US2957123A - Constant current network level selector - Google Patents

Description

United States Patent O CONSTANT CURRENT NETWORK LEVEL SELECTOR John E. Rose, Concord, Calif., assignor to Stauffer Chemical Company, New York, N.Y., a corporation of Delaware Filed Apr. 6, 1959, Ser. No. 804,376

4 Claims. (Cl. 323-76) This invention relates to constant-current networks, and more particularly to level selectors for constant-current networks that are substantially independent of load.

In some circuit applications it is desirable to provide a constant voltage-constant current transformation which is substantially independent of the load into which it operates. One such use is in an electron bombardment melting and casting furnace, such as the one disclosed by Smith, et al., in an article entitled Electron Bombardment Melting, Journal of Metals (Feb. 1959). One of the problems encountered in high vacuum electron beam furnaces is that of maintaining a relatively constant emission current in spite of variations in the cathode-anode resistance in the furnace. The cathodeanode resistive load depends on a plethora of imperfectly understood phenomena. Sufce it to say here that it has been found that not only must variations in the voltage drop across the discharge in the furnace be controlled to regulate emission currents, but that the maximum amount of emission current that can be drawn must be established. This latter depends in part on the electrode configuration, but more importantly on the optimum level of emission current for the different materials to be processed.

For example, in the Smith article referred to above, the writers illustrate by Fig. 5 the wide variations in vapor pressures of materials processed by electron beam furnaces which are assignable to temperature changes. As a result of .these variations, each material to be refined by electron bombardment melting and casting has its own optimum level of constant current. Heretofore in the prior art, these levels have had to be worked out in each case. What applicant has done is to develop a simple and straightforward means by which the output current level of a particular constant voltage-constant current transformation circuit may be changed without otherwise disturbing the network.

The particular constant voltage-constant current transformation employed is one attributed to C. P. Steinmetz and discussed at length in his book, Theory and Calculation of Electrical Circuit (N Y. 1917), pp. 259 et seq. Steinmetz in his early treatise developed a single resonant circuit transformation and a more sophisticated monocyclic transformation both of which are substantially independent of load. The constant-current network ernployed in the exemplary electron bombardment furnace referred to above is an expansion of these transformation networks to accommodate a three-phase constant voltage source. While the degree of independence of load current with respect to variations in the load are similar for various Steinmetz-type circuits, the influence of frequency instability, relative power efficiencies, etc. may differ substantially.

Steinmetz first analyzed that if a constant voltage source is connected to a series resonant circuit, tuned to the frequency of the voltage source, and the output of the circuit is connected across one of the reactances of the series resonant circuit, the absolute output current Patented Oct. 18, 1960 will be substantially independent of output load variations (i.e., ignoring phase changes, power efficiencies, and the like). The Steinmetz circuit, per se, forms no part of the present invention, but his analysis of the effect of the source frequency on output current and the independence of resonant L-C circuits to load variation points up the problem which the present invention seeks to resolve in a simple expedient manner.

An object of the invention, therefore, is to provide a level selective constant current network which maintains the series resonant frequency intact over the selective range.

Another object of the invention is to provide a range of constant-current outputs with a simple switching circuit.

A feature of the invention pertains to the selective application of one or more capacitive reactances across either the inductive or capacitive reactance part of the series resonant transformation circuit.

More particularly, a feature of the invention pertains to the use of at least one reactance connected to the terminal common to the inductive and capacitive reactances of the series resonant transformation circuit, and pro portioned so that it will maintain the networks resonance frequency constant when connected across either the capacitive or inductive reactance, and means for switching the common reactive component from one position to the other.

These and other objects and features will be more fully understood when the following detailed description is read with reference to the drawings, in which Fig. l is a schematic representation of an electron beam furnace with which the present invention may cooperate;

Fig. 2 is a detailed schematic of an exemplary threephase constant voltage-constant current transformation network;

Fig. 3 depicts a single phase of a constant-current network, the output of which is similar to that of Fig. 2; and

Fig. 4 illustrates the relationship between the detached contacts employed on Fig. 2 and the relay coils with which they are associated.

Looking particularly at Fig. l, it can be appreciated that the constant-current network 10 may cooperate with a source of constant-alternating voltage 11, e.g., commercial three-phase supply mains, a rectifier 12, isolation transformer 13, emission limiting circuitry 14 and other parts of an electron beam furnace including a vacuum chamber 15, cathode 16, anode 17 and pump 18. Briefly, the constant-current network is set at a preselected level by a combination of control signal input leads, a, b and c, as will be explained more fully below, and the constant-current output therefrom is rectified by rectiiier 12 and supplied to the cathode-anode load 16-17 in vacuum chamber 15. The pump 1S maintains a vacuum in the range of l micron of Hg in the chamber 15 and emission limiting circuitry 14 acts to sample the effective resistance across the cathode-anode gap (I6-17) and thereby control the emission current. Before a run is started, the proper level of emission current for the stock to be processed is set into the constant-current network 10 by an appropriate combination of signals on leads a, b and c. This may be done automatically or manually. Once set, however, the electron furnace may be operated automatically thereafter.

