U.S. patent number 3,809,980 [Application Number 05/329,748] was granted by the patent office on 1974-05-07 for phase adapter.
Invention is credited to Frank O. Nottingham, Jr..
United States Patent |
3,809,980 |
Nottingham, Jr. |
May 7, 1974 |
PHASE ADAPTER
Abstract
A device is disclosed for converting single-phase power to
three-phase power. The device includes an autotransformer and a
variable impedance device for adjusting the magnitude and phase of
the autotransformer output. A first regulator circuit, including
comparison, amplifier, phase shifting and SCR stages, is provided
to regulate the impedance of the variable impedance device. A
second regulator circuit, including comparison, control and power
output stages, is also included to provide the device with the
capability of compensating for changing power factors in
three-phase loads coupled to it.
Inventors: |
Nottingham, Jr.; Frank O.
(Atlanta, GA) |
Family
ID: |
23286835 |
Appl.
No.: |
05/329,748 |
Filed: |
February 5, 1973 |
Current U.S.
Class: |
318/768; 363/154;
323/204 |
Current CPC
Class: |
H03F
9/02 (20130101); H01F 30/14 (20130101); H02M
5/16 (20130101); H02J 3/1885 (20130101); Y02E
40/32 (20130101); Y02E 40/30 (20130101); H01F
2029/143 (20130101) |
Current International
Class: |
H01F
30/06 (20060101); H01F 30/14 (20060101); H02M
5/02 (20060101); H02M 5/16 (20060101); H02J
3/18 (20060101); H02p 001/44 () |
Field of
Search: |
;318/22R,221R,225R
;321/57 ;323/102,103,112-114 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rubinson; Gene Z.
Attorney, Agent or Firm: Newton, Hopkins & Ormsby
Claims
What is claimed as new and desired to be secured by letters patent
of the
1. A device for converting single-phase power to three-phase power
comprising:
input means for coupling said device to a suitable source of
single-phase power,
transformer means coupled to said input means,
variable impedance means coupled to said transformer means for
adjusting the phase of output current from said transformer means
said variable impedance means including a saturable core reactor
coupled in series with a linear inductor,
first regulator means coupled to said input means and to said
variable impedance means for controlling the impedance of said
variable impedance means in accordance with the magnitude of the
three-phase power demand, and
second regulator means coupled to said first regulator means for
providing
2. A device for converting single-phase power to three-phase power
as in claim 1, wherein:
a first capacitor is coupled in parallel with said series
connected
3. A device for converting single-phase power to three-phase power
as in claim 1, wherein:
said saturable core reactor includes a core having a center leg and
two outer legs, a gate winding wound on said center leg, and a pair
of control windings wound on said outer legs and connected in
series opposition whereby a wide range of reactance variations with
rapid response is
4. A device for converting single-phase power to three-phase power
comprising:
input means for coupling said device to a suitable source of
single-phase power,
transformer means coupled to said input means,
variable impedance means coupled to said transformer means for
adjusting the phase of output current from said transformer
means,
first regulator means coupled to said input means and to said
variable impedance means for controlling the impedance of said
variable impedance means in accordance with the magnitude of the
three-phase power demand and including comparison means for
developing an output error signal representing the difference
between two input signals, amplifying means and for amplifying said
error signal, output means for supplying an output signal,
electronic switching means coupled to said output means for
controlling said output signal, and phase shifting means coupled to
said electronic switching means for controlling the switching phase
thereof, and
second regulator means coupled to said first regulator means for
providing
5. A device for converting single-phase power to three-phase power
comprising:
input means for coupling said device to a suitable source of
single-phase power,
transformer means coupled to said input means,
variable impedance means coupled to said transformer means for
adjusting the phase of output current from said transformer
means,
first regulator means coupled to said input means and to said
variable impedance means for controlling the impedance of said
variable impedance means in accordance with the magnitude of the
three-phase power demand, and
second regulator means coupled to said first regulator means and
including comparison means for developing an output error signal
representing the difference between two input signals for providing
said device with load
6. A device for converting single-phase power to three-phase power
as in claim 5, wherein:
said second regulator means further includes control means
comprising a servo motor and output means including a rotatable
transformer coupled to said servo motor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to phase adapted circuits, and
more particularly to circuits for converting single-phase power to
three-phase power.
