Q. Explain about Direct-current machines?
Generally speaking, conventional dc generators are becoming obsolete and increasingly often are being replaced by solid-state rectifiers in most applications for which an ac supply is available. The same is not true for dc motors. The torque-speed characteristics of dc motors are what makes them extremely valuable in many industrial applications. The significant features of the dc drives include adjustable motor speed over wide ranges, constant mechanical power output or torque, rapid acceleration or deceleration, and responsiveness to feedback signals.
The dc commutator machines are built in a wide range of sizes, from small control devices with a 1-W power rating up to the enormous motors of 10,000 hp or more used in rolling-mill applications. The dcmachines today are principally applied as industrial drivemotors, particularly when high degrees of flexibility in controlling speed and torque are required. Such motors are used in steel and aluminumrollingmills, tractionmotors, overhead cranes, forklift trucks, electric trains, and golf carts.Commutatormachines are also used in portable tools supplied frombatteries, in automobiles as starter motors, in blower motors, and in control applications as actuators and speed-sensing or position-sensing devices.