US2991396A - Voltage surge suppressor - Google Patents

Description

July 4, 1961 c. A. scHURR VOLTAGE SURGE suPPREssoR 2 Sheets-Sheet 1 Filed April ll, 1958 P 0 n OZ .MMM M w j 3 4 :of d 4 LM r/ A f f 5 o o m P. A .mi 2 M j w L 9 F ,f 4 m O 0 O P m8 w OJ. l m .Z f if .JN

July 4, 1961 c. A, scHURR VOLTAGE sum1: suPPREssoR 2 Sheets-Sheet 2 Filed April ll, 1958 Ihwentor /AELE; A SCHUHE' United States Patent O 2,991,396 VOLTAGE SURGE SUPPRESSOR Charles A. Schurr, Warrensville Heights, Ohio, assignor to Square D Company, Detroit, Mich., a corporation of Michigan Filed Apr. 11, 1958, Ser. No. 727,963 1 Claim. (Cl. 317-11) This invention relates to an improved voltage surge suppressor or voltage surge suppressing circuit for limiting the rise in voltage that occurs across an inductive winding when the winding is disconnected from a direct current source and for preventing arcing at the contacts which open to effect the disconnection.

An ideal voltage surge suppressor for these purposes should prevent arcing at the contacts upon opening of the contacts irrespective of the speed o-f opening, and should limit to a relatively low value the rise in volt- -age which occurs across the winding when the contacts open. Furthermore, it is often essential that the suppressor neither effect an increase in the time required for the winding to become substantially deenergized nor .provide a shunt path around the controlling contacts to the winding when the contacts are open. In addition, it is desirable that the suppressor be Irelatively compact and inexpensive, be operative irrespective of the polarity of the voltage applied to it or to the winding, and be so rugged and dependable that it requires no attention for long periods of time. Compactness and ruggedness become very important when it is desired to mount the Suppressors on control panels carried by moving rnachinery.

It is an object of this invention to provide an improved voltage surge suppressor.

Another object is to provide a voltage surge suppressor having all of the foregoing advantages and operating characteristics.

One known type of voltage surge suppressor having some of the foregoing advantages and operating characteristics consists of a suitable valve means connected in parallel with a winding and having the ability to be a good insulator at voltages below a critical value and a good conductor at voltages above that value. One valve means having this ability is a gas-filled diode or glowdischarge tube and another is a dense, non-porous crystalline ceramic material well known in the electrical industry as Thyrite Still another suitable valve means for this purpose consists of a pair of semi-conductor rectiers, usually metallic disc rectiiers containing selenium or other similar material, connected in series with each other in opposed, or back-to-back, relation and in parallel 'with the winding. This latter valve means acts as a surge suppressor because a semi-conductor or a metallic disc rectiiier readily conducts current in one direction and suddenly loses its current blocking action in the opposite direction when a reverse voltage in excess of a predetermined value, known as the Zener voltage, is impressed on it. All of these known valve type surge Suppressors function as voltage limiters when connected across an inductive Winding because a discharge current ilows from the winding through the valve means as soon as the self-induced voltage across the winding reaches a critical voltage, e.g., the breakdown voltage of the tube or the Thyrite or the Zener voltage of the one of the two rectiiers that is poled to normally prevent the flow of current in the direction of the discharge current.

The Thyrite suppressor has been used with both large and small inductive windings for many years with considerable success and the back-toaback rectier suppressor has been used with small direct current relays and con- ICC tactors. However, when any valve type suppressor is used alone with relatively large direct current windings such as those required to operate a contacter having a current rating of 30() amperes or more, a serious disadvantage is present, particularly when the circuit to the winding is interrupted by slowly opening a single pair of contacts. This is because the operation of these known-valve-type surge Suppressors depends upon the voltage across the winding reaching a predetermined value. When the contacts controlling the winding are separated slowly, an arc forms at the contacts, and the induced voltage does not rise to the breakdown value of the valve means. Consequently, no arc suppressing action occurs. Another disadvantage of the valve-type suppressor is that it causes an increase in the time required for the winding to become deenergized. This, of course, is to be expected since the ilow of induced current through the shunt circuit after breakdown maintains the magnetic ilux of the winding. Furthermore, both the tube and the Thyrite Suppressors are fragile and require cumbersome and space-consuming mounting means to protect them from breakage.