2. Description of the Prior Art
In many instances a need exists for converting ordinary
commercially available single-phase power to three-phase power. For
example, many remote areas, such as farms and small shops are
serviced by conventional single-phase power lines. However, in
these locations it is often desirable to operate machine tools,
water pumps, elevating equipment and other devices which are driven
by three-phase electric motors. From an economic point of view it
is much more desirable to convert a fraction of the single-phase
power available at these locations to three-phase power than to
attempt to build three-phase power lines to service these remote
areas. Accordingly, a need exists for a phase adapter suitable for
converting available single-phase power to three-phase power.
Devices for power phase conversion have been designed in the past.
However, known devices have generally been deficient in one or more
areas. For example, some conventional devices have been too
expensive to manufacture commercially, and those that have been
capable of commercial manufacture at a reasonable cost have often
proven to be unreliable. Similarly, other known devices, while
relatively inexpensive and generally reliable have lacked
versatility in that they were incapable of providing large
variations in phase and power factor. In addition, some known
devices are capable of use with only one type of electric motor,
and therefore do not provide a sufficient range of utility to be
commercially successful. Thus a need exists for a novel phase
adapter which is reliable, capable of being manufactured at a
reasonable cost, and capable of efficient operation with various
electric motors and other similar loads over a wide range of phase
angles and power factors.
SUMMARY OF THE INVENTION
Accordingly, one object of this invention is to provide a novel
phase adapter for converting single-phase power to three-phase
power.
Another object of this invention is to provide a novel circuit for
converting single-phase power to three-phase power which is
suitable for use with a variety of electrical motor loads.
Yet another object of this invention is the provision of a novel
phase adapter circuit which is highly reliable, and reasonably
economical to manufacture.
A still further object of this invention is the provision of a
novel phase adapter circuit which is capable of efficient operation
with a wide variety of electrical loads including multiple motor
loads.
Another object of this invention is the provision of a phase
adapter which is capable of reliable and efficient operation with a
variety of loads over a substantial range of power factors.
A still further object of this invention is the provision of a
novel phase adapter circuit including a pair of automatic regulator
circuits.
Another object of this invention is to provide a unique regulator
circuit for use with a novel phase adapter circuit.
Another object of this invention is the provision of two unique
regulator circuits for use with a novel phase adapter circuit.
Briefly, these and other objects of the invention are achieved by
providing a phase adapter circuit including a main autotransformer
having a fixed capacitor coupled in series therewith, and having a
variable inductor coupled in parallel with the fixed capacitor. A
saturable-core reactor is used in the circuit to provide quick
response, and a wide range of reactance values. First and second
regulator circuits are utilized to provide accurate phase and power
factor control.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the
attendant advantages thereof will be readily obtained as the same
becomes better understood by reference to the following detailed
description when considered in connection with the accompanying
drawings, wherein:
FIG. 1 is a phasor diagram of induction motor operation;
FIG. 2 is a phasor diagram of a phase adapter;
FIG. 3 is a phasor diagram for an induction motor supplied from a
phase adapter at 0.866 power factor;
FIG. 4 is a circuit diagram of a simplified phase adapter for
supplying a three-phase motor with a single current at one value of
power factor;
FIG. 5 is a circuit diagram of a variable reactance unit;
FIG. 6 is a circuit diagram of a saturable-core reactor coil
arrangement;
FIG. 7 is a circuit diagram of a basic phase adapter unit;
FIG. 8 is a phasor diagram of a complete phase adapter circuit;
FIG. 9 is a phasor diagram for a 0.50 power factor load;
FIG. 10 is a circuit diagram of the complete phase adapter circuit
of the present invention;
FIG. 11 is a V-curve for a synchronous motor at no load;
FIG. 12 is a circuit diagram of a phase adapter circuit employing
power factor correction, according to the present invention, for
use with a synchronous motor;
FIG. 13 is a circuit diagram of the regulator circuit shown in
block form in FIGS. 7, 10 and 12 as the first regulator unit;
and,
FIG. 14 is a circuit diagram of the regulator circuit illustrated
in block diagram form in FIGS. 10 and 12 as the second regulator
unit.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before discussing the specific embodiments of the present
invention, a consideration of the range of induction motor loads
and the power factors at which motors must operate under these
loads is believed to be in order. In general, for a given
horsepower rating, the higher the synchronous speed of a motor, the
higher the power factor at full load. The normal range of power
factors is from approximately 0.90 for 3,600 rpm motors to
approximately 0.65 for 900 rpm motors. This range in power factors
is equivalent to a range of from approximately 25.83.degree.
