Uninterruptible power supply with plurality of inverters

A uninterruptible power system in which separate inverters are connected to separate d.c. power sources established by independent power supplies, a.c., d.c. or both, and have their a.c. outputs connected to separate transformer primary windings with the primary to secondary turns ratios of the primary windings establishing different effective secondary voltages for determining the power source for normally supplying the power from the system and the sequence in which the other power sources are used on power failure, the magnitudes of the effective voltages determining the power source for initially supplying the power with any other power source supplying the power when its effective voltage is higher than any other available power source.

The present invention relates to an uninterruptible power supply and 
especially to an uninterruptible power supply (UPS) for use in 
communication systems or networks, particularly wide bandwidth systems and 
networks, such as cable TV and telecommunications distribution systems, 
and to other applications in which the power sources of the UPS utilize an 
inverter for providing power to moderate and low power loads, i.e. under 
about 5 kva, although being applicable to uninterruptible power supplies 
generally. 
BACKGROUND OF THE INVENTION 
Back up power systems are available which are based on several different 
topologies. While many of these are sometimes referred to as 
uninterruptible power systems, strictly speaking they are not. In these so 
called UPS systems, power from a first source, most typically an a.c. 
power line, powers the load either directly or through a conditioning or 
regulating device such as a ferroresonant transformer. Upon failure of the 
first source, a second source, typically an inverter powered by batteries, 
is actively switched into the circuit to supply the load. Sometimes the 
first and second sources share a single ferroresonant transformer so that 
the load is always powered by the ferroresonant transformer, which in turn 
is powered by either the first source or, upon failure of the first 
source, the second power source which is switched in to power the 
transformer. Two factors conspire to prevent such designs from providing 
truly uninterruptible power. First, it is necessary to monitor the first 
power source continuously, and quickly determine that it has failed. Given 
the inherent time variant nature of the sinusoidal wave form typical of 
a.c. power sources, this is very difficult to achieve. Further, once it 
has been determined that the first power source has failed, it is 
necessary to switch over to the second power source and to positively 
disconnect the first source to assure that power is not fed back to the 
first source from the second. Typically, this switch over and disconnect 
process is accomplished using an electromechanical relay or contactor, 
which is inherently a slow device. Allowing for the time required to first 
detect failure of the first power source and then accomplish the change 
over to the second source, the output power may be interrupted for several 
tens of milliseconds. This is acceptable for some loads, but not others. 
For sensitive loads, a full time inverter, i.e., a true UPS, is best. In 
this known topology, a single inverter converts d.c. power to a.c. power 
and supplies it to the load on a continuous basis. Generally, power is 
normally supplied from a first a.c. power supply, normally a power line, 
through a rectifier to provide a first d.c. source which normally supplies 
the d.c. power required by the inverter. This same rectifier keeps charged 
batteries which provide a second d.c. source of power. The second d.c. 
source is connected to the inverter in parallel with the first d.c. 
source. Upon failure of the first d.c. source, the inverter continues to 
be supplied with d.c. power from the batteries. Upon restoration of the 
first d.c. source, the rectifier once again powers the inverter and, at 
the same time, recharges the batteries. Because of the parallel connection 
of the first source and the battery across the input to the inverter, the 
inverter always supplies a.c. power to the load with no disturbance or 
interruption of the a.c. output when the first source fails or is 
restored. 
The advantages of a true UPS include a continuously uninterrupted power 
output to the load, as well as an output whose frequency and wave form are 
independent of the input. This allows the UPS to serve as a frequency 
converter, for example, providing stable and accurate 60 Hz power from a 
50 Hz power source, or from a power source having an unstable frequency 
such as an emergency generator. An additional advantage is that no switch 
is required to disconnect the input of the UPS from an a.c. source, since 
the inverter for inverting rectified power from the a.c. source will not 
feed power from other power supplies back through the rectifier. 
This true UPS topology, using a dual conversion approach with the a.c. 
power being converted to d.c. power, and then back to a.c. power, is 
widely used in medium and large UPS systems, or those with output ratings 
above 20 kva or so, as well as small systems used in critical applications 
such as telecommunications. 
The main disadvantage of this true UPS approach at lower power ratings has 
been poor efficiency. The reason for the poor efficiency of the true UPS 
in smaller sizes is the use of batteries, or other power sources, with 
relatively low d.c. voltages. Efficiency at low d.c. voltage is poor, not 
only because of ohmic conduction losses, but also because of losses in the 
semiconductor switching devices of the rectifier and inverter which have a 
relatively constant on-state voltage drop. A typical 1.0 volt drop across 
a conducting transistor, for example, is an insignificant 0.25% loss in a 
400 volt apparatus, but represents a loss of nearly 4.2% in a 24 volt 
system. 
The d.c. voltage utilized in a UPS is typically dictated by the cost of the 
batteries. For a given level of stored energy, a string of relatively few 
large cells is of significantly lower cost than a string of relatively 
many small cells. At the power levels typical of a small, single phase UPS 
in the 1 kva range, for example, batteries are most economical in the 24 
to 36 volt range, but significantly higher efficiency would be achieved at 
a d.c. voltage in the 400 volt range. 
