US2557740A - Apparatus for controlling the timing of periodically actuated switches - Google Patents

Description

reamed June 19, 1951 2,557,740 APPARATUS FOR CONTROLLING THE TIMING OF ATED SWITCHES Alexander Goldstein and Hans Blatter,

PERIODICALLY ACTU- tingen, Switzerland, assignors to Aktiengcsellschaft Brown,

Boveri I; Cie, Baden,

Switzerland, a joint-stock company Application December 1, 1949, Serial No.

In Switzerland Decemberll, 1948 Claims. (cl. car-4a) The present invention relates in general to electrical apparatus such as mechanical converters and the like which include periodically actuated switching devices for controlling the flow of current to a load, and is an improvement upon the converter apparatus described in our copending application Serial No. 664,396, filed April 23, 1946.

In the art of mechanical rectifiers for example, it has been conventional practice to incorporate an auxiliary control device such as a reactor in the primary side of the switch contacts for the purpose of effecting an interval of reduced primary current so that the contacts may be opened under relatively arc-free conditions. As the contacts are opened and closed periodically by a contact bridge actuated by a power drive which is synchronized with the alternations in the alternating current supply, it is obvious that the timing of opening of the contacts is important and that contact opening should occur at the instant the reactors or other current reducing means have produced their optimum efiect.

In the above referred to application, control over the contact opening time is obtained by deriving a measured control quantity the magnitude of which varies as a function of the time interval between the beginning or alternatively the ending of the period of reduced current effected by the reactor and the opening or alternatively the closing of the rectifier contacts, comparing this control quantity with a reference quantity of fixed magnitude representative of the optimum condition to obtain the diiference if any, and utilizing such difierence to readjust the timing of the contacts-until balance between the We compared quantities is reattained. The apparatus for producing the measured control quantity comprises a discharge tube of the grid control type having its anode-cathode circuit closed and opened periodically in timed relation with operation of the rectifier contacts, and its grid bias is likewise varied periodically in timed relation with operation of the reactor element. The arrangement is such that the grid bias, normally at a negative value sufiicient to maintain the tube in a nonconductive state, is driven positive at the beginning of each interval of reduced current and shortly before the rectifier load contacts controlling the anode-cathode circuit of the tube are set to open. Current will thus flow in the anodecathode circuit of the tube from the beginning of the period of reduced current until the rectifier contacts open. The average value of these periodic pulses of anode-cathode current thus become a measure of the time between the instant the reactor begins to produce the interval of reduced current and the instant at which the load contacts open.

While this system is generally satisfactory, it does have one operating disadvantage which stems from the fact that the anode-cathode circuit passes through a switching device formed by one of the stationary rectifier load contacts and the movable contact bridge, and theoretically this anode-cathode circuit is broken at the instant the bridge lifts from the contact. In practice however it has been found that this arrangement for breaking the flow of current in the tube is satisfactory so long as the contacts operate in an arc-free manner. Arc-free operation cannot however always be ob tained especially in applications where the load is subject to rapid fluctuations, and when arcing does occur it is obvious that the anode-cathode circuit of the tube will not be interrupted at the instant of load contact break but rather at a somewhat later instant when the are between the contact and bridge has been extinguished. This then introduces a corresponding error in measurement of the average value of the anode-cathode current for the period desired which is refiected by a like error in response of the mechanism whose function is to adjust the timing of the contacts.

Moreover even under arc-free operating conditions of the load contacts, error in measurement is still possible because the contact bridge may not always lift from both stationary contacts at the same instant with the result that the rectifier load circuit through the contacts may be broken while the anode-cathode circuit of the discharge tube may yet be unbroken.

The object of the present invention is to overcome the foregoing disadvantage and the objective is obtained by providing an extra set of switching contacts operated synchronously with the load contacts of the rectifier and whose sole function is to periodically open and close the anode-cathode circuit of the discharge tube component of the interval measuring device.

One practical embodiment of the invention as applied to a polyphase rectifier system is illustrated in the accompanying drawing which shows the improved system in schematic form.

Referring now to the drawing, the movable contact bridges of a polyphase mechanical rectifier are designated by al, a2 and a3 which coact with opposed sets of stationary contacts vi, 222, v3. The latter are connected on one side to an alternating current network 17 through commutating reactors or chokes el, e2 and e3 and switch S, and on the other side to feed a direct current load system C.

The drive for actuating the several contact bridges is a synchronous motor d connected to the network b. The motor shaft drives cams tl, t2 and t3. which through cam follower rods ul, a2, a3 function to periodically lift the contact bridges al, a2, 03 out of engagement with their associated sets of fixed contacts vi, v2, 123 in the proper sequence as related to the phase sequence of the polyphase power supply I).

The control quantity for regulating the contact timing is obtained through the use of the discharge tube' h which is of the grid control type. Its anode-cathode circuit is arranged to be opened and closed in timed relation with operation of the load contact bridges al, a2, a3, but instead of using the latter also as the switching device for the anode-cathode circuit as is done in the co-pending application Serial No. 664,396, an auxiliary set of contacts and bridge are employed especially for such purpose. The auxiliary contacts are designated :24 and the associated auxiliary contact bridge al. Bridge a4 is actuated periodically in a manner similarly to the other contact bridges associated with the load contacts by means of cam t4 and cam follower rod a4 and is synchronized with the movement of contact bridge 03.