(lagging) to approximately 49.53.degree. (lagging). As is familiar
to those skilled in the art, the power factor at which a given
motor operates varies as the load on its shaft changes. Thus, the
power factor for a particular motor will be highest at full load,
and will drop to its lowest value when no load is applied to the
motor. In practice, the range of power-factors due to loading
extends from approximately 0.90 to approximately 0.50 (lagging).
This means a range in phase-angles from approximately 25.83.degree.
(lagging) to approximately 60.00.degree. (lagging). Accordingly, it
is necessary for a phase adapter of practical utility to operate
reliably throughout these ranges of power factors and phase
angles.
In developing a phase adapter, or single-phase to three-phase
converter, an energy storage element is required to absorb energy
from the single-phase power supply, and to deliver this energy at a
subsequent phase to an appropriate load. The most common energy
storage elements are, of course, the capacitor and the inductor. In
the case of a capacitor, a phase shift will be created such that
the current leads the voltage applied to the unit by nearly
90.degree.. Very low losses are possible with oil-filled
capacitors, so that these devices deviate only slightly from the
ideal phase shift of 90.degree.. Similarly, the current in a well
designed inductor will lag the applied voltage by a large angle,
which may approach 90.degree. in optimum situations. These basic
aspects of energy storage devices must also be taken into
consideration in constructing phase adapter circuits.
Referring now to the drawings, wherein like reference numerals
designate identical or corresponding parts throughout the several
views, and more particularly to FIG. 1 thereof, a phasor diagram
for a conventional induction motor operating from a balanced
three-phase power source is shown. Voltage phasors V.sub.12,
V.sub.23 and V.sub.31 represent the line voltages supplied to motor
terminals 1, 2 and 3 from a three-phase source. If the induction
motor stator is connected in a Y configuration, as is generally the
case, the respective motor phase voltages radiate from a central or
neutral position designated N. In this case the motor phase
voltages are represented by the phasors V.sub.N1, V.sub.N2, and
V.sub.N3. These voltages may be referred to as the line-to-neutral
voltages. The corresponding motor currents designated by the
phasors I.sub.1, I.sub.2 and I.sub.3 are shown lagging the phase
voltages by appropriate phase angles .phi..sub.1, .phi..sub.2 and
.phi..sub.3, respectively. For completely balanced conditions, the
magnitudes of all respective phasors and phase angles are equal,
respectively.
As was pointed out above, the symmetrical phasor diagram of FIG. 1
represents a situation in which a balanced three-phase power source
is available. In the environment of the present invention, it is
assumed that no source of three-phase power is available. Instead,
only one voltage, represented by the phasor V.sub.12, for example,
is supplied by the available power source, such as a public utility
company. In this case, the voltages represented by the phasors
V.sub.23 and V.sub.31 must be derived by means of an auxiliary
apparatus.
FIG. 2 is a phasor diagram representing the voltages which are
desirably supplied by a suitable phase adapter. The precise
relationships among the phasors illustrated in FIG. 2 are set forth
in Table 1 below.
TABLE 1
V.sub.1a = 2 V.sub.12
.theta..sub.1a = 0
V.sub.03 = (0.866) v 90.degree.
v.sub.10 = 0.5 v 0.degree.
v.sub.02 = 0.5 v 0.degree.
v.sub.0a = 1.5 V 0.degree.
V.sub.3a = 1.732 V -30.degree.
V.sub.12 = v
although other relationships are usable with the present invention,
the relationship set forth in the table above is considered
optimum. Similarly, the phasor diagram for an induction motor
supplied from a suitable phase adapter at 0.866 power factor is
shown in FIG. 3. Concerning the phasor diagrams, it is pointed out
that when a three-phase motor receives a balanced set of input
voltages, the motor winding neutral will be located at the center
of the phasor triangle. The voltage from the motor terminal 3 (the
numerals at the corners of the phasor triangles designate motor
input terminals) to the center point of the triangle base 0, that
is V.sub.03, will be at right angles to the base which is the
neutral point of the single-phase supply. Thus, it is clear that
the purpose of a suitable phase adapter is to supply a current to
the third terminal of the motor which is compatible with the
currents supplied by the single-phase source to the remaining two
terminals of the motor. Referring now to FIG. 3, the phasor I.sub.c
represents current supplied from a phase adapter, and adjusted in
phase by means of a suitable capacitor. It will be apparent that
the direction or phase of the current phasor I.sub.c resulting from
the voltage V.sub.3a is parallel to the required motor current for
the illustrated condition. Thus, it is only necessary to adjust the
magnitude of the current phasor I.sub.c to an appropriate magnitude
to properly drive the motor when its power factor is 0.866.
