THREE-PHASE TRANSFORMER FOR CYCLOCONVERTER

Abstract: A three-phase transformer for a cycloconverter comprising three primary windings (21) to which three-phase currents of U-, V- and W-phase are individually applied as inputs, and six secondary windings (22 - 27) which are provided for each of the primary windings (21U - 21W), the three-phase transformer being so arranged that D.C. components contained in six sets of three-phase signals delivered from the secondary windings (22 - 27) are cancelled in the respective phases, thereby to prevent D.C. components from being contained in currents which are supplied to the primary winding (21U - 21W) wound round the main legs of an iron core (13) for the respective phases.

Description: The invention relates to a three-phase transformer for a cycloconverter according to the preamble of claim 1. In more detail, the invention relates to a transformer being used for a cycloconverter having a conversion frequency lower than a source frequency. Heretofore, cycloconverters which can convert a source frequency of 50 Hz - 60 Hz into an output frequency of several to 20 Hz have been employed for speed control on elastic railways, in induction motors and the like. Fig. 3 is a connection diagram showing prior-art three-phase transformers for cycloconverters in the case where a cycloconverter illustrated in, for example, "DENKI-KOGAKU HANDBOOK (HANDBOOK OF ELECTRICAL ENGINEERING)," (issued by Denki Gakkai in 1971), page 714, Section 6. 4. 3, Fig. 168, is adapted to a three-phase system. Referring to Fig. 3, each of three three-phase transformers of identical arrangement 1-3 includes a primary winding 4, and secondary windings 5 and 6 magnetically coupled therewith. Although each winding is depicted as a single winding in Fig. 3, each of the primary winding 4 and the secondary windings 5 and 6 is composed of three windings corresponding to respective phases U, V and W as will be stated later. Three cycloconverter circuits of identical arrangement 7 - 9 are individually connected to the respective three-phase transformers 1 - 3. Each of the cycloconverter circuits 7 - 9 includes a positive group converter 10 and a negative group converter 11 which are respectively formed of thyristor circuits, and a circulating current reactor 12 which is connected in series with both the converters 10 and 11. In addition, three-phase outputs Iu - Iw from the respective cycloconverter circuits 7 - 9 are supplied to a three-phase induction motor (not shown). Fig. 4 is a side sectional view showing the winding structure of one of the three-phase transformers in Fig. 3. Referring to Fig. 4, the iron core 13 of a three-phase three-leg structure has three main legs 13U - 13W which correspond to the U-phase, V-phase and W-phase, respectively. The primary windings 4U - 4W of the respective phases individually wound round the main legs 13U -13W constitute the primary winding 4. The secondary windings 5U - 5W and 6U - 6W of the respective phases wound round the corresponding main legs 13U - 13W constitute the secondary windings 5 and 6. Each of the primary windings 4U - 4W of the respective phases is divided into two sets. These sets are excited in parallel, and one of them is magnetically coupled to the secondary windings 5U - 5W, while the other is coupled to the secondary windings 6U - 6W. The respective phases of each of the secondary windings 5 and 6 are delta-connected or star-connected to construct corresponding terminals which deliver three-phase signals U1 - W1 and U2 - W2. By way of example, the secondary winding 5U of the U-phase, the secondary winding 5V of the V-phase and the secondary winding 5W of the W-phase are delta-connected so as to deliver the three-phase outputs U1 - W1 from the respective nodes of the delta connection. Now, there will be described the operation of the prior-art three-phase transformer for the cycloconverter shown in Figs. 3 and 4. First, when three-phase currents IU - IW at a source frequency of 60 Hz are supplied to the primary winding 4 of the three-phase transformer 1, the primary windings 4U - 4W of the respective phases are excited, and the three-phase signals U1 -W1 and U2 - W2 are respectively delivered from the corresponding secondary windings 5U - 5W and 6U - 6W. The three-phase signals U1 - W1 derived from one secondary winding 5 are supplied to the positive group converter 10, while the three-phase signals U2 -W2 derived from the other secondary winding 5 are supplied to the negative group converter 11. The respective three-phase signals U1 - W1 and U2 - W2 are subjected to rectification and duty factor controls by thyristors included in the corresponding converters 10 and 11. Further, the resulting signals are converted into a desired frequency by the circulating current reactor 12 so as to produce the signal-phase output Iu of the U-phase. Likewise, three-phase signals U3 - W3 and U4 - W4 from the three-phase transformer 