U.S. Pat. No. 5,010,469 issued on Apr. 23, 1991 to Howard H. Bobry (the 
inventor herein) discusses the advantages and the disadvantages of a "true 
UPS". The patent discloses a UPS in which the load is normally supplied 
with power from an a.c. power line, with a battery supplying power upon 
failure of line power. The power line is connected through an isolation 
transformer to a rectifier which provides a relatively high voltage d.c. 
power source. This d.c. power source is connected to a single primary 
winding of a transformer through inverter circuitry having one input 
connected to the relatively high voltage d.c. power source and another 
input of the inverter circuitry connected to a low voltage d.c. power 
source, such as a battery, to effect a connection of the low voltage 
source through the inverter circuitry to a common portion of the 
transformer winding which is common to both d.c. sources. Operation of the 
inverter circuitry at the higher one of the two diverse input voltages is 
achieved through the use of taps on the single primary winding of the 
transformer and is such that the inverter circuitry operates to energize 
the primary winding from the relatively high voltage d.c. source as long 
as it maintains a higher voltage than that of the low voltage power source 
across the common part of the primary winding. Upon a failure of the high 
voltage source to maintain this higher voltage across the common portion 
of the primary winding, the low power source will supply the power to the 
primary winding until the high voltage power source again establishes a 
higher voltage across the common part of the primary winding 
While the UPS of this prior patent achieves operation at two different 
voltages to provide an increase in efficiency, the isolation transformer, 
for the power supply input to the high voltage rectifier, adds to the 
size, weight, and cost of the system and reduces overall efficiency of the 
system. This isolation is needed because of the shared inverter circuitry 
and a common transformer primary winding. Thus an isolation transformer 
for the high voltage power source is required. 
In addition, the change over voltage at which the UPS supplies power from a 
lower order voltage source is dictated by the voltage of the lower voltage 
power source so that a lower voltage source cannot be given preference 
over a higher voltage source for supplying power to the load. Moreover the 
UPS of the patent is not amenable to having any additional d.c. power 
source connected to the common inverter and common transformer winding. 
Among the various objects of the present invention, which will be apparent 
from the description of preferred embodiments, is the provision of a true 
UPS topology which: (1) enables the use of one or more a.c. power 
supplies, including one or more high voltage a.c. power supplies, for 
establishing one or more high voltage d.c. power sources for the UPS; (2) 
enables the order of preference (priority) for d.c. power sources of the 
UPS to be in accordance with or different from the order of the voltage 
levels of the d.c. sources and in accordance with the magnitude of an 
effective voltage established for each d.c. power source; (3) enables the 
establishment of an order of preference for the d.c. power sources which 
have substantially the same voltage level; (4) enables an order of 
preference for the d.c. power sources to be established by effective 
voltages for the d.c. power sources which approximate a desired output 
voltage from the UPS; (5) enables the setting of the effective voltages to 
a voltage higher than a desired output voltage with the output voltages 
being regulated to the desired output voltage; (6) enables the easy 
addition of one or more power supplies to a UPS; (7) enables the 
maximizing of efficiency of the transistors and the inverters of the UPS 
as well as cost reduction in providing isolation for the power supplies 
from each other and the output of the UPS to thus maximize overall system 
efficiency while reducing costs. 
SUMMARY OF THE INVENTION 
The present invention provides a true uninterruptible power supply for 
supplying power to a load from any one of a plurality of independent d.c. 
power sources in a predetermined sequence. The independent power supplies 
may comprise one or more d.c. sources which are established by a battery 
or a battery bank, and one or more d.c. sources which are established by 
rectifying a.c. power from a.c. power lines or from other a.c. power 
supplies. 
The power sources are separately connected to the inputs of the separate 
inverters, one for each power source. The outputs of the inverters are 
separately connected to a voltage converting means for converting the 
voltages of the inverter outputs to selected different predetermined 
effective a.c. voltages which determine the sequence in which the power 
sources are to supply power to the load, with the operating effective 
voltage which is higher than any other establishing a back biasing voltage 
for each power source which produces a lesser converted effective voltage 
whereby the power source and its inverter providing the higher than any 
other effective voltage supplies power for the load. 
In the preferred embodiments of the invention, the output of each d.c. 
power source of the UPS is directly connected to a corresponding inverter, 
which is separate from any other inverter, and the outputs of the 
inverters are each connected to energize a voltage converter having 
separate transformer primary windings, one for each inverter, to provide a 
separate transformed secondary effective voltage for each inverter and its 
corresponding power source. The effective voltages have different 
predetermined magnitudes which may approximate the desired voltage for the 
load with the magnitudes of the effective voltages determining the order 
in which the d.c. power supplies are used to supply power to the load, the 
preferred source for normally supplying the power having an effective 
voltage which is higher than any other effective voltage. 
In the preferred embodiments, the transformer primary windings for the 
inverter outputs have a common secondary configuration. The common 
secondary configuration may comprise, for example, a separate secondary 
winding for each of the primary windings with the secondary windings being 
connected in parallel with each other so that the secondary effective 
voltage higher than any other is established as the secondary voltage for 
all secondary windings. 
