Variable frequency alternating current (AC) drives are used for various applications to deliver power to a load. For example, FIG. 1 illustrates a drive circuit comprising series-connected power cells that include inverters as described in U.S. Pat. No. 5,625,545 to Hammond, the disclosure of which is incorporated herein by reference in its entirety. Referring to FIG. 1, a transformer 110 delivers three-phase, medium-voltage power to a load 130 such as a three-phase induction motor via an array of power cells. The transformer 110 includes a primary winding 112 that excites any number of secondary windings 114-125. Although primary winding 112 is illustrated as having a star configuration, a mesh configuration is also possible. Further, although secondary windings 114-125 are illustrated as having a mesh configuration, star-configured secondary windings are possible, or a combination of star and mesh windings may be used. Further, the number of secondary windings illustrated in FIG. 1 is merely exemplary, and other numbers of secondary windings are possible. The circuit may be used for medium voltage applications or, in some embodiments other applications. A local control system 192 may be used to control each power cell, such as for turning transistors in each cell on or off. In some embodiments, the local control system 192 may be directed by a central control system 195 via a wireless or wired communications network 190.
Each secondary winding is electrically connected to a dedicated power cell 151-153, 161-163, 171-173 and 181-183 so that three phases of series-connected power cells are connected to deliver three-phase power to a load 130.
It should be noted that the number of cells per phase depicted in FIG. 1 is exemplary, and more or less than four cells per phase may be possible in various embodiments. For example, in one embodiment which can be applied to 2300 volts AC (VAC) inductive motor loads, three power cells may be used for each of the three phase output lines. In another embodiment, which may be applied to a 4160 VAC inductive motor load, five power cells may be used for each of the three phase output lines. Such an embodiment may have eleven voltage states which may include approximately +/−3000 volts DC (VDC), +/−2400 VDC, +/−1800 VDC, +/−1200 VDC, +/−600 VDC and zero VDC,
It is desirable to find methods and systems that help to reduce the size and/or cost of circuit configurations that use series-connected inverters, including but not limited to the circuits described in FIG. 1. The disclosure contained herein describes methods and systems that solve one or more of the problems described above.