CA1214825A - Current sensor for ac/dc converter - Google Patents
Current sensor for ac/dc converterInfo
- Publication number
- CA1214825A CA1214825A CA000452984A CA452984A CA1214825A CA 1214825 A CA1214825 A CA 1214825A CA 000452984 A CA000452984 A CA 000452984A CA 452984 A CA452984 A CA 452984A CA 1214825 A CA1214825 A CA 1214825A
- Authority
- CA
- Canada
- Prior art keywords
- current
- converter
- converter circuit
- power
- recited
- 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
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
- G01R19/22—Arrangements for measuring currents or voltages or for indicating presence or sign thereof using conversion of ac into dc
-
- 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
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
- H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
- H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/145—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means
- H02M7/155—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
- H02M7/162—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only in a bridge configuration
- H02M7/1623—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only in a bridge configuration with control circuit
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Rectifiers (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A current sensor for sensing the total DC cur-rent flowing in the DC rails of a multiphase converter circuit. The current sensor comprises two current trans-formers coupled to the AC power lines of the converter and another current transformer coupled to the circuit path of a freewheeling diode connected across the DC rails of the converter. An indication of the total DC current flowing through the converter is obtained by summing an indication of current flowing in the AC power lines of the converter and the instantaneous pulsed DC current flowing in the DC
path of the converter during periods of non-conduction of the switching SCR's in the converter. The combination of the two currents are summed by a summing resistor thereby to generate a voltage signal proportional to the instan-taneous level of the total DC current flowing through the converter.
A current sensor for sensing the total DC cur-rent flowing in the DC rails of a multiphase converter circuit. The current sensor comprises two current trans-formers coupled to the AC power lines of the converter and another current transformer coupled to the circuit path of a freewheeling diode connected across the DC rails of the converter. An indication of the total DC current flowing through the converter is obtained by summing an indication of current flowing in the AC power lines of the converter and the instantaneous pulsed DC current flowing in the DC
path of the converter during periods of non-conduction of the switching SCR's in the converter. The combination of the two currents are summed by a summing resistor thereby to generate a voltage signal proportional to the instan-taneous level of the total DC current flowing through the converter.
Description
~214~;~5 1 50,646 CURRENT SENSOR FOR AC/DC CONVERTER
BACKGROUND OF THE INVENTION
This invention pertains to current transducers useful for measuring DC current, but more specifically, to a current transducer useful for measuring the DC current flow in a multiphase alternating current/direct current converter, such as those used in AC motor controllers.
In a three-phase power system, an AC/DC con-verter typically comprises a three-phase input line which - couples a three-phase power source and a series of con-trolled rectifiers, the switching thereof being controlledin a fashion to supply DC current to an inverter. The invention controls the power supply to an AC motor. Also, a filter network, typically constituted by an inductor coupled in series in a DC path of the converter and a capacitor that couples across the DC lines, together operate to sta~ilize the current and voltage supplied to the invention. Also, a "freewheeling" diode is connected across the DC rails of the converter to provide DC current flow when the converter output voltage becomes zero or negative at certain time periods during the switching operation of the controlled rectifiers. Thus, when an attempt is made to measure the current flowing in the DC
rails of the converter circuit, both the current flowing on the AC side of the converter and the DC current flowing in the DC rails during "zero state" periods of the con-~erter must be accounted. TherefQre, the above-~sntioned 1214~;2,5
BACKGROUND OF THE INVENTION
This invention pertains to current transducers useful for measuring DC current, but more specifically, to a current transducer useful for measuring the DC current flow in a multiphase alternating current/direct current converter, such as those used in AC motor controllers.
