CA1085920A - Static single phase to three phase converter for variable ac loads - Google Patents
Static single phase to three phase converter for variable ac loadsInfo
- Publication number
- CA1085920A CA1085920A CA251,384A CA251384A CA1085920A CA 1085920 A CA1085920 A CA 1085920A CA 251384 A CA251384 A CA 251384A CA 1085920 A CA1085920 A CA 1085920A
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- Canada
- Prior art keywords
- terminals
- phase
- load
- inductor
- reactance
- 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.)
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Classifications
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
- H02M5/02—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc
- H02M5/04—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc by static converters
- H02M5/10—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc by static converters using transformers
- H02M5/14—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc by static converters using transformers for conversion between circuits of different phase number
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Control Of Electrical Variables (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
An apparatus for producing an approximately balanced, three phase load from a single phase load. The apparatus comprises terminals A, B, and C for connection to a three phase power supply. The single phase load is connected across terminals A and C; a first reactance device is connected across terminals A and B; and a second reactance device is connected across terminals B and C. At least one of the first and second reactance devices is a variable reactance device such that its reactance can be adjusted over a range including both capacitive and inductive reactance. Control circuitry is included in order to regulate the reactance of the variable reactance devices.
An apparatus for producing an approximately balanced, three phase load from a single phase load. The apparatus comprises terminals A, B, and C for connection to a three phase power supply. The single phase load is connected across terminals A and C; a first reactance device is connected across terminals A and B; and a second reactance device is connected across terminals B and C. At least one of the first and second reactance devices is a variable reactance device such that its reactance can be adjusted over a range including both capacitive and inductive reactance. Control circuitry is included in order to regulate the reactance of the variable reactance devices.
Description
1~9ZO Case 2265 This invention relates to balanced phase convertors and more particularly to a static convertor for converting a single phase, variable AC load to a balanced three phase load for connection to a three phase power supply.
In many applications o~ single phase loads it is necessary to connect the load to a three phase power supply.
In some instances, such as lighting, it is possible to distribute the load evenly over the three phases; in other cases this is not possible and an unbalanced situation occurs.
This situation can arisefrom the use of equipment such as induction heating furnaces, various types of welders, electric traction equipment such as locomotives in the railway industry, and various other kinds of equipment.
An unbalanced load i3 a serious problem since it can cause, among other things, a negative sequence current and thereby produce unwanted heating and possible damage in AC
motors and generators.
At this time it should be pointed out that by a "balanced" load, it is meant that the currents in each of the phases of the three phase power supply, that supplies the load, are of equal magnitude and they are each separated by a phase angle of 120 degrees.
In order to transform a single-phase load into a balanced three-phase load, a phase balancer is used. Phase balancers of various descriptions are well known in the art.
Previous phase balancers have included methods such as transformer convertors as shown by U.S. Patent 3,375,429 dated March 26, 1968 to F. Pagano; methods involving solid state switching and incorporating frequency conversion such as that shown by Canadian Patent 825,775 dated October 21, 1969 to P.P. Biringer and methods involving switching inductors and capacitors as indicated by U.S. Patent 3,053,920 dated September 11, 1962 to J~P. Seitz.
' .
Case 2265 lOBS9ZO
m ese previous methods of phase balancing have had many limitations and drawbacks. The use of transformers for phase balancers is expensive, gives low power factors and requires mu~h physical space if the loads used are to be large.
me use of inductors acro~s a first phase and capacitors across a second phase of a three phase ~ystem in order to balance the load, connected across the remaining phase, is ~-well known in the prior art. However, with these previous methods, moving parts such as contactors or tap changers have been employed which are subject to problems such as contact corrosion, etc. In addition, only discrete reactance changes are possible, and the reactance in each leg must be predeter-mined as being either inductive or capacitive. Since these methods make use of discrete switched components, they cannot provide accurate phase balancing of loads having a variable magnitude or a variable power factor.
Methods using solid state switching elements~have also been disclosed. These methods, however, have been based upon freguency multipliers, which limits their use to special frequency equipment.
The present invention (in the preferred embodiment) consists of connecting the single phase load across one phase of the three phase supply and connecting, acrO#S each of the two remaining phases, a circuit whose impedance can be varied, both as to magnitude and as to reactance (capacitive or inductive).
