode circuits, as it is possible by this 'means to vary the output voltage of the rectifier, and therefore its power, within the full range from zero to rated value, by means atraves .of acontrol power which is exceedingly small and with an almost instantaneous response.

lInsteadof using a selenium rectifier 155 and a regulator coil 183, it is possible to use an electronic valve (not shown) where the voltage of the rectifier can be controlled by means of the grid control of the valve.

The regulator permits the output voltage of the recti- `fier to vary within a very wide range by means of a regulator coil 182 which acts like a magnetic amplifier with the added advantage of being exceedingly small and the speedof response attained is very high.

The premagnetization of the permanent magnet 163 in Figure 7 replaces the winding 4l) of Figure 1. The permanent magnet 163 is insulated from the pole pieces 126 and 127 by means of thin sheets of insulation 164 and 165. An air gap 190 with very large area or low reluctance by-passes the permanent magnet 163. Thus a high short-circuit current in winding 131 merely increases the flux through the air gap 1% and does not demagnetize the permanent magnet 163.

The electromagnetic switch 125 can work without premagnetization, but then it requires a much higher current to operate.

In the foregoing description, a fundamental circuit was shown in Figure l and an equivalent circuit of slightly different design including voltage regulation was shown in Figure 7. A great many other variations of the fundamental circuit and or" the basic circuit elements are possible.

The saturable transformer 47 in Figure l and 147 in Figure 7 consists of an easily saturable magnetic core 50 or 150 with an air gap 51 or 151. The properties of the magnetic cores 51 and 151 are shown in Figure 6 with pre-excitation and Figure 9 without pre-excitation.

Figure 1l shows another design of a saturable transformer which oliers the same properties when connected in a circuit as in Figure l or 7. Figure lla shows the saturable transformer tcore 250 with 'an .air gap 251. Wound around the core 25d is a winding 252, which is connected in series with an A.-C. generator 223 and an air inductor 258. The voltages on these three elements, i. e. the voltages e223, e258, e252 measured across the elements 223, 25S and 252, respectively, are shown lin Figure 12a, 12b, and 12C.

The generator voltage e223 in Figure 12a is a pure sine wave. The air inductor voltage e258 in Figure 12b is equal to the generator voltage e223 as long as the core 250 is saturated during the intervals t2-t3, and tft-tl. In the intervals tilt2 and te-td, the core 250 is not saturated. Therefore, the coil 252 has a relatively high inductance as determined by the air gap 251. The generator voltage e223 therefore divides itself between the voltages e253 and e252 proportional to the inductances of coils 258 and 252. The inductance of coil 252 being higher than the inductance of coil 258, the voltage e252 is higher than the voltage e258. The sharp cut off at the times t1, t2, t3, t4 is due to the sudden saturation of the core 250. Whenever the coil 25@ is saturated, the coil 252 has a negligible inductance and the voltage e253 equals the voltage e258. The current flowing through this circuit reflects the sudden changes of inductance as during the intervals t1--Z2 and t3-t4 when the inductance is high, the rate of change of the current is low, whereas during the times tZ-t3 and tft-t1 when the inductance is low, the rate ofk change of the current is high. The current ic is shown in Figure 12d.

Figure 11b shows a way of obtaining the same voltage and current curves with a saturable transformer core having no air gap. Highly saturable magnetic cores are usually made without air gap because the change in design required to make a core with air gap is highly detrimental to the magnetic properties, above all the ability to saturate fully at a low current. Figure 1lb is almost identical to Figure 11a except that the air gap core is replaced by the core 260 without an air gap but with a ksecondary vwinding 261 v.which is short-circuited by an inductor 263. Forthe time during which the core-260 is saturated, the circuits 11a and 11b are identical and therefore also the voltages and currents. When the core 260 is unsaturated, it exhibits the properties of a transformer with a very high mutual inductance and a very low leakage inductance. The inductance appearing on the primary is therefore theinductance 263-if the number of turns of coils 261 and 262 are equal to the primary coil. In such a case the circuits 11a and 11b are equivalent, the only difference will be a small internal current iA llowing through the coils 261'and 263 which is shown in Figure 12e. Figure llc shows a simplilied way of representing the equivalence of the two circuits shown in Figures 11a and 11b.