In a three-phase power system, an AC/DC con-verter typically comprises a three-phase input line which - couples a three-phase power source and a series of con-trolled rectifiers, the switching thereof being controlledin a fashion to supply DC current to an inverter. The invention controls the power supply to an AC motor. Also, a filter network, typically constituted by an inductor coupled in series in a DC path of the converter and a capacitor that couples across the DC lines, together operate to sta~ilize the current and voltage supplied to the invention. Also, a "freewheeling" diode is connected across the DC rails of the converter to provide DC current flow when the converter output voltage becomes zero or negative at certain time periods during the switching operation of the controlled rectifiers. Thus, when an attempt is made to measure the current flowing in the DC
rails of the converter circuit, both the current flowing on the AC side of the converter and the DC current flowing in the DC rails during "zero state" periods of the con-~erter must be accounted. TherefQre, the above-~sntioned 1214~;2,5
2 50,646 various systems and techniques for measuring in the DC
rails have been employed to obtain an accurate measurement of the DC current flow. At least one drawback of using the above-~entioned systems and schemes is that they are relatively expensive, require complex interconnection to the converter circuits, require floating power supplies, and/or are susceptible to failure or error in indication of the exact current flow because the current flow is measured indirectly (e.g., magnetic or optical effect~), as opposed to directly.
SUMMARY OF THE INVENTION
In view of the foregoing, a primary objective of this invention is to provide a relatively inexpensive current transducer for sensing the current flow in the DC
rails of a converter.
Another objective of the invention is to provide a method and apparatus for accurately sensing current flow in the DC raiis of a multiphase converter circuit.
A further objective of this invention is to provide a method and apparatus for sensing the combination of the current flow on the AC side of the converter and the internal DC current flow of the converter during periods of non-conduction of the switching rectifiers of the converter, thereby to obtain an accurate representa-tion of the DC current flow.
In accordance with the above and other objec-tives, a method for sensing the DC current in a multiphase converter circuit which employs a plurality of controlled switching devices for controlling current flow in each of the respective alternating current phases comprises the steps of sensing the current flow in the alternating power lines connected to the converter, sensing the current flow in the DC rails of the multiphase converter during periods of non-conduction of the switching devices, and summing these sensed currents thereby to obtain an accurate repre-sentation of the current flowing through the converter.
:~2~4~;~5
rails have been employed to obtain an accurate measurement of the DC current flow. At least one drawback of using the above-~entioned systems and schemes is that they are relatively expensive, require complex interconnection to the converter circuits, require floating power supplies, and/or are susceptible to failure or error in indication of the exact current flow because the current flow is measured indirectly (e.g., magnetic or optical effect~), as opposed to directly.
SUMMARY OF THE INVENTION
In view of the foregoing, a primary objective of this invention is to provide a relatively inexpensive current transducer for sensing the current flow in the DC
rails of a converter.
Another objective of the invention is to provide a method and apparatus for accurately sensing current flow in the DC raiis of a multiphase converter circuit.
A further objective of this invention is to provide a method and apparatus for sensing the combination of the current flow on the AC side of the converter and the internal DC current flow of the converter during periods of non-conduction of the switching rectifiers of the converter, thereby to obtain an accurate representa-tion of the DC current flow.
In accordance with the above and other objec-tives, a method for sensing the DC current in a multiphase converter circuit which employs a plurality of controlled switching devices for controlling current flow in each of the respective alternating current phases comprises the steps of sensing the current flow in the alternating power lines connected to the converter, sensing the current flow in the DC rails of the multiphase converter during periods of non-conduction of the switching devices, and summing these sensed currents thereby to obtain an accurate repre-sentation of the current flowing through the converter.
:~2~4~;~5
3 50,646 An apparatus for accomplishing the above aspect of the invention comprises a current sensor for sensing the magnitude of the DC current flowing through the multi-phase converter circuit, which converter circuit comprises a plurality of controlled switching devices, such as silicon controlled rectifiers for controlling the time period of current flow in the respective alternating current power lines, the current sensor comprising a first curren~ transducer for directly sensing the current in two phases of the alternating current power lines, a second current transducer for sensing the DC current fl~wing through the freewheeling diode of the converter circuit during time periods of non-conduction of the switching devices, a first rectifying means connected to the first current transducer for producing a first DC output, a second rectifier connected to the second current trans-ducer for producing a second DC ou~put indicative of the ~C current flowing through the freewheeling diode and summing means connected to both of the first and second rectifying means for generating a representation of the ma~nitude of the combination of the sensed ~urrent from the first and second current transducers thereby to pro-vide an accurate indication of the DC current flowing through the converter.
