In electric motors, transistors and thyristors are employed in most electronic power converters as electronic relays. The work of the transistor as relay was already addressed when we reported the operation of the electronic regulators. Basically, it is possible to control the cutting / conduction of a high power current with a small current applied to the base.
This is why it is called a fully controlled device. The thyristor behaves as a controlled diode, ie it blocks the reverse current flow, but allows the passage of direct current, provided a voltage pulse is used at its port.
However, the thyristor only stops driving when the current is canceled (except the GTO). This is called a semicontrolled device. The thyristor is also called the Silicon Controlled Rectifier (SCR).
For this reason, the SCR is more suitable for converters that have alternating signals (AC / DC or AC / AC) as input, because zero-voltage switching simplifies thyristor (cut) switching. The SCR is used, for example, in voltage adjustment of permanent magnet alternators (on some motorcycles). https://www.mrosupply.com/electrical/transformers/886003_tdap2000xm_acme/