   #copyright

Alternating current

2007 Schools Wikipedia Selection. Related subjects: Engineering

   City lights viewed in a motion blurred exposure. The AC blinking causes
   the lines to be dotted rather than continuous.
   Enlarge
   City lights viewed in a motion blurred exposure. The AC blinking causes
   the lines to be dotted rather than continuous.

   An alternating current (AC) is an electrical current whose magnitude
   and direction vary cyclically, as opposed to direct current, whose
   direction remains constant. The usual waveform of an AC power circuit
   is a sine wave, as this results in the most efficient transmission of
   energy. However in certain applications different waveforms are used,
   such as triangular or square waves.

   Used generically, AC refers to the form in which electricity is
   delivered to businesses and residences. However, audio and radio
   signals carried on electrical wire are also examples of alternating
   current. In these applications, an important goal is often the recovery
   of information encoded (or modulated) onto the AC signal.

History

   William Stanley, Jr. designed one of the first practical devices to
   transfer AC power efficiently between isolated circuits. Using pairs of
   coils wound on a common iron core, his design, called an induction
   coil, was an early precursor of the modern transformer. The system used
   today was devised by many contributors including Nikola Tesla, George
   Westinghouse, Lucien Gaulard, John Dixon Gibbs, and Oliver Shallenger
   from 1881 to 1889. AC systems overcame the limitations of the direct
   current system used by Thomas Edison to distribute electricity.

   The first long-distance transmission of alternating current took place
   in 1891 near Telluride, Colorado, followed a few months later in
   Germany. Thomas Edison strongly advocated the use of direct current
   (DC), having many patents in that technology, but eventually
   alternating current came into general use (see War of Currents).

   The first modern commercial power plant using three-phase alternating
   current was at the Mill Creek hydroelectric plant near Redlands,
   California in 1893. Its designer was Almirian Decker, a brilliant young
   engineer. Decker's innovative design incorporated 10,000 volt three
   phase transmission and established the standards for the complete
   system of generation, transmission and motors used today. And through
   the use of alternating current, Charles Proteus Steinmetz of General
   Electric was able to solve many of the problems associated with
   electricity generation and transmission.

Transmission, distribution, and domestic power supply

   AC voltage can be stepped up or down by a transformer to a different
   voltage. Modern High-voltage, direct current electric power
   transmission systems contrast with the more common alternating-current
   systems as a means for the bulk transmission of electrical power over
   long distances. However, these tend to be more expensive and less
   efficient than transformers, and did not exist when Edison,
   Westinghouse and Tesla were designing their power systems.

   Use of a higher voltage leads to significantly more efficient
   transmission of power. The power losses in a conductor are a product of
   the square of the current and the resistance of the conductor,
   described by the formula P=I^2 \cdot R \,\! . This means that when
   transmitting a fixed power on a given wire, if the current is doubled,
   the power loss will be four times greater. Since the power transmitted
   is equal to the product of the current, the voltage and the cosine of
   the phase difference φ (P = IVcosφ), the same amount of power can be
   transmitted with a lower current by increasing the voltage. Therefore
   it is advantageous when transmitting large amounts of power to
   distribute the power with high voltages (often hundreds of kilovolts).
   However, high voltages also have disadvantages, the main ones being the
   increased insulation required, and generally increased difficulty in
   their safe handling. In a power plant, power is generated at a
   convenient voltage for the design of a generator, and then stepped up
   to a high voltage for transmission. Near the loads, the transmission
   voltage is stepped down to the voltages used by equipment. Consumer
   voltages vary depending on the country and size of load, but generally
   motors and lighting are built to use up to a few hundred volts between
   phases.

   Three-phase electrical generation is very common. Three separate coils
   in the generator stator are physically offset by an angle of 120° to
   each other. Three current waveforms are produced that are equal in
   magnitude and 120° out of phase to each other.

   If the load on a three-phase system is balanced equally between the
   phases, no current flows through the neutral point. Even in the
   worst-case unbalanced (linear) load, the neutral current will not
   exceed the highest of the phase currents. For three-phase at low
   (normal mains) voltages a four-wire system is normally used. When
   stepping down three-phase, a transformer with a Delta primary and a
   Star secondary is often used so there is no need for a neutral on the
   supply side.

   For smaller customers (just how small varies by country and age of the
   installation) only a single phase and the neutral or two phases and the
   neutral are taken to the property. For larger installations all three
   phases and the neutral are taken to the main distribution panel. From
   the three-phase main panel, both single and three-phase circuits may
   lead off.

