Apparatus and method for balancing electrical currents in a multiple-phase system

A current balancer for balancing one or more currents provided from a three-phase source to a load includes three current transformer/full-wave rectifier bridge sections and one or more saturable core reactors. Each of the current transformer/full-wave rectifier bridge sections is to be connected to a respective one of the phase lines, and a saturable core reactor is to be connected into each phase line whose current is to be controlled. The outputs of all three current transformer/full-wave rectifier bridge sections are connected in electrical parallel to provide a single direct current control current to the saturable core reactor(s).

BACKGROUND OF THE INVENTION 
This invention relates generally to apparatus and methods for balancing 
electrical currents in a multiple-phase electrical system. The invention 
relates more particularly, but not by way of limitation, to an apparatus 
and a method for balancing any of the phase currents in a three-phase 
electrical system in response to a direct current which is proportional to 
the average of the three alternating phase currents. 
In a three-phase electrical power system, for example, to which a 
three-phase motor is connected for energization, many problems can arise 
which degrade the system and the operation and life of the motor. For 
example, when the motor is started, significant electrical and mechanical 
stresses are imposed on the motor because of the high starting currents 
normally used to start the motor. For example, there are electric 
submersible motors which have a starting current of six to eight times the 
nominal rating. Such high starting currents cause the system voltage to 
sag and thereby affect not only the motor, but also other electronic 
circuits on the system. 
Even after a motor is started, significant stresses can be applied to the 
motor due to unbalanced operating currents. These unbalanced currents can 
cause excessive heating and increased power consumption. Also during 
operation, electrical transients can occur in the system, such as from 
lightning or switching surges. These undesirable operating conditions 
adversely affect not only the electrical load, but also the overall 
system. 
The foregoing illustrates the need for an apparatus which eliminates or 
reduces electrical system current imbalances. There is also the need for 
an apparatus to eliminate or reduce electrical transients. There is the 
further need for an apparatus which allows the soft-starting of electrical 
loads, such as motors, to reduce excessive stresses imposed upon such 
loads during start-up. If these needs were met, the operating life and 
dependability of electrical loads could be increased and power consumption 
could be reduced. 
An apparatus which meets such needs should be electrically and mechanically 
dependable and efficient to enhance the structural, operational and 
economic features of such an apparatus. Such an apparatus should also be 
capable of being installed and removed from the power system without 
causing expensive downtime of the system. Such an apparatus should also be 
capable of use without additional step-up transformers and with a minimum 
of special training for installation and maintenance. 
Although the aforementioned needs can be met by my earlier invention 
disclosed in U.S. Pat. No. 4,574,231, there is the further need for an 
improved current balancing apparatus and method which utilize only one 
control current regardless of the number of phases of alternating current 
to be controlled because this would make the control circuit relatively 
simple and inexpensive to manufacture. There is the still further need for 
the control current to be a direct current which has a percent ripple 
substantially lower than other direct current control currents derived 
from less than all of the phases within the multiple-phase system because 
having a relatively lower percent ripple would produce lower power losses 
and permit the use of at least some relatively smaller, less expensive 
components. 
SUMMARY OF THE INVENTION 
The present invention meets the aforementioned needs by providing a novel 
and improved apparatus and method for balancing electrical currents in a 
multiple-phase electrical system. In addition to meeting the needs which 
can be met by my previous invention disclosed in U.S. Pat. No. 4,574,231, 
the present invention utilizes only one control current, which control 
current is a direct current having a relatively small percent ripple. By 
using only one control current regardless of the number of alternating 
current phases to be controlled, the present invention provides a circuit 
which is relatively simple and inexpensive to manufacture. Having a direct 
current control current with substantially lower percent ripple makes the 
power loss in the present invention relatively lower and also permits 
smaller, less expensive components to be used. Specifically for the 
preferred embodiment which utilizes current transformers and saturable 
core reactors having parallel-connected gate windings which carry the 
phase currents, the substantially lower percent ripple direct current 
control current makes the I.sup.2 R power loss in the gate windings lower 
because the parallel-connected gate windings must carry a circulating 
current proportional to the ripple components of the control current as 
well as the alternating current phase or line current. Also, the smoother 
(lower ripple) direct current control current allows the current 
transformers to be smaller because the control winding load (voltamperes) 
is lower than it would be with higher ripple. 
