Electric power inverter with adaptive third harmonic auxiliary impulse commutation

Claim 1

An improved third harmonic auxiliary impulse commutated electric power inverter that is operative to supply variable frequency alternating current to the three different phases of a 3-phase inductive load circuit from a source of relatively smooth direct current, said inverter including at least three pairs of alternately conducting main controllable electric valves arranged in a 3-phase, double-way bridge configuration for interconnecting said source and said load circuit, and a commutation circuit including at least first and second auxiliary controllable electric valves interconnected in series aiding fashion across said source and connected via a precharged commutation capacitor to said load circuit, wherein the improvement comprises: a. bistable first means coupled to said capacitor for sensing the electrical potential difference across said capacitor, said first means being in one state whenever the potential on one side of the capacitor is measurably positive with respect to the other side and being in a different state whenever said potential is measurably negative; b. second means coupled to said 3-phase load circuit for detecting all zero crossings of the fundamental phase-to-phase alternating voltages that are developed at line terminals of the respective phases of said load circuit; c. control means coupled to both said first and second means and having a third harmonic commutation mode of operation in which it cyclically produces a family of periodic firing signals that cause said valves to turn on selectively, said family including a first series of firing signals respectively produced in response to said second means detecting consecutive zero crossings of said phase-to-phase voltages for alternately turning on said auxiliary valves, whereupon load current can immediately transfer from an offgoing main valve to a parallel path including the turned-on auxiliary valve and said capacitor which is first discharged and then recharged with reverse polarity by such current, and said family also including a second series of firing signals respectively produced in delayed response to successive state changes of said first means for turning on said main valves in a predetermined sequence, whereupon load current can then transfer to the oncoming main valve from the turned-on auxiliary valve; and d. said control means including time delay means effective at least after a predetermined initial period of time, which starts when said third harmonic commutation mode of operation commences, for delaying the production of each firing signal in said second series until a programmed interval of time has elapsed following each state change of said first means.