An AC signal has a certain frequency which according to my understanding is the switches between the two extremes of the voltage per second. While my outlets around here typically seem to have a single signal of that form (1 live, 1 neutral, maybe 1 ground) I have read about systems that overlay multiple signals with a phase offset (i.e 3 phase AC output).

Picturing two phases it seems that you will no longer have a alternating voltage between -xV and +xV but by switching between phases you can get a voltage alternating between +xV and 0V at twice the frequency.

Consider the following image: enter image description here

If you switch between red and blue vs neutral at the right interval you will alternate between +xV and 0V (at twice the frequency of each of the phases).

I was wondering, can you theoretically achieve a similar result by increasing the frequency of a single phase?

  • \$\begingroup\$ AC \$\ne\$ DC... 3 phase ac means three sinewaves phase shifted by 120 degrees. \$\endgroup\$ Commented Sep 9, 2020 at 18:12
  • \$\begingroup\$ Yes, obviously? \$\endgroup\$ Commented Sep 9, 2020 at 18:13
  • 1
    \$\begingroup\$ As in you get 3 220V sinewaves alternating between +220V to -220V. \$\endgroup\$ Commented Sep 9, 2020 at 18:15
  • 2
    \$\begingroup\$ You're mixing up a bunch of different things, including the fact that the RMS voltage (eg, your 220) is not the peak excursion in either direction, but rather the equivalent DC voltage which would deliver the same power to a purely restive load. It is true that multi-phase AC provides smoother power delivery; eg, if you have a rectifier and capacitor bank feeding a DC load (such as a programmable frequency inverter in a modern motor drive), you need smaller capacitors if you feed it with 3-phase input rather than single phase. \$\endgroup\$ Commented Sep 9, 2020 at 18:19
  • 2
    \$\begingroup\$ I think you need to brush up on some of the math involved. You can't create a new frequency by adding signals of the same frequency. Here is an interesting tool to help experiment with the ideas: geogebra.org/m/DNbv8gtu \$\endgroup\$
    – mbedded
    Commented Sep 9, 2020 at 18:20

1 Answer 1


it seems that you will no longer have a alternating voltage between -xV and +xV but by switching between phases you can get a voltage alternating between +xV and 0V at twice the frequency.

That is incorrect, but it's an understandable error. It would appear you're perhaps being confused by the "goofy" situation of US domestic power to electric cookers, water heaters, laundry dryers, etc.

First, let's consider the fundamental facts of AC power: the AC component of a signal necessarily alternates around its ground, and such alternation is inherently symmetric as a matter of definition. Any offset or asymmetry from ground is a DC component, and while that might locally exist, it cannot pass through transformers widely present in a grid, so in a conceptual analysis it can be ignored.

So what is present on that clothes dryer outlet?

In a typical US residence, two opposite AC waveforms. When one is high, the other is low, and so on. The voltage between them is 240 volts RMS; the voltage between either and neutral is 120 volts and used for simpler loads.

But many industrial/commercial buildings are actually fed with 3 phase power, as that is more suitable for industrial loads like large induction motors. Instead of the 180 degree phasing of domestic power, industrial/commerical power is distributed at three equal points stepped 120 degrees around the phase circle. Typically in such a setup, each phase to neutral measures 120 volts RMS, so simple single-phase outlets can be provided. But since the phases only differ by 120 degrees instead of 180, if you measure the RMS voltage across any two (which is to say, measure the degree to which they are opposite) you'll only get a trigonometrically reduced 208 volts. Install a domestic model electric cooker or clothes dryer at work, and it may be under-powered, though such simple resistive loads may not particularly care.

(There are also other 3-phase distributions schemes at higher voltage, "wild leg" etc - trying to keep this simple)

  • \$\begingroup\$ Wait! A center-tapped transformer secondary is "goofy?" \$\endgroup\$ Commented Sep 9, 2020 at 19:08
  • 1
    \$\begingroup\$ @Solomon, it's goofy when it's a centre-tap of a delta-wired three-phase transformer system as it gives a "high leg" and destroys the elegant symmetry of a three-phase system.with star-connected neutral. To European eyes it always looks like a hack. \$\endgroup\$
    – Transistor
    Commented Sep 9, 2020 at 22:24
  • \$\begingroup\$ OK, Yeah. There's just one phase of high-voltage feeding my street, so sometimes I forget about that stuff. I know a guy who wanted some extra light over a workbench in a lab in a commercial building where we worked, So he ran out to the DIY store, bought light fixtures, and wired them up himself after the cleaners had gone home. He didn't realize that the overhead lighting in the building was 277 Volts (phase-to-neutral voltage of a 480V, 3-phase, star-wired circuit.) Oops! That was bit much for the 120V fixtures that he bought. You're right. We do have some goofy **** here. \$\endgroup\$ Commented Sep 10, 2020 at 1:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.