Patent GB-598117-A (GB)
Title: Contact converter
IPC: H02M1/26
Filed: 1945-07-18
Abstract: 598,117. Converting. MASCHINENFABRIK OERLIKON. July 18, 1945, Nos. 18408, 18409 and 18410. Convention dates, July 18, 1944, Aug. 2, 1944, and Jan. 8, 1945. [Class 38 (ii)] In a polyphase contact converter for rectifying, inverting or frequency changing, each pair of phases between which commutation takes place, is connected through a variable impedance having its least value during commutation between those phases, and a P.D. is inserted or produced in the circuit formed by the variable impedance and the main contacts in the two phases. In a three-phase converter 4 having contacts 5, 6, 7 and supplied by transformer windings 1, 2, 3, each pair of phases is connected through one of the variable impedances 10, 11, 12 in series with one of the sources of voltage 13, 14, 15 which may be generators. When phase 2 is taking over from phase 1 the current supplied to a load 8, the reactance of impedance 10 is reduced by saturation due to the current through its main or a secondary winding, and voltage source 13 diverts the load from contacts 5 to 6. So that the improvement in commutation is independent of the load value, the voltages inserted at 13, 14, 15 are made dependent on the load as by varying the generator excitation. Alternatively, P.D. may result from the current passing through resistances 13<1>, 14<1>, 15<1> in series with contacts 5, 6, 7. Such resistances may be inserted gradually. Where switches 5, 6, 7 are brushes bearing on a rotating conducting segment 16, Fig. 2, a resistance 18 joined to a number of separate segments 17 at the running- off end is connected in gradually, or the resistance may be provided by the diminution of brush contact area or change of brush pressure. Alternatively, a resistance 20 may be connected across contacts 19 which open a little before the main contacts, Fig. 3. Each variable inductance may have in parallel a fixed impedance such as a resistance and capacity in series to reduce the voltage across the impedance at the end of commutation, or the inductance may be designed to have suitable characteristics including internal capacity. Fig. 7 shows the voltage curve across each inductance during commutation between phases 1, 2. The voltage existing from t, to t2 becomes zero at t2, rises again at t3 when the resistance is introduced and drops to zero at t4 when commutation is completed. The main contacts may be immersed in a fluid dielectric. Each variable inductance may be premagnetized by the difference of constant ampere turns and a smaller number of ampere turns varying with the D.C. load voltage. Thus, as the D.C. voltage falls, the premagnetization rises and the load current needed for saturation is reduced. The asymmetry of the inductance produced by premagnetization is removed by using a double choke wound as in Fig. 11. Main windings 32, 33 on two cores 30, 31 are connected in parallel, and the two premagnetizing windings 34, 35 are connected in series, one opposing and one aiding the main flux. To reduce voltage across the opening contacts, further impedances may be connected across them, alone or together with the subsidiary contacts 19, Fig. 3, and also Fig. 13 (not shown). This impedance may comprise a capacity and resistance in series or an asymmetrical conducting device.