Figure 13a shows a part of Figure l, comprising the saturable transformer 47 and the commutating reactor 44. Figure 13b shows schematically the same circuit except that the core 50 of the saturable transformer has no air gap and is replaced by a reactor 270 as described above.

From Figure l. and Figure 7 it is evident that the coils 46 and 45 have to carry the full load current and must therefore be made of heavy wire. It is possible to combine these two coils into one, as shown in Figure 13e. The new coil 271 is wound around two cores 272 and 273 which are equivalent to the cores 50 and 43 of Figure 13b. The secondary coils 274 and 275, similar to the coils 52 and 53 described above, are wound only around the core 272 and the coil 276, similar to 42 above, only around the core 273. As a potential point between the coils 46 and 45 is not available with this design, an additional primary coil 277 must be added in series with the coil 274. The voltage across the coil 277 is equal to the voltage across the winding 46 described above as it is wound only around the core 272, giving the correct potential to the start of winding 274. Practically, the arrangement shown in Figure 13C is perfectly equivalent to Figure 13a except that it is much cheaper to manufacture.

Single phase full wave rectiers with true valves work either without overlap on capacitive or resistive load or with overlap on inductive load. The overlap is the time during which both valves are carrying current in the same direction with the current in the lirst valve decreasing and the second valve increasing. For resistive or capacitive loads the currents and voltages will be the same as for two single phase half wave rectiiiers working in succession and for inductive loads the make of one valve occurs before the break of the preceding valve. Except for the faster make and break and a shortening of the make and break steps, the operation for inductive load remains unchanged. Practically, the rectier is designed for this latter case which is true commutation and will then be amply sutlicient for the first case. The transition from one mode of commutation to another occurs without disturbance and the electromagnetic valve thus exhibits thc properties of a true valve.

A three phase rectifier circuit using three electromagnetic valves is shown in Figure 14. The operation of this rectifier is essentially the same as the single phase rectiliers described above in reference to Figures l and 7. The A. C. power supply (not shown) is connected to the A. C. terminals 1A, 1B, 1C. The terminals 1A, 1B and 1C are connected to a three phase transformer 300 which is connected in delta in the primary and in Wye in the secondary. The neutral 301 of the transformer which is also the negative terminal of the rectifier is connected to the load 424. The secondary phases A, B and C of transformer 300 have voltages ea, eb, ec as their respective voltages against the neutral potential at 301.

In Figure 15a these voltages are plotted against time as dotted lines and are rectified by the rectifier as is hereinafter described to yield as composite D. C. voltage 11 n shown as a heavy continuous line, also shown in Figure 15a. As the three voltages ea, eb, ec are equal in magnitude and occur during equal time intervals, the rectication takes place in three equal cycles during one cycle of the sine wave of the A. C. line voltage. The D. C. voltage then has a ripple of three times higher frequency than the A. C. frequency of the source.

For easier interpretation of the later gures, a simplied representation of the electromagnetic switch 125, shown in Figure 14, is used. The whole switch is represented in the dotted boxes 325 wherein is maintained only the armature 328, the exciter coil 331 and the iron core 326-326.

The phases A, B, C are connected through the commutating reactor windings 346 and electromagnetic switch 325 to the positive terminal 302.

The coils 346 are wound as described above on the transformer cores 350 and commutating reactor cores 343. The cores 350 also carry the secondary coils 352 which are paralleled by the coils 370. The coils 370 and 352 are connected to the main current carrying positive line 302 shown in heavy lines and to the control commutating reactors 400. The commutating reactors 400 have the coil 382 and 383 wound on the cores 381. The coils 382 connect the coils 352 to the electromagnetic switches 325 through the rectifiers 355. The cores 343 are premagnetized by the windings 342 which are connected in series between main current carrying lines 301 and 302. The cores 350 are pre-magnetized by the windings 353 each connected through an inductor 358 to the negative main line 301 and to a different phase load A, B or C, than the phase the core 350 is in. The choke or inductor 353 is connected to the preceding phase, having the preexcitation voltage leading by 120 degrees to obtain a favorable phase relationship for the preexcitation current.

