GB2138957A - Current sensors for ac/dc converter - Google Patents
Current sensors for ac/dc converter Download PDFInfo
- Publication number
- GB2138957A GB2138957A GB08409620A GB8409620A GB2138957A GB 2138957 A GB2138957 A GB 2138957A GB 08409620 A GB08409620 A GB 08409620A GB 8409620 A GB8409620 A GB 8409620A GB 2138957 A GB2138957 A GB 2138957A
- Authority
- GB
- United Kingdom
- Prior art keywords
- current
- converter
- converter circuit
- recited
- transducer
- 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.)
- Withdrawn
Links
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
Abstract
A current sensor for sensing the total DC current flowing in the DC rails of a multiphase converter circuit, which uses SCR's 30, 32, 34, 36, 38, 40, comprises two current transformers 12, 14 coupled to the AC power lines 18, 20 of the converter and another current transformer 16 coupled to the circuit path of a freewheeling diode 60 connected across the DC rails 48, 50 of the converter. An indication of the total DC current flowing through the converter is obtained by summing signals 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 sum of the two current signals are obtained by a summing resistor 78 thereby to generate a voltage signal proportional to the instantaneous level of the total DC current flowing through the converter. <IMAGE>
Description
SPECIFICATION
Current sensor for AC/DC converter
Background of the invention
This invention pertains generally 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 converter in prior art typically comprises a three-phase input line which couples a three-phase power source and a series of controlled rectifiers, the switching thereof being controlled in a fashion to supply DC current to an inverter. The invention provides an apparatus that 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 stabilize the current and voltage supplied to the invention. Aiso, a "freewheeling" diode is connected in prior art circuits 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 rectifers.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 converter must be accounted. The above-mentioned various prior art systems and techniques for measuring in the DC rails have been employed with some limitations and drawbacks to obtain an accurate measurement of the DC current flow. At least one drawback of using the above-mentioned prior art 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 effects), as opposed to directly.
Summary of the invention
In view of the foregoing, there is need for a relatively inexpensive current transducer for sensing the current flow in the DC rails of a converter. The present invention fulfills such need and also provides a method and apparatus for accurately sensing current flow in the DC rails of a multiphase converter circuit.
Optionally, the invention provides 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 representation of the DC current flow.
The invention resides in a current sensor for sensing the magnitude of DC current flowing through a multiphase converter circuit which is used for converting multiphase alternating power to direct current power, said converter circuit employing a plurality of controlled 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, second transducing means for sensing pulsed
DC current flowing through said converter circuit during periods of non-conduction of said switching devices, and summing means connected to said first and second transducing means for generating a representation of the magnitude of the combination of the sensed current flowing through the converter circuit.
An exemplary embodiment herein describes 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 nonconduction of the switching devices, and summing these sensed currents thereby to obtain an accurate representation of the current flowing through the converter.
A preferred embodiment of the invention in the form of an apparatus comprises a current sensor for sensing the magnitude of the DC current flowing through the multiphase converter circuit, which converter circuit includes 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 current transducer means for directly sensing the current in two phases of the alternating current power lines, a second current transducer for sensing the DC current flowing through the freewheeling diode of the converter circuit during time periods of non-conduction of the switching devices, a first rectifying means connected to th first current transducer for producing a first DC output, a second rectifier connected to the second current transducer for producing a second DC output indicative of the DC 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 magnitude of the combination of the sensed current from the first and second current transducers thereby to provide an accurate indication of the DC current flowing through the converter.
As an additional feature, the invention preferably comprises 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 freewheeling diode connected across the
DC rails of the converter, thereby to sense, without the necessity of a ground reference, the current flowing in both the AC power lines and the output of the converter. A rectifier coupled to the third current transfprmer 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 switch ing devices.To dissipate the stored electromagnetic energy in the winding coupled across the current path of the freewheeling diode, the described apparatus includes a current return circuit costituted 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.
Bythe above method and apparatus, the invention provides for inexpensive, reliable, and accurate measurement of DC current flow.
Brief description of the drawings
A more detailed understanding of the invention can be had from the following description of a preferred embodiment, described by way of example and to be understood in conjunction with the accompanying drawing in which:
Figure I depicts a prior art circuit arrangement overwhich this invention is an improvement foe sensing DC current flow in a converter;
Figure 2 depicts the sensing transducer of an embodiment 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 depicted in Figure 2.
Description of the preferred embodiment
Figure 1 shows a conventional multiphase converter circuit 5 coupled to the AC three-phase power mains. Currenttransformers 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 conductors 18-22 connects 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 conductor 20 couples a node 44 between rectifiers 32 and 38, and line power over conductor 22 couples a node 46 between the rectifiers 30 and 36.In 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 byway 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 1200.
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 duration 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 of the line power supplied over conductor 22 resides above a predetermined minimum level which varies according to the operating parameters determined by the switching controller 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 bytriggenng 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 inverter58. A frewheeling diode 60 also connects across the positive and negative DC rails 48 and 50. The diode 60 permits the continued conduction of DC current through the inductor 54 during periods of nonconduction of the SCR's 30-40. The inverter 58 then may produce a controlled variable frequency, variable amplitude, three-phase power supply for an AC motor.
To measure the magnitude of current flow in the
DC rails 48 and 50, a transducer, such as a Hall detector, an optocoupled transducer, or modulated transformer may be connected to one of the DC rails 48 or 50. Alternatively, a current transducer 10 for measuring AC current only, might be used to approximate the DC current flow in the DC rails 48 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, lack of reliability, and others.The transducer 10 itself senses the current flow in each phase of the AC power lines 18-22 while an auxiliary 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 three-phases of current supplied over the power lines 18-22. The signals are supplied to a diode network generally indicated 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 sustems. The reference numerals of the circuit element of the converter circuit of Figure 1 also correspond to the reference numerals of the circuit elements of the circuit of Figure 2.
