CA1295016C - Power factor correction system - Google Patents
Power factor correction systemInfo
- Publication number
- CA1295016C CA1295016C CA000497852A CA497852A CA1295016C CA 1295016 C CA1295016 C CA 1295016C CA 000497852 A CA000497852 A CA 000497852A CA 497852 A CA497852 A CA 497852A CA 1295016 C CA1295016 C CA 1295016C
- Authority
- CA
- Canada
- Prior art keywords
- power factor
- factor correction
- correction system
- branch
- inductor
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Classifications
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
- H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/30—Reactive power compensation
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P80/00—Climate change mitigation technologies for sector-wide applications
- Y02P80/10—Efficient use of energy, e.g. using compressed air or pressurized fluid as energy carrier
Landscapes
- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Emergency Protection Circuit Devices (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A power factor correction system comprising, in series, a capacitor and a current and voltage limiting circuit. The latter includes a branch comprising, in serial connection, an inductor, a resistor and a first switch. A second switch is mounted in parallel to the branch for short-circuiting same during the steady-state operation of the power factor correction system. The disclosure also describes a fuse assembly having a fusible member mounted within the magnetic field created by the inductor and extending across the magnetic field whereby a force is exerted on the fusible member when a current passes therethrough.
A power factor correction system comprising, in series, a capacitor and a current and voltage limiting circuit. The latter includes a branch comprising, in serial connection, an inductor, a resistor and a first switch. A second switch is mounted in parallel to the branch for short-circuiting same during the steady-state operation of the power factor correction system. The disclosure also describes a fuse assembly having a fusible member mounted within the magnetic field created by the inductor and extending across the magnetic field whereby a force is exerted on the fusible member when a current passes therethrough.
Description
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FIELD OF THE INVENTION
The present invention relates to a power factor correction system and to a rnet:hod for connecting and disconnecting same. The invention also includes a fuse assembly for the power factor correction system of this invention.
BACKGROUND OF THE INVENTION
With the increase of electeicity costs in recent yearsr large electricity consurners have attempted to reduce these costs by improving the power factor of their industrial and commercial establishments. It is well known that a low power factor is created by the presence of induction motors and particularly by motor drives that employ thyristors. The latter are being used more and more because of the ease and accuracy of attaining speed control.
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The power factor of an establishment can be raised to the desired level by installing capacitors.
However, because the power dernand varies throughout the day, the number of capacitors in service must be varied to maintain the power factor at the desired level. As a - 2 - 1295~ ~
result, capacitors must continuously be switched in and out.
When a capacitor is switched across an AC
line it is well known that a large transient current of oscillatory nature flows for a short time, typically - for less than 0.1 second. At the same time, the peak transient voltage across the capacitor can momentarily reach values that are twice as high as those ` 10 encountered in normal operation. The extremely large transient current darnages the switch contacts Furthermore, the large current combined with the overvoltage may cause premature failure of the capacitors.
The peak transient current is particularly large when a capacitor is switched across a line that already has capacitors installed across it. The peak current is then limited only by the resistance and inductance of the leads connecting the incoming capacitor to the line. The resulting peak currents can reach values 100 times greater than normal.
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When the capacitor is disconnected from the line, there is usually no substantial arc while the ` switch contacts are separating. As a result, opening ., the circuit of a capacitor that is directly across the .i .
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FIELD OF THE INVENTION
The present invention relates to a power factor correction system and to a rnet:hod for connecting and disconnecting same. The invention also includes a fuse assembly for the power factor correction system of this invention.
BACKGROUND OF THE INVENTION
With the increase of electeicity costs in recent yearsr large electricity consurners have attempted to reduce these costs by improving the power factor of their industrial and commercial establishments. It is well known that a low power factor is created by the presence of induction motors and particularly by motor drives that employ thyristors. The latter are being used more and more because of the ease and accuracy of attaining speed control.
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The power factor of an establishment can be raised to the desired level by installing capacitors.
However, because the power dernand varies throughout the day, the number of capacitors in service must be varied to maintain the power factor at the desired level. As a - 2 - 1295~ ~
result, capacitors must continuously be switched in and out.
When a capacitor is switched across an AC
line it is well known that a large transient current of oscillatory nature flows for a short time, typically - for less than 0.1 second. At the same time, the peak transient voltage across the capacitor can momentarily reach values that are twice as high as those ` 10 encountered in normal operation. The extremely large transient current darnages the switch contacts Furthermore, the large current combined with the overvoltage may cause premature failure of the capacitors.
The peak transient current is particularly large when a capacitor is switched across a line that already has capacitors installed across it. The peak current is then limited only by the resistance and inductance of the leads connecting the incoming capacitor to the line. The resulting peak currents can reach values 100 times greater than normal.
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When the capacitor is disconnected from the line, there is usually no substantial arc while the ` switch contacts are separating. As a result, opening ., the circuit of a capacitor that is directly across the .i .
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line presents no problem as regards contact wear However, if the switch opens slowly, the interrupted current may be re-established when the contacts are slightly apart. This creates a large transient restrike current which is even greater than the transient current when the capacitor was switched onto the line.
This restrike current may repeat several times, with damaging effect on the switch contacts However,if the switch contacts separate quickly enough! the restrike phenomenon does not occur.
In view of explanations that will be given later on, it must also be stated that if a resistor is connected in series with a capacitor, an arc will be 15drawn when the switch disconnects the capacitor from the AC line. The magnitude of the arc becomes greater, the larger the voltage drop across the resistor at the ~moment the circuit is interrupted. Thus, the contact Jwear on switch opening is greater than if no resistor 20were present. The same remarks apply when an inductance is connected in series with the capacitor.
In view of further explanations that will be given later on, it must be stated that electronic motor 25drives produce harmonic currents in the AC line. These currents have frequencies that are odd multiples of the , .
line frequency. On a 60 Hz line, the principal harmonic ,..
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currents are the 5th , 7 h and 11th, corresponding to 300 ~z, 420 Hz and 660 Hz. Other harmonic currents are also present, but their effect is usually insignificant. The magnitude of the harmonic currents is directly related to the power drawn by the electric drive.
