US3046451A - Switching circuit - Google Patents

Description

K. A. KIESEL SWITCHING CIRCUIT Filed Oct. 1, 1958 INVENTOR Kenneth A. Kiesel ATTORNEY Dropout Current Value For Relay l5 Fig.l.

7 H1! 1| ll L F I! ffiww ggw M1 II II HI i I. l II II I Q, 1 L 5 2 E a m W Q/v! v T. I w B Ml m 5. W W r m July 24, 1962 ilnite 3,046,451 Patented July 24, 1962 3,046,451 SWITCHING CIRCUIT Kenneth A. Kiesel, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Filed Oct. 1, 1958, Ser. No. 764,603 4 Claims. (Cl. 317-11} This invention relates to switching circuit means for controlling supply of alternating current to an alternating current opera-ted device according to the instantaneous alternating current voltage, and more particularly to such switching circuit means as is particularly adapted for embodiment in an Xrray exposure control circuit.

Where a relatively large quantity of alternating current energy is initiated and terminated by a main contactor relay, such as in the control of, for example, a 100 ampere alternating current supply to the primary winding of the high voltage transformer connected to an X-ray tube, it becomes necessary or highly desirable to protect switch contacts of such main coutactor relay, transformer windings, and the X-ray tube from in-rush and pull-out current transients, and this is accomplished by effecting opening and closing of the switch means of the main contactor relay at times when the instantaneous voltage of the alternating current wave form is as near as possible to a zero value.

Apparatus heretofore employed for obtaining the above switching or contacting operation in X-ray exposure control, for example, have comprised a rather elaborate triggering circuit associated with several thyratron tubes, and the necessary power supply for such tubes, for controlling operation of the main contactor relay. Such apparatus, while efiective, can be considered undesirable from the point of view of its bulk, complexity and expense.

The present invention, in general, arrives at a simplification in control of such contacting apparatus by employment of a simple rectifier, in the form of a silicon diode, for example, to provide time-separated direct current pulses from the alternating current source, and a sim ple electro-magnetic low-inertia pilot relay which initially responds only to a flow of direct current from the rectifier which'persists for a selected period of time greater than a quarter-wave period or" the AC. source and less than a half-wave period of such source, to initiate operation .of the main contactor relay via an adjustable choke which affords correlation between the sequential operation .of the two relays. The pilot relay will always pick up initially at substantially the same point in the AC. source wave regardless of the time at which control current from the rectifier is introduced to it, as consequently also will the main ,contactor relay which is brought into accord with a desired point of such source wave by adjustment of the choke. Subsequent drop-out of the main ,contactor relay consistently at a point of the AC. source wave, or within a narrow region .of such point, is obtained by an LR time delay.

Accordingly, in accord with preceding remarks it becomes a prime object of the present invention to provide relatively simple and novel switching circuit means particularly adapted to function as contactor means in an X-ray exposure control circuit.

A further object of the invention resides in the provision of such switching circuit means which is capable of performance .compatibly with timer-controlled current supply intervals as short as A of a second, such as often .finds utility in a highspeed X-ray exposure control.

Other objects and advantages of the invention will become apparent from the following detailed description of such invention when taken in connection with the accompanying drawing, in which:

FIGURE 1 is a schematic representation of an X-ray '7 milliseconds, for example.

tube exposure control circuit embodying the invention; and

FIG. 2 is a diagrammatic view illustrating certain operational features of the invention in correlation with the alternating current source voltage wave form.

