Controlled Rectifier Fed DC Drives:

Controlled Rectifier Fed DC Drives are used to get variable dc voltage from an ac source of fixed voltage. Controlled Rectifier Fed DC Drives are also known as Static Ward-Leonard drives.

Figure 5.25 shows commonly used Controlled Rectifier Fed DC Drives and quadrants in which they can operate on V_a-I_a 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.

Rectifiers of Figs. 5.25(b) and (d) are called Half Controlled Rectifiers as they allow dc voltage control only in one direction and motor control in quadrant I only.

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.