aim of this is to present a fair comparison between two systems, Direct Current
(DC) and Alternating Current (AC) efficiency in the residential homes. AC and
DC clashed with each other in the early days of the electric power systems.
Apparently, it was the ability to transform voltage levels, which would cause
one side to win or the other. If DC systems could have developed this ability,
the power system might have been DC today. The electromagnetic transformers
allowed AC to transform its voltage level and thus AC. won the battle of the
currents. It became the medium for electric power generation, transmission,
distribution and utilization in the form of residential loads.
useful generators of electricity come in two basic forms, alternating current
and direct current. Direct current (dc) comes from generators that do not
change in polarity, always producing a positive charge. In alternating current
(ac) the polarity of the terminals is always changing from positive to
negative. Thus, alternating current flow is left.
current only flows in one direction in a circuit. Because the polarity of a
direct current voltage source is always the same, the flow of current never
changes direction. Batteries are one of the more common direct current voltage
sources. Batteries are good because their voltage is fixed as well as their
polarity. Direct current does not always need to a constant voltage but it must
always stay traveling in the same direction. There are such direct currents
called varying and pulsating that change value but not direction. DC had to
wait for a long time until the development of High Voltage Direct Current
(HVDC) transmission re-introduced DC in the power system. Heavy electric
currents due to line charging/discharging and reactive power losses were
avoided and HVDC transmission became a success. Then DC appeared on the
generation side of the power system in the form of the economically and
environmentally motivated power generation sources namely the renewable energy
had shown that it was much better for transmitting electricity over long
distances. Championed in the last 2 decades of the 19th century by inventors
and theoreticians such as Nikola Tesla and Charles Steinmetz and the
entrepreneur George Westinghouse, AC won out as the dominant power supply
current was chosen early in the 20th century as the North American standard
because it presented fewer risks and promised higher reliability than competing
DC systems of the day. DC is the European standard. Many of DC’s deficiencies were
later corrected, but not until a substantial North American infrastructure had
already been developed.
represented as two wires leading to a device that uses electricity, electric
power distribution requires a circuit. One wire is negative and the other is
either is positive or neutral (ground) in AC current. The two wires take turns
at sending electricity. AC current uses a standard “rhythm” in which
each side gets its turn 60 times each second, thus the 60Hz designation given
to standard AC current in North America. This switching of polarity takes the
form of a rhythmic pulse in the electrical current that occurs within the
normal audible range. This is why you can actually hear this rhythm in circuits
such as fluorescent lighting ballasts and audio equipment as a low buzzing
tone. This buzz is referred to as “sixty cycle hum”. Two AC power
schemes were used in North America prior to the 1970s. One offered energy at
45-50Hz, the other at 60Hz. “Fifty-cycle power”, occasionally
referred to as “rural power”, is now obsolete and the 60Hz standard
is now used throughout North America.
electricity is always the same polarity, which means that in a two-wire
circuit, one “wire”, or side of the circuit, is always negative, and
the negative side is always the one that sends the electricity in DC circuits.
There is no hum because there is no cyclic change in current flow. The cost of
converting DC current to AC is relatively high, so DC is typically
cost-effective only for long-distance transmission but DC current is more
effective for long-distance, high-voltage transmission because it results in
less energy lost in transmission.
devices that convert electricity directly into other forms of energy can
operate just as effectively from AC current as from DC. Light bulbs and heating
elements don’t care whether their energy is supplied by AC or DC current.