Corrective measures using resistors.
In order to reduce the transient current when a capacitor is switched across the line by a switch (Sl), a resistor may be connected in series with the capacitor. The presence of a resistor also eliminates the transient overvoltage across the capacitor, and this method of solving the overcurrent and overvoltage problem is well known. A resistor having a relativity high resistance can reduce the peak transient current to a reasonabIe value. However, such a resistor will cause a substantial arc to be formed when the capacitor is disconnected from the line by opening a first switch (S13. Furthermore, the sustained heat loss and energy loss of the resistor while the capacitor is in service, may not be economically acceptable. As a result, it is common practice to short-circuit the resistor with a second switch (S2) shortly after the capacitor is switched into service.
~2g~6 In -the course of investigation, it has been discovered that when switch (S2) closes, a relatively large transient current is again produced. The magnitude of this current increases with the resistance of the resistor. More precisely, the peak transient current depends upon the voltage drop that existed across the resistor just prior to the moment when switch (S2) was closed. In effect, it is as if the capacitor were switched across a line whose voltage is equal to the voltage drop across the resistor.
'rhe said transient current can damage the contacts of switch (S2). Thus, in order to reduce the contact wear of switch (S2), the resistor must possess a relatively low resistance. But a low resistance tends to raise the peak transient current when switch (Sl) is closed. Thus, a compromise must be struck so that the contact wear of both switch (Sl) and switch (S2) is acceptable. In brief, two transient inrush currents are produced when a resistor/capacitor circuit is used, and the value of the resistor must be selected to optimize ; the contact wear on the two switches.
, Corrective measures using inductors.
An inductor is sometimes used in series with a capacitor to limit the transient current. The - 6 ~ ~ 2~ 6 advantage of the inductor is that it dissipates very little heat (compared to a resistor) and, consequently~
it can be left permanently in series ~ith the capacitor while the latter is in service. However, because the inductor carries the full capacitor current it tends to be large and rather expensive. Furthermore, the presence of the inductor does not, in any way, reduce the transient overvoltage that occurs across the capacitor when it is switched onto the AC line. As a result, the capaci.tor can still suffer damage after repeated switching.
The presence of the inductor can also produce serious resonance effects when the factory or industrial establishment contains electronic motor drives. The reason is that the harmonic currents generated by the drives may be amplified many times depending upon the relative magnitude of the inductors, ; capacitors and other inductive devices (such as electric motors) that happen to be in operation at a given time. This amplification is due to resonance and it can cause large harmonic currents to flow in the capacitors, as well as in other parts of the electrical system of the industrial establishment, notably the service entrance transformer. The large harmonic currents can also produce overvoltages and voltage distortion in the electrical system.
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OBJECTS AND STATEMENT OF T~E I~YE~TIOM
Therefore, it is an object of this invention to provide a power factor correction system with an improved configuration per~nitting to use switches of reduced current capacity than the prior art devices.
Another object of the present invention is to provide a power factor correction system which is compatible with modern electronic motor drives~
Another object is a method for operating the power factor correction system of this invention, which method reduces the erosion of the switch contacts.
Another object is to reduce the size of the inductors and resistors that are used to limit the ~' inrush currents, A further object of the invention is a novel fuse assembly that may be used advantageously with the power factor correction system of the present ~' invention.
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, 25 The above objects are achieved by providing a . power factor correction system comprising a capacitor and a voltage and current limiting circuit (hereinafter .,, - :
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"limiting circuit") for reducing the amplitude of the transient currents and voltages in the capacitor. The limiting circuit includes a branch consisting of a resistor and an inductor connected in series. The branch is active only for a given period of time;
during the steady state operation of the system, it is by-passed.
Preferably, the limiting circuit includes a ; 10 branch consisting of a resistor, an inductor and a first switch (Sl) connected in series, and a second switch (S2) connected in parallel with said branch for de-activating it by establishing a short-circuit.
When the power-factor correction system is ~` energized, (Sl) is closed first. When the resulting transient voltages and currents have essentially ~ subsided and when, therefore, the potential across the ;~ inductor has dropped substantially (S2) is closed so as to short-circuit the branch. The current rating of (Sl) can, therefore, be relatively low since (Sl) carries current only during a short period of time, typically ~ for one or two cycles. The presence of the inductor ;~ allows the use -of a resistor of relatively low resistance and a corresponding low voltage drop across - the resistor. As a result, a relatively low transient ` 1 !
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current is produced when the contacts of (S2) close, which reduces their wear.
To disconnect the power factor correction system, (S1) is opened first, followed by the opening of (S2). If the opposite sequence is followed, the contacts of (S11 erode much faster because the presence of the resistor and the inductor in the branch creates an arc across (Sl) whenever (Sl) opens.
Preferabl~, the power factor correction system of this invention is provided with a fuse in the branch. The fuse comprises a fusible member adapted to melt when a current with a given amplitude passes therethrough for a given time. In order to reduce the size of the fuse and yet allow it to clear a fault quickly and without restriking, the fusible member is mounted within a magnetic field, and substantially cross-wise to the magnetic field. When current circulates through the fusible member, a force is exerted thereon with a direction perpendicular to the ; direction of the current and to the magnetic field.
; When the fuse begins to melt, the force blows both the molten material and resulting arc and so the circuit is interrupted quickly and effectively. The magnetic field may be generated by a coil.
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The power factor correction system of this invention may be used with a single-phase system as well as with a three-phase system. In the latter case, the power-factor correction system includes a capacitor bank in delta or in Y configuration and three limiting circuits connected respectively between the three-phase lines and the capacitor bank.
Therefore, the present invention comprises a power factor correction system which in broad terms includes:
- a capacitor; and - a limiting circuit having a given irnpedance, the limiting circuit limiting the overvoltages and the overcurrents susceptible to occur in the capacitor after the energizing of the power factor correction system, the limiting circuit being connected to one terminal of the capacitor and including:
a) a branch having a resistor serially connected to-an inductor; and b) impedance reducing means operatively connected to the branch for reducing the impedance of the limiting circuit after a given period of time following the energizing of the power factor correction system.