Z5 An additional feature of the invention comprises the use of three single-phase current transformers, two of which are used in sensing current from the three-phase AC
power source and a third to sense current in a freewheel-ing diode connected across the DC rails of the converter, thareby to sense, without the necessity of a ground refer-ence, the current flowing in both the AC power lines and the output of the converter. A rectifier coupled to the third current transformer enables the generation of a DC
current that is proportional to the current flowing in the DC rails during periods of non-conduction of the ~witching devices. To dissipate the stored electromagnetic energy i~ tbe windin~ coupled across the current path of the ~2~41~Z'5
Z5 An additional feature of the invention comprises the use of three single-phase current transformers, two of which are used in sensing current from the three-phase AC
power source and a third to sense current in a freewheel-ing diode connected across the DC rails of the converter, thareby to sense, without the necessity of a ground refer-ence, the current flowing in both the AC power lines and the output of the converter. A rectifier coupled to the third current transformer enables the generation of a DC
current that is proportional to the current flowing in the DC rails during periods of non-conduction of the ~witching devices. To dissipate the stored electromagnetic energy i~ tbe windin~ coupled across the current path of the ~2~41~Z'5
4 50,646 freewheeling diode, the invention includes a current return circuit constituted by, preferably, a zener diode and a diode rectifier connected back to back in series and across the winding. Alternatively, a resistor can replace the zener diode as an energy dissipating device.
By the above method and arrangement, the ~nven-tion provides for inexpensive, reliable, and accurate measurement of DC current flow. Other advantages, as-pects, and features of the invention will become more readily evident upon review of the succeeding disclosure taken in connection with the accompanying drawings. The invention, however, is pointed out with particularity in the appended claims.
BRIEF DESCRIPTION OE THE DRAWINGS
Figure 1 depicts a prior art circuit arrangement over which this invention is an improvement for sensing DC
current flow in a converter;
Figure 2 depicts the sensing transducer of this invention coupled with a multiphase AC converter circuit;
Figure 3 depicts a portion of the circuitry of Figure 2 employing a resistive element, instead of a zener diode; and Figure 4 depicts an alternative embodiment of the sensing circuit of the invention depicted in Figure 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1 shows a conventional multiphase conver-ter circuit S coupled to the AC three-phase power mains.
Current transformers 12 and 14 are used for sensing the current flowing through the AC power lines 18, 20, and 22.
As previously indicated, the converter is useful for controlling the current in AC motor controllers. The power of each phase supplied over conductoræ 18-22 con-nects to a switching network constituted by a series of silicon controlled rectifiers 30, 32, 34, 36, 38, and 40.
Line power supplied over conductor 18 couples a node 42 between the rectifiers 34 and 40, line power over conduc-tor 20 couples a node 44 between rectifiers 32 and 38, and ~2~25 50,646 line power over conductor 22 couples a node 46 between the rectifiers 30 and 36. ~n this arrangement, portions of the positive half-cycles of each phase of the line current are conducted through the rectifiers 30, 32, and 34 to a positive DC rail 48 while the negative half-cycles of the line current are supplied over the conductors 18, 20, and 22 by way of the rectifiers 36-40 to a negative DC rail 50. In a conventional electrical distribution system, the phases of the power supplied over the conductors 18-22 differ from each other by 120.
A conventional switching controller 52 generates triggering pulses which are applied to the control gates of the SCR's 30-40. The time of occurrence and the dura-tion of the triggering pulses generated by the switching controller 52 are such that the line voltage of the power supplied over each of the conductors 18-22 permits the flow of DC current through the respective positive and negative DC rails 48 and 50. In particular, SCR 30 is turned on at a time period during which the voltage cf the 20 . line power supplied over conductor 22 resides above a predetermi~ed minimum level which varies according to the operating parameters determined by the switching control-ler 52. The controller 52 might, for example, respond to load conditions on the AC motor, thereby to generate switching signals which optimize the power efficiency of the motor. Other SCR's 32-40 are similarly controlled by triggering pulses from controller 52.
An inductor 54 and a capacitor 56 connected in the DC circuit path of the converter together aid in stabilizing the DC power supplied to an inverter 58. A
freewheeling diode 60 also connects across the positive and negative ~C rails 48 and 50. The diode 60 permits the continued conduction of DC current through the inductor 54 during periods of non-conduction of the SCR's 30-40. The inverter 58 then may produce a controlled variable fre-~uency, variable amplitude, three-phase power supply for an AC motor.
