Figure 5.25 shows commonly used Controlled Rectifier Fed DC Drives and quadrants in which they can operate on Va-Ia plane. As thyristors are capable of conducting current only in one direction, all these rectifiers are capable of providing current only in one direction.
Rectifiers of Figs. 5.25(a) and (c) provide control of dc voltage in either direction and therefore, allow motor control in quadrants I and IV. They are known as Fully Controlled Rectifiers.
For low power applications (up to around 10 kW) single-phase rectifier drives are employed. For high power applications, three-phase rectifier drives are used. Exception is made in traction where single phase drives are employed for large power ratings.