   Three-wire single phase systems, with a single centre-tapped
   transformer giving two live conductors, is a common distribution scheme
   for residential and small commercial buildings in North America. A
   similar method is used for a different reason on construction sites in
   the UK. Small power tools and lighting are supposed to be supplied by a
   local centre-tapped transformer with a voltage of 55V between each
   power conductor and the earth. This significantly reduces the risk of
   electric shock in the event that one of the live conductors becomes
   exposed through an equipment fault whilst still allowing a reasonable
   voltage for running the tools.

   A third wire is often connected between non-current carrying metal
   enclosures and earth ground. This conductor provides protection from
   electrical shock due to accidental contact of circuit conductors with
   the case of portable appliances and tool.

AC power supply frequencies

   The frequency of the electrical system varies by country; most electric
   power is generated at either 50 or 60 Hz. See List of countries with
   mains power plugs, voltages and frequencies. Some countries have a
   mixture of 50 Hz and 60 Hz supplies, notably Japan.

   A low frequency eases the design of low speed electric motors,
   particularly for hoisting, crushing and rolling applications, and
   commutator-type traction motors for applications such as railways, but
   also causes a noticeable flicker in incandescent lighting and
   objectionable flicker of fluorescent lamps. 16.7 Hz power (approx. ⅓ of
   the mains frequency) is still used in some European rail systems, such
   as in Austria, Germany, Norway, Sweden and Switzerland.

   Off-shore, textile industry, marine, computer mainframe, aircraft, and
   spacecraft applications sometimes use 400 Hz, for benefits of reduced
   weight of apparatus or higher motor speeds.

Effects at high frequencies

   A direct, constant, current flows uniformly throughout the
   cross-section of the (uniform) wire that carries it. With alternating
   current of any frequency, the current is forced towards the outer
   surface of the wire, and away from the centre. This is due to the fact
   that an electric charge which accelerates (as is the case of an
   alternating current) radiates electromagnetic waves, and materials of
   high conductivity (the metal which makes up the wire) do not allow
   propagation of electromagnetic waves. This phenomenon is called skin
   effect.

   At very high frequencies the current no longer flows in the wire, but
   effectively flows on the surface of the wire, within a thickness of a
   few skin depths. The skin depth is the thickness at which the current
   density is reduced by 63%. Even at relatively low frequencies used for
   high power transmission (50–60 Hz), non-uniform distribution of current
   still occurs in sufficiently thick conductors. For example, the skin
   depth of a copper conductor is approximately 8.57 mm at 60 Hz, so high
   current conductors are usually hollow to reduce their mass and cost.

   Since the current tends to flow in the periphery of conductors, the
   effective cross-section of the conductor is reduced. This increases the
   effective AC resistance of the conductor, since resistance is inversely
   proportional to the cross-sectional area in which the current actually
   flows. The AC resistance often is many times higher than the DC
   resistance, causing a much higher energy loss due to ohmic heating
   (also called I^2R loss).

Techniques for reducing AC resistance

   For low to medium frequencies, conductors can be divided into stranded
   wires, each insulated from one other, and the individual strands
   specially arranged to change their relative position within the
   conductor bundle. Wire constructed using this technique is called Litz
   wire. This measure helps to partially mitigate skin effect by forcing
   more equal current flow throughout the total cross section of the
   stranded conductors. Litz wire is used for making high Q inductors,
   reducing losses in flexible conductors carrying very high currents at
   power frequencies, and in the windings of devices carrying higher radio
   frequency current (up to hundreds of kilohertz), such as switch-mode
   power supplies and radio frequency transformers.

Techniques for reducing radiation loss

   As written above, an alternating current is made of electric charge
   under periodic acceleration, which causes radiation of electromagnetic
   waves. Energy that is radiated represents a loss. Depending on the
   frequency, different techniques are used to minimize the loss due to
   radiation.

Twisted pairs

   At frequencies up to about 1 GHz, wires are paired together in cabling
   to form a twisted pair in order to reduce losses due to electromagnetic
   radiation and inductive coupling. A twisted pair must be used with a
   balanced signalling system, where the two wires carry equal but
   opposite currents. The result is that each wire in the twisted pair
   radiates a signal that is effectively cancelled by the other wire,
   resulting in almost no electromagnetic radiation.