A basic phase adapter circuit for developing the appropriately
related current and voltage signals briefly described above is
illustrated in FIG. 4. This circuit includes a pair of input
terminals 10 and 12 which are adapted to be coupled to a suitable
source of single-phase power. The input terminal 10 is coupled
through a main autotransformer 14 and a capacitor 16 to one
terminal (designated terminal No. 3) of a stator winding 18 of an
induction motor 20. The input terminal 10 is also directly coupled
to another terminal of the stator winding 18 (designated terminal
No. 1) through a line 22. Similarly, the input terminal 12 is
directly coupled to the remaining terminal of the stator winding
(designated as motor terminal No. 2) through a line 24. A center
tap 26 of the main autotransformer 14 is also coupled to the line
24, and thus to the motor terminal 2.
In order to control the current delivered to the motor in the
circuit of FIG. 4, a variable capacitor could be used. However at
low frequencies, such as 60 Hz, an extremely large and impractical
capacitor would be required. However, a suitable circuit
arrangement which provides the same effect as a large variable
capacitor is illustrated in FIG. 5. This circuit includes a
variable inductor 28 coupled in parallel with the fixed capacitor
16. The net output response of this circuit is substantially
identical with that of a large variable capacitor. The variable
inductor 28 preferably consists of a saturable core reactor 30
coupled in series with a linear inductor 32 to form a variable
inductance unit.
The details of the saturable core reactor 30 are illustrated in
FIG. 6. Since it is highly desirable to have a saturable core
reactor designed for wide range reactance variations with quick
response, a coil 34 wound on a center leg 36 of a reactor core 38
is used as the "gate" or A.C. winding. A pair of "control" or D. C.
windings 40 and 42 are wound about the outer legs 44 and 46,
respectively, of the reactor core 38. The control windings 40 and
42 are connected in series opposition so that no A. C. voltage
appears across their free terminals. The A. C. winding is designed
so that the peak value of flux density in the core is well below
the saturation value. As a result, a wide range of current
variation is possible. Since there are no short circuited loops in
the A. C. current paths, as there are in saturable core reactors
which have the outer leg coils in parallel as the A. C. winding,
the response of this reactor is much more rapid than conventional
saturable core reactors.
FIG. 7 illustrates a composite circuit wherein the circuits
illustrated in FIGS. 4 and 5 have been combined. In addition, a
first regulator unit 48 is shown coupled to the composite circuit.
The first regulator unit 48, which will be described in more detail
hereinafter, includes a first input terminal 50 coupled to the
input terminal 10, a second input terminal 52 coupled to the input
terminal 12, and a third input terminal 54 coupled to the third
input terminal 3 of the stator winding 18. The first regulator unit
48 also includes a pair of output terminals 56 and 58 which are
coupled across the D.C. input terminal of the saturable core
reactor 30, as illustrated in FIG. 6. The first regulator unit 48
senses the differences between the magnitude of the input voltage
applied across the terminals 10 and 12 and the quadrature voltage
generated in the induction motor 20, and generates a proper D. C.
voltage across its output terminals 56 and 58 in order to maintain
the relationship illustrated in FIG. 2 and set forth in Table 1.
Under these circumstances, if the motor operates at 0.866 power
factor, the voltages at the motor inputs will be balanced, as will
the motor input currents. At other values of power factor
sufficiently near the 0.866 value, the voltages and currents will
be approximately balanced.
As will be apparent from the preceeding discussion, the circuit
illustrated in FIG. 7 is not a completely satisfactory phase
adapter, since additional circuits are required to correct for
power factor variations in the motor as seen by the basic phase
adapter. Referring again briefly to FIG. 2, it will be apparent
that a reactive element, which may change its impedance
characteristics from capacitive to inductive, may be connected
between motor terminals 1 and 3, so that a current is supplied to
terminal 3 which is represented by a phasor lying along the locus
of the voltage phasor V.sub.3a. Since the phasor V.sub.31 is
90.degree. in phase from the phasor V.sub.3a, inductive reactance
will produce a current component into terminal 3 which is opposite
in phase to the phasor V.sub.3a. Similarly, capacitive reactance
will produce a current component at terminal 3 which is in phase
with the phasor V.sub.3a. Thus an auxiliary circuit can be added to
the basic phase adapter illustrated in FIG. 7 which produces a
regulated voltage so that the magnitude of the phasor V.sub.03 is
equal to 0.866 V.sub.12, and the current component produced by the
auxiliary circuit may be used to cause the motor load to appear to
the basic phase adapter as if its power factor is maintained at a
constant value of 0.866. The phasor diagram for this situation is
illustrated in FIG. 8.
