This　paper　proposes　an　analytical　time　and　frequency　domains　evaluation　of　torque　harmonics　produced　by　high-power　load-commutated-inverters　(LCIs)　and　pulsewidth-modulated　(PWM)　voltage-source　inverters　for　adjustable　speed　applications.　These　drive　systems　are　used　in　petrochemical　and　mining　industries,　where　driven　loads　have　multi-inertia　or　long　shafts　that　are　potentially　subject　to　torsional　mode　excitation.　Such　excitation　may　lead　to　accelerated　fatigue,　system　failure,　and　loss　of　production,　and　in　turbine-generator　sets,　to　system　blackout.　The　investigated　power　conversion　topologies　are　multi-pulse　LCIs　(6/6　and　12/12-pulse　systems),　parallel　connected　three-level　neutral-clamped-converters　with　non-interleaved　and　interleaved　PWM　signals,　as　well　as　series-connected　H-bridge　multi-level　inverters,　single　thread　and　four　threads　supplying　a　12-phase　machine　and　parallel　connection　of　four　threads　converter　systems　with　interleaved　PWM　commands.　They　are　the　most　used　variable　frequency　drives　in　the　megawatt　range,　both　with　synchronous　and　induction　machines,　in　the　petrochemical　industry.　The　time　and　frequency　domains　of　the　machine＇s　air-gap　torque　is　calculated　from　the　motor＇s　voltage　and　stator＇s　current　waveforms,　this　proven　method　is　independent　from　the　type　of　ac　machine.　More　than　50　simulated　operating　points　are　discussed　and　experimental　results　for　NPC　and　cascaded　inverters　are　also　presented.　©　1986-2012　IEEE.