This variable reactance circuit comprise# a pair of solid state switching devices, such as SCR's, in an inverse-parallel configuration and connected in series to an inductor; this combinatisn of SCR's and inductor being in parallel relationship with a capacitor. With the SCR's turned off, the capacitor ~`
alone is in the active AC circuit. With the SCR's switched fully on, the capacitor is paralleled by the inductor, and 10~9~ Case 2265 if the impedance of the inductor is made sufficiently small, (so that the magnitude of the current is greater through the inductor than through the capacitor) the inductor will be the main impedance determining element and will thus effectively cancel the capacitor's effects and there will be a predominantly pure inductance in the AC circuit. By controlling how long the SCR's are conducting one can determine the magnitude of the reactance and whether it will be capacitive or inductive.
The magnitude and types of impedances required for balancing the load will be determined according to the size of the single phase load and its power factor.
Stated in another way, the present invention is an apparatus for producing an approximately balanced, three phase load from a single phase load, the apparatus comprising:
terminals A, B, and C for connection to a three phase, alternating current, electric power supply; the single phase load is connected across terminals A and C; a first reactance device is connected across t0rminals A and B: a second reactance device is connected across terminals B and C: at least one of the first and second reactance devices being a variable reactance device:
and control circuitry to regulate the variable reactance devices.
The invention will now be described in more detail with reference to the accompanying drawings, in which:
Figure 1 is a simplified schematic diagram sh~wing the preferred embodiment of the invention for phase balancing;
Figure 2A shows a simplified schematic of Figure 1 when the single phase load has unity power factor;
Figure 2B is a vector diagram relationship of the voltages and currents of Figure 2A:
Figure 3A shows a simplified schematic of Figure 1 when the single phase load has a 0.8 power factor leading;
Figure 3B is a vector diagram relationship of the lO~9ZO Case 2265 of the voltages and currents of Figure 3A;
Figure 4A shows a simplified schematic of Figure 1 when the ~ingle phase load has a 0.8 power factor lagging;
Figure 4B is a vector diagram relationship of the voltage and currents of Figure 4A; and Figure 5 shows a simplified embodiment as in Fig. 1, but power factor control has been added.
Figure 1 shows a variable single phase load 10 connected, according to the invention, to produce a balanced three phase load at terminals A, B and C. The load 10 is shown connected across terminals A and C. Solid state switching devices, such a~ Silicon Controlled Rectifiers (SCR's) 11 and 12, are connected in an inverse-parallel relationship as shown in the Figure. Inductor 13 is connected in series with the SCR 11 ~ ~
and 12 combination, and the whole combination of SCR's 11 and ~ -12 and inductor 13 is connected across terminals A and B as shown in the Figure. Capacitor 16 parallels this circuit and is likewi6e conne~ted to terminal~ A and B. -~
A similar arrangement, cons~oting of SCR~s 14 and 15, inductor 18 and capacitor 17, is connected between the terminals B andC as shown in Figure 1.
To balance a single phase load connected to a three phase power supply it is well known in the art to connect the load to one phase, to connect a capacitor across a second phase and to connect an inductor across the remaining phase. The value of the capacitive and inductive elements are determined by the magnitude and power factor of the single phase load;
different values are necessary for different loads~
me present invention incorporates elements which can be controlled to produce a variable effective reactance and thereby provide the proper value of inductance or capacitance to maintain the load in a balanced state regardless of the 10~920 Cas~ 2265 magnitude or power factor of the single phase load.
One such set of elements in the present invention which serves to vary the reactance is indicated as Variable Reactance Device 24 in Figure 1 and consists of the combination of inverse-parallel SCR's 11 and 12, the inductor 13 and the capacitor 16. A~ i8 readily apparent from Figure 1, if the SCR's were kept in the non-conductive state, the inductor would be eliminated as an active component in the AC circuit and the capacitor 16 would be the only reactive component remaining active in the AC circuitry between terminals A and B.
Alternatively, if the SCR's 11 and 12 were left in the conducting~state continuously, the inductor 13 and capacitor 16 would be in a parallel relationship between terminals A and B. However, if we make the Lmpedance of inductor 13 much smaller than the impedance of capacitor 16, the effect of inductor 13 will dominate the effect of capacitor 16, and we will have essentially a pure inductance means between the terminals A
and B. This is due to the fact that the magnitude of the current conducted through inductor 13 will be much larger than the magnitude of the current conducted through capacitor 16.