. 1214~Zt;
6 50,646 Thus far, the construction and operation of a typical converter has been described. To measure the magnitude of current flow in the DC rails 48 and 50, a prior art transducer, such as a Hall detector, an opto-coupled transducer, or modulated transformer must besomehow connected to one of the DC rails ~8 or 50. Al-ternatively, a current transducer 10 for measuring AC
current only might be used to approximate the DC current flow in the DC rails ~8 and 50. Both types of prior art transducers could also be used together for developing an accurate representation of the current flowing in the DC
rails 48-50, but as previously mentioned, these prior techniques and systems have several drawbacks, such as expense, complicated structure, reliability, and others.
The transducer 10 itself senses the current flow in each phase of the AC power lines 18-22 while an auxiliary prior art current sensor (not shown) senses the current flow in the DC rails during periods of non-conduction of the switching SCR's 30-40.
In further explanation of the present invention, the transducer 10 includes current transformers 12 and 14 coupled to two of the power mains 18 and 20 to generate two respective signals representative of the three phases of current supplied over the power lines 18-22. The signals are supplied to a diode network generally indi-cated at 76 which in turn develops a voltage signal across resistor 78 that is proportional to the instantaneous level of the current flowing in the AC power lines.
Figure 2 depicts a first preferred embodiment of the present invention which uses a current transducer which is supplemental to the current transducer of prior art systems. The reference numerals of the circuit ele-ment of the converter circuit of Figure 1 also correspond to the reference numerals of the circuit elements of the circuit of Eigure 2.
In the preferred embodiment, current trans-formers 12, 14, ~nd 16 comprise sensing e~lements for mea-~2~4~Z5 7 50,646 suring the current flow in the AC power mains and in theD~ rails 48 and 50 during periods of non-conduction of the switching SCR's 30-40. Use of the current transformers does not require a ground reference connection. In struc-ture, the novel arrangement comprises a first currenttransducing means constituted ~y current transformers 12 and 14. Transformers 12 and 14 supply current to a diode network 76 which generates a DC voltage across the resis-tor 78 that is proportional to the instantaneous level of the current flowing through the power mains 18-22. A
second current transducing means constituted by current transformer 16 senses the DC current flowing through the freewheeling diode 60 during periods of non-conduction of the switching SCR's 30-40.
The turns ratio between the primary and secon-dary windings of the three current transformers estab-lishes the level of the pulse current delivered through the secondary windings of the current transformers 12-16.
In practice, an optimum turns ratio is selected to suit-ably match the power dissipating capacity of the resistor 78, the desired sensitivity, and/or the power handling capability of the circuit components, generally. Imped-ance matching by selecting the specific turns ratio is a matter of conventional knowledge within the art.
When all switching SCR's 30-40 are non-conduct-ing, the freewheeling diode 60 momentarily permits the inductor 54 to continue conducting DC current over the DC
rails 48 and 50 from current that is stored in inductor 54. When the switching SCR's 30-40 are turned on, current through the freewheeling diode 60 ceases. Thus, during the operation of the converter circuit 24, a series of pulses of current conducts through the freewheeling diode 60. These current pulses manifest themselves in similar current pulses across the secondary winding of the current transformer 16 which are then supplied to the resistor 78 through a diode 80 thereby to generate a contributing voltage across the resistor 78 that is proportional to the pulsed current flowing through the diode 60.
12~4F~25 8 50,646 To dissipate the magnetic energy developed in the current transformer 16, a current return path is provided by a diodç 82 which permits the flow of current in a direction that is opposite the direction of the current flowing through the diode 80. A zener diode ~4 is coupled back to back with the diode 82 to limit the volt-age level appearing across the transformer 16 to a reason-ably safe level and to provide a mechanism for dissipating the energy stored therein. To dissipate the energy, a resistor may be employed in place of the zener diode 84, as depicted in Figure 3, which shows a portion of the circuitry of Figure 2. Thus, it is seen that the resistor 78, serving as a means for summing the currents in the AC
power mains ~s well as the current in the DC rail during periods of non-conuction of the switching SCR's 30-40, provides a relatively inexpensive, reliable, and accurate means of sensing the magnitude of the DC current in a multiphase converter circuit.