Referring now to FIG. 8, when a motor is operating at a power
factor of 0.50, the motor current is represented by a phasor
I.sub.Nc. As can be seen from FIG. 8, the motor current I.sub.Nc
can be attained by adding together the basic phase adapter current,
represented by the phasor I.sub.Nb and an additional current,
represented by the phasor I.sub.bc, which leads the voltage phasor
V.sub.31 by 90.degree.. On the other hand, if the motor is
operating at a power factor of 0.90, the motor current is
represented by the phasor I.sub.Nc.sub.'. In this case, a current
represented by the phasor I.sub.b.sub.'c.sub.', which lags the
voltage phasor V.sub.31 by 90.degree., must be added to the basic
phase adapter current, represented by the phasor I.sub.Nb.sub.', to
produce satisfactory motor operation. Naturally, this same analysis
can be applied to any desired motor current value and its
associated power factor by means of observing the appropriate
current circle and associated current phasors. Concerning the
various phasor diagrams, it is pointed out that these diagrams do
not constitute a portion of the present invention, but are intended
to aid in the explanation of the present invention. Further
information pertaining to phasor diagrams of the type set forth
herein may be obtained by reference to one or more of the following
texts:
"Principles of Alternating Current Machinery" by: Ralph R. Lawrence
2nd ed. New York, McGraw-Hill 1921 4 ed. -- revised by Henry E.
Richards New York, McGraw-Hill 1953.
"A Course in Electrical Engineering" by: Chester L. Dawes, Vol. II
McGraw-Hill, New York, 4th ed. -- 1947-1952.
"Alternating Current Machines" Puchstein, Lloyd & Conrad New
York, Wiley, 1954.
"Alternating Current Machines" by: Thomas Clair McFarland New York,
D. Von Nostrand Co. 1948.
FIG. 9 represents a phasor diagram analysis which is an alternative
to that illustrated in FIG. 8. The triangular phasor diagram of
FIG. 9 may be obtained from that of FIG. 8 by making the point a in
FIG. 8 correspond with the point 2. If it becomes desirable to
operate a motor having a power factor of 0.866 from the phase
adapter arrangement illustrated in the diagram of FIG. 9, the
required motor current represented by the phasor I.sub.Nc is
obtained by adding the component phasors I.sub.Nb and I.sub.bc, in
essentially the same manner as described with reference to FIG. 8.
However, it should be noted that in FIG. 9 the line of correction
current from the second regulator, which will presently be
described in detail, is 60.degree. from the basic phase adapter
current represented by the phasor I.sub.Nb. This means that the
correction is most favorable when the motor power factor is 0.866,
or a 30.degree. lagging current, since the correction current is at
right angles to the required motor current. For all values of power
factor above 0.50 lagging, the required correction will be in the
same direction. Similarly, for all values of power factor less than
0.50, the correction current required will be in the opposite
direction.
From consideration of the phasor diagrams of FIGS. 8 and 9, it is
apparent that a second regulator must be added to the basic phase
adapter circuit illustrated in FIG. 7 to provide the necessary
power factor correction current. Thus, in FIG. 10, a second
regulator unit 60 is shown coupled to the basic phase regulator
circuit of FIG. 7. More particularly, the second regulator unit 60
includes first, second and third input terminals 62, 64 and 66,
respectively, which are coupled to the first, second and third
input terminals 50, 52 and 54, respectively of the first regulator
unit 48. Similarly, the second regulator unit 60 includes two
output terminals 68 and 70 which are coupled to the D. C. input
terminals of a second saturable core reactor 72. The second
saturable core reactor 72 forms a portion of a second variable
inductor 74 which is coupled in parallel with a second fixed
capacitor 76. The second variable inductor 74 may be identical to
the variable inductor 28, previously described, and the second
fixed capacitor 76 may be identical with the fixed capacitor 16.