In the preferred embodiment, current transformers 12, 14, and 16 comprise sensing elements for measuring the current flow in the AC power mains and in the DC rails 48 and 50 during periods of non-conduction of the switching SCR's 30-40. Use of the current transformers does not necessarily require a ground reference connection. In structure, the novel arrangement ofthe invention includes a first current transducing means constituted by cur renttransfomers 12 and 14. Transformers 12 and 14 supply current to a diode network 76 which gener
ates a DC voltage across the resistor 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 secondary windings of the three current transformers establishes the level of the pulse current delivered through the secondary windings of the current transformers 12-16. In practice, an optimun turns
ratio is selected to suitably match the power dis
sipating capacity of the resistor 78, the desired sensitivity, and/or the power handling capability of the circuit components, generally. Impedance
matching by selecting the specific turns ratio is a matter of conventional knowledge within the art.
When all switching SCR's 30-40 are nonconducting, 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, pulsed 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.
To dissipate the magnetic energy developed in the current transformer 16, a current return path is provided by a diode 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 84, as in Figure 2, is coupled back to back with the diode 82 to limit the voltage level appearing across the transformer 16 to a reasonably 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 as well as the current in the DC rail during periods of nonconduction 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 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. Speci
fically, the current transformers 12 and 14 sense the
AC current flowing through the AC power mains
while the current transformer 16 senses the pulsed
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
shon in Figure 4) of the converter circuit. The zener
diode 84 provides means for dissipating energy
stored in the current transformer 16 after being
pulsed in a forward direction 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
Figure 2. 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 circuit of Figure 4.
In a preferred construction of the current sensor for a 20 and 50 horsepower AC motor controller, 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 comprised IN4001 diodes, the zener diode 84 was a IN4744B rated at 1 watt, and the resistor 78 was 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 applied to any multiple phase system. Also, the arrangement of transformer interconnection may vary according to the number of phases in the distribution system and whether a ground reference is available. 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 artto 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 transformers may be used.
Claims (10)
1. A current sensor for sensing the magnitude of
DC current flowing through a multiphase converter circuit which is used for converting multiphase alternating power to direct current power, said converter circuit employing a plurality of controlled 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,
second transducing means for sensing pulsed DC current flowing through said converter circuit during periods of non-conduction of said switching devices, and
summing means connected to said first and second transducing means 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 the first transducer means comprises a current transformer inductively coupled to the AC power lines of the converter and the second transducer means comprises a current transformer inductively coupled to a DC path of said converter circuit, said first transducer means generating a signal proportional to the current flowing in the AC power lines and said second transducer means generating a signal proportional to pulsed DC current flow in the converter circuit during periods of non-conduction of the switching devices.
3. A current sensor as recited in claim 2 wherein said first transducing means includes frst rectifying means for converting the sensed AC current to a first
DC output current and said second transducing means including second rectifying means for producing a second DC output current, said summing means operative for summing said first and second
DC output currents.
4. A current sensor as recited in claim 1 wherein the first transducer means comprises a current transformer means inductively coupled to the AC power lines of the converter and feeding a bridge rectifier, and wherein the second transducer means comprises a current transformer inductively coupled to a DC path of said converter circuit to conduct current during periods of non-conduction of the switching devices, said first transducer means being adapted to generate a signal proportional to the current flowing in the AC power lines and said second transducer means being adapted to generate a signal proportional to the DC current flow in the converter circuit during periods of non-conduction of the switching devices.
5. A current sensor as recited in claim 4 wherein the first current transducer means includes rectifier
means for rectifying the current therein and said second transducer includes a current return means and a power dissipating means connected in series across the secondary winding of the current transformer for providing a current return path forthe current transformer and for dissipating the energy
stored in said current transformer.
6. A current sensor as recited in claim 5 wherein said current transformer means comprise first and
second current transformers, and said rectifier
means comprises first and second diode bridges.
7. A current sensor as recited in claim 6 wherein
said current return means and said power dissipat
ing means comprise a diode rectifier and a zener
diode connected back to back.
8. A current sensor as recited in claim 7 wherein
said current return means and said power dissipat
ing means comprise, respectively, a diode rectifier
and resistor connected across the secondary wind
ing of the current transformer in said second trans
ducing means.
9. A current sensor as recited in claim 8 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.
10. A current sensor as recited in claim 9 wherein said switching devices comprise silicon controlled rectifiers.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US48964383A | 1983-04-28 | 1983-04-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2138957A true GB2138957A (en) | 1984-10-31 |
Family
ID=23944680
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08409620A Withdrawn GB2138957A (en) | 1983-04-28 | 1984-04-13 | Current sensors 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) |
Cited By (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 |
| FR2696293A1 (en) * | 1992-09-25 | 1994-04-01 | Intelligent Electronic Systems | Current supply process for battery charging system - using HF switching second stage to produce voltage supply for battery charging |
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
Cited By (3)
| 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 |
| US4772996A (en) * | 1986-04-11 | 1988-09-20 | Hitachi, Ltd. | Load current detecting device for pulse width modulation inverter |
| FR2696293A1 (en) * | 1992-09-25 | 1994-04-01 | Intelligent Electronic Systems | Current supply process for battery charging system - using HF switching second stage to produce voltage supply for battery charging |
Also Published As
| Publication number | Publication date |
|---|---|
| CA1214825A (en) | 1986-12-02 |
| FR2545224A1 (en) | 1984-11-02 |
| DE3414145A1 (en) | 1984-10-31 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |