United States Patent [Is] 3,636,423 Jenkins [451 Jan

United States Patent [is] 3,636,423 Jenkins [451 Jan. 18, 1972 ' [54] CYCLOCONVERTER ARRANGEMENTS ‘ [56] ‘ References Cited WHICH PREVENT CIRCULATING ‘ CURRENTS UNITED STATES PATENTS [72] Inventor: John Edward Jenkins, Edinburgh, Scot- 2,264,854 12/1941 Mittag .................................. ..321/7 X land 3,328,660 6/1967 Dunbar . ..321/7 3,470,447 9/1969 Gyugyi et al.. .....32l/7 [73] Assignee: National Research Development Corpora 3,527,995 9/1970 Lee et al. ................................. ..321/7 tion, London, England FOREIGN PATENTS OR APPLICATIONS [22] Filed: Mar. 25, 1970 130,104 12/1960 U.S.S.R. ......................... ..321/7 [211 Appl. No.: 22,600 1,125,082 8/1968 Great Britain ......................... ..321/61 [30] Foreign Application Priority Data Primary Examiner—William H. Beha, Jr. Attorney-Cushman, Darby & Cushman Apr. 2, 1969 Great Britain .................... .. 17,420/69 [57] ABSTRACT [52] U.S.Cl ................................. ..318/227, 318/231, 321/7, A cycloconverter is described in which each supply phase has ,1 321/66 [51] Int. Cl. ..................................... ..I-I02p 5/40, H02m 5/14 a separate circuit path to the load, each path having a separate [58] Field of Search ...................... ..321/7, 60, 61, 65, 66, 69; - pair of oppositely poled controlled recti?ers. This arrange ment prevents circulating currents ?owing and thus removes 318/231, 341, 227 the need for large expensive reactors to limit these currents. Logic circuits for ?ring the controlled recti?ers both at unita ry and at any load power factor are also described. 16_Clgin_|_s_, 1 1 Drawing Figures /0 074 A; /. ‘57 m/vrpaz // 35 70 7' 6" 70 AND 4/ 70 52 PATENTEU JAN I 8 I972 aIéssL423 SHEET 5 OF 7 PATENIEnJmmz ~ vsis-seams SHEET 7 BF 7 3,636,423 1 2 CYCLOCONVERTER ARRANGEMENTS WHICH Since in the cycloconverter according to the invention there PREVENT CIRCULATING CURRENTS - is no path for supply currents except by way of the load the circulating currents mentioned above do not flow. The present invention relates to cycloconverters for provid The cycloconverter usually . feeds an induction motor ing a variable low-frequency electrical supply from a high although in some applications the load could be a synchronous frequency supply. Such converters are particularly, but not ex motor, a transformer, or a travelling-?eld magnet. clusively, useful for controlling the speed of induction motors. Where the cycloconverter is to supply and control an induc For example, drives for o?-highway vehicles require a high tion motor, the motor may have two pairs of windings for each power-to-weight ratio with control at each wheel. The com phase. Each pair is connected in star, together with one pair bination of gas-turbine driven alternator and AC motor wheel 10 from each other phase, to the output terminals of one group. drive is attractive for this application. Other applications of The windings of each pair are wound to give ?uxes of equal cycloconverters include: the generation of travelling magnetic magnitude and in the same direction when equal currents ?ow ?elds for metallurgical uses where alternating magnetic ?elds in the windings towards the star point. A vector representing in molten metal produce currents which heat and stir the the rnaneto-motive force in the rotor of the motor has con~ metal; the control of closely controlled variable low-speed stant magnitude. A similar winding arrangement can be used drives for use for example in steel mills; and the excitation of for the primary windings of a transformer forming the load. the linear induction drive of linear motors for traction‘ systems. Therecti?er means may conveniently comprise pairs of op positely poled controlled recti?ers such as thyristors con The direct-driven multiphase altemator‘runs at a high, rela nected in parallel with each other between one input and one tively ?xed frequency since the single shaft gas turbine must 20 output terminal. operate within a narrow speed range. A solid-state converter The control means may then include a logic circuit, and one can be used to shape the high-frequency wave into a variable AND gate for each thyristor, connected to ?re the thyristor if frequency, variable-voltage motor input. it is not conducting and the logic circuit provides an input The converter may be a recti?er~inverter assembly or a I signal for the gate. cycloconverter. The recti?er-inverter is a two-stage converter 25 In order to avoid the voltage transients mentioned above the which requires auxiliary commutating components. The logic circuit should ?re each thyristor for half-cycles of the cycloconverter is a one-stage converter without commutation high-frequency supply. However, in other circumstances component requirements. It is, potentially, lighter and more delayed angle ?ring may be used for wave-shaping or voltage efficient. ~ reduction. The operation of a cycloconverter is brie?y described by 30 When the load has windings connected in star, as described stating that a high-frequency supply is recti?ed for variable in above, the logic circuit preferably ?res one thyristor in tervals to give successive positive and negative output volt selected recti?er circuits to provide a resultant magnetic flux ages, and the output voltages are used to synthesize a low in a given direction. Another selection of thyristors is then frequency output, whose frequency depends on the duration ?red during the next half-cycle of the supply frequency to pro of the intervals. 