However, one A. C. input terminal of the second variable inductor
74 is coupled to the input terminal 10 and to the first motor input
terminal 1, while the other A. C. input terminal is coupled to the
third motor input terminal 3. It should be noted that the second
variable inductor 74 includes a second linear inductor 78 in
addition to the second saturable core reactor 72.
In operation, the second regulator unit 60, which will be described
in detail hereinafter, senses the differences between the
magnitudes of the voltage phasors V.sub.23 and V.sub.31, and
produces an output signal at terminals 68 and 70 for controlling
the second variable inductor 74 and producing appropriate
correction currents. The regulator preferably includes an
integrating circuit so that when the error voltage is zero, there
will be a proper D.C. current through the second saturable core
reactor to provide the desired correction current.
From the discussion above it is apparent that proper three-phase
motor operating current is obtained by adding an out-of-phase
current component to the output current of the basic phase adapter
shown in FIG. 7.
A second circuit for power factor correction can be developed from
the well-known phenomenon that a synchronous motor with a D.C.
field supply operates at various values of power factor providing
its field excitation is suitably adjusted. This concept may be more
clearly understood by reference to the synchronous motor curve of
FIG. 11. This curve, which is known as a V curve, is clearly
described in the various publications referenced above. The V curve
illustrated in FIG. 11 is the no-load curve for a synchronous
motor, and includes a minimum point 80 at which the motor operates
at a unity power factor. For values of field current lower than
that corresponding to a unity power factor, the motor is magnetized
from the line and the input current will be lagging. Similarly, for
values of field current greater than that required for unity power
factor, the generated voltage of the motor will be greater than the
applied voltage, and the current to the motor will lead the applied
voltage, so that the power factor will be leading. This ability of
the synchronous motor to produce both lagging and leading power
factors may be used to correct the power factor of a total load,
for example induction motors and synchronous motors coupled in
parallel in a manner similar to that described above with reference
to the circuit of FIG. 10. In this case, the second regulator unit
60 merely adjusts the field current of the synchronous motor until
the power factor of the combined load matches the natural power
factor of the basic phase adapter circuit. A circuit of this type
is shown in FIG. 12. The circuit of FIG. 12 is generally similar to
that of FIG. 10, except that the second variable inductor 74 and
second fixed capacitors 76 have been removed, and the induction
motor 20 has been replaced by a synchronous motor 82. The output
signal generated by the second regulator unit 60 is applied over a
pair of lines 84 and 86 to the field windings of the synchronous
motor 82. The control or correction signals generated by the first
and second regulator units 48 and 60, respectively are
substantially the same as those generated in the embodiment
illustrated in FIG. 10. However, as pointed out above, the
synchronous motor provides the necessary correction in power factor
which was performed by the additional variable inductor and fixed
capacitor circuitry shown in FIG. 10. In the embodiment of FIG. 12
properly converted three-phase power is available at the output
terminals of the circuit, designated by the reference numerals 88,
90 and 92.
When a synchronous motor is used for power factor correction, the
rotor of the motor stores mechanical kinetic energy. Since the
motor is synchronous, the energy is fed back into the power system
by a generator action during disturbances to the input. This
property can be very useful for starting induction motors when they
are used as loads. Where extremely heavy starting currents are
required, the synchronous motor may be equipped with a fly-wheel.
Thus, the energy stored in the fly-wheel augments the energy stored
in the rotor of the motor to generate larger starting currents,
when needed.
No starting capacitors are shown in the phase adapter circuits
described above. However, if additional capacitance is needed to
provide higher starting torques, additional capacitors can be added
to the circuit.
Referring now to FIG. 13, the circuit of the first regulator unit
48 is shown in detail. To facilitate description of the first
regulator circuit 48, the circuit has been divided into separate
functional stages, namely a comparison stage 94, and amplifier
stage 96, a phase shifting stage 98 and a SCR stage 100.
Attention is first directed to the comparison stage 94, which
serves to compare the voltage applied to the third motor terminal
(v.sub.30) with the magnitude of the input voltage (V.sub.12),
which is the voltage applied to the terminals 10 and 12 of the
circuit shown in FIG. 10, and corresponds to the voltage applied to
the terminals 50 and 52 of the first regulator unit. More
particularly, the comparison stage 94 produces an error signal at a
pair of terminals 102 and 104 according to the following
equation:
V.sub.error = .vertline.V.sub.30 .vertline. - 0.866
.vertline.V.sub.12 .vertline. (1)
however since the voltages corresponding to the phasors V.sub.30
and V.sub.12 are 90.degree. apart in phase, they cannot be directly
compared. It is therefore necessary to rectify and filter these
voltages, and then compare their D.C. Components, which is the
specific function of the comparison stage 94.