Another ywell known type of voltage surge and arc suppressor consists of a capacitor connected in parallel with a winding. When this type of suppressor is used, disconnection of the winding from the source causes the current passing through the winding to be diverted immediately into the capacitor thereby starting a damped oscillation. When the winding is relatively large, a relatively large capacitor must be used, and the oscillation often results in a current pulse suicient to cause reclosure or partial reclosure of a magnetic armature associated with the winding. Another disadvantage of the use of a shunt capacitor alone as a suppressor is that the capacitor causes the time required for complete deenergization of the winding to be materially increased. If the capacitor size is reduced, the time of deenergization is decreased, but the induced voltage across the winding is increased.

The applicant has discovered, however, that if the characteristics of the rectifier, the size of the capacitor, and the inductance of the winding are properly correlated, the use of both a back-to-back rectiner combination and a capacitor provides a suppressor having not only all of the advantages of each of the prior devices when used alone, but also having important additional advantages and none of the disadvantages of either.

Another object of the invention is to provide an irnproved voltage surge and arc suppressor comprising a voltage limiting valve means connected in `parallel with a capacitor, the parallel connected capacitor and valve means being connected in parallel with an inductive winding.

In accordance with this invention, a voltage surge and arc suppressor for an inductive winding comprises a capacitor connected in parallel with a pai-r of back-toback connected rectitiers and the parallel connected combination of the capacitor and the rectiiiers connected in parallel with the inductive winding. If desired, the rectifiers and the capacitor may be impounded or potted in an otherwise solid block of insulating material to form a compact and rugged suppressor easily mounted in many diverse locations.

Other objects and advantages of this invention will become apparent from the following description wherein reference is made to the drawings, in which:

FIG. l is a wiring diagram illustrating the invention;

FIG. 2 and FIG. 3 are characteristic curves of the rectiers of the circuit of FIG. 1;

FIGS. 4 through 8, and 11 are reproductions of photographs of current and voltage traces made during the operation of the circuit of FIG. 1;

FIGS. 9 and 10 are reproductions of photographs of current and voltage traces made during the operation of prior art circuits;

FIG. 12 is Ka wiring diagram of `a modification of the invention; and

FIG. 13 is a perspective view of a suppressor in accordance with this invention.

Referring to FIG. 1, an inductive winding 10 which may be, for example, the operating coil of an electromagnetic relay or contactor, the shunt field of a motor or generator, or the operating coil of any other electromagnetic device, is arranged to be connected to and disconnected from a source of unidirectional voltage, represented by supply conductors 11 and 12, by circuit making and breaking contacts 13 which may be the contacts of a relay, master switch, or other suitable switching device, and which may be of either the snap-acting or slow-operating type.

In accordance with this invention, a voltage surge and arc suppressor 14 comprises a suitable voltage limiting valve device 15, shown as a pair of rectifiers 16 and 17 connected in a series circuit in back-to-back or opposed relationship with each other, and a capacitor 18 connected in parallel with the series circuit including the rectifiers 16-and 17. The parallel connected combination of the capacitor 18 and the valve means 15 is connected directly across the winding 10 by suitable terminals or conductors 51 and 52.

The rectifiers 16 and 17 may be of any of the commonly used semi-conductor metallic disc types currently available such as selenium, copper-oxide, or germanium, but selenium is preferred because of its greater ability to withstand current flow in the reverse or normally non-conducting direction. Each of the rectifiers 16 and 17 may comprise a number of individual rectifier cells (not shown) connected in series as is well known. If desired, the cells of the rectifiers 16 and 17 may be grouped separately on a common rod or the cells of the rectifiers may be interleaved with each other along the rod.

The rectifier 16 is so poled as to permit current to flow from left to right in its forward or normal conducting direction whereas the rectifier 17 is so poled as to permit current to flow from right to left in its forward or normal conducting direction. Thus, the rectifier 16 substantially prevents current iiow through it from right to left unless its Zener voltage value is exceeded, and the rectifier 17 substantially prevents current flow through it from the leftV to rig-ht unless is Zener voltage value is exceeded.

