Nine-phase step-up/step-down autotransformer
View Patent ↗An irregular polygon connection of a three-phase autotransformer using only five windings per phase provides a source of nine-phase power suitable for an 18-pulse AC to DC power converter. The windings are connected in a manner to limit the amount of third harmonic current circulating in them. When the autotransformer is used to power a nine-phase AC to DC converter its kVA rating is typically less than 50% of the DC load kW. An additional tap on one of the five windings allows a wide range of AC output voltages to be obtained. Voltages greater than, less than, or equal to the AC input voltage can be obtained. Other multi-phase outputs are feasible. Additional isolated windings can provide means for the invention to operate as an efficient double-wound transformer.
1. An autotransformer for converting three-phase AC power to nine-phase AC power comprising:
a. three coils each containing serial windings and a plurality of non-serial windings;
b. said serial windings and plurality of non-serial windings being magnetically coupled;
c. said windings on said coils containing a start end and a finish end such that current entering the start end produces a magnetic flux in one direction whereas a current entering the finish end produces a magnetic flux in the opposite direction;
d. said windings being possible of at least two methods of interconnection;
e. with a first method of interconnection obtained by joining the finish end of a non-serial winding on a third coil with the start end of a serial winding on a first coil; with the finish end of said serial winding being connected to the finish end of a non-serial winding on the third coil; with the start end of said non-serial winding being connected to the start end of a non-serial winding on the first coil;
f. said interconnections of the first method of interconnection being repeated sequentially for all three coils until all windings are interconnected to form a nonagon;
g. with a second method of interconnection obtained by joining the start end of a non-serial winding on a second coil with the start end of a non-serial-winding on a first coil; with the finish end of said non-serial winding being connected to the start end of a non-serial winding on a third coil; with the finish end of said non-serial winding being connected to the start end of a serial winding on the first coil; with the finish end of said serial winding being connected to the finish end of a non-serial winding on a third coil;
h. with said interconnections of the second method of interconnection being repeated sequentially for all three coils until all windings are interconnected to form a dodecagon;
i. said nonagon and dodecagon topologies including symmetrically disposed nodes;
j. said symmetrically disposed nodes providing points for applying three-phase AC input voltage and extracting AC output voltages of predetermined amplitude and phase.
2. The autotransformer of claim 1 wherein the input voltage is a three-phase AC supply and the output is a nine-phase AC supply with voltages nominally equal in amplitude and typically spaced by 40°.
3. The autotransformer of claim 1 with an additional isolated winding on each coil such that three phase AC input power may be applied through such isolated windings to convert the autotransformer to a double wound transformer.
4. The autotransformer of claim 1 wherein specific amplitude AC output voltages may be less than greater than or equal to that of the three-phase AC supply.
5. The autotransformer of claim 1 wherein a similar winding on each coil is tapped to provide additional input nodes and a wide range of output voltages.
6. The autotransformer of claim 2 wherein a nine-phase output is connected to a nine-phase rectifying device so as to produce an 18-pulse converter with low output voltage ripple on the DC voltage and with low harmonic distortion of the AC input harmonic current.
7. The autotransformer of claim 2 wherein series reactance is included in the three phase AC input power source as to further attenuate high frequency harmonic currents that may be associated with a multi-pulse Ac to DC converter.