The apparatus of the present invention for balancing current to a load in 
an electrical system which provides multiple phases of alternating current 
comprises: control current means, responsive to all the multiple phases of 
alternating current, for providing a single control current proportional 
to an average of all the multiple phases of alternating current; and 
variable impedance means, connected to the control current means, for 
providing a current conductive path for a phase of alternating current, 
the current conductive path having an impedance automatically variable in 
response to the single control current so that a phase of alternating 
current flowing along the current conductive path is balanced relative to 
the remaining phases of alternating current in response to the variable 
impedance. 
An additional feature of the present invention is a variable resistance 
electrically connected to three parallel-connected rectifier means in 
electrical parallel therewith, which rectifier means and variable 
resistance are included within the control current means. Another feature 
of the present invention includes capacitance means, electrically 
connected between the variable impedance means and electrical ground, for 
providing capacitance in combination with an inductance provided by the 
control current means and the variable impedance means to define filter 
means for impeding high frequencies of current and voltage. Still another 
feature includes transient suppressor means for electrically grounding 
high voltage transients above a predetermined voltage threshold, which 
transient suppressor means is electrically connected between the 
capacitance means and electrical ground. Another feature of the present 
invention is resistance means, electrically connected between the variable 
impedance means and electrical ground, for discharging to electrical 
ground direct current voltage between any phase and ground and between 
phases. A still further feature is that the variable impedance means 
operates in a saturation mode in response to the control current being at 
at least a predetermined ratio to the controlled phase current and the 
control means has means for preventing the control current from being at 
at least the predetermined ratio to the controlled phase current when the 
controlled phase current is at a predetermined value. 
The present invention also provides a method of energizing a motor with 
balanced currents from a three-phase current source, which motor is 
connected to three phase lines of the current source. The method comprises 
the steps of: generating a first direct current in response to a first 
phase current from the current source; generating a second direct current 
in response to a second phase current from the current source; generating 
a third direct current in response to a third phase current from the 
current source; combining the first, second and third direct currents into 
a single control current; and controlling each of an impedance in a first 
phase line of the current source, an impedance in a second phase line of 
the current source and an impedance in a third phase line of the current 
source in response to the single control current so that the first, second 
and third phase currents of the first, second and third phase lines are 
balanced in response to the single control current. 
In a preferred embodiment, each step of generating includes limiting the 
respective direct current to a predetermined maximum in response to the 
respective phase current exceeding a predetermined threshold magnitude; 
and the step of controlling includes changing, in response to limiting the 
direct currents, the impedance of the first phase line, the impedance of 
the second phase line and the impedance of the third phase line so that a 
phase voltage of each of the first, second and third phase lines is 
reduced; and which preferred embodiment method further comprises starting 
the motor with phase currents exceeding the predetermined threshold 
magnitude to apply reduced phase voltages to the motor in response to the 
step of changing the impedances of the phase lines so that a phase 
voltage-responsive starting torque of the motor is reduced and the motor 
is soft-started. 
Therefore, from the foregoing, it is a general object of the present 
invention to provide a novel and improved apparatus and a novel and 
improved method for balancing electrical currents in a multiple-phase 
electrical system. Other and further objects, features and advantages of 
the present invention will be readily apparent to those skilled in the art 
when the following description of the preferred embodiments is read in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As represented in FIG. 1, the present invention provides a current balancer 
2 for balancing one or more alternating phase currents to a three-phase 
induction motor 4. The alternating phase currents which are balanced are 
provided from a conventional three-phase electrical system or source 6. 