Instead of separate secondary windings for each primary winding which are 
connected to provide a common secondary, the primary windings for the 
outputs of the inverters may have a common core with a single secondary 
for all primaries. Any other equivalent secondary or transformer structure 
to that described may be utilized. 
In accordance with the preferred embodiments, the desired magnitudes for 
effective voltages for the respective power sources are obtained by 
setting the primary to secondary turn ratios to provide effective voltages 
having relative magnitudes which establish the desired order in which the 
power sources are to be used to supply power to the load. The magnitude of 
the effective secondary voltage of each power source being such that, when 
a power source is supplying the load power, its secondary effective 
voltage establishes a primary voltage on the primary windings of the other 
power sources which is sufficient to back bias any power source with a 
lesser effective voltage against supplying load power to its corresponding 
inverter. When the back-biasing voltage on the primary winding for any 
power source drops to or slightly below the voltage output of the power 
source to the inverter for the winding, that power source will supply 
power to the load until it is again back biased by the operation of a 
higher order power source. 
To provide the desired secondary effective voltages, the primary to 
secondary turns ratios for the transforming of the outputs of the 
inverters are set to establish secondary effective voltages with relative 
magnitudes corresponding to the desired order in which the d.c. sources 
are to be used to supply power to the load. The predetermined magnitudes 
for the effective voltages are also such that each is sufficient to 
establish the back biasing of any power source having a lesser effective 
voltage. When the secondary voltage of the preferred source drops to or 
slightly below the effective voltage of the next preferred d.c. power 
source, the next preferred power source will no longer be back biased and 
will start supplying the UPS power for the load. 
In operation, all inverters of the UPS operate simultaneously. The 
effective voltages of the power sources preferably approximate each other 
to enable a following source to quickly assume the function of supplying 
the load when the preceding power source is failing. 
The independent inverters, one for each power source, are synchronized and 
operate continuously and in parallel with each other. 
In the illustrated embodiments of the invention, one uses two or more 
transformers, one for each of the inverters. Another embodiment has a 
single transformer incorporating two or more primary windings, one for 
each of the inverters.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The preferred embodiments of the invention for an uninterrupted power 
supply, are described and illustrated using certain inverter designs for 
the preferred modes of operation. Those skilled in the art will fully 
understand from the structure and operation of the preferred embodiments 
disclosed and described, and will appreciate, that the present invention 
can be practiced using many different inverter designs which convert d.c. 
power to a.c. power. Such inverter designs include the H-bridge inverter 
as illustrated herein, as well as center tapped, parallel, half-bridge, 
and other inverter designs as are known in the art or may be later 
developed. Similarly, while the use of field effect transistors (FETs) is 
shown, for simplification of the drawings, other switching devices such as 
bipolar transistors, insulated gate bipolar transistors (IGBT S), or any 
other device having appropriate characteristics for the intended 
application may be used. 
In the embodiment of FIG. 1, a UPS apparatus 10 is shown as having two 
independent a.c. power supplies. Power lines 12, 14 from a first a.c. 
power supply provide power input to a rectifier circuit 16 to provide a 
first d.c. power source. The rectifier 16, may be a conventional rectifier 
circuit such as a switchmode power supply, a phase controlled rectifier, a 
controlled ferroresonant rectifier, a power factor controlled boost 
circuit, or any other suitable rectifier known in the art. The rectifier 
need not incorporate isolation means, as isolation of the power supplies 
from each other and the output will be provided by the UPS apparatus of 
the invention as will be appreciated from the further description of the 
preferred embodiments. 
The d.c. output current of rectifier 16 is conducted through diode 18 to 
the input of an inverter A comprised of transistors 20, 22, 24, and 26. 
Inverter A is of conventional design. In inverter A, transistors 20 and 26 
are rendered conductive to provide one half cycle of an a.c. output of the 
inverter and transistors 22, 24 are rendered conductive to provide the 
alternate half cycle. The purpose of diode 18 is to prevent a backwards, 
or reverse, flow of current from the inverter to the rectifier, and may 
not be necessary with some rectifier designs. Where diode 18 is not 
required, it may simply be omitted. Diodes 28, 30, 32, and 34 provide 
paths for reverse current flow through the inverter, thus allowing 
operation in all four quadrants so that the inverter may power reactive 
loads, as is well known and understood. These diodes may be separate and 
discrete components, or may be integrated with transistors 20, 22, 24, and 
26. 
The inverter A drives a transformer primary 36, which has an associated 
transformer secondary 90. The primary 36 and the transformer secondary 90, 
while shown spaced in the schematic, as is conventionally done, have a 
common transformer core 91, the core being schematically shown along both 
the primary 36 and the secondary 90. 
A second a.c. power supply comprises a.c. power lines 38, 40. In a manner 
similar to that described for the first power source, the power lines 38, 
40 are connected to the input of a rectifier circuit 42 to provide a 
second d.c. power source. The output of the rectifier circuit 42, supplies 
d.c. current, through (optional) diode 44, to the input of an inverter B 
comprised of transistors 46, 48, 50, and 52, and diodes 54, 56, 58, and 60 
which operate as described for the corresponding transistors 20, 22. 24, 
and 26, and corresponding diodes 28, 30, 32, and 34 of inverter A. 
