Direct Current (DC) System

Motor used in Direct Current System –

1. Dc Series Motor
2. Dc Compound Motor (as regenerative braking is difficult for Dc series motor)

• Dc series motor has high starting torque and easy speed control.
• Size of Dc series motor is less than A.C. series motor of same HP.
• High acceleration (Quick starting) and high retardation (Quick stopping). Hence, this system is used in urban. (Cities, developed areas) and sub. Urban (area near city) areas.

Voltage used in Direct Current System –

• 600V to 750V Dc for suburban lines
• 1500V to 3000V Dc for urban and Main line.

3 phase supply (at 11 KV) comes to substation and it is first step-down to 600 V to 3000 V (as requirement) and then it is converted into DC by using rectifier.

Rectifier used in Direct Current System –

• Mercury arc rectifiers— (old and outdated)
• Using diodes, SCR etc. modern electronic devices (Bridge rectifier, centre tap etc.).