Although FIG. 1 illustrates the present invention with respect to a 
three-phase induction motor, such as a submersible pump motor, the current 
balancer 2 can be used for other three-phase applications where balanced 
currents are desirable even though the three phase voltages are not 
balanced or the load impedance is not balanced. Furthermore, the present 
invention has utility in a multiple-phase system; however, the present 
invention will be described in its preferred embodiments with reference to 
a three-phase system which is used to energize a three-phase induction 
motor. 
With respect to a three-phase system, the principle of operation of the 
present invention is to connect in one or more of the three phase lines, 
between the source 6 and the motor 4, a respective automatically variable 
impedance through which the respective phase current must flow. In the 
preferred embodiments, each automatically variable impedance includes a 
conventional saturable iron core reactor. The impedance of each saturable 
core reactor is controlled by a direct current which is proportional to 
the average of the three alternating currents flowing from the source 6 to 
the motor 4. The term "direct current" as used herein generally means a 
current which can have alternating current components, but which has a net 
average value other than zero. More specifically, however, the "direct 
current" control current of the subsequently described preferred 
embodiments is the current derived from the sum of the rectified currents 
from three full-wave rectifiers fed by three current transformers, each of 
which is disposed in a respective one of the three phase lines. These 
features of the present invention will be more fully described with 
reference to FIGS. 2-4, and additional features by which the basic 
preferred embodiments shown in FIGS. 2-4 can be modified will then be 
described with reference to FIGS. 5-8. 
As in each preferred embodiment described herein, the preferred embodiment 
shown in FIG. 2 includes control current means 8, responsive to all the 
multiple phases of alternating current, for providing a single control 
current proportional to an average of all the multiple phases of 
alternating current. The FIG. 2 embodiment also includes variable 
impedance means 10, connected to the control current means 8, for 
providing a current conductive path for a phase of alternating current, 
which current conductive path has an impedance automatically variable in 
response to the single control current so that a phase of alternating 
current flowing along the current conductive path is balanced relative to 
the remaining phases of alternating current in response to the variable 
impedance. For the embodiment shown in FIG. 2, the control current means 
is connected to each of three phase lines or conductors, designated A--A', 
B--B' and C--C', of a three-phase electrical system. The variable 
impedance means 10 is connected into the phase line B--B'. 
The control current means 8 includes three control or converter sections 
12, 14, 16, each of which is connected to a respective one of the three 
phase lines. Because each of the sections 12, 14, 16 are identical, only 
the section 12 will be described in detail with the like components of the 
sections 14, 16 being designated by like reference numerals but followed 
by the lower case letter corresponding to the respective alphabetical 
designations of the respective phase line into which it is connected as 
shown in FIG. 2. 
The control section 12 converts the phase of alternating current flowing in 
the phase line A--A' into a direct current. This is done by means of a 
current transformer 18a and a full-wave rectifier bridge 20a . 
The current transformer 18a includes a primary winding 22a which has means 
for connecting it in electrical series into the phase line A--A' so that 
the alternating current conducted through to the phase line A--A' flows 
through the primary winding 22a when the current transformer 18a is 
connected as shown in FIG. 2. The current transformer 18a also includes a 
secondary winding 24a inductively coupled to the primary winding 22a. The 
winding 24a has terminals connected to input junctions 26a, 28a of the 
full-wave rectifier bridge 20a. 
The full-wave rectifier bridge 20a also includes output junctions 30a, 32a. 
The outputs 30a, 32a are electrically connected in electrical parallel to 
the corresponding outputs of the full-wave rectifier bridges 20b, 20c of 
the sections 14, 16, respectively, as indicated by the schematically 
illustrated electrical connection 34. 
