HSC PHYSICS > MOTORS AND GENERATORS| AC & DC
Gather secondary information to discuss the advantages and disadvantages of AC and DC generators and relate these to their use.
The slip rings of an AC generator have a continuous surface that allows the brushes to remain in contact with the ring’s surface. Thus the brushes in an AC generator do not wear out as fast as in a DC generator as they do not create an electric short circuit every half-turn. Therefore they required less maintenance and is more reliable that DC generators. AC’s voltage can be changed by transformers. It is more readily transformed to different voltages than DC. Because of this property, AC can travel longer distances on high voltage (low current) with minimal power loss. Transformers can step-down the voltage to the adequate amount to be used in homes and buildings.
Power is lost through heat on transmission lines due to inductance and resistance which causes voltage drops. To minimise power loss, long distance transmission is run on high voltage (low current).
Applications of AC
a a a a Most commercial motors are AC motors. In the 1960s, solid state diodes enabled AC to be converted to DC. AC generators (aka alternators) have replaced DC in in cars. Automotive alternators have advantages over DC generators as they use slip ring which provides an extended brush life over a split-ring commutator. Alternators allow the current to be changed to DC when needed.
An advantage of a DC generator is that its output can be made smoother by the arranging many coils in a regular pattern around the armature. The brushes are arranged to make contact only with the commutator bars corresponding to the coils producing the greatest emf at a particular time. The result is an output voltage that “ripples” about a mean value rather than fluctuating between zero and the maximum twice per revolution. The more... [continues]
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