The output of the second inverter B drives a separate primary 62 of a 
second transformer, the primary 62 having an associated transformer 
secondary 92. As before, the transformer primary winding and the 
transformer secondary winding have a common transformer core 93. This 
second transformer is separate and distinct from the previously described 
first transformer driven by the first inverter. 
A third inverter circuit C follows the structure of the other inverters and 
is comprised of transistors 70, 72, 74, and 76, and diodes 78, 80, 82, and 
84. The input to the third inverter circuit C is connected to the output 
of a third power supply comprising a battery 64. The connection from the 
battery to the input of the inverter circuit is shown as having a diode 
68. This inverter drives a third transformer primary 86 of a separate 
transformer, the primary having a common core 95 with a transformer 
secondary winding 94. 
A battery charger circuit 66 maybe be connected across the diode 68 to be 
powered by current flowing from the primary winding 86 through the third 
inverter to the battery charger when the UPS is powered by either the 
first or the second a.c. power source. As explained in more detail 
hereafter, when the UPS supplies power through either the first or second 
inverter, the primary winding 86 will have a secondary to primary 
transformed voltage thereon which will back bias power flow from the 
battery. This back biasing voltage will also operate through the inverter 
C to supply power to the battery charging circuit to keep the battery 
charged. This powering of the battery charging circuit is such that it 
does not interconnect the inverters or their power supplies, nor does it 
affect the isolation of the inverters and the UPS output since the back 
biasing voltage is established on the primary 86 by transforming the 
output voltage of the secondary as will be well understood, from the 
description herein and the drawings, by those in the art. As an alternate, 
a separate battery charger operating independently of the inverter 
circuits may be provided (not shown). 
All three inverter circuits are driven in synchronization with each other 
by a control and drive circuit 88 in a manner well known in the art so 
that each inverter produces the identical wave form as the other two 
inverters, differing only in magnitude. The inverter wave forms may be a 
simple square wave, or may be a pulse width modulated wave form which is 
controlled to provide a regulated output voltage and/or a desired output 
wave shape. A filter circuit 96 may be used to further control the shape 
of the output wave form produced at output terminals 98. The filter 
circuit may be a simple LC circuit comprised of a series inductor and a 
parallel capacitor, or may be a more complex circuit as required by the 
specific application. For some applications, the filter circuit may be 
omitted. 
The control and drive circuit 88 may be isolated from the inverters and the 
UPS output via the use of isolation trnasformers, optical couplers, or 
other suitable means well known in the art. Similarly, isolating means may 
be used to supply power to the control and drive circuit 88 from each of 
the power supplies for the UPS. As is conventional practice, connections 
(not shown) are made to the power lines 12, 14, and 38, 40 for the 
rectifiers 16 and 42 and to the battery 64 to establish individual d.c. 
sources (not shown) for powering the control and drive circuit. The 
outputs of these d.c. sources are ORed in a well known manner to supply 
power to the control and drive circuit whenever one or more of the power 
supplies for the UPS are available, i.e., are functional to supply power. 
While inverters A, B, C, have been described as having the same design, and 
operating in the same manner, it will be understood that the present 
invention allows the transistors and the inverter designs for the 
inverters to be different to maximize the efficiency of the respective 
inverters at the applied voltage. 
In accordance with the present invention, the secondary transformed 
voltages constitute effective voltages for the d.c. power sources and the 
transformation ratios for the primaries are such as to provide secondary 
effective voltages of magnitudes which are different from each other and 
which approximate the desired output voltage for the UPS. The relative 
magnitudes of the effective voltages will determine the order of 
preference in which the respective d.c. power sources operate to supply 
load power from the UPS. The most preferred d.c. source, i.e. the one 
chosen for normally supplying power from the UPS, has a transformed 
voltage higher than that of any other and which operates to reverse bias 
all other d.c. sources, with each other d.c. power source assuming the 
function of supplying power to the load through the UPS when its effective 
voltage becomes higher than any other and it will continue to supply power 
for the load until a d.c. power source of higher order reestablishes a 
higher effective voltage. 
As noted above, the UPS output will, at any time, normally be powered by 
the preferred inverter, when it is available, because it is the one having 
an effective voltage higher that any other d.c. power source. This 
effective voltage will cause all other primaries to reverse bias their 
power sources, whether rectifiers or batteries, or any other type of d.c. 
source, and prevent the power sources from supplying current to their 
respective inverters. If the preferred power source fails, another 
inverter and its power source becomes the most preferred and automatically 
and naturally, without any action by a control circuit, provides the power 
for the load. This operation is similar to the use of diodes to "OR" d.c. 
power sources together to supply power to an inverter so that the power 
source having the highest voltage will supply power to the load but upon 
failure of that voltage source the voltage source having the next highest 
voltage will assume the load. By using an inverter for each power supply 
and effectively OR-ing together the transformer outputs for the inverters, 
the power supplies or sources for the inverters may be isolated from each 
other as well as from the a.c. output of the UPS to allow, for example, 
the use of low voltage batteries as one power source which are safely 
isolated from a high voltage power line used as another voltage supply. As 
will be understood by those skilled in the art from the foregoing, this 
isolation may be accomplished since each primary winding is dedicated to 
one d.c. power source and the inverter for that power source and there 
need not be any connections between inverters. In addition, the rectifiers 
will block any feed back from the primary windings when a primary is back 
biasing its corresponding d.c. source. 