The current transformer 18a transforms the phase current flowing in phase 
line A--A' into a proportional alternating current induced in the 
secondary winding 24a. This induced alternating current is rectified by 
the full-wave rectifier bridge 20a into a rectified direct current. This 
direct current is provided through the output junctions 30a, 32a and 
defines a component of the single control current provided by the control 
current means 8 of the present invention. The remaining components of the 
single control current are provided by the other control sections defining 
the remainder of the control current means 8, which other sections for the 
preferred embodiment shown in FIG. 2 include the sections 14, 16. The 
respective control current components provided at the respective outputs 
of the respective full-wave rectifier bridges are combined into the single 
control current by parallel connecting the outputs of the full-wave 
rectifier bridges as designated by the parallel connection 34. Thus, 
through the parallel connection 34, each of the separately generated 
direct current components is provided in combination with the others to 
the variable impedance means 10. 
The variable impedance means 10 includes in the preferred embodiment a 
saturable core reactor 35b which includes two gate winding sections 36b, 
38b. As shown in FIG. 2, the gate winding sections 36b, 38b are connected 
in electrical parallel and include means for connecting with the phase 
line B--B'; however, it is contemplated that the two gate winding sections 
can be connected in electrical series, but in either event it is preferred 
to use two gate winding sections wound in a known manner to cancel any 
alternating current induced from the phase line back into the control 
circuit. The gate winding sections 36b, 38b are electrically connected in 
electrical series with the phase line B--B' so that the gate winding 
sections 36b, 38b provide within the saturable core reactor 35b a 
conductive path for the phase current. This current conductive path has an 
impedance which is variable in response to the control signal flowing 
through control winding sections 40b, 42b of the saturable core reactor 
35. This variable impedance is a known characteristic of a saturable core 
reactor. 
The control winding sections 40b, 42b are inductively connected to the gate 
winding sections 36b, 38b, respectively. The control winding sections 40b, 
42b are electrically connected in electrical series and are electrically 
connected to the parallel-connected outputs of the full-wave rectifier 
bridges 20a, 20b, 20c so that the single control current comprising the 
three direct current components output from the rectifier bridges flows 
through the serially connected control winding sections 40b, 42b . Thus, 
the single control current, comprising three combined but separately 
rectified direct current components, flows through the single conductive 
path which includes the serially connected control winding sections 40b, 
42b. 
For the preferred embodiment shown in FIG. 2, only the phase current 
flowing in the phase line B--B' is controlled. The current in this phase 
line is held at the average of the three phase currents in response to the 
single control current defined by the direct current components created by 
each of the current transformer/full-wave rectifier bridge circuits of the 
sections 12, 14, 16. This type of single phase current control is useful 
in cases wherein the uncompensated current in phase line B--B' would be 
higher than the phase currents in lines A--A' and C--C' and where the 
phase currents of these other two lines are equal. Open delta transformers 
cause this condition. Also long, flat three-conductor motor cables used 
with submersible pumps cause this condition. 
Another preferred embodiment of the present invention is shown in FIG. 3. 
The apparatus shown in FIG. 3 includes the same control current means 8 as 
in the previously described embodiment, but the FIG. 3 embodiment includes 
two sections of the variable impedance means 10. One of the sections of 
variable impedance means 10 is connected into the phase line A--A' and the 
other section is connected into the phase line C--C' for the circuit 
illustrated in FIG. 3. Because the components of the control current means 
and the variable impedance means illustrated in FIG. 3 are the same as 
these described hereinabove with reference to the FIG. 2 embodiment, no 
further description thereof is made other than to indicate that the like 
components are designated by like reference numerals followed by the 
respective lower case alphabetic designation corresponding to the 
respective phase line into which it is connected as shown in FIG. 3. 
In the FIG. 3 embodiment, the two phase currents of the phase lines A--A' 
and C--C' flow through the saturable core reactors 35a, 35c, respectively. 
These two phase currents are held at the average of the three phase 
currents in response to the single control current provided by the control 
current means 8 in the same manner as described hereinabove with reference 
to FIG. 2. This single control current flows through the serially 
connected control winding sections 40a , 42a , 40c , 42c of the saturable 
core reactors 35a, 35c. The embodiment illustrated in FIG. 3 is useful 
where for any reason the current in the phase line B--B', when 
uncompensated, is lower than either the phase current of line A--A' or the 
phase current of line C--C'. 