2 are converted into the signal-phase output Iv of the V-phase by the cycloconverter circuit 8, and three-phase signals U5 - W5 and U6 - W6 from the three-phase transformer 3 are converted into the signal-phase output Iw of the W-phase by the cycloconverter circuit 9. The single-phase output Iu - Iw form three-phase outputs having phase differences of 120° from one another, and are used for the speed control of the induction motor. Fig. 5 is a waveform diagram for explaining the process in which one of the single-phase outputs is obtained. When an A.C. input voltage as shown in Fig. 5 is supplied as a three-phase current IU - IW, signals depicted as output voltages (hatched parts) are provided from the respective converters 10 and 11. These output voltages are accordingly derived through the circulating current reactor 12, whereby a single-phase output as indicated by an output fundamental wave voltage is obtained. In this case, the output frequency fo of each of the single-phase outputs Iu - Iw becomes 1/3 of the input frequency fi of the three-phase currents IU - IW. Therefore, assuming by way of example that the input frequency fi is 60 Hz, the output frequency fo becomes 20 Hz. With the operation control system employing such three-phase outputs Iu - Iw, however, when the output frequency fo is to be set at about 1/2 of the input frequency fi, D.C. components are respectively generated in the three-phase signals U1 - W1, ... and U6 - W6, and hence D.C. components also add to the three-phase currents IU - IW to be supplied to the primary windings 4U - 4W of each of the three-phase transformers 1 - 3. Accordingly, each of the three-phase transformers 1 - 3 undergoes D.C. excitation, and the main legs 13U - 13W of the iron core 13 become saturated, with the result that the primary windings 4U - 4W and the secondary windings 5U - 5W and 6U - 6W wound round these main legs 13U - 13W are adversely affected electromagnetically and mechanically by overheating of the iron core 13, rush currents, etc. In order to prevent this drawback, the system has heretofore been operated so that the output frequency fo for the speed control may have the following relation to the input frequency fi: fo < fi/2 and may become 0 - 25 Hz or so. In a case where the speed control range of a load such as an induction motor is to be expanded, speed changes have been performed using gears. As thus far described, prior-art three-phase transformers for cycloconverters have produced the single-phase outputs Iu - Iw of the respective phases by the use of the three three-phase transformers 1 - 3. Accordingly, there has been the problem that, when it is intended to obtain the three-phase outputs Iu - Iw whose output frequency fo is higher than 25 Hz, the D.C. excitation causes the magnetic saturation in the iron cores 13 of the respective three-phase transformers 1 - 3 and renders the operation difficult. Another problem has been that, when the speed range of the controlled system is to be broadened, gears must be used incurring increases in the cost of the overall apparatus and increases in labor for the maintenance of the moving parts. It is the object of the invention to improve a three-phase transformer as described before such that no D.C. excitation is generated in the main legs within the iron core even when operated so that the output frequency of the cycloconverter becomes 1/2 of the input frequency thereof. The afore-mentioned problem is solved by applying the resultant current, based on the secondary windings, to respective primary windings wound around main legs of a single iron core such that the D.C. components contained in six sets of three-phase signals delivered from said secondary windings are cancelled in their respective phases. Prefered embodiments of the invention are described in the sub-claims. In the following an embodiment according to the invention is described in some more detail by drawings in which Fig. 1 is a connection diagram showing an embodiment of this invention; Fig. 2 is a side sectional view showing the winding structure of a three-phase transformer in Fig. 1; Fig. 3 is a connection diagram showing a prior-art three-phase transformer for a cycloconverter; Fig. 4 is a side sectional view showing the winding structure of a three-phase transformer in Fig. 3; and Fig. 5 is a waveform diagram for explaining the operation of a conventional cycloconverter circuit. DESCRIPTION OF THE PREFERRED EMBODIMENT Now, an embodiment of this invention will be described with reference to the drawings. Fig. 1 is a connection diagram showing one embodiment of this invention, while Fig. 2 is a side sectional view showing the winding structure of a three-phase transformer in Fig. 1. Portions 7 - 13 in these figures are similar to the respective constituents described before. A three-phase transformer 20 has seven windings, which comprise one primary winding 21 and six secondary windings 22 - 27 