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This invention also relates to a m~thod for energizing a power factor correction system comprising:
- a capacitor; and - a limiting circuit having a given S impedance, the limiting circuit being connected to one terminal of the capacitor and limiting the overvoltages and the overcurrents susceptible to occur in the capacitor after the power factor correction system is energized, the limiting circuit including:
a) a branch including in series a resistor, an inductor and a first switch means; and b) a second switch means mounted in parallel with the branch for short-circuiting the branch after a given period of time from the energizing of the power factor correction system, the first and the second switch means being in opened position before the energizing of the power factor correction system.
The method for energizing the power factor correction system, in general terms, consists of the following sequential steps:
- closing the first switch means thereby allowing current to circulate through the branch, the current creating a relatively high potential across the inductor; and ~25 - closlng the second switch means when the ~potential has dropped substantially.
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The present invention further relates to a method for disconnecting the power factor correction system, the method consisting, in broad terms, of the following sequential steps:
- opening the first switch means; and - opening the second switch means.
This invention also comprises a combined fuse and inductor and resistor circuit for use in a power factor correction system, the fuse and inductor and resistor circuit comprising in its most general aspects:
- an inductor adapted to generate a magnet.ic field with a given direction when current circulates therethrough; and - a fusible member serially connected to the inductor and being mounted within the magnetic field and extending across the magnetic field, current with a given direction being adapted to circulate through the fusible member, when current circulates through the fusible member and through the inductor a force being exerted on the fusible member with : a d.irection perpendicular to the direction of the magnetic field and to the direction of the current in the fusible member.
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BRIEF DESCRIPTIOI~ OF DRAWINGS
A detailed description of preferred embodiments of the present invention will be given hereinafter with reference to the annexed drawings in which:
Figure 1 is a diagram of a prior art power factor correction system;
Figure 2 is a diagram of a single-phase power factor correction system according to this invention;
Figure 3 is a diagram of a three-phase power ` 15 factor correction system according to this invention;
: ~ ~ Figure 4 is a diagram illustrating the ~, principle of operation of a fuse assembly according to this invention; and Figure 5 is an exploded view, partly sectional, of an embodiment of the resistor-inductor ~ f ~;t~ and fuse assembly of this invention.
,~ Z5 Figure~ 1 illustrates schematically a prior art power factor correction system such as the one .~ : making the subject of US patent 1 939 064 issued . December 12, 1933 to T. Kopczynski.
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PRIO~ A~T SYSTEM
The power factor correction system, designated generally by reference numeral 1, is connected to a single-phase line 2 The correction system 1 comprises a capacitor 3 connected to a resistor 4. A main switch 5 is in series with capacitor 3 and is utilized to switch correction system 1 across the power line. A second switch 6 is mounted in parallel with resistor 4. When the power-factor correction system is to be energized, switch 5 is closed first and resistor 4 limits the transient current in capacitor 3. After a predetermined period of time sufficiently long so that the capacitor current and voltage have become stable, switch 6 is closed, thus short-circuiting resistor 4 and avoiding continual power losses.
However, when switch 6 closes, and as previously explained in the disclosure, the voltage drop Er across resistor 4 gives rise to a large transient current which flows through the contacts of switch 6, increasing the wear thereof.
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Further~nore, during the steady-state operation of the power factor correction system 1, both switches 5 and 6 carry the full current of the - 1s - ~,.~5~
capacitor and consequently they must both have a relatively high current rating.
DESCRIPTION OF PREFERRED EMBODI~ENTS
Referring now to figure 2, a single-phase power factor correction system 7, according to this invention, comprises a capacitor 8, and a main fuse 9 connected in series with a current-limiting circuit 10.
The latter includes switch (S2) and a branch 11 comprisiny in serial connection a switch (Sl), a resistor 12, a fuse 13 and an inductor 14. Switch (S2) is connected in parallel with branch 11 for short-circuiting same during the steady state operation of the power factor correction system 7.
Figure 3 illustrates a three-phase power factor correction system, according to the present invention, connected across the three-phase lines 15 of an industrial or commercial establishment. To each phase line A, B, C is connected a limiting circuit 1~, already described. A capacitor bank 16 co~posed of three capacitors 8 is connected to the limiting circuits 10. The capacitor bank has a Y configuration.
The capacitor bank may also have a delta configuration.
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'' ~2~5 Figure 4 illustrates schematically an embodiment of branch 11 which comprises the inductor 14 formed by a coil of conduc~ive copper wire 17 having a given intrinsic resistance which constitutes resistor 12. The resistance of resistor 12 may be changed by varying ~he cross section of wire 17, as is well known in the art.
Fuse 13 is in serial connection with inductor 14 and comprises a fusible member 18 adapted to melt when the current passing therethrough exceecls a predetermined I2t limit. Fusible member 18 lies within and across the magnetic field generated by inductor 14.
The direction of the magnetic field is designated by the arrow M when current I passes in the direction shown through fusible member 18. When current I flows in the direction designated by arrow I, a force F is exerted on fusible member 18. According to Fleming's right-hand rule, the direction of force F is perpendicular to the direction of the current in the fusible member and also perpendicular to the direction of the magnetic field. As a result, fusible member 18 is pushed away from the observer, and into the page of ~ Figure ~. Whenever an excessive current flows for a :,. .
sufficient length of time through fusible member 18, ~ the force F will blow away the molten parts of fusible ;~ member 18 as well as the resulting arc. The arc becomes ~ . ' stretched, allowing the fault to clear rapidly and completely, thus eliminating the possibility of an arc restrike between the terminals 19, 20 of the fusible member. The inductor thus fills a dual role: it limits the peak transient current as previously described, and provides the magnetic field to blow ou- the fuse.
Resistor 12 may also be constituted by an independent resistor cQnnected in series with inductor 14. Fuse 13 may also be constituted by an independent fuse of commercial make but not located in the magnetic field oE inductor 14.
