Method and apparatus for three-phase sensing using two current transformers

A method and apparatus is disclosed for complete three-phase sensing using two current transformers. Two phases of a three-phase system are used as the primary windings in each current transformer such that each current transformer produces a phasor representation of a sum of the two phases. A three phase sensor is provided to add the phasor representations produced by each of the current transformers and to produce a third phasor representation from the summation. The three phasor representations can then be applied to any conventional overload relay control to monitor each phase and produces a trip signal to open the three-phase system when a phasor representation exceeds some predetermined value, or the magnitudes of the phasors differ by more than a predefined value.

BACKGROUND OF THE INVENTION 
The present invention relates generally to phase sensing of alternating 
currents in a polyphase distribution system, and more particularly to a 
method and apparatus for three-phase sensing using two current 
transformers. 
Current transformers (CTs) are used for sensing AC electrical currents in 
load control and in protection devices. For example, CTs are used in 
sensing electrical currents through contactors, motor starters and 
controllers, circuit breakers, monitors and analyzers, and in general, 
electrical distribution systems. In many such applications, these products 
are polyphase, or more particularly three-phase, and generally require a 
CT for each phase. 
Most modern prior art attempts at monitoring overload and fault conditions 
in a load supplied by a multiphase, or polyphase power supply, use a 
current transformer for each separate phase of the three-phase power 
distribution system. For example, U.S. Pat. No. 4,967,304 discloses a 
digital circuit interrupter applicable for use on a three-phase power 
distribution system wherein a separate current transformer is required for 
each separate phase of the distribution system. One attempt at using two 
current transformers to detect phase failure and overload is disclosed in 
U.S. Pat. No. 2,202,998. However, the two CTs monitor only two of the 
three phases, and the third phase is only indirectly monitored. That is, a 
failure or overload on the unmonitored third phase is detected by the 
reaction it has on the two monitored phases. The third phase itself is not 
monitored. Further, a phase loss in the unmonitored phase will go 
undetected until the two CTs detect the resulting higher currents in the 
two monitored phases, which may be too late to protect modern loads having 
very tight thermal tolerances. 
There is therefore a need for a method and apparatus for monitoring AC 
currents in a multiphase system using fewer CTs than the number of phases. 
Such a system would be particularly useful in low cost products where 
having a CT for each phase is cost prohibitive. Therefore, it would be 
desirable to have a method and apparatus for three-phase sensing using two 
current transformers that solves the aforementioned problems. 
SUMMARY OF THE INVENTION 
The present invention accomplishes the foregoing by providing a device for 
monitoring all three phases of a polyphase power supply with only two 
current transformers. The present invention reduces the cost associated 
with the current transformers by one-third from that of most commercially 
available overload devices using three such CTs, and reduces the overall 
cost of such devices by approximately 10%. 
In accordance with one aspect of the invention, a device for monitoring a 
polyphase system connected to a load, for example a motor, has two current 
transformers, each one in operable association with two phases of the 
polyphase system. In this manner, each current transformer monitors two 
phases of the three phase system and thereby produces a phasor 
representation of the negative sum of the two phases monitored. The result 
of this summation, occurring in each current transformer, is the phasor 
representation of a third phase. A three phase sensor is connected to each 
of the current transformers for summing the phasor representations 
produced by each of the two current transformers to extract the third 
phasor representation. 
The results can then be used in any conventional monitoring device to 
monitor each of the three phases. The device may produce a trip signal 
when a phasor representation is not within some predefined range, or a 
comparison of the magnitudes can be made to trip on any phase loss, 
unbalance or jam. For example, a thermal overload relay can be used 
wherein the three phasor representations are used to compute the RMS value 
for the current and integrate the RMS value to create a total heat 
accumulation value. After subtracting a cooling factor, a net heat 
accumulated value is produced which can then be compared to a set point. A 
trip signal is produced when the net heat exceeds the set point. In this 
manner, all three phases can be monitored with the two current transformer 
device. The present invention is equally applicable to such monitoring 
devices as meters and other measuring or monitoring apparatus. 
In accordance with another aspect of the invention, a method of monitoring 
a polyphase system is also disclosed. The method includes detecting the 
current flow in each leg of a polyphase system, then creating a first 
phasor representation from the current flow in the first and second legs 
of the polyphase system and creating a second phasor representation from 
the current flow in the second and third legs of the polyphase system. The 
process next includes calculating a third phasor representation from the 
first and second phasor representations, and then monitoring each phasor 
representation. The method can also include preventing current flow in 
each leg in response to at least one of the phasor representations varying 
by more than a predetermined value, or one phasor value varying by more 
than a predetermined amount over that of another phasor. 
Various other features, objects and advantages of the present invention 
will be made apparent from the following detailed description and the 
drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is described herein in connection with a motor, a 
motor starter, and a thermal overload relay. However, it is understood 
that the application of the invention in this configuration is for 
illustration only, and it will be appreciated by those skilled in the art 
that the present invention is equally applicable to any load connected to 
a polyphase distribution system with an appropriate overload, fault 
detection, and/or metering control. 
Referring to FIG. 1, an electric motor 10 is energized by a polyphase AC 
distribution system 12 connected to a power source 14. The polyphase AC 
distribution system 12 has three-phase conductors 16, 18, and 20 
interruptible by a switch having contacts 22, 24, and 26 connecting the 
power source 14 to the motor 10 through the three-phase conductors 16, 18, 
and 20. 
A motor starter 28 includes a contactor 30 incorporating a coil 32 which 
when energized, closes the switching contacts 22, 24, and 26 in conductors 
16, 18, and 20 to connect the motor 10 20 to the power source 14. The 
motor starter 28 also has an overload relay 34 to receive analog signals 
from a three phase sensor 36 on lines 38, 40, and 42. 