Additionally, by controlling how long a period the SCR's 11 and 12 conduct, we can obtain any value of reactance between :
our two extremes of essentially all capacitance and essentially all inductance. The same effect is produced across terminals B and C by the elements located therebetween; namely the SCR's 14 and 15, the inductor 18 and the capacitor 17.
The SCR's can be controlled manually or by any method known in the art; the method indicated herein is but one of many and is sho~n solely to exemplify the operation of the invention.
Current transformers 21, 22 and 23 are connected to the three lines of the balanced load as shown in Figure 1. The current transformers are used to measure the current flowing in each 108S9Z~ Case 2265 pha~e of the balanced three phase load and to feed this informa-tion to the Current Detection Circuitry 20. The Current Detection Circuitry 20 feeds the Control Circuitry 19 which controls the firing of the SCR's, and accordingly the impedance across phases A-B and B-C, in order to maintain the balanced load in a balanced state.
The effect of varying the impedance in two legs of the load can best be seen by studying an example. Accordingly, Figure 2A shows a simplified version of the invention shown in Figure 1. The load 30 is fixed and is purely resistive, inductor 32 is used across one phase of the three phase supply and capacitor 31 is used across the remaining phase as shown in Figure 2A. m e voltages (E) and currents (I) used in this example are shown on the same Figure.
Figure 2B i8 a vector diagram showing vectorially the relationship between the voltages and currents of Figure 2A and how the addition of capacitor 31 and inductor 32 results in a balanced load at terminals A, B and C.
Figure 2B will now be described in more detail with Figure 2A being used for definition of symbols and general background reference. The vectors El, E2 and E3 represent the voltages of each of the phases of the power supply. They are equal in magnitude and are separated in phase by 120 degrees; since they are produced by a constant voltage source (i.e. the power supply) they will maintain this relationship whether the load is balanced or not, and accordingly they provide our frame of reference for the rest of the discussion.
Since lo~ad 30 is a pure resistance, the current I1 passing through it will be in phase with the voltage El across the load 30. Accordingly, Il is shown as being colinear to, and in the same direction as~ El in the vector diagram of Figure 2B.
The current I2 passing through the inductor 32 will ` ~0~9ZO Case 2265 lag, by 90 degrees, the voltage E2 across the inductor 32 Accordingly, the vector I2 in the vector diagram i9 shown 90 degrees clockwise of the vector E2.
The current I3 through the capacitor 3I will lead, by 90 degrees, the voltage E3 across the capacitor 31. Therefore, the vector I3 in the diagram is shown 90 degrees counterclockwise of the vector E3.
Referring to Figure 2A, the currents flowing towards the terminals A, B and C are Il-I2, I2-I , and I -Il respectively. These currents, shown vectorially in Figure 2B, can be obtained by vector subtraction of the currents involved.
As can be seen in Figure 2B, the currents Il-I2, I2-I3, and I3-Il are all of equal magnitude and are all separated by angles of 120 degrees. We have obtained a balanced three phase load.
Figure 3A shows another simplified version of Figure 1. This time, however, load 40 has a leading power factor of 0.8. Accordingly, capacitor 41 and 42 are connected across -~ -the remaining two phases to balance the three phase supply as shown.
Figure 3B is a vector diagram showing vectorially -~
the relationship between the voltages and currents of Figure 3A
and how the addition of capacitors 41 and 42 results in a balanced load at terminals A, B and C. As before, the vectors El, E2 and E3 represent the voltages of each of the phases~
They are equal in magnitude and are separated in phase by 120 degrees; they are produced by a constant voltage source and thus will be constant, and accordingly, provide our frame of reference for the remainder of the discussion. Since the load 40 has a leading power factor of 0.8, the current I4 passing t~rough it will lead the voltage El across the load. Accordingly, I4 is shown in Figure 3B counterclockwise of the vector El.
.' ' ~ " ~ .
10 ~ 9 ~ Case 2265 The current I5 passing through the capacitor 41 wi:Ll lead, by 90 degrees, the voltage E2 across the capacitor 41. Accordingly, the vector I5 in the vector diagram i8 ~hown 90 degrees counterclockwise of the vector E2. ~-The current I6 through the capacitor 42 will lead, by 90 degrees, the voltage E3 across the capacitor 42.