Whenever the voltage of a phase exceeds the voltage on the direct current load by a certain positive amount, the electromagnetic switch of this phase will close. Whenever the current through an electromagnetic switch 325 decreases towards zero, the switch will open, operating exactly the same way as a half wave valve described above in reference to Figures 1 and 7.

The similarity of the operation is seen by comparing Figures 8 and l5; taking into consideration that Figure 8 concerns a half wave rectier operating on a mostly resistive load whereas Figure l concerns a three phase rectier operating on a load 424 with exceedingly high inductance shown as 425 in Figure 14. A high inductance such as 425 inserted into the direct current circuit i will maintain the direct current almost perfectly constant. The sum of the phase currents ia, b, ic, is then constant where each of these currents consists of a series of pulses with a fiat top equal to the direct current. Due to the three phase rectification, the direct voltage is more continuous, as shown in Figure a as a solid line than the voltage e124 in Figure 8. During the changeover from one phase to another, the direct voltage assumes the average voltage of both phases as during the times t3-t4 and rS-tt in Figure 15 which is a common property of all rectiers.

In Figure 15e, the pre-excitation current i353a of the saturable transformer of phase A is shown as a dotted line. Comparing Figures 15C with 15a shows that i353a is leading in phase on the voltage ea of phase A. Figure 15e also shows the voltage e346a, which is the voltage across the common winding 346 of the ,phase A. 'In the time interval tf1-t4, the pre-excitation current z'353a is positive and keeps the transformer core 350 saturated in the forward direction, therefore, the transformer core 350 must be fully demagnetized and reversed before the line current can decrease to zero. A longer break step is then caused during the intervals t6-z9, due to the cores 350 and 353. These steps are represented by the voltage time areas enclosed by the voltage 346:1 in Figure V15e, which is the YcommutatingV reactor voltage of the Aphase A.

output of the rectier itself.

When the voltage ea of phase A reaches a positive value high enough to overcome the direct voltage between 301 and 302 at time t0 in Figure 15, a current i355a starts owing through the windings 352, 370, 382, the selenium rectier 355 and the main winding 321 of the electromagnetic switch 325. Due to the easily saturable core 381, the current is limited to a very small value approximately a few milliamperes. The entire voltage difference between the point A and the point 320 appears on the winding 382 as the voltage e382a shown in Figure 15D. This voltage also appears across the open contact 328 of the armature shown as e328a in Figure 15B. When the core 381 of the regulator coil 382 finally saturates, the voltage e382 disappears and the current i355a rises freely as is shown in Figure 15G at the time t1. The current i355a also ows through the windings 321 of the electromagnetic switch 325 and eects the closing of the switch 325 at the time t2 when it reaches the critical value to close the switch. A current i328a starts owing through the winding 346 and the armature 328 demagnetizing the commutating core 343. During the demagnetizing period of the core 343 or the make step which is the time t2-t3, the voltage difference between the points A and 302 appears on the winding 346 shown as e346a in Figure 15C. The current 328a is maintained at the value of the magnetizing current of the core 343 which is less than one ampere as is shown in Figure ISF. During this time the current i135a still rises because the voltage difference between A and 302 still exits. At the time t3, the core 343 is saturated and the current i355a rises rapidly to the `value of the full direct current as shown in Figure 15F whereas i355a decreases toward zero as shown in Figure 15G. The sum of i328a and i355a is the line current ia shown in Figure 15E.

The rise of current in phase A is delayed by the time required to saturate, by the voltage e382 the regulator core 381. Depending on the amount of ux to be changed in the core 381, the commutation between the phases is more or less delayed giving a lower or higher average direct voltage output. This voltage regulation can be effected by means of the ux reversal of the Core 381 which is obtained with an auxiliary current i383 in the coil 383. The coils 383 are connected to selenium Arectiiiers 430 which prevent thel current i383 from reversing through limiting resistors 431. The rectifiers 430 are connected to the secondaries 432 which control the amount of flux reversal to be effected. It is the phase voltage ea which is essentially used to perform the flux reversal.