Referring now to Figure 5, the inductor, resistor and fuse of the present invention are mounted within a housing 21 comprising a capsule 22 of plastic material into which is embedded inductor 14. As stated earlier, resistor 12 is constituted by the intrinsic resistance of the wire forming inductor 14. The fuse terminals, 19 and 20, respectively project upwardly from capsule 22, terminal 20 being connected to terminal 23 of inductor 14. Between terminals 19 and 20 extends fusible member 18. Two leads 25 and 24 projecting from capsule 22 are in serial connection with ~erminal 26 of inductor 14 and fuse terminal 19, respectively.
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, j On the top face of capsule 22 is mounted a spacing ring 27 of plastic material receiving a transparent disc member 28 having openings 29 Ring 27 and disc 28 define a fuse chamber into which is located fusible member 18. When fusible member 18 blows, the : excess pressure generated in the fuse chamber is eliminated through openings 29. Since disc member 28 is transparent, the condition of fuse member 18 may be easily inspected.
The power factor correction system of this invention operates as follow.s. Referring to Fig~lre 2, when the system is switched on, switch (Sl) is closed first and a relatively large transient current I
begins to circulate through branch 11 and capacitor 8.
The peak value of transient current Il is limited to an acceptable value by inductor 14 and resistor 12.
Compared to the voltage drop across resistor 12, the transient voltage drop El across inductor 14 is relatively high immediately after the closing of (Sl) : because the current in branch 11 is changing very ~ rapidly. In order to prevent an excessive translent ; overvoltage across capacitor 8 during this period, it is important to select appropriate values for both the `1` 25 resistance of resistor 12 and the inductance of inductor 14 in relation to the capacitance of capacitor :~. 8.
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Switch (S2) is closed when the voltage drop El across inductor 14 has become negligible, which occurs shortly after the closure of (S1). In effect, inductor 14 and resistor 12 rapidly damp the transient current Il in branch 11. When the transient has died out, the potential be-~ween the contacts of (S2) is constituted almost entirely oy the voltage drop Er across resistor 12 which is of lesser value than the potential across resistor 4 of the prior art device shown in Figure 1. The reason is that, in the present invention, inductor 14 permits the use of a resistor 12 of smaller resistance without affecting the current limiting properties of branch 11.
Therefore, at the closure of (S2) the peak ; transient current I2 is smaller than it would be if the inductor was not present and only a resistor was used to limit current Il.
From the above, it is now clear that the resistance of resistor 12 should be kept as low as possible in order to limit the peak transient current I2. This may be achieved, without increasing the peak value of transient current Ill by increasing the i lnductance of inductor 14. However, care must be taken to choose an inductance that is not too high, which may create damaging transient overvoltages across capacitor . . ..
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As an example, the following table illustrates the typical relationship between currents Il, I2 and the elements of branch 11.
I1 peak I2 peak a) resistor only600 A 800 A
b) resistor and inductor 300 A 800 A
c) resistor , and inductor, the resistor being of lesser value than in case b) 400 A 500 A
In case a) only a resistor is used in branch 11 of the current-limiting circuit. The resistor has a sufficiently large resistance to limit the peak ,~ ~ transient current Il to 600 A, which is assumed to be ~; an acceptable value. However, the steady-state voltage ` i drop Er across the resistor is relatively high and so ~' when switch (S2) closes a large transient current I2 ;' ~ having a peak of 800 A is drawn.
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In case b) an inductor having negligible resistance is added to branch 11. The presence of the inductor reduces the current Il to 300 A, but has no significant effect on current I2.
In case c) the value of the resistor has been reduced relative to case b), causing an increase of current Il from 300 A to 40~ A, which is assumed to be acceptable. However, the peak transient current I2 is thereby reduced to 500 A which, in turn, reduces the wear of switch (S2).
In a typical installation, the transient inrush current Il is essentially terminated within five milliseconds. Accordingly, on a 50 Hz or 60 HZ system, switch (S2) can be closed one or two cycles after (Sl) closes. Now when (S2) closes, the current Il falls to a negligible value because branch 11 is then short-circuited. This implies that a relatively small switch ~Sl) is adequate because it carries current for a only very short time. By the same token, resistor 12 and inductor 14 can be made physically very small. The conductor size of the inductor is typically 100 times smaller than whak it would be if the inductor had to permanently carry the rated current of the capacitor.
However, the small size of the inductor, ,, . .
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which has an intrinsic resistance equal to that of resistor 12, makes the use of a fuse protection in series with the inductor, preferable. One reason is that i~ switch (S2) should, for any reason, fail to close, the current that continues to flow through the inductor would cause the latter to burn up in less than a minute. To eliminate the danger of such a failure, a fuse 13 is included in branch 11.
In order to disconnect the power factor correction system of the present invention, switch (Sl) opens first followed by switch ~S2). If the opposite sequence is followed, the presence of inductor 14 and resistor 12 will create an arc across the contacts of switch (Sl) which will shorten its useful life.
The above description has been given only as an example and should not be considered as limiting in any sense. The scope of this invention is defined in the annexed claims.
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line presents no problem as regards contact wear However, if the switch opens slowly, the interrupted current may be re-established when the contacts are slightly apart. This creates a large transient restrike current which is even greater than the transient current when the capacitor was switched onto the line.
This restrike current may repeat several times, with damaging effect on the switch contacts However,if the switch contacts separate quickly enough! the restrike phenomenon does not occur.
In view of explanations that will be given later on, it must also be stated that if a resistor is connected in series with a capacitor, an arc will be 15drawn when the switch disconnects the capacitor from the AC line. The magnitude of the arc becomes greater, the larger the voltage drop across the resistor at the ~moment the circuit is interrupted. Thus, the contact Jwear on switch opening is greater than if no resistor 20were present. The same remarks apply when an inductance is connected in series with the capacitor.
In view of further explanations that will be given later on, it must be stated that electronic motor 25drives produce harmonic currents in the AC line. These currents have frequencies that are odd multiples of the , .
line frequency. On a 60 Hz line, the principal harmonic ,..
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currents are the 5th , 7 h and 11th, corresponding to 300 ~z, 420 Hz and 660 Hz. Other harmonic currents are also present, but their effect is usually insignificant. The magnitude of the harmonic currents is directly related to the power drawn by the electric drive.