Referring to FIG. 1, the X-ray exposure control circuit in which the novel switching circuit means of the invention is shown embodied for sake of illustration, comprises an X-ray tube 1 which is connected through the medium of the usual rectifier means 2to the secondary winding 3 of-a high voltage transformer 4, the primary winding 5 of which is adapted to be energized with alternating current from an alternating current source (not shown) by way of leads 6 and 7, the usual autotransformer 8, for example, source leads 9 and it the usual line switch means 11, for example, and the source lines iii The novel switching apparatus as embodied in the X-ray exposure control circuit, comprises relay means, in the form of a main contactor relay 115 and a pilot relay 16 including normally open contacts 17, for controlling pickup and dropout of normally open contacts 13 in the energizing circuit for the primary winding 5 of the high voltage transformer 4; rectifier means 20, in the form of a silicon diode, for example, to provide unipolar direct current pulses for operation of the relay means 15, re; voltage divider means in the form of resistors 22 and 23 for feeding the direct current pulses from the rectifier means 20 to the coil of pilot relay 16; rectifier means 24 connected across the coil of pilot relay 16 to prevent chattering of such relay in the presence of the pulsating direct current; inductor means in the form of a choke 26 which is preferably adjustable to regulate response of the main contactor relay 15 relative to operation of the pilot relay 16 as well as providing for presentation of an inductive kick to such main contactor relay for rapid operation thereof; damper means 30, 31 in the form of a rectifier 30 and adjustable resistor 31 connected in parallel with the coil of the main contactor relay 15 to prevent chattering of such relay during the longer exposure periods as well as to provide means which affords regulation or adjustment of the time at which such relay will drop out with respect to the instantaneous value of the alternating current source voltage wave; exposure timing means 35, including normally closed contacts 36, under control of normally open contacts 37 associated with the main contactor relay 15, for determining the time that contacts 18 of the main contactor relay 15 will remain closed; and manually operable switch means 38 for initiating operation of the switching circuit means.

The main contactor relay 15 may be in a commercially available form having contacts 18 capable of handling the necessary current, which may be in the vicinity of amperes, and a directcurrent coil for operation with a volt source.

The variable resistor 31 associated with the coil of the main contactor relay 15 may be variable from 0 to ohms, for example, and it has been found by practice that an adjustment of the resistor 31 to provide 75 ohms of resistance in shunt with the coil of the main contactor 15 having a resistance value of 150 ohms, provides satisfactory operation of the switching circuit.

The pilot relay 16 is a small low inertia relay which is capable of closing in a period greater than one-fourth and equal to or less than one-half of the cyclic period of the alternating current source voltage, approximately A suitable relay has been found to be, for example, Model No. 41R5000S, as manufactured by Sigma Instruments Inc., and having a 15 volt, 5000 ohm coil. Resistors 22 and 23 having values of 1500 ohms and 500 ohms, respectively, have been found to provide for the necessary energization of the coil of this particular relay.

The inductor means in the form of adjustable choke 26 may vary withi the range 50 mh. to 125 mh., and it has been found that such choke when adjusted to provide approximately 75 rnh. affords satisfactory operation of a main contactor relay 15 having the characteristics specified hereinbefore.

Rectifiers 2t), 24 and 30 may all be of the silicon diode type. Rectifier 28 will be chosen to suitably operate at 0.5 ampere and 115 volts; rectifier 24 will be rated at approximately 15 volts and .01 ampere; while rectifier 31D will be chosen to operate suitably at 115 volts 0.1 ampere.

The timing means 35 is preferably of the electronic type such as that associated with the well-known milliarnpere second timer which is capable of responding within 5 milliseconds, for example, to energization of a control circuit including normally open contacts 37 to open the normally closed contacts 36 for effecting an X-ray exposer of approximately of a second, for example, to provide the higher speed exposures, and which is adjustable to provide for opening of the normally closed contacts 3s at greater intervals of time following closure of contact 37 to afiord X-ray exposures of longer duration.

Operation Referring to PEG. 1, assume that it is desired to effect energization of the X-ray tube 1 for a duration of time as determined by adjustment of the timing means 35. The operator will then effect closure of the normally open switch means which will enable the alternating current of a particular polarity to flow from the line via time: contact as through the rectifier 2-9 as half-wave pulsating direct current which flows through the coil of pilot relay 16 to the line 1%? by way of resistor 22. During the flow of the first pulse of half-wave direct current through the coil of pilot relay rs, such relay will pick up and effect closure of the normally open contact 17, while current flow through such coil via the rectifier 24 caused by the collapsing field during the interval between successive pulses of half-wave direct current from rectifier will maintain the relay picked up and non-chattering so long as the switch means and contact eniain closed. By choosing a relay 16 having response time greater than a quarter-wave period and less than a half-wave period, it will be appreciated that upon closure of the switch means 38 to initiate energization of the X-ray tube 1, such relay 16 will always pick up to close contact 17 at substantially the same point in the direct current half-wave pulse, corresponding, in the circuit illustrated, to the positive half-cycle wave form of the alternating current source voltage, irrespective of the time at which such switch means 38 is closed. For example, if switch means 38 is closed at a time in the half period of a direct current pulse when less than half of such half period remains for energization of the coil of pilot relay 16, then the relay will not pick up during such half period but will wait for the succeeding half period pulse for the required euergization and pickup at the desired point with respect to the wave form of the alternating source voltage.