As noted, adjustment of transforming ratios allows any desired order of 
preference for the power sources to be set by design, even to render as 
the most preferred power source, one which has a lower voltage than a 
lesser preferred power source. For example, a most preferred power source 
may be from a first a.c. power line which is of lower voltage than a 
second a.c. power line of higher voltage but which is preferred only as an 
alternate to AC first power line. 
It will also be understood that the voltage of a d.c. power source may be 
changed, for example, by boosting the voltage when rectifying a.c. power, 
to change the secondary effective voltage for the d.c. power source. This 
provides flexibility not only when designing a UPS in accordance with the 
present invention, but also facilitates later modifying the order of 
preference in the UPS as well as the addition of power sources, 
particularly when using power supplies of substantially the same voltage. 
An example may provide useful clarification of circuit operation. Assume 
that the first rectifier circuit 16 produces a regulated d.c. output of 
400 volts, (the rectifier may provide d.c. power either lower or higher 
than the a.c. supply). Further assume that the UPS output is to be a 60 
volt square wave, typical of that required for powering broad band 
communications networks over coaxial cable. The inverter comprised of 
transistors 20, 22, 24, and 26 will provide a 400 volt square wave across 
transformer primary 36, while a 60 volt square wave is desired across 
transformer secondary 90. This is accomplished by setting the primary to 
secondary turns ratio in accordance with the transformer equation: 
EQU Vp/Vs=Rps, 
where V is voltage and subscript p indicates primary, s indicates 
secondary, R is turns ratio and ps indicates primary to secondary turns 
ratio. Accordingly the Rps for the first transformer with 400 volts on the 
primary, and a secondary voltage of 60 volts, will be 400/60, or 6.66/1. 
Assume that the second rectifier circuit 42 produces a regulated d.c. 
output of 360 volts. The turns ratio between the second transformer 
primary 62 and the second transformer secondary 92 should be 360/60, or 
6.00/1. Similarly, given the nominal voltage of the battery 64 as 36 
volts, the turns ratio between the third transformer primary 86 and the 
third transformer secondary 94 should be 36/60, or 0.60/1 so that a 60 
volt output can be supplied from the 36 volt battery. 
It will be recognized that all three transformer secondaries, 90, 92, and 
94 are connected in parallel across the secondary output connections 97a, 
97b, and thus all have the same voltage, i.e. the secondary effective 
voltage which is higher than any other. With the first a.c. power supply 
available, the first rectifier circuit 16 provides a 400 volt d.c. source 
for the first inverter, which in turn produces a 400 volt a.c. square wave 
across transformer primary 36, resulting in a 60 volt a.c. square wave 
across all three transformer secondaries. With a secondary voltage of 60 
volts across the secondary winding of the second transformer, the voltage 
across the primary 36 of the second transformer, as given by the above 
equation, will be 60.times.Rps or 360 volts, the assigned primary to 
secondary turns ratio having been 6.00/1, Thus, with the assigned turns 
ratio the voltage will be the same voltage as its d.c. source, i.e. the 
rectifier circuit 42. A slight difference in voltage at either the first 
rectifier circuit 16 or the second rectifier circuit 42 would determine 
which of the two sources would supply the load, but it is preferred that 
the first power source supply power for the load when it is available. 
This can be assured by adjusting the turns ratio of our second transformer 
slightly. By making the turns ratio 6.10/1, rather than 6.00/1, the 
secondary voltage for 360 volts on the primary 62 will be 360/6.10, or 59 
volts. This is lower than the 60 volts of the first transformer but when 
the first power source supplies the power, the secondary voltage of 60 
volts of the first transformer, is now transformed by the second 
transformer with a secondary to primary turns ratio of 1/6.1, and the 
above equation now becomes Vp=60.times.6.1, and a voltage of 366 volts is 
established across the primary for the second power source. This exceeds 
the 360 volt level of the output of the second rectifier circuit 42, so no 
current will flow from this second rectifier circuit. All of the power 
required will be supplied by the first power source, as desired. If the 
first power source fails, power will then flow from the second power 
source, via second rectifier circuit 42. With the primary/secondary turns 
ratio of the second transformer now at 6.10/1, the secondary effective 
voltage of the second power source will be 360/6.10, or 59 volts and the 
second power source will operate to supply power for the load when the 
voltage on its secondary is 59 volts. This does not provide the 60 volts 
desired for the load, but we will discuss this in more detail later. 
Similarly, the 60 volt level from the first transformer, when it is 
available, will be across the third transformer secondary 94. With the 
assumed battery voltage of 36 volts and an assigned primary to secondary 
ratio of 0.60/1, 60 volts on the secondary of the third transformer would 
transform to 36 volts across the primary winding for the battery, which is 
again the same as the assumed nominal voltage of the d.c. power source, 
i.e. the battery. This again needs to be adjusted slightly to prevent 
power flow from the battery at 36 volts as was done for the second power 
source. By making the primary to secondary turns ratio 0.62/1, the 
secondary voltage from the battery will be 58 volts. With this 
primary/secondary turns ratio, when the secondary for the battery 
transformer has 60 volts across it from the first transformer, the voltage 
across the third transformer primary 86 will be transformed with a 
transformation factor of 0.62 to provide 37.2 volts across its primary. 
