At ωt = 0, the input supply voltage becomes positive, thyristor T1, becomes forward biased. It turns ON when gate pulse Ig is applied at ωt = α, where a is called firing angle or delay angle. The angle after input voltage starts positive until the thyristor is first fired is called delay angle.
During the period 0 < ωt < α, the thyristor blocks the supply voltage and the load voltage remains zero. Hence no current flows during this interval, soon after gate pulse Ig is applied to the thyristor at ωt = α it turns ON. The voltage across thyristor reduces to zero and full supply voltage appears across the load. From this point onwards, the load voltage follows supply voltage.