Fig. 2 discloses the details of an exemplary three qb constant-current network 10 and the means whereby the emission current levels may be changed. Before considereing Fig. 2, however, it will be helpful to consider the basis for the level selector circuit with a single-phase network. This may be done with reference to Fig. 3

which illustrates a 'series resonant circuit having an input voltage e0, a variable output load, and an output current i owing through the load. With the arrangement shown, Steinmetz demonstrated that rso/Z0, where Z equals the reactive impedance in series with the load. If the series resonant circuit included only X2 and X3, Z0 would equal X2=|X3I- But in the instant case, in addition to inductive reactance X2 and capacitive reactance X3, inductive reactance X2 is shunted by a ct.- pacitive reactance X1. lt should be remembered that in the ensuing discussion it is assumed that the leads and the reactances have no resistive components. While this is not entirely true, it approximates the facts suihciently well to not introduce any substantial errors.

With the arrangement shown in Fig. 3, if the total capacitance of the circuit, C14-C3, is maintained constant and equal to 1/w2L, proportioning :of the total capacitance between X1 and X3 will change the output current i owing in the load without changing the resonant frequency of the circuit. For the circuit of Fig. 3, it is desired to maintain Z1 2Z3 for resonance conditions, but

Transposing, substituting and clearing:

the voltage source is 60 c.p.s., CI+C3=580M2 and some exemplary output currents might be as follows:

Cri-C3:

Switch C1, Gs, Z1 Zi-z Z3 Output Position pf. pf. (ohms) (ohms) (ohms) Current 0 580 U0 4. 6 4. 6 218 en 120 460 22. 1 5. 8 5. 8 172 en 480 1GO 5. 5 26. 4 26. 4 .038 6o Before proceeding to the constant current network of Fig. 2, it may be well to clarify the symbols used therewith to identify contacts associated with relays A, B, and C. Fig. 4 relates the operation of relays A, B, and C of Fig. 2 to the detached contact designations used therein. For example, the X across lead 1 2 with the designation A-1 adjacent thereto means lthat a path is completed between terminals 1 and 2 through make or front contact 1 @of relay A with the relay operated. Contrariwise, the Yvertical line with accompanying designation A-2 means a path is completed between terminals 3 and 4 through back or break contact 2 of relay A with the relay released. vSimilar notations are used with respect to relays B and C.

Turning to Fig. 2, now that the rationale for the present level selector circuit is understood, it is a simple matter to provide switching circuits to give it effect. Fig. 2 exemplarily discloses one possible system. In this circuit, constant current network is a three-phase variant of the single-phase system described in connection with Fig. 3. The constant voltage supply 11 is connected via terminals 31, 32 and 33 to the A-connected transformation circuit, each leg of which includes a series L-C combination resonant to the input source frequency. The output leads 34, 35 and 36 of the network are connected intermediate the capacitor and inductance forming each leg of the A.

For each phase, capacitors 40, 41 and 42 have one set of their terminals connected intermediate the capacitor and inductors of each series resonant pair, and their other terminals connected respectively to independent swingers associated with relays A, B, and C. For each phase of the network and with relays A, B, and C released, capacitors 40, 41 and 42 are in parallel with capacitor C of its cooperating series resonant pair by virtue of respective back contacts of relays A, B, and C. Any one of the capacitors 40, 41 and 42 may be switched across its associated inductor L by operation of its associated relay.

Thus, capacitors 40 in the three phases are normally connected in parallel with respective capacitors C in each phase through respective back contacts A2, A4 and A6 of relay A. Upon operation of relay A, however, capacitors 40 are switched via their respective cooperating front contacts A1, A3 and A5 in parallel with this respective series resonant inductors L. In a similar manner, the operation of relay B causes capacitors 41 to be switched into parallel relation with respective inductors L via respective contacts B-1, B-3 and B-S. Alternatively, when relay B is released, capacitors 41 are connected in parallel with their respective capacitors C through respective back contacts B-2, B-4 and B6. Capacitors 42 are switched by relay C in a manner similar to that of capacitors 40 and 41 by relays A and B. They are switched by respective front contacts C-l, C-3 and C-S of relay C to positions across respective inductors L and their normally quiescent connections, when relay C is released, are in parallel with respective capacitors C via respective back contacts C-Z, C-4 and C-6.

lt should be apparent from the foregoing that numerous other output current levels may be selected, either by adding additional capacitors (keeping the sum of the capacitances equal to l/wZL), by using combinations of capacitors 40, 41 and 42, or both. Even with the exemplary embodiment of Fig. 2, eight different combinations may be obtained depending on the states of relays A, B and C.