Therefore, the vector I6 in the diagram is shown 90 degrees counterclockwise of the vector E3.
Referring to Figure 3A, the currents flowing towards the terminals A, B and C are I4-I5, I5-I6 and I6-I
respectively. These currents, shown vectorially in Figure 3B, can be obtained by vector subtraction of the current involved.
As can be seen from Figure 3B these currents are all of equal magnitude and are all separated by angles of 120 degrees.
Figures 4A and 4B show a simplified version of Figure 1 and the associated vector diagram, respectively, for a load with a power factor of 0.8 lagging. It can be shown that this example also produces a balanced load condition by reference to Figures 4A and 4B and by using the same logic a9 was applied in the preceding examples for unity and leading power factor loads.
Accordingly, the present invention (in the preferred embodiment) controls the effective impedance across two phases of a three phase supply in order to produce a balanced load condition regardless of how the single phase load across the remaining phase of the supply varies in magnitude or power factor.
The foregoing has been a description at the preferred embodiment of the invention as envisioned by the inventor. In one variation of the preferred embodiment, the capacitors 16 and 17 [as well as the capacitor in variable reactance device 35, yet to be described) can be combined in series with tuning . . : -, ~
1~920 Case 2265 reactors to produce low impedance filters to minimize the flow of harmonic currents from the load 10 or the phase balancer to the power system.
A further variation, not shown in any of the Figures, is to replace one of the variable reactance devices with a fixed value reactance device. For example, the variable reactance device 24 of Figure 1 could be replaced by a capacitor (or an inductor, depending upon the particular circumstances), leaving the remaining variable reactance device across terminals B and C to compensate for changes in load 10.
_ 9 _ .
- . . . .
.
In many applications o~ single phase loads it is necessary to connect the load to a three phase power supply.
In some instances, such as lighting, it is possible to distribute the load evenly over the three phases; in other cases this is not possible and an unbalanced situation occurs.
This situation can arisefrom the use of equipment such as induction heating furnaces, various types of welders, electric traction equipment such as locomotives in the railway industry, and various other kinds of equipment.
An unbalanced load i3 a serious problem since it can cause, among other things, a negative sequence current and thereby produce unwanted heating and possible damage in AC
motors and generators.
At this time it should be pointed out that by a "balanced" load, it is meant that the currents in each of the phases of the three phase power supply, that supplies the load, are of equal magnitude and they are each separated by a phase angle of 120 degrees.
In order to transform a single-phase load into a balanced three-phase load, a phase balancer is used. Phase balancers of various descriptions are well known in the art.
Previous phase balancers have included methods such as transformer convertors as shown by U.S. Patent 3,375,429 dated March 26, 1968 to F. Pagano; methods involving solid state switching and incorporating frequency conversion such as that shown by Canadian Patent 825,775 dated October 21, 1969 to P.P. Biringer and methods involving switching inductors and capacitors as indicated by U.S. Patent 3,053,920 dated September 11, 1962 to J~P. Seitz.
' .
Case 2265 lOBS9ZO
m ese previous methods of phase balancing have had many limitations and drawbacks. The use of transformers for phase balancers is expensive, gives low power factors and requires mu~h physical space if the loads used are to be large.
me use of inductors acro~s a first phase and capacitors across a second phase of a three phase ~ystem in order to balance the load, connected across the remaining phase, is ~-well known in the prior art. However, with these previous methods, moving parts such as contactors or tap changers have been employed which are subject to problems such as contact corrosion, etc. In addition, only discrete reactance changes are possible, and the reactance in each leg must be predeter-mined as being either inductive or capacitive. Since these methods make use of discrete switched components, they cannot provide accurate phase balancing of loads having a variable magnitude or a variable power factor.
Methods using solid state switching elements~have also been disclosed. These methods, however, have been based upon freguency multipliers, which limits their use to special frequency equipment.
The present invention (in the preferred embodiment) consists of connecting the single phase load across one phase of the three phase supply and connecting, acrO#S each of the two remaining phases, a circuit whose impedance can be varied, both as to magnitude and as to reactance (capacitive or inductive).
