Alternating current

Alternating current (AC) is an electric current in which the direction of flow and the magnitude of the current vary periodically with time. This contrasts with direct current (DC), in which electric charge flows steadily in a single direction. AC is the standard form in which electrical power is generated, transmitted and supplied to homes, offices, industries and public facilities. When appliances such as televisions, kitchen equipment, fans and lamps are plugged into wall sockets, they are supplied with alternating current. The abbreviations AC and DC are also used in phrases such as AC voltage and DC voltage to distinguish between voltages that vary with time and those that remain essentially constant.

Definition and basic characteristics

In most power systems the instantaneous value of alternating current follows a sine wave. In such a waveform:

• The positive half-cycle corresponds to current flowing in one direction.
• The negative half-cycle corresponds to current flowing in the opposite direction.
• One complete sequence of a positive half and a negative half is called a cycle.
• The number of cycles per second is the frequency, measured in hertz (Hz).

In a sinusoidal system, the average value of current over a full cycle is zero, but the root-mean-square (r.m.s.) value is used to represent the effective heating or power-delivering capability of the current. Rated domestic supply voltages, such as 230 V in many countries, refer to this r.m.s. value.
Although “alternating current” most commonly refers to power distribution at mains frequency, many other electrical signals are technically alternating, including audio-frequency and radio-frequency currents, in which the direction of current reverses at much higher frequencies.

Waveforms and signal applications

The standard waveform for power distribution is a sinusoidal wave because it is easy to generate with rotating machines, and it leads to simpler design of transformers and AC machines. However, other waveforms are widely used in electronics and specialised power applications, such as:

• Square waves – widely used in digital electronics, switching power supplies and some low-cost inverters.
• Triangle and sawtooth waves – used in signal generators, audio synthesis and certain control circuits.

Audio and radio signals carried on wires or through space are also alternating currents. These signals often carry information (sound, video, data) by modulation of a higher-frequency carrier AC signal in amplitude, frequency or phase.
In these applications, the purpose of the alternating current is not primarily to deliver power, but to convey information. The frequencies involved are typically far higher than those used in power systems; for example, audio signals range from about 20 Hz to 20 kHz, and radio signals extend from kilohertz into gigahertz.

Transmission, distribution and domestic power supply

Electrical energy is distributed as alternating current chiefly because AC voltages can be increased or decreased easily using transformers. This property makes long-distance transmission of power efficient and practicable.
The power loss in a transmission line is given approximately byP₍loss₎ = I²R,where I is the current and R is the resistance of the conductor. For a given amount of power P = VI (assuming negligible phase difference between current and voltage), transmitting at a higher voltage allows the same power to be delivered with a lower current, reducing I²R losses in the lines. For example, if the transmission voltage is doubled for the same power, the current is halved and the resistive loss becomes one quarter.
In a typical power system:

• Power is generated in a power station at a convenient voltage for the generator design.
• Step-up transformers increase this voltage to tens or hundreds of kilovolts for long-distance transmission on overhead lines or underground cables.
• Near consumption areas, step-down transformers reduce the voltage to distribution levels (for example, tens of kilovolts).
• Further local transformation brings the voltage down to utilisation levels, usually between about 100 V and 240 V for domestic and commercial use, depending on the national standard.

High voltages, however, bring disadvantages, such as:

• The need for more robust insulation and larger clearances.
• Increased complexity in switchgear and safety procedures.
• Greater risk in the event of insulation failure.

Despite these challenges, the efficiency gains justify the use of high-voltage AC transmission. In parallel, high-voltage direct current (HVDC) systems have become increasingly viable for very long distances and submarine cables, where their particular advantages outweigh the difficulty and cost of converting between AC and DC.

Three-phase and split-phase systems

For large-scale generation, transmission and industrial loads, three-phase AC systems are standard. A three-phase generator typically has three identical coils in the stator, mechanically spaced by 120° around the rotor. As the magnetic field rotates, each coil produces a sinusoidal voltage equal in magnitude but displaced in phase by 120° from the others. The resulting system has three line voltages and currents, each phase carrying power.
Advantages of three-phase systems include:

• Smoother and more constant power transfer than single phase.
• More efficient use of conductors for a given total power.
• The ability to drive three-phase motors that are self-starting and have good torque characteristics.

