Electrical Bus

Rectification is the only step required for dc drives.Therefore, they have the advantage of relatively simple control sys-tems. Compared with ac drive systems, the dc drive offers a widerspeed range and higher starting torque. However, purchase and main-tenance costs for dc motors are high, while the cost of power electronicdevices has been dropping year after year. Thus, economic considera-tions limit use of the dc drive to applications that require the speed andtorque characteristics of the dc motor.Most dc drives use the six-pulse rectifier shown in Fig Largedrives may employ a 12-pulse rectifier. This reduces thyristor current

duties and reduces some of the larger ac current harmonics. The two largest harmonic currents for the six-pulse drive are the fifth and seventh. They are also the most troublesome in terms of system response. A 12-pulse rectifier in this application can be expected to eliminate about 90 percent of the fifth and seventh harmonics, depending on system imbalances. The disadvantages of the 12-pulse drive are that there is more cost in electronics and another transformer is generally required.