The grid of tube h is normally biased negatively from an auxiliary voltage source i and the grid circuit extends from the source 2 through a winding f inductively associated with the reactor e3. The use of reactors in the primary of the rectifier is well known and hence the theory of their operation need not be detailed here. It is suflicient to say that at the beginning of the interval of reduced current effected by reactor e3, the inductive effect inherent therein is sufflcient to induce a potential pulse in winding 13 which is so connected in the grid circuit of tube h that the grid bias is swung positive and renders the tube conductive. As this action takes plate at a time shortly before the contact bridges a3 and a4 operating synchronously are set for lifting from the respective sets of fixed contacts v3 and v4, current will flow in the anodecathode circuit of tube h from the beginning of the interval of reduced current in the phase served by load contacts 03 until the latter and also contacts 04 open. Thus as the rectifier operates and the load and auxiliary contacts periodically open and close, periodic pulses of direct current of constant amplitude flow in the anode-cathode circuit of tube It and hence it now becomes obvious that the average fiow of current in this tube circuit, which can be observed by an ammeter g placed in the circuit, is a measure of the time between the instant at which the choke e3 begins to produce its interval of reduced current and the instant at which load contact bridge a3 lifts from its fixed contacts 03.

To measure the average current in the tube circuit a resistance It is connected therein and the drop in voltage across the resistance will hence vary with the current.

The potential across resistance It is used to regulate the timing of the rectifier contacts so that they always open at the optimum condition, i. e. substantially maximum current reduction in the primary line. For this purpose the voltage drop at resistance It is fed into an amplifying device I which is of conventional construction and therefore has been illustrated in block schematic only. Connected across the output of amplifier l is a resistor m that is connected to a selected portion of a manually adjustable voltage dividing resistor n in such manner as to derive a voltage which the respective voltage sistors.

An instrument type relay 9 of the crossed coil type is used in the control and the contacts of this relay control the operation of a reversible motor q. Coil pl is connected directly to an auxiliary source of direct current power indicated by conventional symbols and hence current through it remains constant. However coil :22 is connected in the circuit between opposed resistors 1n and n and hence its current depends upon the unbalance between the voltage drop across these two resistors. The arrangement is such that when the current through coil pl of the instrument's moving system is equal to the current through coil p2 which is at right angles to coil pl, the contact arm p3 will take a position between and spaced from the stationary contacts 114, p5 and the motor q will stand still since the circuits to both of the motor fields qi and 112 are open. If however the two coil currents are unbalanced, the contact arm 93 will be moved into engagement with one or the other of contacts pl, p5, dependent upon the direction of the unbalance, and hence will cause motor q to run in one direction or the other as determined by the alternatively energizable motor fields qi and :12.

A control mechanism 1' actuated by the shaft of motor q functions to adjust the timing of the contact bridges al-al relative to their respectively associated phase voltages of the polyphase power supply by changing the positions of the cam follower contact bridge actuating rods u l-ul relative to their actuating cams to tl-tl in one direction in which motor q is rotated.

The various operating components are adjusted so that when the load contacts of the rectifier are operating when optimumn condition, i. e. without arcing, the drop across resistance 1: and hence the resulting drop across resistor m is such that the current through relay coil p2 is equal to that through coil pi and hence motor q will stand still. However should the desired timing of the rectifier contact bridge a3 relative to the operation of the commutating reactor e3 shift in one direction or the other which would therefore cause the contact bridge a3 to are as it lifts from the contacts '03, such shift will appear as a change in the conducting time of tube It and hence change the voltage drop across resistor k. In turn the current through instrument relay coil :12 is changed and this causes relay p to effect rotation of motor q in such direction as will readjust the timing of the contacts to the optimum operating condition. When the correct timing is reached, the voltage drops across resistor R: and m will have gained a rebalance and motor q will stop. The regulating action is thus entirely automatic and constantly maintains correct timing of the contact bridges.

Moreover since a separate set of switching contacts operating synchronously with one of the sets of load contacts on the rectifier are utilized solely to open and close the anode-cathode circuit of the control tube It, correct measurement of the interval between the beginning of the period of reduced load current and the actual drops across the two rerepresents the difference in Another possible modification would be to associate the winding 1 with reactor e2 or el instead of e3 as now illustrated leaving the auxiliary set of contacts a4 v4 synchronized with contacts a3 03 of e3. In such case however a mean value proportional to a constant value if el) plus the time between measuring instrument 9.

We claim:

1. A mechanical current converter comprising a set any deviation in the average anode-cathode circuit of said periodically in a predetermined sequence to peri odically open and close the several phases connected to supply a common load, a control device in series with each set of load contacts for effect ing periodic intervals of reduced current to minito restore the average current to said selected value.

control device controlling the grid bias of said tube.

5. A mechanical converter as defined in claim REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA ENTS Number Name Date 1,529,341 Burnham Mar. 10, 1925 2,279,729 Bedford Apr. 14, 1942 2,465,682 Goldstein Mar. 29, 1949

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