When the voltage across the secondaries is from the second power source, 
59 volts, the voltage on the primary for the battery will be 36.6 volts. 
Either voltage would block current flow from the battery to its primary 
winding. 
Thus far the circuit of FIG. 1 provides an output voltage which will vary 
from 58 volts (battery) to 60 volts (first source), depending upon the 
power sources. This is satisfactory for many applications. However, some 
applications need a better regulated output voltage including those which 
require adjustments for the nominal 36 volt battery which thus far has 
been assumed to be operating at 36 volts. This battery will not operate at 
a constant 36 volts. Battery voltage will, in fact, typically vary from 
about 42 volts at full charge to about 32 volts when fully discharged. A 
feature of the present invention, is that the turns ratios may be further 
adjusted so that the third transformer will have no power flow from the 
battery when its voltage is as high as 42 volts, and the voltage across 
transformer secondaries 90, 92, and 94 is as low as 59 volts. 
This adjustment results in a turns ratio for the third transformer of 
0.72/1. With this turns ratio, operation from a fully charged battery at 
42 volts would result in a secondary voltage of 58.3 volts. However, when 
operating from a discharged battery at 32 volts this would result in a 
secondary voltage of 44.4 volts. The output voltage of the UPS could thus 
vary from 60 volts to 44.4, depending on which power source is being used 
and the state of the battery. 
A regulated output, constant regardless of the power source or state of 
charge of the battery, may be provided through a voltage regulator. The 
UPS of FIG. 1 utilizes a voltage regulator 100 which utilizes pulse width 
modulation. Using pulse width modulation in a well known manner, the 
voltage regulator circuit 100 monitors the output voltage of the UPS and 
controls the duty cycle of the inverters to maintain a constant and well 
regulated output voltage. 
FIG. 2 illustrates the inverter voltage wave forms resulting from the use 
of pulse width modulation. As is conventional, each half cycle of the wave 
form may be comprised of a single pulse, either positive or negative on 
alternating half cycles with the instantaneous voltage of the inverter 
being controlled by changing the width of the pulse whereby the 
instantaneous voltage during each half cycle of the inverter is either 
positive or negative, or zero as shown in FIG. 2. 
With reference to FIG. 2, a voltage wave form 110 as shown would appear 
across transformer secondaries 90, 92, and 94 when the UPS is operating 
from battery 64 at its low voltage, such as 32 volts in the given example. 
Wave form 112 of FIG. 2 is a typical voltage wave form which would appear 
across the transformer secondaries when the UPS is operating from the 
second power source, and wave form 114 is a typical voltage wave form 
which would appear across the transformer secondaries when the UPS is 
operating from the first power source. While the three voltage wave forms 
shown in FIG. 2 differ in shape, voltage regulator circuit 100 is used to 
adjust pulse width so as to maintain a constant voltage output at 
terminals 98. Such regulator circuits are well known and no further 
description is necessary to those working in the art. Filter circuit 96 
will function to assure that the output voltage wave form approximates a 
square wave, or a sinusoidal wave form, or such other wave form as may be 
desired in a specific application. It will be understood by those versed 
in the art that the desired regulation by pulse width modulation may also 
be achieved by varying the widths of multiple pulses per half cycle, as is 
well known. 
Returning to our example, the turns ratio for the battery 64 based on a 
fully charged battery was adjusted to 0.72/1 to have no power flow from a 
fully charged battery voltage of 42 volts. However, with the ratio of 
0.72/1, the battery 64 at its minimum voltage of 32 volts, and the wave 
form 110 of maximum pulse width, as shown in FIG. 2, the UPS output 
voltage will be 44.4 volts. The output voltage when operating from either 
the first or second power source could be regulated down to that same 44.4 
volt figure by the use of pulse width modulation of FIG. 2. But this would 
provide a regulated 44.4 volt UPS output when a 60 volt output is 
specified. This can be achieved by making a further adjustment to the 
transformer turns ratios to boost the UPS output voltage to 60 volts. To 
do this, all of the turns ratios are adjusted by dividing them by a factor 
of 60/44.4, i.e. 1.35. This factor is the desired output voltage for the 
UPS (60 volts) divided by the effective voltage of 44.4 for the battery 
when operating at a low charge (32 volts) with a transformer 
primary/secondary turns ratio of 0.72/1), which is the turns ratio set for 
the battery at full charge (42 volts). Dividing all the turns ratio by 
1.35, the turns ratio of 6.66/1 for the first transformer is now 4.93/1; 
the ratio 6.10/1 for the second transformer is now 4.52/1 and the third 
transformer ratio 0.72/1 is now 0.53/1. This sets the primary turns ratios 
for all power supplies lower to provide higher than the 60, 59, and 58 
volts for the secondary voltages of the first and second sources, and the 
battery when operating at 42 volts, the effective voltage for a battery 
operating at 32 volts being slightly above the desired 60 volts for the 
secondary voltage. 
The secondary voltages for the transformers will now be about 81.1 for the 
first transformer, about 79.7 for the second transformer, and about 79.3 
for the third transformer with the battery operating at 42 volts and about 
60.4 volts when the battery is operating at 32 volts. 
It will be noted that the effective secondary voltage for the second 
transformer is more than one volt lower than that of the first transformer 
and only 0.3 volt higher than that of the battery effective voltage when 
at 42 volts. Depending on the voltage regulation of the first and second 
d.c. power sources it may be advantageous to use a 4.5/1 turns ratio for 
the primary of the second transformer for the second power source to 
provide an effective voltage of 80 volts which is about 1.1 volts below 
that for the first power source and about 0.7 volt higher than that for 
the battery source when operating at 42 volts. 
These effective voltages, all of which exceed the desired 60 volts for the 
UPS, are, by using voltage regulation as described, regulated down to the 
desired 60 volts to maintain the desired output voltage for the UPS for 
all power sources. The order of the magnitudes for the effective voltages 
of the power sources are maintained so that the order of preference for 
supplying power from the UPS apparatus is also maintained. It will be 
noted that for the voltage regulation described, the power source which is 
the less preferred of all power sources is assigned an effective voltage 
which is less than any other effective voltage and the primary/secondary 
turns ratios are set so that the power source which is preferred less than 
any other has an effective secondary voltage which is a little higher than 
the desired load voltage, with all effective voltages higher than the 
desired load voltage being regulated down to the desired voltage. 
FIG. 3, illustrates a modification of the UPS of FIG. 1 and is a preferred 
embodiment of the present invention. The embodiment utilizes the same 
power supplies, rectifiers inverters and diodes of the embodiment of FIG. 
1. These function as in FIG. 1 to energize an individual primary for each 
inverter. Accordingly, the circuitry has been given the same reference as 
in FIG. 1. However, the three transformer primaries 36, 62, and 86 shown 
in FIG. 1 are replaced with three transformer primaries 102, 104, and 106. 
Whereas the three transformer primaries of FIG. 1 are each closely coupled 
magnetically to an individual one of associated transformer secondaries 
90, 92, and 94 respectively using three separate transformer cores, the 
three transformer primaries of FIG. 3 are closely coupled magnetically to 
each other with a single transformer core 107 having a single transformer 
secondary 108 constituting a common secondary for the primary windings. 
The secondary 108 is connected to the filter circuit 96 and thence to the 
output terminals 98. All components of the embodiment of FIG. 3 operate in 
the same manner as in FIG. 1, but have a common core 107 for the primary 
and secondary windings with a single secondary 108 replacing the three 
parallel connected secondary windings of the first embodiment, this being 
the full equivalent to the parallel connected secondaries as is well known 
by those in the art. The turns ratios for the primary windings and voltage 
regulation are the same in both embodiments. 
The transformer secondary of the embodiment of FIG. 3 need not be closely 
coupled magnetically to the transformer primaries. The transformer may in 
fact comprise a ferroresonant transformer, or a controlled ferroresonant 
transformer, both well known in the art, in which case the functions of 
the filter circuit 96 and the voltage regulator circuit 100 are provided 
by the transformer, so the separate filter circuit and voltage regulator 
circuit may be omitted. As an alternate, the voltage regulation circuit 
may be retained for use with a ferroresonant transformer, thus regulating 
the primary voltage of the transformer and enhancing its efficiency. 
Further, it is understood that a UPS according to the present invention may 
be operated to provide any output frequency, voltage, or wave form 
suitable for the intended application, and with the addition of a bridge 
rectifier across the output terminals 98, as is well known, a d.c. output 
may be obtained. 
While UPS embodiments comprising three inverters have been described, it is 
understood that these are exemplary of the best modes, and that any number 
of inverters greater than one may be used. Most typically, a UPS according 
to the present invention would use two inverters, one operating from a 
first power source such as the commercial power line, and the other 
operating from a second power source such as a battery. It is understood 
that additional power sources, either a.c. or d.c., may be incorporated 
using a separate inverter and transformer primary for each added power 
source. A UPS may, for example, be comprised of a first inverter for 
operation from a first power source such as an a.c. power line, a second 
inverter for operation from a second power source such as an a.c. 
generator, a third inverter for operation from a third power source such 
as a fuel cell, a fourth inverter for operation from a fourth power source 
such as an array of photo voltaic cells, a fifth inverter for operation 
from a fifth power source such as a battery, and so on. Preferably, for 
efficiency, the UPS will be set to naturally and automatically select the 
available power source which allows operation via the inverter having the 
highest input voltage available, thus resulting in maximum efficiency at 
all times consistent with the power sources available. The apparatus can 
be designed such that it is of modular construction, allowing additional 
inverters and primary windings to be added as additional power sources are 
made available, and allowing inverters to be removed for maintenance or as 
power sources become unavailable. 
The present invention thus provides a true uninterruptible power supply 
having a plurality of inverters for operation from a plurality of power 
sources, the power sources being selected naturally and automatically to 
operate the UPS from available power sources in a sequential predetermined 
order of preference which may differ from the sequential order of the 
voltage magnitudes of the power sources, with the order of preference 
being typically determined so as to maximize efficiency. 