Other embodiments may well be constructed in accordance with the above teachings. For example, Figure 4 depicts the current sensing portion of the circuit of Figure 2 wherein diode bridges 86 and 88 are employed as a substitute for the diode arrangement 76 for rectifying the current sensed on the AC power mains. The operation of 2S the sensing circuit of Figure 4 essentially is the same as that described with reference to Figure 2. Also, some of the circuit components therein are similarly numbered.
Specifically, the current transformers 12 and 14 sense the AC current flowing through the AC power mains while the current transformer 16 senses the current flowing in the DC rails during periods of non-conduction of the switching SCR's 30-40. A summing means constituted by the resistor 78 generates a voltage signal proportional to the total current flowing through the DC rails 48 and 50 ~not shown 3S here) of the converter circuit. The zener diode 84 pro-vides means for dissipating energy stored in the current transformer 16 after being pulsed in a forward direction 12~5 50,646 dictated by the polarity of diode 60. In the diode bridge arrangement 88 of Figure 4, the diode 92 passes forward current from the current transformer 16 to the summing resistor 78, just as diode 80 did to summing resistor 78 of Eigure ~. Likewise, the diode 94 of the diode bridge 88 provides a return current path for dissipation of the energy stored in the current transformer 16 during reverse oscillations thereof after being pulsed in the forward direction. As depicted in Figure 3, a resistor as well could be used in place of the zener diode 84 in the cir-cuit of Figure 4.
A preferred construction of the current sensor was constructed for a 20 and 50 horsepower AC motor con-troller. In that construction, the current transformers 12-16 comprised three current transformers commercially known as Vectrol Part No. 9010-204, the diodes of the diode network 76, 80, and 82 are IN4001 diodes, the zener diode 84 is a lN4744B rated at 1 watt, and the resistor 78 is a ten-ohm, one-percent tolerance resistor rated at 1/4 watt.
It is apparent that several alternative forms of the invention can be constructed in view of the above teachings. In particular, although a three-phase system is shown and described, the teachings hereof can be ap-plied to any multiple phase system. Also, the arrangementof transformer interconnection may vary according to the number of phases in the distribution system and whether a ground reference is avail~ble. Accordingly, it is not the intent to limit the invention to exactly what is shown and described, but to include all such modifications and variations as may come to those skilled in the art to which this subject matter pertains. This invention is not limited to an AC arrangement in which only two separate current transformers are used on the AC side. Three separate current trans~ormers may be used.
By the above method and arrangement, the ~nven-tion provides for inexpensive, reliable, and accurate measurement of DC current flow. Other advantages, as-pects, and features of the invention will become more readily evident upon review of the succeeding disclosure taken in connection with the accompanying drawings. The invention, however, is pointed out with particularity in the appended claims.
BRIEF DESCRIPTION OE THE DRAWINGS
Figure 1 depicts a prior art circuit arrangement over which this invention is an improvement for sensing DC
current flow in a converter;
Figure 2 depicts the sensing transducer of this invention coupled with a multiphase AC converter circuit;
Figure 3 depicts a portion of the circuitry of Figure 2 employing a resistive element, instead of a zener diode; and Figure 4 depicts an alternative embodiment of the sensing circuit of the invention depicted in Figure 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1 shows a conventional multiphase conver-ter circuit S coupled to the AC three-phase power mains.
Current transformers 12 and 14 are used for sensing the current flowing through the AC power lines 18, 20, and 22.
As previously indicated, the converter is useful for controlling the current in AC motor controllers. The power of each phase supplied over conductoræ 18-22 con-nects to a switching network constituted by a series of silicon controlled rectifiers 30, 32, 34, 36, 38, and 40.