Relays A, B and C in the exemplary embodiment are operated by placing ground on the proper leads a, b or c, since each relay winding is connected to a source battery 39 at its other terminals. By placing ground on a combination of these leads a, b and c, the eight combinations may be obtained. While ground may be applied to leads a, b or c through a manual key, there is no reason why the current limiting circuitry 14 of the exemplary electron beam furnace of Fig. l could not control the emission current output level in response to characteristics of the material being processed. In any event, what is important is that the present invention provides a simple technique for changing constant current output levels without laffecting the resonant circuits forming a part of the constant voltage-constant current transformation.

While the present invention has been described in some detail with respect to single and three-phase networks, it should be apparent to those skilled in the art that numerous other arrangements are possible without departing from the scope of the invention.

What is vclaimed is:

1. A circuit for converting a source of constant voltage of a predetermined frequency to one of a plurality of constant currents substantially independent of the output load connected to the circuit comprising in combination, a source of constant voltage having output terminals, a series circuit including capacitive and inductive reactances connected across said constant voltage output terminals, said capacitive and inductive reactances tuned to said predetermined frequency to form a series resonant circuit and said capacitive reactance including at least two discrete valued capacitors connected in parallel relation, an output load having at least one terminal connected to the junction of said inductive and capacitive reactances, and means to switch at least one of said capacitors into parallel relation with said inductive reactance whereby the magnitude of the output current flowing through said load is changed while maintaining the resonant condition,

2. A circuit for converting a source of three-phase constant voltage having a known frequency to one of a plurality of three-phase constant current values, all of which are substantially independent of the load into which the circuit works comprising, in combination, a source of three-phase constant voltage having three output terminals, three series resonant circuits each including capacitive and inductive reactances and each tuned to the known frequency of said voltage source, said series resonant circuits connected across the output terminals of said constant voltage source to form a delta transformation network and said capacitive reactance in each of said series resonant circuits consisting of a plurality of discrete valued capacitors connected in parallel, three output terminals for said transformation network, each one connected intermediate the capacitive and inductive reactances of one of said series resonant circuits, a load connected across said output terminals, and means to switch dilerent combinations of said capacitors forming said plurality from said parallel relation with each other to parallel relation with said inductive reactance of their series resonant circuit whereby the magnitude of the output current flowing through said load is changed without disturbing the resonant tuning of the three-phase transformation network.

3. Electrical apparatus for supplying alternating current of adjustable, xed amplitude to a load of varying impedance, comprising a source of alternating-current electric power of substantially constant voltage and fixed frequency, an inductor and a capacitor connected in series across said source, a load connection from the circuit junction between said inductor and said capacitor, at least one other capacitor, and switching means for connecting said other capacitor in parallel with either said inductor or the rst-mentioned capacitor selectively, the sum of the capacitances of said capacitors being substantially equal to 1/ w2L, where w is 211- times said fixed frequency and L is the inductance of said inductor, whereby alternating current of substantially xed amplitude is supplied through said load connection, said amplitude being adjustable by said switching means.

4. Electrical apparatus for supplying alternating current of adjustable, fixed amplitude to a three-phase load of varying impedance, comprising a source of three-phase, alternating-current electric power fof substantially constant voltaffe and fixed frequency, three similar, seriesresonant circuit branches connected in closed delta conliguration to said source, each of said branches consisting essentially of an inductor and a plurality of capacitors all connected together at a junction intermediate the ends of the series-resonant branch, the sum of the capacitances of said capacitors of each branch being substantially equal to 1/ wZL, where w is 2jr times said ixed frequency and L is the inductance of said inductor of the same branch, three-phase load connections from said junctions of the three series-resonant branches, and switching means in each of said three branches for individually connecting each of a plurality of said capacitors either in series or in parallel selectively with the inductor of the same branch, whereby alternating current of substantially Xed amplitude is supplied through said load connections, said amplitude being adjustable by said switching means.

References Cited in the tile of this patent UNITED STATES PATENTS 729,234 Steinmetz May 26, 1903 2,148,301 Kuyper Feb. 21, 1939 2,771,568 Steigerwald Nov. 20, 1956 2,792,281 Steigerwald May 21, 1957 2,860,251 Pakswer et al Nov. 11, 1958 FOREIGN PATENTS 367,430 Great Britain Feb. 22, 1932 473,368 Great Britain July 29, 1936 1,141,535 France Mar. 18, 1957

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