Corrective measures using resistors.
In order to reduce the transient current when a capacitor is switched across the line by a switch (Sl), a resistor may be connected in series with the capacitor. The presence of a resistor also eliminates the transient overvoltage across the capacitor, and this method of solving the overcurrent and overvoltage problem is well known. A resistor having a relativity high resistance can reduce the peak transient current to a reasonabIe value. However, such a resistor will cause a substantial arc to be formed when the capacitor is disconnected from the line by opening a first switch (S13. Furthermore, the sustained heat loss and energy loss of the resistor while the capacitor is in service, may not be economically acceptable. As a result, it is common practice to short-circuit the resistor with a second switch (S2) shortly after the capacitor is switched into service.
~2g~6 In -the course of investigation, it has been discovered that when switch (S2) closes, a relatively large transient current is again produced. The magnitude of this current increases with the resistance of the resistor. More precisely, the peak transient current depends upon the voltage drop that existed across the resistor just prior to the moment when switch (S2) was closed. In effect, it is as if the capacitor were switched across a line whose voltage is equal to the voltage drop across the resistor.
'rhe said transient current can damage the contacts of switch (S2). Thus, in order to reduce the contact wear of switch (S2), the resistor must possess a relatively low resistance. But a low resistance tends to raise the peak transient current when switch (Sl) is closed. Thus, a compromise must be struck so that the contact wear of both switch (Sl) and switch (S2) is acceptable. In brief, two transient inrush currents are produced when a resistor/capacitor circuit is used, and the value of the resistor must be selected to optimize ; the contact wear on the two switches.
, Corrective measures using inductors.
An inductor is sometimes used in series with a capacitor to limit the transient current. The - 6 ~ ~ 2~ 6 advantage of the inductor is that it dissipates very little heat (compared to a resistor) and, consequently~
it can be left permanently in series ~ith the capacitor while the latter is in service. However, because the inductor carries the full capacitor current it tends to be large and rather expensive. Furthermore, the presence of the inductor does not, in any way, reduce the transient overvoltage that occurs across the capacitor when it is switched onto the AC line. As a result, the capaci.tor can still suffer damage after repeated switching.
The presence of the inductor can also produce serious resonance effects when the factory or industrial establishment contains electronic motor drives. The reason is that the harmonic currents generated by the drives may be amplified many times depending upon the relative magnitude of the inductors, ; capacitors and other inductive devices (such as electric motors) that happen to be in operation at a given time. This amplification is due to resonance and it can cause large harmonic currents to flow in the capacitors, as well as in other parts of the electrical system of the industrial establishment, notably the service entrance transformer. The large harmonic currents can also produce overvoltages and voltage distortion in the electrical system.
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OBJECTS AND STATEMENT OF T~E I~YE~TIOM
Therefore, it is an object of this invention to provide a power factor correction system with an improved configuration per~nitting to use switches of reduced current capacity than the prior art devices.
Another object of the present invention is to provide a power factor correction system which is compatible with modern electronic motor drives~
Another object is a method for operating the power factor correction system of this invention, which method reduces the erosion of the switch contacts.
Another object is to reduce the size of the inductors and resistors that are used to limit the ~' inrush currents, A further object of the invention is a novel fuse assembly that may be used advantageously with the power factor correction system of the present ~' invention.
` ?
, 25 The above objects are achieved by providing a . power factor correction system comprising a capacitor and a voltage and current limiting circuit (hereinafter .,, - :
- 8 - ~ ~ ~ S~ 1~
"limiting circuit") for reducing the amplitude of the transient currents and voltages in the capacitor. The limiting circuit includes a branch consisting of a resistor and an inductor connected in series. The branch is active only for a given period of time;
during the steady state operation of the system, it is by-passed.
Preferably, the limiting circuit includes a ; 10 branch consisting of a resistor, an inductor and a first switch (Sl) connected in series, and a second switch (S2) connected in parallel with said branch for de-activating it by establishing a short-circuit.
When the power-factor correction system is ~` energized, (Sl) is closed first. When the resulting transient voltages and currents have essentially ~ subsided and when, therefore, the potential across the ;~ inductor has dropped substantially (S2) is closed so as to short-circuit the branch. The current rating of (Sl) can, therefore, be relatively low since (Sl) carries current only during a short period of time, typically ~ for one or two cycles. The presence of the inductor ;~ allows the use -of a resistor of relatively low resistance and a corresponding low voltage drop across - the resistor. As a result, a relatively low transient ` 1 !
' .
current is produced when the contacts of (S2) close, which reduces their wear.
To disconnect the power factor correction system, (S1) is opened first, followed by the opening of (S2). If the opposite sequence is followed, the contacts of (S11 erode much faster because the presence of the resistor and the inductor in the branch creates an arc across (Sl) whenever (Sl) opens.
Preferabl~, the power factor correction system of this invention is provided with a fuse in the branch. The fuse comprises a fusible member adapted to melt when a current with a given amplitude passes therethrough for a given time. In order to reduce the size of the fuse and yet allow it to clear a fault quickly and without restriking, the fusible member is mounted within a magnetic field, and substantially cross-wise to the magnetic field. When current circulates through the fusible member, a force is exerted thereon with a direction perpendicular to the ; direction of the current and to the magnetic field.
; When the fuse begins to melt, the force blows both the molten material and resulting arc and so the circuit is interrupted quickly and effectively. The magnetic field may be generated by a coil.
i - 1 o - $;~5~
The power factor correction system of this invention may be used with a single-phase system as well as with a three-phase system. In the latter case, the power-factor correction system includes a capacitor bank in delta or in Y configuration and three limiting circuits connected respectively between the three-phase lines and the capacitor bank.
Therefore, the present invention comprises a power factor correction system which in broad terms includes:
- a capacitor; and - a limiting circuit having a given irnpedance, the limiting circuit limiting the overvoltages and the overcurrents susceptible to occur in the capacitor after the energizing of the power factor correction system, the limiting circuit being connected to one terminal of the capacitor and including:
a) a branch having a resistor serially connected to-an inductor; and b) impedance reducing means operatively connected to the branch for reducing the impedance of the limiting circuit after a given period of time following the energizing of the power factor correction system.