A source 436 of a direct current i436 of very low power (approximately one watt) essentially determines the direct current output voltage of the rectifier. The source 436 feeds through a resistor which limits the control current i436 to the primary windings 434 of each phase. The windings 432 and 434 are wound on the cores 433 made of an easily saturable material. When the control current i436 is zero, the auxiliary current i383 is limited in the forward direction only by the resistor 431. A high current i383 flows during the time that the phase voltage ea is negative, which reverses the flux of the core -381 and thus delays the current i355a, delaying in turn the closing of the switch 325 so that the rectifier delivers a very small direct voltage or none at all.

When the control current i436 is high enough to magnetize the cores 433 which require only a very small current, the auxiliary current i383 in turn is delayed by the flux reversal of the cores 433. The current i383 is unable to rise to a sizeable value during the timeA that the phase voltage ea is negative, and therefore the magnetic -13 flux in the core 381 remains unchanged. This in turn permits the current i355a to rise immediately when the phase voltage ea exceeds the direct voltage, thus closing the switch 325 and allowing the highest possible direct voltage output of the rectifier.

It is obvious that an intermediate amount of the control current 436 permits regulation of the'output voltage of the rectifier to any desired value.

.The above described procedure is similar to the one described by means of Figures 7 and 8, except that a new core 433 was introduced. The control core 433 permits the regulation of the rectifier by means of a direct current instead ofa pulsed current and requires a control power which can be made as low as one millionth of the power output of the rectifier.

The aggregate of the control coil 433 and associated windings together with the selenium rectifier 430 forms a self-saturating magnetic amplifier. Similarly, the aggregate of the regulatorcoil 400 except that its output current 355a appears as double pulses acts as an amplifier. .The direct current control is not used immediately on the regulator coil 400, as it would greatly hamper the correct interruption of the contact by introducing a flux reversal during the time when the switch 325 is closed and i355a is zero.

Comparing the output current of the rectifier to the control current i436 it appears that it exhibits the properties of an amplifier with the advantages of a high efficiency, small weight as the control elements are submitted to small currents only, fast response and an exceedingly high rate of amplication.

To open the electromagnetic switch, the same procedure is applied as in the examples described above in reference to Figures 1 and 7. The current ia in the phase A is Aequal to the direct current of the output until phase B starts carrying current by the same process as described above for phase A. When the current ib in phase B rises to the full value of the direct current during the time t5 to t6, the current in phase A decreases correspondingly, as shown in Figure E. At the time t6,

' the current ia has reached the point at which the saturable transformer core 350 unsaturates. `The commutating voltage between the phases A and B appears on the winding 346 of phase A, shown as e346a in Figure 15C. The current ia is limited by the inductance of the saturable transformer core 350 and decreases in a less rapid slope as shown in Figure 15E. The Voltage e346a is transformed into the Winding 352 which has more turns than the winding 346 and is so proportionally higher. The transformer voltage is the cause of a new rise of the current i355a as shown in Figure 15G. Inasmuch as i355a is rising, the current through the armature i328a decreases as shown in Figure 15F. At the time t7, the current i328a reaches zero, and now the core 343 unsaturates as well, keeping i323a at the zero value, as shown in Figure lSF. The current i355a through the parallel path is now equal to the line current shown in Figures 15E and 15G and continues to decrease until it reaches zero at the time t9. At the time t8 the line current ia which is :355e reaches the limit of the holding current of armature 328 by means of the coil 321 and interrupts only the small magnetizing current allowed by the core 343 in coil 346.

The opening of the switch 325 always happens when there is no current flowing through it and at the natural cut o time of the electromagnetic valve.

The examples treated in the foregoing description are only a few of many possible connections as, for example, bridge connections, multiphase connection with interphase transformers, etc., may be used.

It is also possible to replace the selenium rectifier 355 in all these examples by a gas discharge tube, and control the output voltage of the rectifier by means of the grid control of this tube.

As described above in reference to Figure 13a, the sat- -urable transformer 47 and the commutating reactor 44 can be altered, Without affecting their operation, to simplify manufacture. VThe core 50'of the saturable transformer, -with the air gap, presents an yalmost unsurmount- -able obstacle, as it must be made of an easily saturable magnetic material Yand also have an air gap of definite dimensions. If an air gap is cut into such a core, or the core is made .in two halves and assembled,fmost of the desirable characteristics of the material are lost.

