5. AC Vs DC
The relative advantages and disadvantages of AC and DC generators relate to two features of their design: DC generators use a split-ring commutator, while AC generators use slip rings; and in DC generators the output current is induced in the rotor, whereas the roles of the rotor and the stator can be reversed in an AC generator.
The commutator of a DC generator consists of a number of metal bars separated by narrow gaps filled with insulating material. As the brushes remain in contact with the commutator under spring pressure, they are constantly striking the leading edge of each successive bar. This wears the brushes and they need to be replaced regularly. The commutator bars also wear down until the insulating material between them prevents the brushes from making proper contact with the bars, reducing the efficiency of the generator. Pieces of metal worn from the commutator bars can become lodged in the gaps, causing a short between bars and reducing the output of the generator.
In contrast, the slip rings of an AC generator have continuous, smooth surfaces, allowing the brushes to remain continuously in contact with the slip ring surface. Thus the brushes in an AC generator do not wear as fast as in a DC generator. There is no possibility of creating an electrical short circuit between segments in an alternator because the slip rings are already continuous. An AC generator therefore requires less maintenance and is more reliable than a DC generator. Most commercial generators are AC generators.
In a DC generator the current is generated in the rotor and is then drawn from the windings through the commutator and out via the brushes. The larger the current required, the heavier the rotor coils must be, placing high demands on bearings and supporting structures. In addition, drawing large currents through the commutator-brush connection increases the likelihood of electric arcs forming as the brush breaks contact with each bar in turn. This reduces the efficiency of the generator and creates radio “noise”. This limits the usefulness of DC generators to relatively low current applications.
In an AC generator designed for high current applications, such as in a power station, the current is produced in the stator windings rather than in the rotor. The rotor is used to create the field magnetization that induces the AC current in the stator when the rotor is rotated. It is much easier to draw the current through a fixed connection in the stator rather than through a commutator from a moving rotor. Thus AC generators are better suited to high current demands than DC generators.
An advantage of AC generators is that they can easily be designed to produce three-phase electricity by the use of six stator poles and a single electromagnet rotor. The coils are mounted in opposing pairs spaced evenly around the stator, and connected in pairs to the three phases of the power supply. The rotor induces alternating current in successive pole pairs. The sinusoidally varying voltages are then 120 degrees out of phase with each other. AC generators are ideal for generating electricity on a large scale for distribution over a wide area.
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