The comparison stage 94 includes a first transformer 106 for
measuring the input voltage V.sub.12 applied across the terminals
50 and 52. The output of the first transformer 106 is fed through a
padding resistor 108 and a potentiometer 110 to a full wave
rectifier including four diodes 112, 114, 116, and 118. The output
of the full wave rectifier is then applied to a filter capacitor
120 having a bleeder resistor 122 coupled in parallel
therewith.
A second transformer 124 is coupled to the input terminal 54 and to
a primary center tap 126 of the first transformer 106, setting up a
Scott Connection. A loading resistor 128 is coupled across the
output of the second transformer 124 so that the two input
voltages, which are 90.degree. apart, will be balanced at the
tranformer output. The transformer output is coupled to a second
full wave rectifier including four diodes 130, 132, 134 and 136,
which are in turn coupled to a second filter capacitor 138 having a
bleeder resistor 140 coupled in parallel therewith. This combined
circuit provides an output which is proportional to the voltage
V.sub.30 between the terminal 104 and a terminal 142. It is pointed
out that the terminal 142 is the common negative terminal of each
of the above described rectifier circuits, which are connected in
opposition. Thus, the voltage between the terminals 102 and 104
will be the difference between the respective rectifier output
voltages. Accordingly, the voltage between the terminals 102 and
104 will be given by the following equation.
V.sub.102, 104 = k .vertline.V.sub.12 .vertline. -
.vertline.V.sub.30 .vertline. (2)
variation of the constant k is made by adjusting the potentiometer
110. By appropriate adjustment of the potentiometer 110, the error
voltage V.sub.102, 104 can be made equal to zero for desired values
of V.sub.30.
The amplifier stage 96 includes a third transformer 144 having its
output coupled to a third full-wave rectifier including four diodes
146, 148, 150 and 152. A pair of resistors 154 and 156, along with
a filter capacitor 158 supply direct current to the amplifier
circuit 160. The amplifier circuit 160 includes a pair of
interconnected transistors 162 and 164. The transistor 162 is
preferably of the NPN type, while the transistor 164 is preferably
of the PNP type. The collector of the transistor 162 is coupled to
the base of the transistor 164 to provide two stages of
amplification. The input to the amplifier circuit 160 is the error
voltage defined by equation (2) above, and is applied to the
amplifier circuit 160 through a blocking diode 166 and a voltage
divider consisting of resistors 168 and 170. The blocking diode 166
insures that no negative current reaches the base of transistor
162. Appropriate loading resistors 172 and 174 are coupled to the
collectors of the transistors 162 and 164, respectively. A dropping
resistor 176 and a zener diode 178 are coupled in series across the
amplifier circuit 160. The zener diode 178 maintains a circuit
point 180 at a constant positive reference potential with respect
to a circuit point 182. Whenever the transistors 162 and 164 are
not conducting, a circuit point 184 will be at the same potential
as the point 182, and the point 180 will be positive with respect
to either of the other two points. Thus, the voltage between point
184 and point 180 will be the negative of the value of the
reference voltage across the zener diode 178. Accordingly, when
transistors 162 and 164 are conductive, it is possible that the
voltages between the circuit points 184 and 182, 180 and 182, and
184 and 180 are equal to zero. In addition, whenever conduction of
the transistors 162 and 164 causes the voltage drop across the
resistor 174 to be greater than the reference voltage across the
zener diode 178, the voltage between the points 184 and 182 will be
greater than the voltage between the points 180 and 182. Thus, the
voltage between the points 184 and 180 will be equal to the
difference between the voltages between the points 184 and 182 and
180 and 182. In addition, this difference will be positive.
Accordingly it will be apparent that negative voltages do not
affect the amplifier circuit 160.
The phase shifting stage 98 includes a pair of conventional phase
shifting RC networks, including a pair of resistors 186 and 188,
and a pair of capacitors 190 and 192. An isolation transformer 194
is coupled to the combined phase shifting networks to isolate the
power circuit from the output circuit of the phase shifter. The
isolation transformer is also preferably a step-down transformer to
increase the output current. The output of the isolation
transformer 194 is coupled through a pair of diodes 196 and 198 to
a pair of protective resistors 200 and 202. A pair of anti-floating
resistors 204 and 206 are coupled across the diode outputs.
