Three Phase Half Wave Silicon Controlled Rectifier with R Load and RL Load

What triggers a silicon controlled rectifier?
The control circuit or triggering circuit not required in which rectifiers?
The angle of conduction in silicon-controlled rectifiers circuit changed by which voltage?
How many thyristors are used in three phase half wave controlled rectifier?
A three-phase half-wave controlled converter is fed from a 3-phase, 400 V source and is connected to a load which takes a constant current of 36 A. Find,the value of average output voltage and average current rating of SCR for a firing angle of 30°?