Advantages and Disadvantages of Ad and Dc

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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.

AC generators

Advantages
 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.

Disadvantages
 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.

DC generators

Advantages
 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 coils, the smoother the output DC voltage ripple.  This is an advantage for use with equipment that needs a steady voltage rather than a sinusoidally varying voltage like in AC.  DC generators do not cause losses through magnetic induction.

Disadvantages
 DC’s voltages cannot be altered.  Because of this, DC electricity for distribution could only be generated at the voltages at which it was used by customers in homes and buildings. This means that DC need to have a great current to distribute to cities.  As power loss is given by the equation , an increase in current would result in an exponential increase in energy loss through heat in the conductors. The large current needed for DC distribution would result in excessive energy loss. Thus, many power stations and an unattractive abundance of wires to carry the required current were needed in order to supply DC electricity to a large city.

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HSC PHYSICS > MOTORS AND GENERATORS| AC & DC

 DC generators require a split-ring commutator that wears out quicker than AC’s slip ring commutator as each disconnection between the brushes and commutator creates an electric short circuit every half-turn.  The high maintenance is a major disadvantage as DC generators wear out quicker and require regular replacement.  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.

Applications of DC
d d DC has a low voltage (normally 12V, 24V) which prevents the risk of electrocution. Therefore it’s commonly used in low-voltage hand help appliances, especially batteryoperated ones (as batteries run on DC) – such as cameras, cell phones, torches, children’s toys and other small appliances. Due to DC’s high power loss, it is typically used in appliances that run on a short circuit – such as battery operated lawn-mowers.

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HSC PHYSICS > MOTORS AND GENERATORS| AC & DC

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