In order to-obtain the desired magneticy characteristic, the core 50 should be a toroid, as `described above. Windings aroundsuch. a toroidal core are difficult and therefore expensive to make. It is possible to reduce the number of coils utilized in the system. These alterations are shown in Figure 16.

In Figure 16, .the coils 352 and 353k of Figure 14 are combined into one coil 454 and the former coil 370 has as its equivalent a transformer 450. The transformer 450 has a primary winding 457 and a secondary winding 455 with air gaps 460 in the iron core 458. The air gaps 460 limit the inductance of the secondary 455 to the same value as of coil370 in Figure 14. The coil 456 vand the transformer 450 are mounted together and provided with adjustable air gapsl 460 and 463. The adjustable airgaps 460 kand-463 permit the amount and the slope of the current 355a, which is shown in Figure 15G, to be adjusted. These adjustments are required to compensate for the individual differences of the electromagnetic switches.

Comparing the circuitrof Figure 16 to the circuit of Figure 1, it is apparent that the coils 46 and 45 of Figure l have been ymerged into coil 45t) in Figure 16; thecoils 52 .and 53, together with the air gap 51, have been replaced `by a single coil 454. The coil 42 becomes coil 453 and the coil `58 becomes coil 456. Thetransformer 450 is an additional unit, but issmall and easy to manufacture and so its expense -is insignificant yas comparedto the savings in the other items.

Referring now to Figure 17, a source of alternating voltage 500 provides a direct current to a load 527 through an electromagnetic Valve 570 formed by the movable armature 519 and the fixed contacts 515 and 517. T hev xed contacts 515 and 517 areidentical with the two legs or poles of the electromagnetic valve 570. An armature 519 is fastened to a spring 520 which is supported by insulators 521.

A combined commutating reactor 571 having a coil 508, a reactor core 507 and a saturable transformer core 504 is connected in series with the electromagnetic valve 570. The reactor core 507 has a pre-excitation Winding 506 which is supplied with a small direct current from the battery 505. The saturable transformer core 504 has a secondary winding 563 which is connected to a transformer 572 having an air gap 502 and a current limiting reactor 501.

A regulator 573 is connected in series between the electromagnetic valve 570 and the commutating reactor assembly 571. The regulator has a regulator coil 512 with a main currentV saturating coil 509, a core 510 and a control coil 511. The control coil 511 is supplied with a direct current from' an external source 513. One end of the regulator ycoil 512 is connected to the coil 503, described above, and the other end to the electromagnetic valve 570, as is hereinafter described.

When a direct current of sufficient magnitude ows through the coil 514 or the coil 518, which are Wound the electromagnetic valve 57i).

current in the circuit, the direction of the current will .n

v across the switch 570 and corresponds exactly to the parts 166 and 171 of Figure 7. They form a Spark Suppressor circuit which has a low impedance immediately after the opening of the armature 519, a higher impedance when the voltage is reversed against the direction of the direct current, and also a high impedance for prolonged application of the forward voltage which occurs when the closing time of the electromagnetic valve is delayed for output voltage control.

The junction of the rectifier 522 and the capacitor S23 is connected to a discharge resistor 524, which corresponds to resistor 170 in Figure 7. Theresistor 524 is, in turn, connected to a source of direct voltage 52S.

When the armature 519 is closed and carrying the load current, the battery 525 and the dry cell rectitier 522 carry a very small forward current. This current is sufficient to overcome the initial forward voltage drop of the dry cell rectifier 522, and provide a small negative charge to capacitor S23. When the armature 519 opens, the residual forward current flowing through it will find a Spark Suppressor, parallel path 522 and 523 which has an exceedingly low resistance, which is much lower than such a circuit without the battery 525. Since the forward potential barrier of the dry cell rectifier 522 is Y very small (i. e. one fiftieth of the inverse voltage), and the current required to overcome it is also very small (i. e. one tenth of an ampere); the required direct current supply 525 can be made Very small and inexpens1ve.

In order to save the material and expense of a coil of heavy wire with many turns, an additional coil 518, described above, is wound on the magnet pole 517. The coil 518 carries only the current owing through the parallel valve 526, shown as 1'55 in Figure 4e and so can be made from very thin wire. The coil 514, however, carries the current flowing through the armature 5.19, shown as 1'28 in Figure 4f, which is high enough to provide enough magnetization even if coil 514 has only a few turns. The coil 514, therefore, is made of heavy wire, but has only a few turns.