Referring now to the SCR stage, it will be seen that this stage
includes first and second silicon controlled rectifiers (SCR's) 208
and 210 having a free-wheeling diode 212 coupled between them. The
SCR's 208 and 210 and the diode 212 are coupled to the D.C. winding
of the saturable core reactor 30. The output of the amplifier
circuit 160 is coupled to the cathode of both SCR's 208 and 210,
and also to the cathode of the diode 212. The outputs of the phase
shifting stage 96 are coupled to the gate electrodes of the SCR's
208 and 210. Thus, the amplifier and phase shifting stages control
the phase and duration of the conductive period of each SCR.
Consequently, the current fed to the D. C. winding of the saturable
reactor 30 by the SCR stage 90 is controlled by the amplifier and
phase shifting stages. Since the signal developed by the comparison
stage 94 controls the output of the amplifier stage 96, the SCR
stage 100 also operates under the control of the comparison stage
94. By proper selection of circuit parameters in conventional
manners known to those skilled in the art, the current applied to
the D. C. winding of the saturable core reactor 30 is accurately
regulated in accordance with the requirements set forth
hereinabove.
The purpose of the R-C networks is to produce voltages at diodes
196 and 198 which lag the anode voltages of the SCRs 208 and 210 by
a suitable angle, usually 90.degree..
Attention is now directed to FIG. 14 wherein the circuit of the
second regulator units 60 is shown in more detail. As in the case
of the first regulator unit, the second regulator unit will be
described in terms of three functional stages, namely a comparison
stage 213, a control stage 215 and a power output stage 217.
The comparison stage 213 is substantially identical with the
comparison stage 94 of the first regulator unit. More particularly,
the comparison stage 213 includes first and second transformers 214
and 216. The input terminals of the transformer 214 are coupled to
input terminals 62 and 66 of the second regulator 60, while the
input terminals of the transformer 216 are coupled to terminals 64
and 66 of the second regulator unit. The output of the transformer
215 coupled to a full-wave rectifier, consisting of diodes 218,
220, 222 and 224, which are in turn coupled to a filter capacitor
226 shunted by a bleeder resistor 228. Similarly, the output of the
transformer 216 is coupled to a full-wave rectifier comprised of
diodes 230, 232, 234 and 236, which are coupled to a filter
capacitor 238, which in turn is shunted by a bleeder resistor 240.
The basic purpose and functioning of the comparison stage 213 is
also substantially identical to that of the comparison stage 94 of
the first regulator unit, with the exception that different input
voltages are supplied to the comparison stage 213.
The control stage 215 includes a two-phase induction motor 242
having a permanent splitter capacitor 244 and a surge surpressing
resistor 246 coupled thereto. A pair of gate controlled full-wave
A.C. silicon switches, such as Triacs (TM) 248 and 250 are coupled
to the two-phase induction motor for controlling the operation
thereof. For example, if Triac 248 is conducting, the motor 242
will rotate clockwise, while if Triac 250 is conducting the motor
will rotate counterclockwise. A diode and resistor network,
including resistors 252, 254, 256 and 258 and diodes 260, 262, 264
and 266 is coupled to the gate electrodes of the Triacs 248 and 250
for controlling the operation thereof.
The power output stage 217 includes a rotatable transformer 268
which is mechanically coupled to and rotated by the output shaft of
the two-phase induction motor 242. The output voltage of the
rotatable transformer 268 is measured by shaft rotation from a
reference position for a given maximum angle. If the transformer
windings are coupled in series, the output voltage varies from 0 to
twice the input voltage for a 180.degree. shaft rotation. A pair of
limit switches 270 and 272 are interposed in the phase leads of the
motor 242, and are mechanically actuated by cams (not shown) on the
rotatable transformer shaft to limit rotation of this shaft to a
predetermined maximum allowable angular range. The output of the
rotatable transformer 268 is applied to a diode rectifier including
diodes 274, 276, 278 and 280. This rectifier then applies power to
the field of the synchronous motor 82 shown in FIG. 12, or the D.
C. windings of the second saturable core reactor 72 shown in FIG.
10.
Naturally, the regulator circuits illustrated herein are considered
to represent exemplary circuits for performing the necessary
regulative functions required according to the present invention.
It will be apparent to those skilled in the art that other types of
circuits for performing the same functions may be constructed.
Obviously, numerous additional modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *