CURRENT SENSOR APPARATUS AND METHOD FOR UNINTERRUPTIBLE POWER SUPPLY
Background of the Invention
[0001] The field of the invention relates to current sensors for
uninterruptible power supplies (UPS).
[0002] Typically, in a UPS with power factor correction, the current flowing
into the front-end rectifier is made to be sinusoidal and in-phase with the AC supply 12 voltage Vj. Fig. 1 illustrates, for example, a single-phase, boost converter-based UPS 10 which has front-end rectifier 15 wherein the current lj flowing through the input inductor Li is measured by the current sensor 20. The switch S1 is controlled so that the current through the inductor li follows a sinusoidal shape and is in phase with the AC supply 12 voltage Vi. However, the configuration of the UPS circuit 10 fails to account for the non-linear current drawn by other circuits in the UPS system, such as a battery charger 18, which are connected between the AC supply 12 and current sensor 20.
[0003] U.S. Patent 4980812 also describes a single phase UPS having a
through-going neutral and power factor correction. The patented UPS achieves a unity power factor when the current through the line inductor is substantially similar to and in phase with the line voltage. However, the UPS described in US 4980812 does not include any additional UPS components, such as a battery charger, connected to the source (mains) voltage. Thus, the UPS described by US 4980812 also does not account for the non-linear currents of other circuits in a UPS system when attempting to improve the power factor.
[0004] Accordingly, a need exists for a UPS which is capable of operation
at a unity power factor while accounting for and including elements on the UPS circuit in addition to a rectifier.
Brief Description of the Invention
[0005] In one embodiment of the invention, the placement of a current
sensing device in an uninterruptible power supply (UPS) is such that the net current drawn from the AC supply is of the desired wave shape and phase with respect to the AC supply voltage. The net current drawn is the sum of the current drawn by the input rectifier and the battery charger of the UPS. In one mode of operation, the UPS is operated at unity power factor so that the current drawn from the utility is sinusoidal and in phase with the AC supply, even though the individual current components flowing through the battery charger and the input rectifier are not sinusoidal or in phase with the AC supply. The invention is applicable to three phase as well as single phase UPS circuits.
[0006] In a further embodiment of the invention, the current being drawn
by the battery charger circuit and the current drawn by the main power circuit of the UPS are such that the net current is sinusoidal and in phase with the AC supply voltage.
[0007] The various features of novelty which characterize the invention are
pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and benefits obtained by its uses, reference is made to the accompanying drawings and descriptive matter. The accompanying drawings are intended to show examples of the many forms of the invention. The drawings are not intended as showing the limits of all of the ways the invention can be made and used. Changes to and substitutions of the various components of the invention can of course be made. The invention resides as well in sub-combinations and sub-systems of the elements described, and in methods of using them.
Brief Description of the Drawings
[0008] Fig. 1 is a circuit diagram of a prior art uninterruptible power supply
having a current sensor;
[0009] Fig. 2 is a circuit diagram of an embodiment of the present
invention; and
[0010] Fig. 3 is a graph of current and voltage versus time displaying
representative waveforms in the circuit of Fig. 2.
Detailed Description of the Invention
[0011] Referring now to the drawings in which like reference numerals are
used to indicate the same or related elements, Fig. 2 illustrates a front end portion of a single phase UPS 100 in which battery charger 18 is connected to the AC power supply 12 in parallel with main power circuit 50. A battery booster
19 may be connected in series with the battery charger 18 if desired. Main
power circuit 50 includes diode bridge 15 connected in series with boost inductor
Li Boost inductor Li is in turn connected in series with the AC power supply 12.
[0012] The battery charger 18 has a conventional diode bridge front end
that draws a highly distorted current. If the current through the boost inductor Li is measured and the switch S1 controlled such that the current through the inductor Li is sinusoidal and in phase with the AC power supply 12, the net supply current Iin, which is the sum of the boost inductor current li and the battery charger current lb, will not be sinusoidal and will have a slight phase shift with respect to the AC power supply 12. The battery charger 18 draws a slightly leading current due to the presence of the capacitive filter on the output of its front-end diode bridge. The various waveforms are shown in Fig. 3.
[0013] The total current lin flowing from the AC supply 12 is measured by a
current sensor 20. Current sensor 20 may be a current transformer, a Hall device or a shunt resistor, or other structure which provides current measurement information. The current sensor 20 is positioned to be in series with the AC power supply 12. Accordingly, the current input to the current sensor
20 includes the current flowing through the main power circuit 50 and the battery
charger 18. This measured current lin is used as a feedback in the current loop
of the control circuit (not shown). The UPS controller circuit then forces the
current lin through the current sensor 20 to be sinusoidal and in phase with the
AC power supply voltage Vi to produce unity power factor operation, such as by operation of switch S1. Switch S1 is controlled based on the feedback from the current sensor 20 such that the net current lin, which is the sum of the boost inductor current Ii and the battery charger current lb, is sinusoidal and in phase with the AC power supply 12. Unity power factor operation of the UPS 100 is achieved even though the individual components of the UPS 100 do not exhibit sinusoidal currents themselves.
[0014] The various waveforms of the input voltage and component
currents are shown in Fig. 3. As illustrated in Fig. 3, mains voltage Vi of the AC power supply 12 has a conventional sinusoidal waveform. Ii and lb comprise the component elements of mains current lin. As seen in Fig. 3, Ii and lb are not perfectly sinusoidal, but do sum to form sinusoidal current lin. That is, as the battery charging current lb changes from zero, Ii, the current through the boost inductor Li changes from sinusoidal to reflect the subtraction of the battery charging current lb. As the battery charges, the current lb to the battery charger 18 reduces and the current Ii to main power circuit 50 becomes substantially the same as current li.
[0015] As will be appreciated, moving the current sensor 20 to a position in
series with the input voltage source 12 and ahead of any UPS circuit components, such as the main power circuit or an accessory circuit, ensures that the net current lin is in phase with the input voltage Vi to provide unity power factor operation. Unity power factor operation is achieved regardless of the components connected with the UPS main power circuit 50.
[0016] While the present invention has been described with references to
preferred embodiments, various changes or substitutions may be made on these embodiments by those ordinarily skilled in the art pertinent to the present invention with out departing from the technical scope of the present invention. Therefore, the technical scope of the present invention encompasses not only those embodiments described above, but all that fall within the scope of the appended claims.

What is claimed is:
1. An uninterruptible power supply (UPS) operating at a unity power factor
comprising:
an input power supply;
a main power circuit connected to the input power supply;
at least one accessory circuit connected to the input power supply in parallel with the main power circuit; and
a current sensor connected in series with the input power supply.
2. An uninterruptible power supply (UPS) according to claim 1, wherein the at least one accessory circuit comprises a battery charger.
3. An uninterruptible power supply (UPS) according to claim 1, wherein the main power circuit includes a controlled switch, an input current lin measured by the current sensor providing feedback for controlling the controlled switch.
4. An uninterruptible power supply (UPS) according to claim 1, wherein an input current lin of the current sensor is in phase with a voltage Vi of the input power supply.
5. An uninterruptible power supply (UPS) according to claim 1, further comprising a boost inductor in series with the main power circuit connected to the current sensor.
6. A method of configuring an uninterruptible power supply (UPS) to operate at unity power factor comprising:
providing an input power supply;
providing a main power circuit;
providing at least one accessory circuit;
connecting the main power circuit and at least one accessory circuit in parallel to the input power supply;
connecting a current sensor in series with the input power supply for measuring the net current in each of the main power circuit and at least one accessory circuit;
connecting a controlled switch in the main power circuit to receive an input current linmeasured by the current sensor as feedback for controlling the controlled switch during operation of the UPS to achieve unity power factor operation.
7. A method according to claim 6, wherein the at least one accessory circuit comprises a battery charger.