Coax cables

   At frequencies above 1 GHz, unshielded wires of practical dimensions
   lose too much energy to radiation, so coaxial cables are used instead.
   A coaxial cable has a conductive wire inside a conductive tube. The
   current flowing on the inner conductor is equal and opposite to the
   current flowing on the inner surface of the outer tube. This causes the
   electromagnetic field to be completely contained within the tube, and
   (ideally) no energy is radiated or coupled outside the tube. Coaxial
   cables have acceptably small losses for frequencies up to about 20 GHz.
   For microwave frequencies greater than 20 GHz, the dielectric losses
   (due mainly to the dissipation factor of the dielectric layer which
   separates the inner wire from the outer tube) become too large, making
   waveguides a more efficient medium for transmitting energy.

Waveguides

   Waveguides are similar to coax cables, as both consist of tubes, with
   the biggest difference being that the waveguide has no inner conductor.
   Waveguides can have any arbitrary cross section, but rectangular cross
   section are the most common. With waveguides, the energy is no longer
   carried by an electric current, but by a guided electromagnetic field.
   Waveguides have dimensions comparable to the wavelength of the
   alternating current to be transmitted, so are only feasible at
   microwave frequencies.

Fibre optics

   At frequencies greater than 200 GHz, waveguide dimensions become
   impractically too small, and the ohmic losses in the waveguide walls
   become large. Instead, fibre optics, which are a form of dielectric
   waveguides, can be used. For such frequencies, the concepts of voltages
   and currents are no longer used.

Mathematics of AC voltages

   A sine wave, over one cycle (360°). The dashed line represents the root
   mean square (RMS) value
   Enlarge
   A sine wave, over one cycle (360°). The dashed line represents the root
   mean square (RMS) value

   Alternating currents are accompanied by alternating voltages. An AC
   voltage v can be described mathematically as a function of time by the
   following equation:

          v(t)=V_\mathrm{peak}\cdot\sin(\omega t) ,

   where
     * V[peak] is the peak voltage (unit: volt),
     * ω is the angular frequency (unit: radians per second)
          + The angular frequency is related to the physical frequency, f,
            which represents the number of oscilations per second (unit =
            hertz), by the equation ω = 2\,\pi\, f .
     * t is the time (unit: second).

   The peak-to-peak value of an AC voltage is defined as the difference
   between its positive peak and its negative peak. Since the maximum
   value of sin(x) is +1 and the minimum value is −1, an AC voltage swings
   between +V[peak] and −V[peak]. The peak-to-peak voltage, usually
   written as V[pp] or V[P-P], is therefore (+V[peak]) − (−V[peak]) = 2 ×
   V[peak].

   AC voltage is usually expressed as a root mean square (RMS) value,
   written V[rms]. For a sinusoidal voltage:

          V_\mathrm{rms}=\frac{V_\mathrm{peak}}{\sqrt{2}}

   V[rms] is useful in calculating the power consumed by a load. If a DC
   voltage of V[DC] delivers a certain power P into a given load, then an
   AC voltage of V[peak] will deliver the same average power P into the
   same load if V[rms] = V[DC]. Because of this fact, RMS is the normal
   means of measuring AC voltage.

Example

   To illustrate these concepts, consider a 240 V AC mains supply. It is
   so called because its RMS value is (at least nominally) 240 V. This
   means that it has the same heating effect as 240 V DC. To work out its
   peak voltage (amplitude), we can modify the above equation to:

          V_\mathrm{peak}=\sqrt{2}\ V_\mathrm{rms}

   For our 240 V AC, the peak voltage V[peak] is therefore 240 V × √2,
   which is about 339 V. The peak-to-peak value V[P-P] of the 240 V AC
   mains is even higher: 2 × 240 V × √2, or about 679 V.

   Note that non-sinusoidal waveforms have a different relationship
   between their peak magnitude and effective (RMS) value. This is of
   practical significance when working with non-linear circuit elements
   that produce harmonic currents, such as rectifiers.

   The European Union (including the UK) has now officially harmonized on
   a supply of 230 V 50 Hz. However, it made the tolerance bands very wide
   at ±10%. Some countries actually specify stricter standards than this;
   for example, the UK specifies 230 V +10% −6%. Most supplies to the old
   standards therefore conform to the new one and do not need to be
   changed.

   Retrieved from " http://en.wikipedia.org/wiki/Alternating_current"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
