THREE PHASE LOAD CONTROLLING SYSTEM

摘要

1532031 Control of AC motors; controlling thyristors HITACHI Ltd 6 Jan 1976 [6 Jan 1975] 00306/76 Headings H2J and H2F. A load 15, for example a three-phase induction motor, is controlled by switches S1, S2 in two of its connections to an A.C. supply UVW, the anti-parallel-connected thyristors 11, 12 and 21, 22 of these switches being controlled by trigger circuits 41, 42 receiving from a transformer 30 signals in synchronism with the frequency of supply UVW and from a control unit (not shown) a signal CS representative of a desired firing angle, the circuits 41, 42 being supplied respectively by transformer secondary windings S U , S V in series and by secondary winding S W , whereby the firing signals applied to thyristors 11, 12 are 30 degrees in advance of the line-to-neutral voltage of phase U and the firing signals applied to thyristors 21, 22 are in phase with the line-toneutral voltage of phase W. This arrangement reduces the period of single phase current flow occurring at large firing angles of the thyristors and also enables a maximum current to be produced by conduction of the thyristors for the whole 180 degree period of their respective phases. Operation and waveforms are discussed for signal CS setting a firing angle of 165 degrees, Fig. 3a (not shown) and 45 degrees, Fig. 3b (not shown), operation at a 180 degrees firing angle producing a 30 degree period of single phase current flow and operation at maximum current being produced by advancing signal CS until thyristors 21, 22 conduct for the full 180 degree period, thyristors 11, 12 having previously reached this condition by virtue of their 30 degree phase advance. In Fig. 4 the motor is reversible by contactors 51, 52, which have auxiliary contacts 511, 512, 521, 522 controlling the connections of pairs of transformer secondary windings S U1 , S U2 ; S V1 , S V2 ; S W1 , S W2 to the circuits 41, 42, whereby the output signals of circuits 41, 42 are transposed when contactors 52 close to connect the switches S1 and S2 to the W and U phases respectively. The transformer 30 of Fig. 1 may be replaced by a three-phase star-connected impedance network, Fig. 5 (not shown) and this modification may also be applied to a reversible motor, the auxiliary contacts 511, 512, 521, 522 controlling the connections of circuits 41, 42 to the impedance network, Fig. 6 (not shown).