Preferably, the rectifiers 16 and 17 are electrically identical and each has a voltage-current characteristic like that of FIG. 2. It is understood, however, that surge suppression is obtained even though the rectifiers are not exactly electrically identical. A curve 19 in FIG. 2 shows how the effective resistance of either one of the rectifiers 16 and 17 varies when a voltage is impressed across it in its forward direction, whereas a curve 20 shows how the effective resistance of either one of the rectifiers 16 and 17 varies when the voltage across it is in its reverse direction. It will be noted from the curve 19 that, when the forward voltage is increased above a relatively low value, for example, live volts, a very large forward current fiows through the rectifier. In the case of a reverse voltage, however, as shown by the curve 20, the reverse voltage must exceed a relatively high value, such as 400 volts, before any appreciable reverse current flow occurs. The voltage at which material current flow starts in the reverse direction is the Zener voltage.

When the rectifiers 16 and 17 have identical reverse current characteristics and are connected in series in opposed or back-toeback relationas in FIG. l', the operation of the series-connected rectifier combination is as shown by curves 21'andf 22 in FIG. 3. Curves 21 and 22 are images of each other and show that little current can flowineither direction until theZener voltage-is reached.

It is to be noted that the series-connected combination of the rectifiers 16 and 17 acts as a high resistance until the voltages exceeds about 400 volts 4at either polarity. When the voltage is above this value, the reverse resistance of one of the rectifiers decreases suddenly, and the rectifier combination approaches a constant voltage oper- -ating condition.

The numerical values of voltage and current given in FIGS. 2 and 3 are for comparison purposes only and, of course, will depend upon the type and size of the rectiiiers used and the suply voltage. The examples given are based upon tests made using for the rectifier combination an assembly of eighteen, one inch square, selenium rectifier cells with nine cells connected to conduct at low impressed voltage in one direction and with the other cells connected to conduct at low impressed voltage in the other direction.

Because of the symmetry of the circuit of FIG. 1, it is apparent that it is immaterial which of the conductors 11 and 12 is positive and which is negative.

FIGS. 4 through 8, which are reproductions of photographs taken of a cathode ray oscillcgraph, show the fluctuations of the various currents and voltages in the circuit of FIG. 1 upon opening of the contacts 13. A curve 24 in FIG. 4 applies to the current in the winding 10 upon opening of the contacts 13 and shows that this current decreases from its full value to zero in a time t1 and then oscillates at a relatively low value of a time t2. A curve 28 in FIG. 5 shows how the current in the circuit branch between points 29 and 30y in FIG. 1 varies while the current in the winding 10 is changing in accordance with the curve 24 of FIG. 4. It is to be noted that the current indicated by the curve 28 is for all practical purposes the same as the current indicated by the curve 24, and that no current is therefore flowing across the contacts 13. With no current across the contacts 13, there is, ofcourse, no arcing at those contacts.

A curve 31 in FIG. 6 shows how the current in the capacitor 18 varies while the current in the winding 10 is changing in accordance with the curve 24. A curve 32 in FIG. 7 shows how the current through the rectifiers 16 and 17 varies while the current in the Winding 10 is changing in accordance with the curve 24. Asis apparent from the curve 32, the rectiiier current suddenly reaches a relatively high value and then decreases gradually to zero. It is to be noted Afrom a comparison of the curves 24, 31 and 32 that the damped oscillation, indicated bythe curve 24, results from the oscillation of the current flowing through the capacitor 18 as shown by the curve 28.

A curve 34 of FIG. 8 shows how the Volt-age across the winding 10 changes during the deenergization times t1 and t2. Before the contacts 13 open, the voltage has a value indicated at 35 which may be assumed to be in a positive sense from the conductor 111 to the conductor 12. As soon as the contacts 13 open, this voltage reverses and rises to a maximum value in the opposite direction indicated at 36 which is slightly above the Zener voltage of the rectifier 16. Current then starts to ow through the rectifiers 16 and 17 and the voltage decreases along the portion of the curve indicated by 34a and reaches zero in the time t1 plus t2.

