8.1 THE DC MACHINE

Applications. The most important role played by the dc generator is the power supply for the important dc motor. It supplies essentially ripple-free power and precisely held voltage at any desired value from zero to rated. This is truly dc power, and it permits the best possible commutation on the motor because it is free of the severe waveshapes of "dc" power from rectifiers. It has excellent response and is particularly suitable for precise output control by feedback control regulators. It is also well suited for supplying accurately controlled and responsive excitation power for both ac and dc machines.

The dc motor plays an ever-increasing vital part in modern industry, because it can operate at and maintain accurately any speed from zero to its top rating. For example, high-speed multistand steel mills for thin steel would not be possible without dc motors. Each stand must be held precisely at an exact speed which is higher than that of the preceding stand to suit the reduction in thickness of the steel in that stand and to maintain the proper tension in the steel between stands.

General Construction. Figure 8-1 shows the parts of a medium or large dc generator. All sizes differ from ac machines in having a commutator and the armature on the rotor. They also have salient poles on the stator, and, except for a few small ones, they have...