This variable reactance circuit comprise# a pair of solid state switching devices, such as SCR's, in an inverse-parallel configuration and connected in series to an inductor; this combinatisn of SCR's and inductor being in parallel relationship with a capacitor. With the SCR's turned off, the capacitor ~`
alone is in the active AC circuit. With the SCR's switched fully on, the capacitor is paralleled by the inductor, and 10~9~ Case 2265 if the impedance of the inductor is made sufficiently small, (so that the magnitude of the current is greater through the inductor than through the capacitor) the inductor will be the main impedance determining element and will thus effectively cancel the capacitor's effects and there will be a predominantly pure inductance in the AC circuit. By controlling how long the SCR's are conducting one can determine the magnitude of the reactance and whether it will be capacitive or inductive.
The magnitude and types of impedances required for balancing the load will be determined according to the size of the single phase load and its power factor.
Stated in another way, the present invention is an apparatus for producing an approximately balanced, three phase load from a single phase load, the apparatus comprising:
terminals A, B, and C for connection to a three phase, alternating current, electric power supply; the single phase load is connected across terminals A and C; a first reactance device is connected across t0rminals A and B: a second reactance device is connected across terminals B and C: at least one of the first and second reactance devices being a variable reactance device:
and control circuitry to regulate the variable reactance devices.
The invention will now be described in more detail with reference to the accompanying drawings, in which:
Figure 1 is a simplified schematic diagram sh~wing the preferred embodiment of the invention for phase balancing;
Figure 2A shows a simplified schematic of Figure 1 when the single phase load has unity power factor;
Figure 2B is a vector diagram relationship of the voltages and currents of Figure 2A:
Figure 3A shows a simplified schematic of Figure 1 when the single phase load has a 0.8 power factor leading;
Figure 3B is a vector diagram relationship of the lO~9ZO Case 2265 of the voltages and currents of Figure 3A;
Figure 4A shows a simplified schematic of Figure 1 when the ~ingle phase load has a 0.8 power factor lagging;
Figure 4B is a vector diagram relationship of the voltage and currents of Figure 4A; and Figure 5 shows a simplified embodiment as in Fig. 1, but power factor control has been added.
Figure 1 shows a variable single phase load 10 connected, according to the invention, to produce a balanced three phase load at terminals A, B and C. The load 10 is shown connected across terminals A and C. Solid state switching devices, such a~ Silicon Controlled Rectifiers (SCR's) 11 and 12, are connected in an inverse-parallel relationship as shown in the Figure. Inductor 13 is connected in series with the SCR 11 ~ ~
and 12 combination, and the whole combination of SCR's 11 and ~ -12 and inductor 13 is connected across terminals A and B as shown in the Figure. Capacitor 16 parallels this circuit and is likewi6e conne~ted to terminal~ A and B. -~
A similar arrangement, cons~oting of SCR~s 14 and 15, inductor 18 and capacitor 17, is connected between the terminals B andC as shown in Figure 1.
To balance a single phase load connected to a three phase power supply it is well known in the art to connect the load to one phase, to connect a capacitor across a second phase and to connect an inductor across the remaining phase. The value of the capacitive and inductive elements are determined by the magnitude and power factor of the single phase load;
different values are necessary for different loads~
me present invention incorporates elements which can be controlled to produce a variable effective reactance and thereby provide the proper value of inductance or capacitance to maintain the load in a balanced state regardless of the 10~920 Cas~ 2265 magnitude or power factor of the single phase load.
One such set of elements in the present invention which serves to vary the reactance is indicated as Variable Reactance Device 24 in Figure 1 and consists of the combination of inverse-parallel SCR's 11 and 12, the inductor 13 and the capacitor 16. A~ i8 readily apparent from Figure 1, if the SCR's were kept in the non-conductive state, the inductor would be eliminated as an active component in the AC circuit and the capacitor 16 would be the only reactive component remaining active in the AC circuitry between terminals A and B.
Alternatively, if the SCR's 11 and 12 were left in the conducting~state continuously, the inductor 13 and capacitor 16 would be in a parallel relationship between terminals A and B. However, if we make the Lmpedance of inductor 13 much smaller than the impedance of capacitor 16, the effect of inductor 13 will dominate the effect of capacitor 16, and we will have essentially a pure inductance means between the terminals A
and B. This is due to the fact that the magnitude of the current conducted through inductor 13 will be much larger than the magnitude of the current conducted through capacitor 16.