Line power supplied over conductor 18 couples a node 42 between the rectifiers 34 and 40, line power over conduc-tor 20 couples a node 44 between rectifiers 32 and 38, and ~2~25 50,646 line power over conductor 22 couples a node 46 between the rectifiers 30 and 36. ~n this arrangement, portions of the positive half-cycles of each phase of the line current are conducted through the rectifiers 30, 32, and 34 to a positive DC rail 48 while the negative half-cycles of the line current are supplied over the conductors 18, 20, and 22 by way of the rectifiers 36-40 to a negative DC rail 50. In a conventional electrical distribution system, the phases of the power supplied over the conductors 18-22 differ from each other by 120.
A conventional switching controller 52 generates triggering pulses which are applied to the control gates of the SCR's 30-40. The time of occurrence and the dura-tion of the triggering pulses generated by the switching controller 52 are such that the line voltage of the power supplied over each of the conductors 18-22 permits the flow of DC current through the respective positive and negative DC rails 48 and 50. In particular, SCR 30 is turned on at a time period during which the voltage cf the 20 . line power supplied over conductor 22 resides above a predetermi~ed minimum level which varies according to the operating parameters determined by the switching control-ler 52. The controller 52 might, for example, respond to load conditions on the AC motor, thereby to generate switching signals which optimize the power efficiency of the motor. Other SCR's 32-40 are similarly controlled by triggering pulses from controller 52.
An inductor 54 and a capacitor 56 connected in the DC circuit path of the converter together aid in stabilizing the DC power supplied to an inverter 58. A
freewheeling diode 60 also connects across the positive and negative ~C rails 48 and 50. The diode 60 permits the continued conduction of DC current through the inductor 54 during periods of non-conduction of the SCR's 30-40. The inverter 58 then may produce a controlled variable fre-~uency, variable amplitude, three-phase power supply for an AC motor.
. 1214~Zt;
6 50,646 Thus far, the construction and operation of a typical converter has been described. To measure the magnitude of current flow in the DC rails 48 and 50, a prior art transducer, such as a Hall detector, an opto-coupled transducer, or modulated transformer must besomehow connected to one of the DC rails ~8 or 50. Al-ternatively, a current transducer 10 for measuring AC
current only might be used to approximate the DC current flow in the DC rails ~8 and 50. Both types of prior art transducers could also be used together for developing an accurate representation of the current flowing in the DC
rails 48-50, but as previously mentioned, these prior techniques and systems have several drawbacks, such as expense, complicated structure, reliability, and others.
The transducer 10 itself senses the current flow in each phase of the AC power lines 18-22 while an auxiliary prior art current sensor (not shown) senses the current flow in the DC rails during periods of non-conduction of the switching SCR's 30-40.
In further explanation of the present invention, the transducer 10 includes current transformers 12 and 14 coupled to two of the power mains 18 and 20 to generate two respective signals representative of the three phases of current supplied over the power lines 18-22. The signals are supplied to a diode network generally indi-cated at 76 which in turn develops a voltage signal across resistor 78 that is proportional to the instantaneous level of the current flowing in the AC power lines.
Figure 2 depicts a first preferred embodiment of the present invention which uses a current transducer which is supplemental to the current transducer of prior art systems. The reference numerals of the circuit ele-ment of the converter circuit of Figure 1 also correspond to the reference numerals of the circuit elements of the circuit of Eigure 2.
In the preferred embodiment, current trans-formers 12, 14, ~nd 16 comprise sensing e~lements for mea-~2~4~Z5 7 50,646 suring the current flow in the AC power mains and in theD~ rails 48 and 50 during periods of non-conduction of the switching SCR's 30-40. Use of the current transformers does not require a ground reference connection. In struc-ture, the novel arrangement comprises a first currenttransducing means constituted ~y current transformers 12 and 14. Transformers 12 and 14 supply current to a diode network 76 which generates a DC voltage across the resis-tor 78 that is proportional to the instantaneous level of the current flowing through the power mains 18-22. A
second current transducing means constituted by current transformer 16 senses the DC current flowing through the freewheeling diode 60 during periods of non-conduction of the switching SCR's 30-40.
The turns ratio between the primary and secon-dary windings of the three current transformers estab-lishes the level of the pulse current delivered through the secondary windings of the current transformers 12-16.
