Direct Current Drives

Although generally more expensive and less reliable than AC induction motors, DC motors have advantages with many applications. DC motors produce high starting torque and offer more precise speed control down to 1 percent of the motor's nominal speed.

The primary weakness of the DC motor is its commutation subsystem. Friction and arcing wear-out the brushes and copper blade terminals. For this reason, DC motors require frequent maintenance and are not suitable for use in explosive or corrosive environments. In addition, because of the complexity of the rotor, DC motors are substantially more expensive than induction motors. DC motors have traditionally been used in applications where high starting torque is required, such as traction devices and cranes, or where accurate speed control is needed, such as rolling mills, lathes, paper machines, and winders.

Using DC motors for speed control requires the conversion of available AC power to DC. Historically, this was done using either a motor-generator set or a rectifier.

And Furthermore . . .