Patent GB-1239457-A (GB)
IPC: H02M5/45
Filed: 1968-10-29
Abstract: 1,239,457. "Electric converting systems; control of A.C. motors. GENERAL ELECTRIC CO. 29 Oct., 1968 [2 Nov., 1967], No. 51242/68. Headings H2F and H2J. Main power for an inverter (Fig. 1) is derived from a variable voltage D.C. source 11, while commutating energy is derived from a fixed voltage source 23, the voltage of which is equal to or higher than that of source 11, and is thus independent of variations in the inverter output voltage. SCR1 and SCR4 conduct alternately to pass current through a load 12; current limiting choke 18 and saturable inductors 27 or 28, and are commutated off by an inductor 20 and capacitor 21 in series forming an oscillatory circuit having a frequency (e.g. 9300 Hz) which is higher than the highest frequency at which turn-on signals are supplied to SCR1 and SCR4. Commutating thyristors SCR1A, SCR4A are alternately switched on to energize the oscillatory circuit from source 23 and in synchronism with and slightly in advance of the turn-on signals for the opposite main SCR, the turn on sequence being: SCR1, SCR1A, SCR4, SCR4A. Commutation operation.-Initially a steady state condition is assumed, with SCR1 and diode D1 conducting, and C21 fully charged to potential of junction 22, being negative with respect to the centre tap 18a. The load is assumed to be inductive. SCR1A is turned on at time t1 and conducts allowing C21 to discharge through L20, L18, diodes D1A, Diode D1 and conductor 25. Quickly the magnitude of discharge current rises above the load current and the forward current of SCR1 is reduced to zero. C21 proceeds to discharge completely and then recharge with opposite polarity and current in the LC circuit 20, 21 remains higher than the load current for a sufficient time to turn off SCR1. Immediately afterwards, while some current still flows in D1A, SCR4 is triggered at time t2 and current is commutated from the upper to lower half of L18 so diodes D1A, D1 stop conducting. C21 is now recharged by source 23 via SCR1A, L20, C21, lower half L18, SCR4 and conductor 26, junction 22 now being positive with respect to 18a. The oscillatory current will oscillate to zero naturally whereupon SCR1A turns off. SCR4 takes up reverse load current conduction. At time t3 the SCR4A is triggered to turn off SCR4 in like manner. Saturable reactors 27, 28 are provided to impede transfer of current from the commutating energy source 23 to the main source 11 during the commutation period. These reactors are normally saturated but change abruptly to unsaturated condition when reverse current tends to flow back into the main source 11. In lieu of the saturable reactors 27, 28, the feedback diodes D1A, D4A can be replaced by controlled rectifiers, operated to prevent reverse current flow. Three single-phase inverters (as Fig. 1) may be combined to supply a 3-phase variable-speed motor as illustrated in Fig., 2, also Fig. 3 (not shown), the main invertor trigger signals being supplied from a variable frequency oscillator via logic or ring counter circuits- or digital techniques (not described). The oscillator<SP>:</SP> frequency control and the source voltage control may be ganged to give constant torque characteristics. One of the inverter sections may be used to energize an auxiliary load from a stand-by D.C. source (48), (Fig. 3, not shown). This load may be the commutating circuits plus the gate control circuits.