In practice, an optimum turns ratio is selected to suit-ably match the power dissipating capacity of the resistor 78, the desired sensitivity, and/or the power handling capability of the circuit components, generally. Imped-ance matching by selecting the specific turns ratio is a matter of conventional knowledge within the art.
When all switching SCR's 30-40 are non-conduct-ing, the freewheeling diode 60 momentarily permits the inductor 54 to continue conducting DC current over the DC
rails 48 and 50 from current that is stored in inductor 54. When the switching SCR's 30-40 are turned on, current through the freewheeling diode 60 ceases. Thus, during the operation of the converter circuit 24, a series of pulses of current conducts through the freewheeling diode 60. These current pulses manifest themselves in similar current pulses across the secondary winding of the current transformer 16 which are then supplied to the resistor 78 through a diode 80 thereby to generate a contributing voltage across the resistor 78 that is proportional to the pulsed current flowing through the diode 60.
12~4F~25 8 50,646 To dissipate the magnetic energy developed in the current transformer 16, a current return path is provided by a diodç 82 which permits the flow of current in a direction that is opposite the direction of the current flowing through the diode 80. A zener diode ~4 is coupled back to back with the diode 82 to limit the volt-age level appearing across the transformer 16 to a reason-ably safe level and to provide a mechanism for dissipating the energy stored therein. To dissipate the energy, a resistor may be employed in place of the zener diode 84, as depicted in Figure 3, which shows a portion of the circuitry of Figure 2. Thus, it is seen that the resistor 78, serving as a means for summing the currents in the AC
power mains ~s well as the current in the DC rail during periods of non-conuction of the switching SCR's 30-40, provides a relatively inexpensive, reliable, and accurate means of sensing the magnitude of the DC current in a multiphase converter circuit.
Other embodiments may well be constructed in accordance with the above teachings. For example, Figure 4 depicts the current sensing portion of the circuit of Figure 2 wherein diode bridges 86 and 88 are employed as a substitute for the diode arrangement 76 for rectifying the current sensed on the AC power mains. The operation of 2S the sensing circuit of Figure 4 essentially is the same as that described with reference to Figure 2. Also, some of the circuit components therein are similarly numbered.
Specifically, the current transformers 12 and 14 sense the AC current flowing through the AC power mains while the current transformer 16 senses the current flowing in the DC rails during periods of non-conduction of the switching SCR's 30-40. A summing means constituted by the resistor 78 generates a voltage signal proportional to the total current flowing through the DC rails 48 and 50 ~not shown 3S here) of the converter circuit. The zener diode 84 pro-vides means for dissipating energy stored in the current transformer 16 after being pulsed in a forward direction 12~5 50,646 dictated by the polarity of diode 60. In the diode bridge arrangement 88 of Figure 4, the diode 92 passes forward current from the current transformer 16 to the summing resistor 78, just as diode 80 did to summing resistor 78 of Eigure ~. Likewise, the diode 94 of the diode bridge 88 provides a return current path for dissipation of the energy stored in the current transformer 16 during reverse oscillations thereof after being pulsed in the forward direction. As depicted in Figure 3, a resistor as well could be used in place of the zener diode 84 in the cir-cuit of Figure 4.
A preferred construction of the current sensor was constructed for a 20 and 50 horsepower AC motor con-troller. In that construction, the current transformers 12-16 comprised three current transformers commercially known as Vectrol Part No. 9010-204, the diodes of the diode network 76, 80, and 82 are IN4001 diodes, the zener diode 84 is a lN4744B rated at 1 watt, and the resistor 78 is a ten-ohm, one-percent tolerance resistor rated at 1/4 watt.
It is apparent that several alternative forms of the invention can be constructed in view of the above teachings. In particular, although a three-phase system is shown and described, the teachings hereof can be ap-plied to any multiple phase system. Also, the arrangementof transformer interconnection may vary according to the number of phases in the distribution system and whether a ground reference is avail~ble. Accordingly, it is not the intent to limit the invention to exactly what is shown and described, but to include all such modifications and variations as may come to those skilled in the art to which this subject matter pertains. This invention is not limited to an AC arrangement in which only two separate current transformers are used on the AC side. Three separate current trans~ormers may be used.
