Alternating Current

Alternating Current as a power source, what is it and what are the advantages?

AC sine wave

Example of an AC sine wave

Alternating Current is the power you get out of a wall socket at home and at work.  It is often abbreviated as AC.

The reason it's called Alternating Current is the voltage is constantly changing back and forth.  If you were to plot the voltage on a graph it would look like a sine wave, where the x-axis is time and the y-axis is voltage.


How fast the voltage goes back and forth is called the frequency.  In the United States the voltage goes back and forth 60 times per second, while in Europe and most of Asia it goes back and forth 50 times per second.

There's an easier way to say "goes back and forth 60 times per second."  A unit called "Hertz" stands for the number of cycles per second.  The abbreviation is Hz.  The phrase "60 cycles per second" can be shortened to 60Hz, while "50 cycles per second" can be shortened to 50Hz. The unit of Hertz was named after the German physicist Heinrich Hertz.

Throughout the world most countries have adopted either 50Hz or 60Hz as the standard frequency for their electrical grid.  Overall most countries use 50Hz.

50 hz and 60 hz Sine wave graphs

50 Hz vs. 60 Hz alternating current sine waves


The voltage level is also a value that has been standardized to different values in different countries.  Even within a country the voltage level can be different depending on the situation.  In the United States, household voltage is 120VAC while industrial voltage is 208VAC, 240VAC, 480VAC and higher.  (VAC is an abbreviation for "Volts AC").

Why is alternating current used in houses and for power distribution instead of direct current?

AC voltage local distribution pole armThe first part of the answer is Alternating Current has a big advantage over Direct Current in a very important area.  It's much easier to change the voltage level of AC than it is for DC.  All you need is a transformer - one of the simplest electrical devices to make.

The second part of the answer is it's much more efficient to transmit high voltages rather than low voltages.  The higher the voltage, the less is lost during transmission.  This is why neighborhood transmission lines have voltages over 10,000V and long distance lines have much higher voltages at 250,000V to 500,000V.  However it would be very dangerous to have this much voltage in a house.  This is where transformers come into play.  They take the high voltage and bring it down to 120VAC or 240VAC before it comes into the house or building.  Even for most industrial uses the voltage is brought down to safer levels.

In short AC can give the best of both worlds, it can transmit power efficiently when boosted to higher voltages and it can be safer to use when the voltage is reduced to lower levels.  With DC power there isn't an easy and cost effective way to switch the voltage up and down so you would have to make a compromises on safety, efficiency and cost.


AC voltage can be delivered as single phase and three phase.

Single phase AC is typically used for households and for commercial and office applications.  It uses two wires to transmit power plus a ground wire.  The advantage is it's safer, easy to use, and the wiring costs less.

Three phase AC is used for industrial applications.  It uses three wires to transmit power.  The advantage is it delivers constant power and does it more efficiently.


AC Power and Servo Drives

  • ADVANCED Motion Controls has a large selection of AxCent and DigiFlex® Performance drive models with options to operate with single and three phase AC as well as a number of voltages found throughout the world including 120, 208, 240 and 480VAC.
  • DC power supplies that accept both single and three phase AC are also offered

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