;
2~s~ ~ ~
This invention also relates to a m~thod for energizing a power factor correction system comprising:
- a capacitor; and - a limiting circuit having a given S impedance, the limiting circuit being connected to one terminal of the capacitor and limiting the overvoltages and the overcurrents susceptible to occur in the capacitor after the power factor correction system is energized, the limiting circuit including:
a) a branch including in series a resistor, an inductor and a first switch means; and b) a second switch means mounted in parallel with the branch for short-circuiting the branch after a given period of time from the energizing of the power factor correction system, the first and the second switch means being in opened position before the energizing of the power factor correction system.
The method for energizing the power factor correction system, in general terms, consists of the following sequential steps:
- closing the first switch means thereby allowing current to circulate through the branch, the current creating a relatively high potential across the inductor; and ~25 - closlng the second switch means when the ~potential has dropped substantially.
.' ~
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The present invention further relates to a method for disconnecting the power factor correction system, the method consisting, in broad terms, of the following sequential steps:
- opening the first switch means; and - opening the second switch means.
This invention also comprises a combined fuse and inductor and resistor circuit for use in a power factor correction system, the fuse and inductor and resistor circuit comprising in its most general aspects:
- an inductor adapted to generate a magnet.ic field with a given direction when current circulates therethrough; and - a fusible member serially connected to the inductor and being mounted within the magnetic field and extending across the magnetic field, current with a given direction being adapted to circulate through the fusible member, when current circulates through the fusible member and through the inductor a force being exerted on the fusible member with : a d.irection perpendicular to the direction of the magnetic field and to the direction of the current in the fusible member.
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BRIEF DESCRIPTIOI~ OF DRAWINGS
A detailed description of preferred embodiments of the present invention will be given hereinafter with reference to the annexed drawings in which:
Figure 1 is a diagram of a prior art power factor correction system;
Figure 2 is a diagram of a single-phase power factor correction system according to this invention;
Figure 3 is a diagram of a three-phase power ` 15 factor correction system according to this invention;
: ~ ~ Figure 4 is a diagram illustrating the ~, principle of operation of a fuse assembly according to this invention; and Figure 5 is an exploded view, partly sectional, of an embodiment of the resistor-inductor ~ f ~;t~ and fuse assembly of this invention.
,~ Z5 Figure~ 1 illustrates schematically a prior art power factor correction system such as the one .~ : making the subject of US patent 1 939 064 issued . December 12, 1933 to T. Kopczynski.
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PRIO~ A~T SYSTEM
The power factor correction system, designated generally by reference numeral 1, is connected to a single-phase line 2 The correction system 1 comprises a capacitor 3 connected to a resistor 4. A main switch 5 is in series with capacitor 3 and is utilized to switch correction system 1 across the power line. A second switch 6 is mounted in parallel with resistor 4. When the power-factor correction system is to be energized, switch 5 is closed first and resistor 4 limits the transient current in capacitor 3. After a predetermined period of time sufficiently long so that the capacitor current and voltage have become stable, switch 6 is closed, thus short-circuiting resistor 4 and avoiding continual power losses.
However, when switch 6 closes, and as previously explained in the disclosure, the voltage drop Er across resistor 4 gives rise to a large transient current which flows through the contacts of switch 6, increasing the wear thereof.
`
Further~nore, during the steady-state operation of the power factor correction system 1, both switches 5 and 6 carry the full current of the - 1s - ~,.~5~
capacitor and consequently they must both have a relatively high current rating.
DESCRIPTION OF PREFERRED EMBODI~ENTS
Referring now to figure 2, a single-phase power factor correction system 7, according to this invention, comprises a capacitor 8, and a main fuse 9 connected in series with a current-limiting circuit 10.
The latter includes switch (S2) and a branch 11 comprisiny in serial connection a switch (Sl), a resistor 12, a fuse 13 and an inductor 14. Switch (S2) is connected in parallel with branch 11 for short-circuiting same during the steady state operation of the power factor correction system 7.
Figure 3 illustrates a three-phase power factor correction system, according to the present invention, connected across the three-phase lines 15 of an industrial or commercial establishment. To each phase line A, B, C is connected a limiting circuit 1~, already described. A capacitor bank 16 co~posed of three capacitors 8 is connected to the limiting circuits 10. The capacitor bank has a Y configuration.
The capacitor bank may also have a delta configuration.
~, ~
'' ~2~5 Figure 4 illustrates schematically an embodiment of branch 11 which comprises the inductor 14 formed by a coil of conduc~ive copper wire 17 having a given intrinsic resistance which constitutes resistor 12. The resistance of resistor 12 may be changed by varying ~he cross section of wire 17, as is well known in the art.
Fuse 13 is in serial connection with inductor 14 and comprises a fusible member 18 adapted to melt when the current passing therethrough exceecls a predetermined I2t limit. Fusible member 18 lies within and across the magnetic field generated by inductor 14.
The direction of the magnetic field is designated by the arrow M when current I passes in the direction shown through fusible member 18. When current I flows in the direction designated by arrow I, a force F is exerted on fusible member 18. According to Fleming's right-hand rule, the direction of force F is perpendicular to the direction of the current in the fusible member and also perpendicular to the direction of the magnetic field. As a result, fusible member 18 is pushed away from the observer, and into the page of ~ Figure ~. Whenever an excessive current flows for a :,. .
sufficient length of time through fusible member 18, ~ the force F will blow away the molten parts of fusible ;~ member 18 as well as the resulting arc. The arc becomes ~ . ' stretched, allowing the fault to clear rapidly and completely, thus eliminating the possibility of an arc restrike between the terminals 19, 20 of the fusible member. The inductor thus fills a dual role: it limits the peak transient current as previously described, and provides the magnetic field to blow ou- the fuse.
Resistor 12 may also be constituted by an independent resistor cQnnected in series with inductor 14. Fuse 13 may also be constituted by an independent fuse of commercial make but not located in the magnetic field oE inductor 14.