Additionally, by controlling how long a period the SCR's 11 and 12 conduct, we can obtain any value of reactance between :
our two extremes of essentially all capacitance and essentially all inductance. The same effect is produced across terminals B and C by the elements located therebetween; namely the SCR's 14 and 15, the inductor 18 and the capacitor 17.
The SCR's can be controlled manually or by any method known in the art; the method indicated herein is but one of many and is sho~n solely to exemplify the operation of the invention.
Current transformers 21, 22 and 23 are connected to the three lines of the balanced load as shown in Figure 1. The current transformers are used to measure the current flowing in each 108S9Z~ Case 2265 pha~e of the balanced three phase load and to feed this informa-tion to the Current Detection Circuitry 20. The Current Detection Circuitry 20 feeds the Control Circuitry 19 which controls the firing of the SCR's, and accordingly the impedance across phases A-B and B-C, in order to maintain the balanced load in a balanced state.
The effect of varying the impedance in two legs of the load can best be seen by studying an example. Accordingly, Figure 2A shows a simplified version of the invention shown in Figure 1. The load 30 is fixed and is purely resistive, inductor 32 is used across one phase of the three phase supply and capacitor 31 is used across the remaining phase as shown in Figure 2A. m e voltages (E) and currents (I) used in this example are shown on the same Figure.
Figure 2B i8 a vector diagram showing vectorially the relationship between the voltages and currents of Figure 2A and how the addition of capacitor 31 and inductor 32 results in a balanced load at terminals A, B and C.
Figure 2B will now be described in more detail with Figure 2A being used for definition of symbols and general background reference. The vectors El, E2 and E3 represent the voltages of each of the phases of the power supply. They are equal in magnitude and are separated in phase by 120 degrees; since they are produced by a constant voltage source (i.e. the power supply) they will maintain this relationship whether the load is balanced or not, and accordingly they provide our frame of reference for the rest of the discussion.
Since lo~ad 30 is a pure resistance, the current I1 passing through it will be in phase with the voltage El across the load 30. Accordingly, Il is shown as being colinear to, and in the same direction as~ El in the vector diagram of Figure 2B.
The current I2 passing through the inductor 32 will ` ~0~9ZO Case 2265 lag, by 90 degrees, the voltage E2 across the inductor 32 Accordingly, the vector I2 in the vector diagram i9 shown 90 degrees clockwise of the vector E2.
The current I3 through the capacitor 3I will lead, by 90 degrees, the voltage E3 across the capacitor 31. Therefore, the vector I3 in the diagram is shown 90 degrees counterclockwise of the vector E3.
Referring to Figure 2A, the currents flowing towards the terminals A, B and C are Il-I2, I2-I , and I -Il respectively. These currents, shown vectorially in Figure 2B, can be obtained by vector subtraction of the currents involved.
As can be seen in Figure 2B, the currents Il-I2, I2-I3, and I3-Il are all of equal magnitude and are all separated by angles of 120 degrees. We have obtained a balanced three phase load.
Figure 3A shows another simplified version of Figure 1. This time, however, load 40 has a leading power factor of 0.8. Accordingly, capacitor 41 and 42 are connected across -~ -the remaining two phases to balance the three phase supply as shown.
Figure 3B is a vector diagram showing vectorially -~
the relationship between the voltages and currents of Figure 3A
and how the addition of capacitors 41 and 42 results in a balanced load at terminals A, B and C. As before, the vectors El, E2 and E3 represent the voltages of each of the phases~
They are equal in magnitude and are separated in phase by 120 degrees; they are produced by a constant voltage source and thus will be constant, and accordingly, provide our frame of reference for the remainder of the discussion. Since the load 40 has a leading power factor of 0.8, the current I4 passing t~rough it will lead the voltage El across the load. Accordingly, I4 is shown in Figure 3B counterclockwise of the vector El.
.' ' ~ " ~ .
10 ~ 9 ~ Case 2265 The current I5 passing through the capacitor 41 wi:Ll lead, by 90 degrees, the voltage E2 across the capacitor 41. Accordingly, the vector I5 in the vector diagram i8 ~hown 90 degrees counterclockwise of the vector E2. ~-The current I6 through the capacitor 42 will lead, by 90 degrees, the voltage E3 across the capacitor 42.