Still another preferred embodiment of the present invention is shown in 
FIG. 4. This embodiment includes variable impedance means 10 in all of the 
phase lines. As indicated by the like reference numerals, the components 
of each of the sections shown in FIG. 4 are the same as corresponding ones 
shown in FIGS. 2 and 3. The control winding sections 40a , 42a , 40b, 42b, 
40c , 42c of the saturable core reactors 35a, 35b, 35c are electrically 
connected in electrical series, which series of windings is connected 
across the parallel-connected full-wave rectifier bridges 20a, 20b, 20c of 
the three control sections 12, 14, 16 of the control current means 8. 
Thus, in the FIG. 4 embodiment, all three phase currents flow through 
saturable core reactors which are controlled by the single control current 
provided by the parallel-connected full-wave rectifier bridges so that the 
phase current in each phase line is held at the average of the three phase 
currents. 
With reference to FIG. 5, a modification of the embodiment shown in FIG. 4 
will be described. Inherently created by the connection of the control 
current means 8 and the variable impedance means 10 into the three phase 
electrical system is an inductance. By combining a capacitance with this 
inductance, an inductive-capacitive (L-C) filter can be provided to filter 
high frequencies. Such a filter is constructed in the FIG. 5 embodiment 
wherein each of three capacitors 44a , 44b , 44c is connected between a 
respective one of the phase lines and an electrical ground of the 
electrical system. The use of L-C filters in electrical systems to impede 
or filter high frequencies is well known; however, in the present 
invention such filtering is obtained simply by adding capacitors in 
combination with the inherent inductance provided by the current 
transformers and saturable core reactors used in the preferred embodiments 
of the present invention described hereinabove. 
A different modification, but one similar to the modification shown in FIG. 
5, is illustrated in FIG. 6. In the FIG. 6 embodiment, the capacitors 44a 
, 44b, 44c are used; however, each has an end connected in common to a 
transient suppressor means 46 for electrically grounding high voltage 
transients above a voltage threshold which is predetermined by the nature 
of the means 46. As shown in FIG. 6, the transient suppressor means 46 is 
connected between the commonly connected ends of the capacitors 44a , 44b 
, 44c and electrical ground. An example of a suitable embodiment of the 
transient suppressor means 46 is a metal oxide varistor of a type as known 
to the art. This type of device has been known to be connected directly 
between a phase line and electrical ground to provide the aforementioned 
transient suppression for protecting against over-voltage conditions such 
as can occur when lightning strikes the system, for example. 
Whereas the modifications illustrated in FIGS. 5 and 6 are useful in 
impeding unwanted alternating current and transient currents and voltages, 
the modification illustrated in FIG. 7 discharges direct current 
potentials that exist between the load circuit and electrical ground and 
between the phase lines. This is accomplished by connecting resistors 48a 
, 48b , 48c between respective phase lin0650 es and electrical ground. As 
indicated by the dash-line drawings in FIG. 7, the resistors can be placed 
anywhere along the system and can be combined with the modifications 
illustrated in FIGS. 5 and 6. 
It is to be noted that any of the foregoing modifications can be used with 
the other preferred embodiments shown in FIGS. 1 and 2. 
Another modification which can be used with any of the other preferred 
embodiments is shown in FIG. 8. This modification includes a variable 
resistance 50 electrically connected in electrical parallel with the 
parallel-connected full-wave rectifier bridges 20a, 20b, 20c and with the 
series-connected control windings 40a, 42a, 40b, 42b, 40c, 42c of the 
variable impedance means. The variable resistance 50 is adjusted to 
compensate for variations in the characteristics of either the current 
transformers of the control current means 8 or the saturable core 
reactor(s) of the variable impedance means. 