When the contact '7 of the pilot relay 16 closes, the half-wave pulsating direct current from the rectifier it} is applied to the coil of the main contactor relay through the adjustable choke 26 for closing contacts 18 and 37 to effect energization of the X-ray tube 1 and initiation of operation of the timer 35, respectively. The point of closure of the contacts 18 for effecting energization of the X-ray tube 1 will be consistent with respect to the wave form of the alternating current source voltage since the contact 17 of pilot relay 1d initiates energization of the coil of the main contactor relay 15 at a fixed point with respect to such wave form. Choke 2 6 provides an inductive kick or transient which causes main contactor relay 15 to pull in very rapidly thereby acting in behalf of insuring consistent closure of its contacts with respect to the wave form of the alternating current source voltage. Choke also provides a time delay which is adjusted to assure that the time of closure of the contact 13 for energizing the X-ray tube 1 occurs at a time when the alternating current voltage of the source appearin lines it and '7 is at or within a few electrical degrees or zero, in accord with the primary object of the inVention, as set forth hereinbefore.

During successive intervals between half-wave direct current pulses supplied to the coil 15 of the main contactor relay 15 by way of the choke 2d, and during exposures embracing a number of alternating current cycles, for example, the collapsing field of such coil will cause current how by way of the adjustable resistor 31 and rectifier to flow through such coil in a direction which will maintain relay 15 picked up during such intervals,

'le at the same time preventing such relay from chattfirriig.

Closure of contacts 37 simultaneously with closure of contacts 18 to initiate energization of the Y-ray tube 1 will establish an operating circuit for the timing means 35 which will respond according to a preselected adjustment to efifect opening of contacts 36 to deenergize the main contactor relay 15. Deenergization of relay 15 will open the contacts 18 and 37 to terminate supply of power to the X-ray tube 1 and to deactivate the timing means 35. In accord with a feature of the invention, the main contactor relay 15 will drop out and its contacts 15 will open consistently at a time when the alterhating current voltage in lines 6 and 7 is at or within a few electrical degrees of the Zero voltage point. By way of illustration, it may be assumed that the current through the choke 26 which is supplied to the coil of the relay 15 when continuing, for example, for several cycles as shown in FIG. 2, will lag the alternating current source voltage 90 electrical degrees, for example. The variable resistor means 31 may be adjusted to provide, in cofunction with the inductance of the coil of relay 15, an LR time delay which permits decay of the energizing current through the coil of relay 15 from its maximum value, corresponding to the peak current passing through choke 26, to the drop-out current value for such relay in approximately one cycle of the alternating current source voltage wave. The curve I15 in FIG. 2 illustrates the current passing through the coil of relay 15 under the exemplified conditions. Referring to FIG. 2,

it can be seen that if timer contacts 36 are opened anywhere between points a and b, representing 270 electrical degrees or 75% of a cycle of the alternating current voltage wave form, current through the coil of the main contactor relay 15 will decay exponentially along a portion 0 including dash line d to the dropout current value for relay 15 at a point f in the alternating current source voltage wave form which is at or within several electrical degrees of a zero value. if timer contacts 36 open between points It and a, within the remaining 90 electrical degrees or the remaining 25% of the alternating current source voltage cycle, the drop-out point for the relay 15 will be either at a point near point a which is also a zero-value point of the AC. source voltage, or it will vary between point 1 corresponding to zero source voltage and point j, for example, which may be electrical degrees from point f. The reason for this is that if the timer contacts open within about 45 electrical degrees after point It, the relatively small increase of the current -15 within this interval has practically no effect and the situation is as if the timer had opened at point h. On the other hand, if the timer contacts open within about 45 electrical degrees preceding point a, the decay line c is just slightly lower, but less steep than shown in FIG. 2 so that, as already stated, there is merely a variation of the dropout point between point f and point j.

For a particular set of relays 15 and 16, once having established the proper inductance to be provided by the adjustable choke 26 to assure that relay 15 will pick A at \fr ,relays and 16.