Claims (7)
1. A current sensor for sensing the magnitude of DC current flowing through a multiphase converter circuit used for converting multiphase alternating power to direct current power, said converter circuit employing a plurality of control-led switching devices for controlling the time and duration of current flow in each phase of the alternating current lines, said current sensor comprising:
first transducing means for sensing the current in the alternating current lines comprising a current transformer inductively coupled to the AC power lines of the converter and first rectifying means for converting the sensed AC current to a first DC output current, second transducing means for sensing DC current flow-ing through said converter circuit during periods of nonconduc-tion of said switching devices comprising a current transformer inductively coupled to a DC path of said converter circuit and second rectifying means for producing a second DC output current, said first transducer means generating a signal pro-portional to the current flowing in the AC power lines and said second transducer means generating a signal proportional to the DC current flow in the converter circuit during periods of non-conduction of the switching devices, summing means connected to said first and second transducing means for summing said first and second DC output currents and for generating a representation of the magnitude of the combination of the sensed current flowing through the converter circuit.
first transducing means for sensing the current in the alternating current lines comprising a current transformer inductively coupled to the AC power lines of the converter and first rectifying means for converting the sensed AC current to a first DC output current, second transducing means for sensing DC current flow-ing through said converter circuit during periods of nonconduc-tion of said switching devices comprising a current transformer inductively coupled to a DC path of said converter circuit and second rectifying means for producing a second DC output current, said first transducer means generating a signal pro-portional to the current flowing in the AC power lines and said second transducer means generating a signal proportional to the DC current flow in the converter circuit during periods of non-conduction of the switching devices, summing means connected to said first and second transducing means for summing said first and second DC output currents and for generating a representation of the magnitude of the combination of the sensed current flowing through the converter circuit.
2. A current sensor as recited in claim 1 wherein said second transducer includes a current return means and a power dissipating means connected in series across the secon-dary winding of the current transformer for providing a current return path for the current transformer and for dissipating the energy stored in said current transformer.
3. A current sensor as recited in claim 2 wherein said rectifier means comprises diode bridges.
4. A current sensor as recited in claim 2 wherein said current return means and said power dissipating means comprise a diode rectifier and a zener diode connected back to back.
5. A current sensor as recited in claim 2 wherein said current return means and said power dissipating means comprise, respectively, a diode rectifier and resistor con-nected across the secondary winding of the current transformer in said second transducing means.
6. A current sensor as recited in claim 1 wherein said summing means comprises a resistor which receives the sums current from said first and second transducer means thereby to generate a signal proportional to the DC current flowing through said converter circuit.
7. A current sensor as recited in claim 6 wherein said switching devices comprise silicon controlled rectifiers.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US48964383A | 1983-04-28 | 1983-04-28 | |
| US489,643 | 1983-04-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1214825A true CA1214825A (en) | 1986-12-02 |
Family
ID=23944680
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000452984A Expired CA1214825A (en) | 1983-04-28 | 1984-04-27 | Current sensor for ac/dc converter |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1214825A (en) |
| DE (1) | DE3414145A1 (en) |
| FR (1) | FR2545224A1 (en) |
| GB (1) | GB2138957A (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2190754A (en) * | 1986-04-11 | 1987-11-25 | Hitachi Ltd | Load current detecting device for pulse width modulation inverter |
| FR2696293B1 (en) * | 1992-09-25 | 1994-12-16 | Intelligent Electronic Systems | Power supply method with sinusoidal or quasi-sinusoidal current absorption and device for its implementation. |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1252799B (en) * | ||||
| GB1069626A (en) * | 1962-12-07 | 1967-05-24 | English Electric Co Ltd | Improvements in electrical overload-protection systems |
-
1984
- 1984-04-13 GB GB08409620A patent/GB2138957A/en not_active Withdrawn
- 1984-04-14 DE DE19843414145 patent/DE3414145A1/en not_active Withdrawn
- 1984-04-24 FR FR8406380A patent/FR2545224A1/en active Pending
- 1984-04-27 CA CA000452984A patent/CA1214825A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB2138957A (en) | 1984-10-31 |
| FR2545224A1 (en) | 1984-11-02 |
| DE3414145A1 (en) | 1984-10-31 |
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