35 vide flux in the same direction. This process is continued for Conventional ' phase-controlled cycloconverters have alternate supply cycles for an interval determined by the out several disadvantages such as relatively heavy reactors are put frequency, and the further thyristors in the recti?er cir required to limit circulating currents; motor control is dif?cult cuits are ?red for alternate supply cycles to provide a resultant at low-current values; voltage transients, are impressed on the magnetic flux at an angle to the given direction. After a similar vsystem when the ?ring delay of controlled recti?ers in the 40 interval other recti?ers are again ?red to increase the angle, cycloconverter is large; and ?ring delays reduce the power and in this way the resultant magnetic ?ux is made to resolve. factor of the cycloconverter operation. The logic circuit may include a shift register having a Cycloconverters have been developed which eliminate number of stages equal to the number of intervals required to some of these difficulties. For example, the “practical rotate the resultant magnetic ?ux by 211 radians. Thus when cycloconverter” prevents intergroup circulating currents by 45 the register contains a binary “one" and clock pulses are ap blanking off groups of thyristors. However, control of motor plied thereto, the “one" progresses round the register, and the speed may be lost during blanking. The “practical cyclocon position of the “one” determines which AND gates receive an verter” is described by L. J. Lawson, in I.E.E.E. Transactions enabling signal from the logic circuit. on Industry and General Applications, Vol. [GA-4, No. 2, Certain embodiments of the invention will now be described March/April 1968. 50 by way of example, with reference to the accompanying According to the present invention there is provided drawings, in which: cycloconverter apparatus for converting a high-frequency FIG. 1 is a block diagram of a ?rst embodiment of a electrical supply having a ?rst predetermined number of cycloconverter according to the invention. phases to a low-frequency supply having a second predeter FIGS. 2(a), (b) and (c) are vector diagrams used in explain mined number of phases, including a number of controlled 55 ing the operation of the cycloconverter of FIG. 1, recti?er means at least equal to the larger of the said predeter FIG. 3(a) shows the waveform of one phase voltage of an al mined numbers, each controlled-recti?er means being capa ternator of FIG. 1, ble of passing current selectively in either direction and con FIG. 3(b) shows the voltage applied to each coil of a motor trol means for providing control signals for the controlled of FIG. 1, recti?er means to synthesize a low-frequency output having 60 FIG. 4(a) shows the waveform of currents applied in a the same number of phases as the load, the controlled-recti?er phase belt of the motor when the load power factor is unity, means being connected to provide a number of bidirectional FIG. 4(b) shows the waveform of currents applied in the red current paths, one path for currents ?owing in each combina phase belt of the motor when the load power factor is 0.5 tion of load phase and supply phase and particular to that lagging, ' combination, and the only paths for substantial currents from 65 FIG. 4(0) shows a waveform from a square wave generator the supply, when the load and supply are connected to the‘ used in controlling the ?ring of thyristors, cycloconverter apparatus, being by way of the load, even if all FIG. 4(d) shows a wavefonn from a synchronizing pulse the recti?er means were to receive control signals enabling generator used in controlling the ?ring of the thyristors, them to conduct. FIG. 5 is a block diagram of the control circuit of FIG. 1, The term “bidirectional current path" means either a single FIG. 6 shows the torque-speed characteristics of the motor path in which current can ?ow in either direction or a path in controlled by the cycloconverter of FIG. 1, which there are parallel parts for currents in opposite FIG. 7 is a block diagram of a second embodiment of directions; for example, where oppositely poled thyristors are cycloconverter according to the invention, connected in parallel.

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