DIRECT CURRENT (DC) GENERATORS BASIC AND TUTORIALS

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 Of DC Generator.

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 commutating poles between the main poles.

Construction and Size.

Small dc machines have large surface-to-volume ratios and short paths for heat to reach dissipating surfaces. Cooling requires little more than means to blow air over the rotor and between the poles. Rotor punchings are mounted solidly on the shaft, with no air passages through them.

Larger units, with longer, deeper cores, use the same construction, but with longitudinal holes through the core punchings for cooling air. Medium and large machines must have large heat-dissipation surfaces and effectively placed cooling air, or “hot spots” will develop.

Their core punchings are mounted on arms to permit large volumes of cool air to reach the many core ventilation ducts and also the ventilation spaces between the coil end extensions.

Design Components.

Armature-core punchings are usually of high-permeability electrical sheet steel, 0.017 to 0.025 in thick, and have an insulating film between them. Small and medium units use “doughnut” circular punchings, but large units, above about 45 inches in diameter, use segmental punchings shaped as shown in Fig. 8-2, which also shows the fingers used to form the ventilating ducts.

Main- and commutating-pole punchings are usually thicker than rotor punchings because only the pole faces are subjected to highfrequency flux changes. These range from 0.062 to 0.125 in thick, and they are normally riveted.

The frame yoke is usually made from rolled mild steel plate, but, on high-demand large generators for rapidly changing loads, laminations may be used. The solid frame has a magnetic time constant of 1/2 s or more, depending on the frame thickness. The laminated frame ranges from 0.05 to 0.005 s.

The commutator is truly the heart of the dc machine. It must operate with temperature variations of at least 55 DEG C and with peripheral speeds that may reach 7000 ft/min. Yet it must remain smooth concentrically within 0.002 to 0.003 in and true, bar to bar, within about 0.0001 in.

The commutator is made up of hard copper bars drawn accurately in a wedge shape. These are separated from each other by mica plate segments, whose thicknesses must be held accurately for nearly perfect indexing of the bars and for no skew.

This thickness is 0.020 to 0.050 in, depending on the size of the generator and on the maximum voltage that can be expected between bars during operation. The mica segments and bars are clamped between two metal V-rings and insulated from them by cones of mica.

On very high speed commutators of about 10,000 ft/min, shrink rings of steel are used to hold the bars. Mica is used under the rings.

Carbon brushes ride on the commutator bars and carry the load current from the rotor coils to the external circuit. The brush holders hold the brushes against the commutator surface by springs to maintain a fairly constant pressure and smooth riding.