Patent GB-1518934-A (GB)
Title: Polyphase inverter/load arrangements
IPC: H02M7/523
Filed: 1975-08-14
Abstract: 1518934 Inverting GENERAL ELECTRIC CO Ltd 14 Aug 1975 [14 Aug 1974] 35779/74 Heading H2F A polyphase inverter/load arrangement comprises, for each phase, a pair of controlled rectifiers in series between D.C. supply terminals, each controlled rectifier having an uncontrolled rectifier in reverse parallel therewith, each interconnection between a said pair of controlled rectifiers providing a respective phase output of the inverter, each output being connected in series with a capacitor to a respective phase of a polyphase inductive load, the series connection of capacitor and load providing a series resonant circuit, and firing means for firing said pair of controlled rectifiers alternately and at a frequency, less than the resonant frequency of the resonant circuit, such that a firing pulse for each controlled rectifier of the pair occurs during the "overswing" half cycle of load current resulting from the preceding firing of the other one of the pair, the maximum firing frequency being such that conduction of each controlled rectifier of the pair ceases just before the other one is fired, and the pairs of controlled rectifiers being fired in rotation to provide a symmetrical polyphase system. In Fig. 1 the inverter comprises three pairs of series-connected thyristors T1, T4; T3, T6; T5, T2, which are connected across a D,C, supply +V, -V and in inverse parallel with respective diodes D1-D6, the common points 8, 9, 10 of the thyristor pairs being connected through capacitors 12 to the windings 13 of an induction motor 11. The values of the capacitors 12 are chosen so that they form resonant circuits with the windings 13, whereby a substantially sinusoidal current is produced through each thyristor pair as they are sequentially fired, a typical sequence for the pair T1, T4 being as follows: (1) T1 is fired and passes a current pulse determined by the resonant circuit. (2) When this pulse falls to zero, T1 becomes non- conductive and the resonant circuit generates an "overswing" current which flows through diode D1, the resultant voltage drop across this diode accelerating the turn-off of T1. (3) T4 is fired soon after T1 has turned off, the resultant current pulse being similar but of opposite polarity to that for T1 and causing diode D1 to be reverse biased. (4) When this pulse falls to zero, T4 becomes non-conductive and diode D4 carries the "overswing" current to accelerate turn-off of T4, subsequent to which T1 is fired to repeat the cycle. Typical waveforms are discussed for the above operations, Fig. 2 (not shown), and for three phase operation, Fig. 3 (not shown), the thyristors being fired in the order T1-T6 and the neutral point of the motor oscillating between potential levels of 2V/3 and -2V/3 in steps of V/3. Capacitors 12 also provide power factor correction and, when the motor is a linear induction motor with a resultant large leakage inductance, provide a compensating impedance to permit a larger starting current. The capacitor value may be changed to alter the resonant frequency of any desired value, for example by using switched capacitors. The inverter firing frequency is correspondingly adjusted and also may be varied over a range below its maximum possible value to vary the power supplied to the motor.