Nearly all modern electronic devices require direct current for their
operation, however. Alternating current is still used to deliver electricity to
the device, and usually at much, lower than the supplied voltage so that
electronic devices can use it a transformer is included with these devices to
convert AC power to DC power.
that constantly changes in amplitude, and which reverses direction at regular
intervals is alternating current. You learned previously that direct current
flows only in one direction, and that the amplitude of current is determined by
the number of electrons flowing past a point in a circuit in one second. The
amplitude of direct current in the wire is one ampere if, for example, a
coulomb of electrons moves past a point in a wire in one second and all of the
electrons are moving in the same direction. If half a coulomb of electrons
moves in one direction past a point in the wire in half a second, then reverses
direction and moves past the same point in the opposite direction during the
next half-second, a total of one coulomb of electrons passes the point in one
second, similarly. The amplitude of the alternating current is one ampere.
disadvantages in using direct current in the home became apparent when
commercial use of electricity became widespread in the United States. The
voltage must be generated at the level (amplitude or value) required by the
load if a commercial direct-current system is used. The dc generator must
deliver 240 volts to properly light a 240-volt lamp, for example. A resistor or
another 120-volt lamp must be placed in series with the 120-volt lamp to drop
the extra 120 volts if a 120-volt lamp is to be supplied power from the
240-volt generator. An amount of power equal to that consumed by the lamp is
wasted when the resistor is used to reduce the voltage.
the direct current (I) from the generating station must be transmitted, a long
distance over wires to the consumer another disadvantage of the direct-current
system becomes evident. A large amount of power is lost due to the resistance
(R) of the wire when this happens. The power loss is equal to I2R. However,
this loss can be greatly reduced if the power is transmitted over the lines at
a very high voltage level and a low current level. This is not a practical
solution to the power loss in the dc system since the load would then have to
be operated at a dangerously high voltage. Practically all modern commercial
electric power companies generate and distribute alternating current (ac)
because of the disadvantages related to transmitting and using direct current.
voltages can be stepped up or down in amplitude by a device called a
TRANSFORMER, unlike direct voltages. Use
of the transformer permits efficient transmission of electrical power over
long-distance lines. The transformer output power is at high voltage and low
current level at the electrical power station. The voltage is stepped down by a
transformer to the value required by the load at the consumer end of the
transmission lines. Alternating current has replaced direct current in all but
a few commercial power distribution systems due to its inherent advantages and
now know that there are two types of current and voltage, that is, direct
current and voltage and alternating current and voltage. The dc voltage has constant
amplitude. Some voltages go through periodic changes in amplitude. The pattern,
which results when these changes in amplitude with respect to time, is known as
current is more superior to direct current. The main reason for this is that it
can be controlled in terms of voltage by the use of a transformer. As we have
seen, alternating current is more common and useful, in electromagnets, as well
as home appliances.
has been one of the major factors used to judge if DC is better than AC. to present
a comparative efficiency study of AC and DC residential power distribution
systems. The DC power transfer, although, given up a long time ago; is
witnessing a comeback in the system and for the particular case of residential
power distribution, its efficiency was found comparable to that of AC. However,
if AC power has to be given up in favor of DC, then DC should not only match
the feasibility of AC, it should exceed this to provide a strong reasoning for
making this huge change in the power system. At the present, this doesn’t
appear to be the case as far as system efficiency is concerned. Even the
increase of DC power demand in buildings via the use of air-conditioning was
found to make a small contribution to the overall system performance. From the
point of view of efficiency, an increase in demand of DC power may not be a
justification/suggestion for opting DC power distribution as long as energy has
to flow through power electronic converters.
the residential side, the tremendous increase of modern electronic loads has
created a significant demand for DC power. Besides the usual household
electronic loads, the modern concept of Light Emitting Diodes (LED) lighting is
creating yet another consumer of DC electric energy. Apart from these, the Variable
Speed Drives based air-conditioning (cooling and heating) leads to the
conversion of the input AC power to DC, which is then again converted to AC and
supplied to the compressor motor. If these loads are also included as loads
demanding DC power, then the overall demand of DC power may exceed the demand
of AC power for the modern residential buildings.
best current is to use is both AC and DC current. Some devices perform better
and have better efficiency with AC current and other devices perform better
with DC current then AC current would. Nevertheless, for right now, AC current is
in the Lead and DC current is catching up.
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