The three phase sensor 36 is connected to two current transformers 44 and 
46, each in operable association with two legs of the polyphase AC 
distribution system 12. Each of the current transformers 44, 46 include a 
toroidal core 44a, 46a and a secondary winding 44b, 46b, respectively. The 
conductors 16, 18, and 20 pass through the toroids 44a, 46a to form the 
primary winding of the current transformer. In the preferred embodiment of 
FIG. 1, conductors 16 and 18 pass through toroid 44a to form the primary 
winding of current transformer 44, and conductors 18 and 20 pass through 
toroid 46a to form the primary winding of current transformer 46. The 
secondary winding 44b generates analog signals representative of the 
currents in the associated conductors 16 and 8, and the secondary winding 
46b generates analog signals representative of the currents in the 
associated conductors 18 and 20. The analog signals are relayed to the 
three phase sensor 36 via leads 44c and 46c, respectively. 
In this manner, each current transformer 44 and 46 monitors two phases in 
the polyphase system 12 to produce a phasor representation of a sum of the 
two phases monitored. For example, current transformer 44 monitors phases 
A and B on conductors 16 and 18 and produces the phasor representation -C 
as a result of the vector addition. Similarly, current transformer 46 
monitors phases B and C on conductors 18 and 20 to produce the phasor 
representation -A from the vector summation of B and C, and supplies that 
result on leads 46c to the three phase sensor 36. 
The three phase sensor 36 is connected to each current transformer 44 and 
46 for summing the phasor representations produced by each transformer and 
extracts a third phasor representation from the result. In this case, the 
three phasor sensor 36 adds the phasor representation -C from current 
transformer 44 to the phasor representation -A from current transformer 46 
to derive the third phasor representation B. The three phase sensor 36 can 
then supply all three phasor representations in analog form on lines 38, 
40, and 42 to analog-to-digital (A/D) convertor 48 in overload relay 34 to 
digitize the phasor representations. 
As is known, a control or microprocessor 50 monitors an associated value of 
each digitized signal and when a given value exceeds some predetermined 
value, or the magnitudes differ by more than a predefined value, the 
microprocessor control 50 produces a trip signal to contactor 30 of motor 
starter 28 to open switching contacts 22, 24, and 26, thereby 
disconnecting motor 10 from power source 14. In a thermal overload 
application, the three phasor representations are used to compute the RMS 
value for the current and integrate the RMS value to create a total heat 
accumulation value. After subtracting a cooling factor, a net heat 
accumulated value is produced which can then be compared to a set point. A 
trip signal is produced when the net heat exceeds the set point. It is 
understood that the A/D converter 48 and microprocessor 50 can be 
equivalently replaced with a microcontroller. 
Referring to FIG. 2, one embodiment of the three phase sensor 36 is shown 
in circuit schematic form. The secondary winding of the current 
transformer 46 is shown as inductor 60 sensing phases B and C, and the 
secondary winding of the current transformer 44 is shown as inductor 62 
sensing phases A and B. Burden resistors 64 and 66 are matched precision 
resistors connected in parallel with the secondary windings, or inductors 
60 and 62, respectively. RC network 68 and 70 provide high frequency 
filtering and current limiting to the microcontroller. Resistors 72 and 74 
are each in series with inductors 60 and 62 and form a voltage divider 
between the two voltage sources 76 and 78 to create a summing junction at 
node 80. 
To provide consistent voltage levels to the microcontroller on lines 38, 
40, and 42, resistors 68a and 70a are of equal value, and in a preferred 
embodiment, are 1.00 k ohms. The voltage divider resistors 72 and 74 are 
then twice the value of resistor 68a and 70a, and in this preferred 
embodiment are then 2.00 k ohms. Capacitor 82 provides high frequency 
filtering to the third phase node at 80, and is of equal value to that of 
capacitors 68b and 70b of RC network 68 and 70, respectively. In the 
preferred embodiment, capacitors 68b, 70b, and 82 are 0.01 .mu.F. 
As will be understood by those skilled in the art, the inductor 60 of 
current transformer 46 monitors phases B and C to provide a phase 
representation of -A at 76, and the inductor 62 of current transformer 44 
monitors phases A and B to produce phase -C at 78. The voltage divider of 
resistors 72 and 74 add the phase representation -A from voltage source 76 
with -C from voltage source 78 to provide phase B at summing junction 80. 
In this manner, all three phases are determined and supplied to the 
microcontroller on lines 38, 40, and 42 with the use of only two current 
transformers. 
The invention therefore also includes a method of monitoring a polyphase 
distribution system. The method includes the steps of detecting current 
flow in each leg of a polyphase system, then creating a first phasor 
representation from the current flow in a first and second leg and 
creating a second phasor representation from the current flow in a second 
and third leg of the polyphase system. In using a common leg for 
determining the first two phasor representations, the method can then 
calculate a third phasor representation from the first and second phasor 
representations. Now that all three phasor representations have been 
determined, the method also includes monitoring each of the phasor 
representations. The step of monitoring can include preventing current 
flow in the polyphase system when an overload is detected in any of the 
phasor representations. The step of calculating can encompass the step of 
summing the first and second phasor representations as previously 
discussed. 
The present invention has been thoroughly described herein as applied to a 
motor, a motor starter, and a thermal overload. However, it is understood 
that the invention is well suited for use in any polyphase distribution 
system and is not limited to the application described herein and shown in 
the drawings. 
The present invention has been described in terms of the preferred 
embodiment and it is recognized that equivalents, alternatives, and 
modifications, aside from those expressly stated, are possible and within 
the scope of the appending claims.