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3.FIRING MODESQ7000 power thyristor units can be controlled with one of the following thyristor firing types:• a series of supply voltage cycles with zero crossing firing.Firing Modes3.1. BURST MODE - CT ≥ 2‘Burst mode’ firing is a proportional cyclewhich delivers a series ofwhole supply cycles to the load.Thyristor firing and cut-off is synchronisedwith the supply and occurs atzero crossing.LoadVoltagesT FT MThyristor firing for one of the phases, in ‘Burst mode’T NFtThyristor firing in ‘Burst mode’ can be described bythe :firing time (T F ) ,non-firing time (T NF ) andthe modulation time (T M )where T M = T F + T NFThe power delivered to the load is defined by the dutyratio η = T F / T MFiring in ‘Burst mode’ is defined by theBase Cycle Time (T B ).The Base Cycle Time is equal to thenumber of cycles firing at50% of the duty ratio(or 50% of the power supplied to the load):T B = T F = T NF .Modulation Time8 T B6 T B4 T B2 T BT F = T NF = T BT B0 25 % 50 % 75 % 100 %Duty Ratio η10 T BT M‘Burst mode’ modulation time dependingon setpointThe Base Cycle time is equal to 16 cycles for code C16and 64 cycles for code C64.The control system adjusts the modulation time to retain the same precision for all duty ratios η (power requested).3.2. SINGLE-CYCLE (code FC1) - CT = 1‘Burst mode’ firing with a single firing or non-firingcycle is known as ‘Single-cycle’.25%powertFor example, with a setpoint of 50% (correspondingto a duty ratio η = 50%) the modulation comprises 1firing cycle and 1 non-firing cycle.T FT NFT MT NC = 3 T FFor duty ratios η < 50% the firing time remainsunchanged (1 cycle) and the non-firing timeincreases.50%powertFor duty ratios η > 50% the non-firing timeremains unchanged (1 cycle) and the firing timeincreases.75%powerT FT MT NFT NF = T F = T BtT FT MT NFT F = 3 T NFTypical firing in ‘Single-cycle’ mode for various duty ratiosQ7000 User Manual3-2