Referring now to Figure 5, the inductor, resistor and fuse of the present invention are mounted within a housing 21 comprising a capsule 22 of plastic material into which is embedded inductor 14. As stated earlier, resistor 12 is constituted by the intrinsic resistance of the wire forming inductor 14. The fuse terminals, 19 and 20, respectively project upwardly from capsule 22, terminal 20 being connected to terminal 23 of inductor 14. Between terminals 19 and 20 extends fusible member 18. Two leads 25 and 24 projecting from capsule 22 are in serial connection with ~erminal 26 of inductor 14 and fuse terminal 19, respectively.
.
, j On the top face of capsule 22 is mounted a spacing ring 27 of plastic material receiving a transparent disc member 28 having openings 29 Ring 27 and disc 28 define a fuse chamber into which is located fusible member 18. When fusible member 18 blows, the : excess pressure generated in the fuse chamber is eliminated through openings 29. Since disc member 28 is transparent, the condition of fuse member 18 may be easily inspected.
The power factor correction system of this invention operates as follow.s. Referring to Fig~lre 2, when the system is switched on, switch (Sl) is closed first and a relatively large transient current I
begins to circulate through branch 11 and capacitor 8.
The peak value of transient current Il is limited to an acceptable value by inductor 14 and resistor 12.
Compared to the voltage drop across resistor 12, the transient voltage drop El across inductor 14 is relatively high immediately after the closing of (Sl) : because the current in branch 11 is changing very ~ rapidly. In order to prevent an excessive translent ; overvoltage across capacitor 8 during this period, it is important to select appropriate values for both the `1` 25 resistance of resistor 12 and the inductance of inductor 14 in relation to the capacitance of capacitor :~. 8.
/
- 19 - ~
Switch (S2) is closed when the voltage drop El across inductor 14 has become negligible, which occurs shortly after the closure of (S1). In effect, inductor 14 and resistor 12 rapidly damp the transient current Il in branch 11. When the transient has died out, the potential be-~ween the contacts of (S2) is constituted almost entirely oy the voltage drop Er across resistor 12 which is of lesser value than the potential across resistor 4 of the prior art device shown in Figure 1. The reason is that, in the present invention, inductor 14 permits the use of a resistor 12 of smaller resistance without affecting the current limiting properties of branch 11.
Therefore, at the closure of (S2) the peak ; transient current I2 is smaller than it would be if the inductor was not present and only a resistor was used to limit current Il.
From the above, it is now clear that the resistance of resistor 12 should be kept as low as possible in order to limit the peak transient current I2. This may be achieved, without increasing the peak value of transient current Ill by increasing the i lnductance of inductor 14. However, care must be taken to choose an inductance that is not too high, which may create damaging transient overvoltages across capacitor . . ..
~, i ., .
- 20 ~ 9 EXAMPLES
As an example, the following table illustrates the typical relationship between currents Il, I2 and the elements of branch 11.
I1 peak I2 peak a) resistor only600 A 800 A
b) resistor and inductor 300 A 800 A
c) resistor , and inductor, the resistor being of lesser value than in case b) 400 A 500 A
In case a) only a resistor is used in branch 11 of the current-limiting circuit. The resistor has a sufficiently large resistance to limit the peak ,~ ~ transient current Il to 600 A, which is assumed to be ~; an acceptable value. However, the steady-state voltage ` i drop Er across the resistor is relatively high and so ~' when switch (S2) closes a large transient current I2 ;' ~ having a peak of 800 A is drawn.
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In case b) an inductor having negligible resistance is added to branch 11. The presence of the inductor reduces the current Il to 300 A, but has no significant effect on current I2.
In case c) the value of the resistor has been reduced relative to case b), causing an increase of current Il from 300 A to 40~ A, which is assumed to be acceptable. However, the peak transient current I2 is thereby reduced to 500 A which, in turn, reduces the wear of switch (S2).
In a typical installation, the transient inrush current Il is essentially terminated within five milliseconds. Accordingly, on a 50 Hz or 60 HZ system, switch (S2) can be closed one or two cycles after (Sl) closes. Now when (S2) closes, the current Il falls to a negligible value because branch 11 is then short-circuited. This implies that a relatively small switch ~Sl) is adequate because it carries current for a only very short time. By the same token, resistor 12 and inductor 14 can be made physically very small. The conductor size of the inductor is typically 100 times smaller than whak it would be if the inductor had to permanently carry the rated current of the capacitor.
However, the small size of the inductor, ,, . .
, .
. ' ' - 22 ~ t~
which has an intrinsic resistance equal to that of resistor 12, makes the use of a fuse protection in series with the inductor, preferable. One reason is that i~ switch (S2) should, for any reason, fail to close, the current that continues to flow through the inductor would cause the latter to burn up in less than a minute. To eliminate the danger of such a failure, a fuse 13 is included in branch 11.
In order to disconnect the power factor correction system of the present invention, switch (Sl) opens first followed by switch ~S2). If the opposite sequence is followed, the presence of inductor 14 and resistor 12 will create an arc across the contacts of switch (Sl) which will shorten its useful life.
The above description has been given only as an example and should not be considered as limiting in any sense. The scope of this invention is defined in the annexed claims.
. - .
' ~'. .
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Claims (14)
1. A power factor correction system comprising:
- a capacitor; and - a limiting circuit, having a given impedance, for limiting overvoltages and overcurrents susceptible to occur in said capacitor after energizing of said power factor correction system, said limiting circuit being connected to one terminal of said capacitor and including:
a) a branch having a resistor serially connected to an inductor; and b) impedance reducing means operatively connected to said branch for reducing the impedance of said limiting circuit after a given period of time following the energizing of said power factor correction system.
- a capacitor; and - a limiting circuit, having a given impedance, for limiting overvoltages and overcurrents susceptible to occur in said capacitor after energizing of said power factor correction system, said limiting circuit being connected to one terminal of said capacitor and including:
a) a branch having a resistor serially connected to an inductor; and b) impedance reducing means operatively connected to said branch for reducing the impedance of said limiting circuit after a given period of time following the energizing of said power factor correction system.