In large generators, more poles may be used. For instance, a 2-pole machine running at 3,600 rpm and a 12-pole machine running at 600 rpm can both generate 60 Hz. For larger machines, lower rotational speed is preferable for mechanical reasons, so higher pole numbers are common.
If the load across the three phases is perfectly balanced, the sum of the instantaneous currents is zero and no current flows in the neutral conductor. Even with unbalanced linear loads, the neutral current does not exceed the highest phase current. However, non-linear loads, such as switch-mode power supplies, can generate significant harmonic currents, especially in the third harmonic and its multiples, which add in the neutral. In such cases, the neutral conductor may experience currents exceeding those in the phase conductors, so it is sometimes oversized in modern installations.
For low-power customers, supply arrangements vary:

• Many domestic customers receive single-phase plus neutral from a three-phase distribution network.
• Larger consumers, such as industrial facilities, receive all three phases (and often neutral) so that both three-phase and single-phase loads can be supplied.

A related system is split-phase supply, common in North American residential distribution. A single centre-tapped transformer secondary winding provides two live conductors and a neutral. The voltage between either live and neutral is typically 120 V, while the voltage between the two lives is 240 V. This arrangement allows both low-voltage lighting and higher-voltage loads (such as ovens and air-conditioners) to be supplied efficiently from the same service.
In the United Kingdom and some other regions, construction sites often use local centre-tapped transformers providing 110 V between the two live conductors and 55 V to earth on each conductor. This reduces the risk of electric shock from accidental contact with one live conductor while still providing adequate voltage between the two for portable tools.

Earthing, neutral and protection

In AC systems, earthing (grounding) and bonding are critical safety measures. A protective earth conductor is connected to non-current-carrying metal parts such as enclosures, frames and tool casings. Its purposes are to:

• Provide a low-impedance path for fault current if a live conductor accidentally contacts exposed metal.
• Ensure that protective devices (fuses, circuit-breakers, residual current devices) operate rapidly to disconnect the supply.
• Keep exposed conductive parts at or near earth potential, reducing the risk of electric shock.

All earth and bonding conductors are usually connected to a common earth point at the main service panel. The neutral conductor is also bonded to earth at this point in many systems, so that the neutral potential remains close to earth potential. This arrangement stabilises the system voltages with respect to earth and ensures that faults involving live conductors and earthed metal cause high enough currents to clear protective devices quickly.

AC power supply frequencies

The utility frequency of AC mains supply varies by country and sometimes within a country. Most power systems operate at either 50 Hz or 60 Hz. For example, many European, Asian and African countries use 50 Hz, while North America and parts of other regions use 60 Hz. Some countries have a mixture of both frequencies in different regions or for different legacy systems.
Frequency selection involves a compromise:

• Lower frequencies simplify the design of some rotating machines, particularly large motors for hoisting, crushing and rolling, and early commutator traction motors.
• However, low frequencies cause noticeable flicker in incandescent and arc lamps and make transformers larger and heavier.

Historically, many early systems used lower frequencies, such as 25 Hz, to suit heavy industrial and traction loads. The original Niagara Falls generators produced 25 Hz power, which was adequate for motors but caused flicker in lighting. Over time, most such systems were converted to the modern standard frequencies. In parts of Europe, specialised railway power networks still use about 16.7 Hz (formerly 16⅔ Hz) for traction, supplied separately from the public 50 Hz grid.
In contrast, certain specialised applications use high frequencies, typically around 400 Hz. These include aircraft power systems, some military and marine installations, and certain industrial and computer applications. At higher frequency:

• Transformers and motors can be made smaller and lighter for the same power rating.
• Ripple in rectified DC supplies is easier to filter, allowing more compact power conversion equipment.

Historically, large mainframe computer systems sometimes used 400 Hz or similar frequencies internally to reduce the size of power supplies and smoothing components.

Effects at high frequencies

The behaviour of conductors under AC differs from DC, especially at high frequency. In a homogeneous conductor carrying direct current, the current density is uniform across the cross-section. Under alternating current, particularly at higher frequencies, the current tends to be forced towards the outer surface of the conductor. This phenomenon is known as the skin effect.
Consequences of the skin effect include:

• The effective cross-section available to carry current is reduced, increasing the effective resistance of the conductor at higher frequencies.
• Increased resistive losses occur in high-frequency conductors compared with DC or low-frequency AC.

To mitigate these effects at higher frequencies and in high-current applications, conductors may be:

• Made from stranded or litz wire, in which many thin, insulated strands are woven to equalise current distribution.
• Constructed as tubular or hollow conductors in some high-current AC busbars, since most of the current flows near the surface.