magnetically coupled therewith. The primary winding 21 and the secondary windings 22 - 27 are respectively composed of three primary windings 21U - 21W and secondary windings 22U - 22W, ... and 27U - 27W which correspond to phases U, V and W, and which are wound round the main legs 13U - 13W of the respective phases as shown in Fig. 2. Further, each of the primary windings 21U - 21W of the respective phases is divided into two sets. These sets are excited in parallel, and one of them is magnetically coupled to the secondary windings 22 - 24, while the other set is coupled to the secondary windings 25 - 27. Further, the respective phases of each of the secondary windings 22 - 27 are delta-connected, and the nodes of the delta connections of the respective windings 22 - 27 construct output terminals for three-phase signals U1 - W1, ... and U6 - W6. Next, there will be described the operation of the embodiment of this invention shown in Figs. 1 and 2. First, when three-phase currents IU - IW at a source frequency of 60 Hz are supplied to the primary winding 21, the U-phase primary winding 21U through the W-phase primary winding 21W are excited, and the three-phase signals U1 - W1, ... and U6 - W6 are respectively delivered from the secondary windings 22 - 27. These three-phase signals U1 - W1, ... and U6 - W6 are supplied to the corresponding cycloconverter circuits 7 - 9, and are converted into three-phase outputs Iu - Iw at a desired frequency as in the prior-art described before. In general, D.C. components are contained in the three-phase signals U1 - W1, ... and U6 - W6. However, since all these signals are three-phase balanced currents, the resultant currents thereof do not contain any D.C. components. In this case, the resultant currents based on the secondary windings 22U - 27U, 22V - 27V and 22W - 27W are respectively applied to the main legs 13U, 13V and 13W of the iron core 13, so that no D.C. excitation develops. For this reason, no D.C. components appear in the respective three-phase currents IU - IW to be supplied to the primary windings 21U - 21W, either, so that the corresponding main legs 13U - 13W are not subjected to D.C. excitation at all. Accordingly, even when the three-phase transformer 20 is operated so as to establish an output frequency fo equal to 1/2 of the input frequency fi, it does not give rise to the obstacles as explained before, and hence, it is an economical setup suited to the operation of the cycloconverter. Although the embodiment has been described in reference to a three-phase three-legged iron core 13, an equal effect is achieved even with a three-phase five-legged iron core having side legs. Moreover, although the respective phases of each of the secondary windings 22 - 27 have been delta-connected so as to obtain the three-phase signals U1 - W1, ... and U6 - W6, the secondary windings 22U - 22W, ... and 27U - 27W may well be star-connected respectively. As described above, according to this invention, each of three primary windings, to which three-phase currents are individually applied as inputs, is furnished with six secondary windings, and D.C. components contained in respective three-phase signals delivered from the secondary windings are cancelled to zero, so as to prevent any D.C. component from being contained in currents which are supplied to the primary winding wound round main legs of the iron core for respective phases. The invention is therefore effective for economically providing a three-phase transformer for a cycloconverter which is free from D.C. excitation without regard to the frequency of outputs from cycloconverter circuits.

Claims: A three-phase transformer for a cycloconverter comprising three primary windings (21U - 21W) to which three-phase currents (IU, IV, IW) of U-, V- and W-phases are individually applied as inputs, and six secondary windings (22U - 22W ... 27U - 27W) which are provided for each of said primary windings (21U - 21W), characterized by applying the resultant current, based on the secondary windings (22U ... 27W), to respective primary windings (21U ... 21W) wound around main legs (13U ... 13W) of a single iron core (13) such that D.C. components contained in six sets (22 - 27) of three-phase signals delivered from said secondary windings (22U ... 27W) are cancelled in their respective phases. The three-phase transformer according to claim 1, characterized in that each of the primary windings (21U - 21W) of the respective phases is divided into two sets which are excited in parallel. The three-phase transformer according to claim 2, characterized in that the first set of primary windings (21U - 21W) is magnetically coupled to a first group of secondary windings (22 - 24), while the second set is coupled to a second group of secondary windings (25 - 27).

Inventors: ASAKURA, MASAKATSU
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1987
Main classification: H01F33
IPC codes: H01F30, H02M5