Wherein, as illustrated by way of example, the time constant of the damping circuit including resistor means 31 and the coil of relay 15 is adjusted to provide a drop-out delay following opening of contacts 36 equal approximately to a full cycle of the alternating current source voltage, it will be apparent that the contacts 18 of relay 15 cannot be opened and closed in less than of a second when such alternating current source voltage is 60 cycles per second. Under the circumstances where it is desired to energize the X-reiy tube 1 for an exposure of A of a second, the well-known expedient of providing switch means in association with the full-Wave rectifier means 2 to convert same to half-wave rectification may be employed. It will be apparent also that where it is desired that the relay 15 close and open contacts 18 within of a second, the timing means must be'capable of responding to-closure of contacts 37 to instantly open contacts 36 in order that the main contactor relay 15 will become effectively deenergized at the proper time.

Although the invention has been described in connection with a specific embodiment, it will be apparent to those skilled in the art that various changes in the form and arrangement of parts might be made to suit requirements without departing from the spirit and scope of the invention.

I claim as my invention:

1. In a switching apparatus for establishment of electrical connection between a source of alternating current voltage and a load according to the instantaneous value of such voltage, said apparatus comprising rectifier means to produce equal-time-separated unipolarity half-Wave direct current pulses from said alternating current voltage source, electro-magnetic pilot relay means including an operating coil effectively energizable by direct current pulses from said rectifier means, said electro-magnetic relay means being responsive exclusively to energization of its operating coil for a certain period greater than a quarter-wave period and less than a half-wave period to effect the aforesaid establishment, and switch means controlling supply of energizing current from said rectifier means to said operating coil.

2. Switching apparatus for selective establishment of electrical connection between a source of alternating current voltage and a load according to the instantaneous value of such voltage, said apparatus comprising rectifier means to produce equal time-separated half-wave direct current pulses from said alternating current voltage source, first electro-magnetic relay means operable to effect the aforesaid establishment, second electro-magnetic relay means including an operating coil energizable effectively by direct current pulses from said rectifier means, said elecno-magnetic relay means being responsive exclusively to energization of its operating coil for a certain period greater than a quarter-wave period and less than a halfwave period to effect operation of said first relay means, Witch means controlling supply of pulsating current from said rectifier means to said second relay means, and time delay means interposed between the two electro-magnetic relay means for correlating operation of said first relay means with respect to an instantaneous value of the aforesaid alternating current voltage.

3. Switching apparatus for controlling establishment connection between a source of alternating current voltage and a load according to the instantaneous value of such voltage, said apparatus comprising half-wave rectifier means to produce time-separated direct current pulses from said alternating current voltage source, electro-magnetic relay means including an operating coil energizable by direct current pulses from said rectifier means, said electro-magnetic relay means being'responsive exclusively to energization of its operating coil for a certain period greater than a quarter-Wave period and less than a halfwave period to effect the aforesaid establishment, switch means controlling supply of pulsating ctu'rent from said half-wave rectifier means to said electro-magnetic relay means, and current decay control means connected to said operating coil for maintaining effective energization thereof between successive direct current pulses of full half- Wave period.

4. Switching apparatus for establishment and disestablishment of electrical connection between a source of alternating current voltage and a load according to an instantaneous value of such voltage, said apparatus comprising rectifier means to produce time-separated direct current pulses from said alternating current voltage source, first electro-magnetic relay means energizable and deenergizable to effect the aforesaid establishment and disestablishrnent, respectively, second electro-magnetic relay means responsive to direct current energization from said rectifier means exclusively for a certain period greater than a quarter-wave period and less than a half-wave period to effect supply of energizing current to said first electro-magnetic relay means, time delay means interposed between the two relay means for correlating operation of said first electro-magnetic relay means with respect to an instantaneous value of the aforesaid alternating current voltage, normally open switch means operable to elfect supply of pulsating current from said rectifier means to said second electro-magnetic relay means, and timing switch means operable upon effective energization of said first electro-magnetic relay means to terminate supply of energizing current thereto according to predetermined time intervals.

References Cited in the file ofthis patent UNITED STATES PATENTS 1,708,384 Fox Apr. 9, 1929 2,225,763 Bayha Dec. 24,1940 2,412,092 Mayle Dec. 3, 1946 2,424,329 Penney July 22, 1947 2,809,296 Godbarsen Oct. 8, 1957

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