AC FULL FORM| What does AC stand for?


Definition:Alternating Current
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What does AC stand for?

Alternating current (AC, ac) is a constant flow of electric charge that reverses in direction. Direct current (DC) only allows electric charge to flow in one direction.

What is Alternating Current?

Chapter 1 – Basic AC Theory

The majority of students studying electricity start with direct current (DC), which refers to electricity flowing in a constant direction and/or having a voltage that is constant polarity.

DC refers to the type of electricity that is made by a battery with definite positive or negative terminals. It can also be the charge created by rubbing certain materials against one another.

Alternating Current vs. Direct Current

While DC can be very useful and easy to understand, it’s not the only type of electricity that is in use. Some sources of electricity, such as rotary electromechanical generators, naturally produce voltages that alternate in polarity and reverse over time.

This “kind” is also known as Alternating current (AC). It can be used as either a voltage switching direction or as a current switch direction back and forth.

direct and alternating current (AC and DC)

Direct vs. alternating current

AC Voltage source

Whereas the familiar battery symbol is used as a generic symbol for any DC voltage source, the circle with the wavy line inside is the generic symbol for any AC voltage source.

It is possible to wonder why AC would be useful. In some cases, AC is more practical than DC.

For applications in which electricity is used for heat dissipation, it doesn’t matter what polarity or direction the current flows. As long as the load has enough voltage and current, heat can be produced (power dissipation).

 It is possible to create AC electric generators, motors, and power distribution systems that perform far better than DC. This is why AC is used primarily in high-power applications around the globe.

A little background knowledge on AC is needed to explain why this is the case.

AC Alternators

A machine that rotates a magnetic field around a series of stationary wire coils using the turning of a shaft will produce AC voltage as the shaft rotates, according to Faraday’s Law of electromagnetic Induction.

This is the operating principle of an AC generator. Also known as an alternator, it can be seen in Figure below.

Alternator operation

Alternator operation

You can see how the polarity between the coils of wire reverses when the opposing poles of a rotating magnet pass by.

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This reversing voltage direction will be created when the circuit is connected to a load. The alternator’s shaft must be turned faster to spin the magnet, creating an alternating current and voltage that changes direction more frequently in a certain time.

Although DC generators operate on the same principle of electromagnetic induction as AC generators, their construction is more complicated than AC.

The coil of wire is attached to the shaft, where the magnet is located on the AC alternator. Electrical connections are made via stationary carbon brushes that contact copper strips on rotating shaft.

This is all that’s required to change the coil’s output polarity to an external circuit, so that the external circuit has a constant polarity.

DC generator operation

DC generator operation

e generator in the above picture will produce

The generator in the above picture will produce two pulses per revolution of the shaft. Both pulses are in the same direction (polarity). Multiple coils are used to make intermittent contact with the brushes in order for a DC generator not only to produce constant voltage, but also to generate constant voltage.

This diagram is slightly simplified than the one you might see in real life.

It is important to recognize the problems associated with electrical contact with moving coils (sparking, heat), particularly if the shaft of a generator is rotating at high speeds. Spark-producing brush contacts can be even more difficult if the surrounding atmosphere contains explosive or flammable vapors.

An AC generator (alternator), does not need brushes or commutators in order to function. Therefore, it is immune from the problems that DC generators can have.

AC Motors

Electric motors also reflect the advantages of AC over DC in terms of generator design.

DC motors need brushes to make contact with moving wire coils, but AC motors don’t. AC and DC motor designs look very similar to generator counterparts.

They are almost identical (identical for this tutorial), with the AC motor dependent upon the reversing magnet field created by alternating current through stationary coils of wire to turn the rotating magnet around on its shaft.

The DC motor dependent on brush contacts making and breaking the connections to reverse current through rotating coil every half rotation (180 degrees).


We know that AC generators/AC motors are simpler than DC generators/DC motors. This relative simplicity leads to greater reliability and lower manufacturing costs. What else is AC useful for? There must be more than just the design details of generators or motors. Yes, there is.

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The mutual induction effect is an electromagnetism phenomenon that involves two or more coils placed in such a way that the changing magnetic field in one coil induces a voltage the other. If two coils are mutually inductive, and one coil is energized with AC, the AC voltage will be created in the other coil. This device is also known as a transformer.

Transformer “transforms” AC voltage and current.