The composite graph of FIG. 9 shows how the circuit of FIG. 1 operates with the rectifiers 16 and 17 omitted. Curves 38 and 39 illustrate the change in the current, including a current reversal, in the winding 10 and in the capacitor 18, respectively, upon opening of the contacts 13. A curve 40 shows how the voltage across the winding 1t) rises to several times the peak voltage of curve 43 and then decreases during the deenergization interval.

The composite graph of FIG. 10 shows how the circuit of FIG. 1 operates with the capacitor 18 omitted. Curves 41 and 42 show the changes in the current in the winding 10 and in the rectifiers 16 and 17, respectively, upon openingof thecontacts 13., A curve 43 illustrates the variation in the voltage across the winding at this time.

For comparison with the curves 38, 39 and 40 of FIG. 9 and with the curves 41, 42 and 43 of FIG. l0, the operation of the complete circuit of FIG. 1 with the same circuit parameters as in FIGS. 9 and 10 is shown by the curves 44, 45, and 46 of FIG. 11. The curves 44 and 45 are the currents in the winding 10` and in the branch 29-30, respectively, whereas the curve 46 is the voltage across the winding 10. Note, by comparing FIG. 11 with FIGS. 9 and 10, that when the rectiiiers 16 and 17 and the capacitor 18 are all used as in FIG. 1, the peak Voltage `is low, there is no signicant reversal of the coil current, and the deenergization time is small. The reduction in deenergization time provided by the suppressor 14 is quite pronounced. In a comparative test on an operating winding for a relatively large contacter, the drop-out time was decreased from 0.150 second without any surge suppressor to 0.117 second with the suppressor 14. The theory of operation of the circuit of FIG. l appears to be that upon opening the contacts 13, a charging current of very short duration ows into the capacitor .=118. This prevents the forming of any arc at the contacts 13. During this charging interval, the voltage across the coil has been increasing. At about the instant the capacitor 18 becomes charged, the circuit through the rectiers 16 and 17 breaks down and starts to conduct. The conduction prevents a further rise in coil voltage, and since no arc has been formed, no current can now ow through the contacts.

FIG. 12 illustrates the same embodiment as FIG. l but with the addition of a small resistor R which may be connected between the winding l10 to be protected and the suppressor 14 to limit the inrush of current to the suppressor. The resistor R is connected in this instance between the points 29 and 30. Operation of the modication is very similar to that of the embodiment illustrated in FIG. 1.

A pair of back-to-back connected rectiliers 16 and 17 and a capacitor 18 suitably correlated and electrically connected to form a suppressor 14 to provide surge protection for a winding of known resistance can be molded within a block of insulating material 50 asf shown in FIG. 13. Partly embedded in and extending outwardly from the block 50 are the suitable terminals 511 and 52 which connect the suppressor 14 electrically to a winding 10. If desired, the block may also be provided with a mounting bracket 53 which is partly embedded therein to secure the block 50, and thereby the suppressor 14, to a suitable supporting structure.

Having thus described my invention, I claim:

An electrical circuit comprising a direct current source, a switch, an inductive winding, circuit means connecting said switch and said winding in series with each other across said source, whereby upon closure of said switch a direct current ows from said source through said switch and said winding, a capacitor connected in parallel with said winding, a pair of identical rectifiers connected in series with each other in back-to-back relation to define a bypass circuit, the reverse breakdown voltage rating of each of said rectifiers being greatly in excess of the voltage of said source, means connecting said bypass circuit in parallel with said `capacitor and said inductive winding, the volt-age of said source, the capacitance of said capacitor and the inductance of said winding being so related to each other that, upon opening of said switch, the consequent rise in voltage across said winding causes a charging current of short duration to flow into said capacitor with no arc being formed at said switch, and said reverse breakdown voltage rating of said rectiers being so related to the voltage of said source, the capacitance of said capacitor and the inductance of said winding that the voltage across said capacitor just prior to its becoming fully charged is sufficient to break down one of the said rectifiers in the reverse direction, thereby to prevent an arc from forming at said switch after the capacitor becomes charged.

References Cited in the iile of this patent UNITED STATES PATENTS OTHER REFERENCES AIEE, Technical Paper 48-215, August 1948; Sensitive Relay Contact Protection Systems.

Electrical Manufacturing, vol. 71, No. 2, page 166, February 1952.

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