The components of the foregoing preferred embodiments are of conventional 
types and can be selected as to specific electrical characteristics as 
desired. However, with respect to the current transformers 18 and the 
saturable core reactors 35, a specific embodiment of the present invention 
contemplates selecting these to have characteristics by which the 
three-phase induction motor 4 of the exemplary environment illustrated in 
FIG. 1 can be softstarted. In general, each saturable core reactor 35 is 
selected so that it operates in a saturation mode in response to the 
control current being at at least a predetermined ratio to the respective 
phase current flowing through the saturable core reactor, which selection 
is made in combination with a selection of the current transformers which 
are designed to prevent the control current from being at at least such 
predetermined ratio to the phase currents when the phase currents are at a 
predetermined value. By way of a specific example, current transformers 
can be selected to keep the saturable core reactor saturated up to 
approximately 150% of rated current of the balancer device 2. For any 
current above this rated current, the respective current transformer can 
no longer hold a constant ratio between its secondary voltage and the 
respective phase current because the control voltage required is above the 
burden rating of the current transformer. For such a high phase current, 
the saturable core reactor is not kept in saturation. This causes the 
three-phase voltage supplied to the motor 4 to sag approximately 15%, 
thereby sagging the motor torque approximately 30%. Thus, by appropriately 
selecting the current transformers and the saturable core reactors, a 
voltage sag and correspondingly a starting torque sag can be imposed at 
the high starting currents which are drawn when starting the motor 4. 
Again referring to the exemplary environment illustrated in FIG. 1, the 
preferred embodiment of the method of the present invention will be 
described. This method is particularly adapted for energizing the motor 4 
with balanced current from the three-phase current source 6. This method 
comprises generating respective direct currents in response to each of the 
three phase currents flowing from the current source 6 to the motor 4. 
These three direct currents are combined into a single control current, 
which control current controls each of the variable impedances in the 
three phase lines so that the three phase currents of the three phase 
lines are balanced in response to the single control current. Utilizing 
the foregoing method to soft-start the motor 4 further includes: limiting 
the respective direct current of a phase to a predetermined maximum in 
response to the respective phase current exceeding a predetermined 
threshold magnitude; and changing, in response to limiting the direct 
currents, the impedances of the three phase lines so that a phase voltage 
of each of the three phase lines is reduced; and starting the motor with 
phase currents exceeding the predetermined threshold magnitude to apply 
reduced phase voltages to the motor in response to the step of changing 
the impedances of the phase line so that a phase voltage-responsive 
starting torque of the motor is reduced and the motor is soft-started. 
From the foregoing, it is apparent that phase current balancing of one or 
more phase currents is accomplished with the present invention by using 
only a single control current regardless of the number of phase currents 
to be controlled. The direct current components making up the single 
control current are derived in the same manner from each phase so that 
identical electric components can be used with each phase line. This makes 
the control circuit simple and inexpensive to manufacture. Additionally, 
creating the single control current from a combination of rectified direct 
currents from all of the available phase lines creates a control current 
with ripple which is substantially lower than if the control current were 
derived from less than all of the phases. For example, it is believed that 
a single phase rectified control current might have about 70-80 percent 
ripple, whereas it is believed that the three-phase control current of the 
preferred embodiments of the present invention might have only about 15 
percent ripple. This makes the I.sup.2 R power loss in the gate windings 
of the saturable core reactors relatively lower because the gate windings 
must carry a circulating current proportional to the ripple component of 
the control current as well as the phase current. Also, a smoother direct 
current control current allows the current transformers to be smaller 
because the control winding load is lower than it would be with higher 
ripple. 
Thus, the present invention is well adapted to carry out the objects and 
attain the ends and advantages mentioned above as well as those inherent 
therein. While preferred embodiments of the invention have been described 
for the purpose of this disclosure, changes in the construction and 
arrangement of parts and the performance of steps can be made by those 
skilled in the art, which changes are encompassed within the spirit of 
this invention as defined by the appended claims.