Transformer “transforms AC voltage and current.”

A transformer’s ability to change the voltage between the unpowered and powered coils is the most important feature. The AC voltage generated in the unpowered (or secondary) coil is equal to that across the primary coil. This is multiplied by the ratio between secondary coil turns and primary coil turns.

The secondary coil powers a load by supplying current. This is the reverse: secondary coil current multiplied with the ratio of primary and secondary turns. This relationship is very similar to a mechanical analogy. Torque and speed are used to represent voltage or current.

Speed multiplication gear train steps torque down and speed up. Step-down transformer steps voltage down and current up.

Speed multiplication gear train speeds torque down and speed upward. Step-down transformer changes the voltage and current.

If the winding ratio of the primary coil is reversed, so that it has fewer turns than its secondary coil (primary coil), the transformer “steps up” voltage from the source to a higher level at load.

Speed reduction gear train steps torque up and speed down. Step-up transformer steps voltage up and current down.

The speed reduction gear train moves torque up and down. Step-up transformer increases voltage and decreases current.

AC has an advantage over DC in terms of power distribution, as shown in figure below. The transformer can easily step AC voltage up and down with ease.

When transmitting electrical power over long distances, it is far more efficient to do so with stepped-up voltages and stepped-down currents (smaller-diameter wire with less resistive power losses), then step the voltage back down and the current back up for industry, business, or consumer use.

Transformers enable efficient long distance high voltage transmission of electric energy.

Transformers allow for long-distance high voltage transmission of electric power.

Long-range distribution of electric power has become possible thanks to transformer technology. It would be prohibitively expensive to build power systems that are not close to the ground (within a few hundred miles) without being able to step voltage up or down.

Transformers are useful, but they work only with AC and not DC. Transformers will not work with DC because mutual inductance is dependent on magnetic fields changing. Direct current (DC), however, can only produce stable magnetic fields.

Direct current can be interrupted (pulsed through the primary winding) to create a changing magnet field. This is similar to what is used in automotive ignition systems to generate high-voltage spark-plug power from low-voltage DC batteries. However, pulsed DC is no different than AC.


This is probably the main reason AC is so popular in power systems.


  • DC stands for Direct Current, which is a voltage or current that has a constant polarity over time.
  • AC is an acronym for Alternating Current. It refers to voltage or current that changes direction or polarity over time.
  • AC electromechanical generators, known as alternators, are of simpler construction than DC electromechanical generators.
  • AC and DC motor design follows respective generator design principles very closely.
  • transformer consists of two mutually-inductive coils that transmit AC power from one to the other. The number of turns in each coil can be adjusted to produce a voltage increase/ decrease between the primary (powered) and secondary (unpowered) coils.
  • Secondary voltage = Primary voltage (secondary turns / primary turns)
  • Secondary current = Primary current (primary turns / secondary turns)


Alternating current

Alternating currentAbbreviationACFlow ofElectric chargeIt reverses itself periodically. It begins at zero, increases to a maximum, decreases back to zero, reverses direction, reaches a maximum opposite to its original value, then returns to the original value.

This cycle continues indefinitely. The time interval between two consecutive cycles that reach a definite value is called thePeriodThe number of cycles, or periods per second, is theFrequencyThe maximum value in either direction is theamplitudeThe alternating current. Low frequencies, such 50 to 60 cycles per second (hertzThese are used to provide domestic and commercial power.

However, alternating currents with frequencies of around 100,000,000 cycles per sec (100 megahertz), are used inTelevisionAnd those with several thousand megahertz inradarOrmicrowaveCommunication. Cellular phones operate at frequencies around 1,000 megahertz (1 gigahertz).

Asynchronous current (AC), which transmits power over long distances with little loss of energy, has been a popular choice over direct current (DC). 

The current times the voltage equals the power transmitted. However, the resistance times square of the current equals the power lost. Changing voltages was very difficult with the first DC electric power grids in the late 19th century. 

These grids were unable to transmit power over long distances due to power loss. They used low voltages in order maintain high current. DC power transmission was soon supplanted by AC systems that transmit power at very high voltages (and correspondingly low current) and easily use transformers to change the voltage.

 However, current DC systems are capable of changing voltages. AC systems send power from generators at thousands of volts. They then use transformers to reduce the voltage to 120 volts for individual customers. See also electric current.

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