Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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.
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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
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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
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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.
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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-
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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.
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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
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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.