2. A power factor correction system as defined in claim 1, wherein said branch further comprises a first switch means in series with said resistor and said inductor.
3. A power factor correction system as defined in claim 2, wherein said impedance reducing means comprises a second switch means mounted in parallel with said branch for short-circuiting said branch.
4. A power factor correction system as defined in claim 1, 2 or 3, wherein said period of time is such to allow a relatively high potential created across said inductor, after the energizing of said power factor correction system, to drop substantially.
5. A power factor correction system as defined in claim 1, wherein said branch further comprises a fuse in series with said resistor and said inductor.
6. A power factor correction system as defined in claim 5, wherein said inductor is adapted to generate a magnetic field with a given direction, said fuse including a fusible member through which is adapted to circulate a current with a given direction, said fusible member being mounted within said magnetic field and extending substantially across thereto, wherein a force is created on said fusible member with a direction perpendicular to the direction of said magnetic field and the direction of said current in said fusible member.
7. A power factor correction system as defined in claim 6, wherein said inductor is constituted by a coil of conductive wire, said wire having an intrinsic resistance constituting said resistor.
8. A power factor correction system as defined in claim 6, wherein said inductor and said fuse are mounted in a housing having a transparent window allowing a visual inspection of said fusible member.
9. A power factor correction system as defined in claim 8, wherein said housing comprises a plurality of pressure release openings for eliminating the excess pressure generated in said housing when said fusible member melts when an excessive current passes therethrough for a given period of time.
10. A method for energizing a power factor correction system which includes:
- a capacitor; and - a limiting circuit having a given impedance, said limiting circuit being connected to one terminal of said capacitor and limiting overvoltages and overcurrents susceptible to occur in said capacitor after said power factor correction system is energized, said limiting circuit including:
a) a branch including in series a resistor, an inductor and a first switch means; and b) a second switch means mounted in parallel with said branch for short-circuiting said branch after a given period of time from the energizing of said power factor correction system, said first and said second switch means being in opened position before the energizing of said power factor correction system, said method comprising the sequential steps of:
i) closing said first switch means for allowing current to circulate through said branch, immediately after the closing of said first switch means said current creating a relatively high potential across said inductor; and ii) closing said second switch means when said potential has dropped substantially.
- a capacitor; and - a limiting circuit having a given impedance, said limiting circuit being connected to one terminal of said capacitor and limiting overvoltages and overcurrents susceptible to occur in said capacitor after said power factor correction system is energized, said limiting circuit including:
a) a branch including in series a resistor, an inductor and a first switch means; and b) a second switch means mounted in parallel with said branch for short-circuiting said branch after a given period of time from the energizing of said power factor correction system, said first and said second switch means being in opened position before the energizing of said power factor correction system, said method comprising the sequential steps of:
i) closing said first switch means for allowing current to circulate through said branch, immediately after the closing of said first switch means said current creating a relatively high potential across said inductor; and ii) closing said second switch means when said potential has dropped substantially.
11. A method for disconnecting a power factor correction system which includes:
- a capacitor; and - a limiting circuit having a given impedance, said limiting circuit being connected to one terminal of said capacitor and limiting the overvoltages and the overcurrents susceptible to occur in said capacitor after said power factor correction system is energized, said limiting circuit including:
a) a branch including in series a resistor , an inductor and a first switch means; and b) a second switch means mounted in parallel with said branch for short-circuiting said branch after a given period of time from the energizing of said power factor correction system, said first and second switch means being in closed position before the disconnection of said power factor correction system, said method comprising the sequential steps of:
i) opening said first switch means; and ii) opening said second switch means.
- a capacitor; and - a limiting circuit having a given impedance, said limiting circuit being connected to one terminal of said capacitor and limiting the overvoltages and the overcurrents susceptible to occur in said capacitor after said power factor correction system is energized, said limiting circuit including:
a) a branch including in series a resistor , an inductor and a first switch means; and b) a second switch means mounted in parallel with said branch for short-circuiting said branch after a given period of time from the energizing of said power factor correction system, said first and second switch means being in closed position before the disconnection of said power factor correction system, said method comprising the sequential steps of:
i) opening said first switch means; and ii) opening said second switch means.
12. A three-phase power factor correction system to be used with a three-phase system, said system having three power supply lines, said power factor correction system comprising:
- a capacitor bank having three output terminals; and - three limiting circuits connected respectively between an output terminal of said capacitor bank and a power supply line of said system, each limiting circuit having a given impedance and limiting overvoltages and overcurrents susceptible to occur in said capacitor bank after said power factor correction system is energized, each limiting circuit comprising:
a) a branch having a resistor serially connected to an inductor; and b) impedance reducing means operatively connected to said branch for reducing the impedance of said limiting circuit after a given period of time following the energizing of said power factor correction system.
- a capacitor bank having three output terminals; and - three limiting circuits connected respectively between an output terminal of said capacitor bank and a power supply line of said system, each limiting circuit having a given impedance and limiting overvoltages and overcurrents susceptible to occur in said capacitor bank after said power factor correction system is energized, each limiting circuit comprising:
a) a branch having a resistor serially connected to an inductor; and b) impedance reducing means operatively connected to said branch for reducing the impedance of said limiting circuit after a given period of time following the energizing of said power factor correction system.
13. A three-phase power factor correction system as defined in claim 12 wherein said capacitor bank comprises three capacitors connected in Y.
14. A three-phase power factor correction system as defined in claim 12 wherein said capacitor bank comprises three capacitors connected in delta.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000497852A CA1295016C (en) | 1985-12-17 | 1985-12-17 | Power factor correction system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000497852A CA1295016C (en) | 1985-12-17 | 1985-12-17 | Power factor correction system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1295016C true CA1295016C (en) | 1992-01-28 |
Family
ID=4132108
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000497852A Expired - Fee Related CA1295016C (en) | 1985-12-17 | 1985-12-17 | Power factor correction system |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1295016C (en) |
-
1985
- 1985-12-17 CA CA000497852A patent/CA1295016C/en not_active Expired - Fee Related
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