Thyristors are switches. We can control exactly when the Thyristor is switched on. The Thyristor then stays on until the voltage across it is zero. When used with 50 or 60 Hz ac voltage, the Thyristor can be switched on for part of a sine wave. This results in a voltage wave that follows the downward slope of a sine function and resembles a triangle. Turning on the Thyristor earlier increases the size of this triangle. See Wikipedia Thyristor History. Combining six Thyristors with 3-phase ac produces a voltage that is mostly dc, but has triangles imposed on it.
Thyristors are limited in frequency of response by the line frequency. With 3-phase power this results in six pulses per cycle (360 Hz for 60 Hz power). A torque regulator adjusts the voltage to the motor to control torque. A speed or velocity regulator adjusts the torque demand to hold speed. The dc drive is much simpler than the ac drive.
A dc drive has couple of nice simple characteristics. Current is proportional to torque and voltage is proportional to RPM.