In the Figure 7 the regulator core 181 is demagnetized by a coil 183, in which flows a pulsed direct current. This control current must be unidirectional to provide only a flux reversal, and variable in amplitude according to the desired output voltage. lt must also be zero during the time the load current i128 in Figure 8 flows through the armature 12S (Figure 7) and not through the valve 155. Using a continuous direct current would be muchfsirnpler and cheaper. A direct current, however, must be prevented from reversing the flux of the regulator core 181 during the time the load current flows through the line and the armature 28, or the de-saturated core 183 would prevent the current 1'55 from flowing at theA end of the cycle and therefore prevent the Valve from opening.

In Figure 17 the regulator coil 512 with the core 51) and the control coil 511 corresponds to coil 182 in Figure 7. An additional coil S619 connected in series with the main line of current and having only a few turns, magnetizes the core 516 with the main current itself. The main current will overcome the demagnetizing eiect of the control current, during the time the main current flows through the armature 519. Thus it is permissible to operate the control coil 511 Witha continuous direct current which may be provided by any desired external source 513.

Although the modification as shown in Figure 17 permits an appreciable saving of space, material and 'lost energy, the principle of operation of the electromagnetic valve remains unchanged.

When the voltage of the generator 50i) is positive in the left hand terminal and negative in the right hand terminal, it is positive in direction of the flow of rcurrent through lf this voltage causes a be positive according to the arrow as shown.

When the voltage of generator 500 turns from the negative direction into the positive direction, a current will start flowing through the coils 503, 512, S18, the rectifier 526 and the load 527.

This Ycurrent isw limited by the impedance of the coil 512 to a very small value, which is too small to operate the electromagnetic switch 570. The voltage of the generator 500 appears on the coil 512 which is the highest impedance in the circuit and remains there until the core 510 saturates. The time interval required is given by the amount of tlux which was displaced by the control current in the coil 511 during the previous half cycle. In other words, the starting moment for the ow of current through the magnetic valve 57i) is determinedby the saturation of the core 510, ,which in turn is determined by the small direct controlcurrentowing through the coil 511.

If the control current is zero, the core 510 remains always saturated in the positive direction, and therefore no delay of current flow in coil 512 will ensue and the v output voltage will be the highest obtainable. If the control current is equal to or higher than the magnetizing current of the regulator core 510, then the full delay which is proportional to the iron area of the core 510 will ensue. By proper design, this delay can be made sulcient to stop the operation of the rectifier altogether, and the output voltage will be zero. For any intermediate control current, the output voltage will assume an intermediate value.

As the main winding 512 of the regulator coil carries only a small rms current, it can be wound with small wire. Thus, a relatively small coil will control the voltage and hence the power of a much larger system.

Once the core 510 is saturated, `the impedance of the coil 512 becomes very low and the current through the circuit increases rapidly. When it reaches a value high y enough to magnetize (by means of coil 518), the poles 51S and 517, the armature 519 is moved establishing a low resistance path through the main line. Y

The current through the main line cannot increase immediately as it is limited by the impedance of the coil 508, due to the non-saturated core S07. Only after saturating core 507 can the main current rise freely. The delay required to saturate core 507 is called the make step, which prevents excessive current throughrarmature 519 before it is fully seated. 'Y

During the time period described above, the transformer core S04 is saturated by'aV current owing from transformer 572 into coil 503 and hence does not contribute to the impedance of the coils 503 and 598.

Once the armature 519 is firmly closed and the core 507 Y' is fully saturated, the current through the main line can rise to the full value determined only by the voltage of 500 and the impedance of 527. The parallel circuit through the elements 503-512-5155 and 526, which 1 initiated the operation and which carried enough current through coil 518 to close the electromagnetic switch 570, is now short circuited. Thus the current in coil 514 rises while it decreases in coil 518. The ultimate value of the current in coil 514 is higher than the current in coil 518, as the main line has a much lower impedance requiring fewer turns of the coil 514. With increasing line current, the force on the armature 519 increases, reducing the Contact resistance.

The coil 506 fed with a direct current from the source 59S pre-magnetiz'es the core 507 in such a way as to have

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