Therefore, the vector I6 in the diagram is shown 90 degrees counterclockwise of the vector E3.
Referring to Figure 3A, the currents flowing towards the terminals A, B and C are I4-I5, I5-I6 and I6-I
respectively. These currents, shown vectorially in Figure 3B, can be obtained by vector subtraction of the current involved.
As can be seen from Figure 3B these currents are all of equal magnitude and are all separated by angles of 120 degrees.
Figures 4A and 4B show a simplified version of Figure 1 and the associated vector diagram, respectively, for a load with a power factor of 0.8 lagging. It can be shown that this example also produces a balanced load condition by reference to Figures 4A and 4B and by using the same logic a9 was applied in the preceding examples for unity and leading power factor loads.
Accordingly, the present invention (in the preferred embodiment) controls the effective impedance across two phases of a three phase supply in order to produce a balanced load condition regardless of how the single phase load across the remaining phase of the supply varies in magnitude or power factor.
The foregoing has been a description at the preferred embodiment of the invention as envisioned by the inventor. In one variation of the preferred embodiment, the capacitors 16 and 17 [as well as the capacitor in variable reactance device 35, yet to be described) can be combined in series with tuning . . : -, ~
1~920 Case 2265 reactors to produce low impedance filters to minimize the flow of harmonic currents from the load 10 or the phase balancer to the power system.
A further variation, not shown in any of the Figures, is to replace one of the variable reactance devices with a fixed value reactance device. For example, the variable reactance device 24 of Figure 1 could be replaced by a capacitor (or an inductor, depending upon the particular circumstances), leaving the remaining variable reactance device across terminals B and C to compensate for changes in load 10.
_ 9 _ .
- . . . .
.
Claims (2)
1. An apparatus for producing an approximately balanced, three phase load from a single phase load, comprising: terminals A, B and C for connection to a three phase, alternating current, electric power supply; said terminals A and C also being adopted for connection to said single phase load; a first variable reactance device connec-ted across terminals A and B and comprising two thyristor devices connected in an inverse-parallel configuration and in series with an inductor, and a capacitor connected in parallel with the series connection of the two thyristor devices and the inductor; a second variable reactance device connected across terminals B and C and comprising two thyristor devices connected in an inverse-parallel configuration and in series with an inductor, and a capacitor connected in parallel with the series connection of the two thyristor devices and the inductor; and a control circuit to regulate said first and second variable reactance devices to provide impedances between terminals A and B and between terminals B and C to produce an approximately balanced three phase load.
2. An apparatus as defined in claim 1 and further including a third variable reactance device connected across terminals A and C in parallel with said single phase load, said third variable reactance device being controllable to adjust the power factor of the single phase load.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA251,384A CA1085920A (en) | 1976-04-29 | 1976-04-29 | Static single phase to three phase converter for variable ac loads |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA251,384A CA1085920A (en) | 1976-04-29 | 1976-04-29 | Static single phase to three phase converter for variable ac loads |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1085920A true CA1085920A (en) | 1980-09-16 |
Family
ID=4105819
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA251,384A Expired CA1085920A (en) | 1976-04-29 | 1976-04-29 | Static single phase to three phase converter for variable ac loads |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1085920A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2170623A (en) * | 1985-02-06 | 1986-08-06 | Ass Elect Ind | A c power supply system |
| CZ306041B6 (en) * | 2015-09-30 | 2016-07-07 | Technická univerzita v Liberci | Device to symmetrize three-phase electric network |
-
1976
- 1976-04-29 CA CA251,384A patent/CA1085920A/en not_active Expired
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2170623A (en) * | 1985-02-06 | 1986-08-06 | Ass Elect Ind | A c power supply system |
| WO1986004751A1 (en) * | 1985-02-06 | 1986-08-14 | Associated Electrical Industries Limited | A.c. power supply systems |
| US4689735A (en) * | 1985-02-06 | 1987-08-25 | Associated Electrical Industries Limited | Power supply for single-phase traction system |
| AU587801B2 (en) * | 1985-02-06 | 1989-08-31 | Associated Electrical Industries Limited | A.c. power supply systems |
| CZ306041B6 (en) * | 2015-09-30 | 2016-07-07 | Technická univerzita v Liberci | Device to symmetrize three-phase electric network |
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