It will be understood from the foregoing that the separate outputs of the 
inverters are connected to a common power output by circuitry which 
establishes an effective voltage for each inverter with the effective 
voltages being of different magnitudes and preferably approximating the 
desired voltage for the load. In the described circuitry, the output from 
the parallel connected secondary windings of FIG. 1 and the single 
secondary winding of FIG. 3 each provide a common power output for the 
preferred embodiment. 
It is also to be understood that the effective voltages described herein 
are those voltages which would actually exist from an operating power 
source at the common output of the transformer secondary if there is no 
modification as by voltage regulation. However, the effective voltages can 
differ from the desired load voltage within the range of voltage 
regulation as illustrated in the example in which the turns ratios are 
adjusted to provide a desired effective secondary voltage for a battery at 
its fully charged voltage and a different desired effective voltage at its 
fully discharged voltage, with all effective voltages being regulated down 
to the desired load voltage. Moreover, the effective voltage, which is 
that determined by the primary to secondary turns ratio for a given 
voltage of the d.c. source, will vary with voltage variations of the d.c. 
source. Preferably, the primary to secondary turns ratios for the 
effective voltages are set to accommodate the normal voltage variations of 
the d.c sources for supplying power and to provide the described back 
biasing voltages for lower order power sources. 
It can also be seen from the foregoing that the present invention ORs the 
separate simultaneously operating inverters for the separate power 
supplies to establish one of the inverters as the inverter for supplying 
power to a load. This ORing is accomplished by providing separate 
inverters for the power sources and connecting the outputs of the 
inverters to separate transformer primaries to establish different 
effective secondary voltages for the inverters, the secondary voltages 
having a common secondary for ORing the inverters so as to supply load 
power from one inverter, i.e., the inverter with an effective secondary 
voltage higher than any other inverter with an operative power supply, the 
other operable power supplies of lower order being back biased by the 
higher effective secondary voltage. It may also be seen that by using 
separate inverters for the power sources and separately transforming the 
outputs of the separate inverters, the UPS of the present invention 
maintains isolation between the power sources and between the power 
sources and the load. 
Various aspects of the invention will also be appreciated from the 
foregoing description of preferred embodiments. Among others, it will be 
appreciated that the present invention enables the use of one or more a.c. 
power supplies in a UPS having an a.c. output, including one or more high 
voltage a.c. sources. Further, it enables the order of preference, or 
priority, for the d.c. power sources of the UPS to be different from the 
order of the voltage levels of the d.c. sources. It also enables an order 
of preference for d.c. power sources having substantially the same voltage 
level to be established. Moreover, it enables each d.c. power source to 
establish an effective secondary voltage which approximates the desired 
voltage for the load. And with voltage regulation, the effective voltages 
may be set sufficiently higher than the desired output from the secondary, 
as described herein, to effect voltage regulation to the desired secondary 
output voltage for all d.c. sources. It also reduces the cost of and 
losses in the UPS system by reducing isolation costs and loses associated 
there as well as enabling the inverters to be designed for the voltages of 
the individual power sources. 
Moreover the present invention also provides an improved method of 
supplying continuous uninterruptible power to a load. In the method, load 
power is supplied from one of a plurality of independent d.c. sources 
which have nominal voltages and which are to be used in a predetermined 
order initially and on power failure. In the method of the preferred 
embodiments, a primary to secondary transforming ratio is established for 
each power source for transforming the inverted voltage from the power 
source to an effective secondary voltage with the primary to secondary 
transformation ratios for the power sources being such that the effective 
voltages of the power sources have different magnitudes with the 
magnitudes defining a sequential order corresponding to the sequential 
order in which the d.c. power sources are to be used to supply power to 
the load, and the magnitudes of the effective voltages for the power 
sources are such that each power source higher in the order of preference 
than another has an effective secondary voltage of a magnitude which will 
back bias the next lower order power source against supplying power, 
thereby establishing the operative d.c. power source having a secondary 
effective voltage higher than any other as the power source which supplies 
load power. 
It will be understood that the effective secondary voltage for a power 
source and its inverter and primary is the secondary voltage which would 
be produced using primary to secondary turns ratios as described. If there 
is no voltage regulation, the secondary output voltage is the effective 
secondary voltage for each power source when it is supplying the power. 
As will be understood by those in the art, when pulse width modulated 
voltage regulation as described is used to regulate the secondary output 
of the transformers to a desired load voltage, the regulated secondary 
voltages contain instantaneous voltages of the magnitude of the effective 
voltages so that the instantaneous voltages of the effective voltage 
higher than any other will establish the back biasing of the lower order 
power sources. Other known voltage regulators equivalent to that described 
may also be used. 
As generally used, high voltage power sources are generally power sources 
having a voltage of at least about 170 volts, but normally 200 volts or 
over, with a low voltage source having a voltage of up to about 70 volts 
and a moderate voltage source being between the high and low voltage 
sources. 
While the invention has been shown and described with respect to specific 
embodiments thereof, it has been for the purpose of illustration rather 
than limitation, and other variations and modifications of the specific 
embodiments herein shown and described will be apparent to those skilled 
in the art and within the scope of the invention claimed.