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In a 3-phase system, when one phase hits the peak and is about to go down, the next phase starts hitting the peak, and there would be a total of six peaks as compared to two for a single phase system (from 0 to 360 degrees).

I am trying to figure out how 3-phase provides more power. Those six peaks are from three different sine waves. It is not a single sine wave that is giving us those six peaks. I am making the following two guesses.

The equipment that is using 3-phase supply is designed to somehow use only the peaks. Like use positive peak from phase A and as soon as it decreases, pick the next positive peak from phase B and so on? But if this is the case, it means when a single phase gets faulty or disconnected for some reason, there would be problems. Also, this doesn't seem right because we are neglecting the negative peaks and using only three positive peaks which does provide more consistent power, but not very efficient it seems. If we are picking those negative peaks too then this guess is not valid.

My second guess is that 3-phase rectification and some filtering is a must because that would give a smoother wave like a DC. But this also makes me think that not all of those equipment are DC-operated. So how a 3-phase system provides more power. The plots below are for full wave rectifiers for single and 3-phase systems.


Single phase rectification


3-phase rectification

Image1 source

Image2 source.

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4 Answers 4

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A three-phase system provides 3 times the power. That's simple math. At the same time, you usually need 5 conductors instead of 3 (and of those, usually 3 are carrying significant current and consequently heating up the whole cable, while in a two-phase system 2 are carrying significant current and heating up the whole cable).

So this is a more efficient use of copper to start with, assuming an equal distribution of power along the phases. What kind of appliances will have such distribution? Stuff like heaters are one. Another are rotating devices which benefit greatly from three-phase current because they have a dedicated direction of rotating magnetic fields.

With regard to DC bridge rectification of a 3-phase source: that delivers a higher voltage than 2-phase rectification because a peak on one phase coincides with opposing voltages on the other two phases.

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  • \$\begingroup\$ 3 times the power with only 1.5 times the number of conductors (for the same current, and same voltage between conductors). \$\endgroup\$
    – user16324
    Jun 25, 2023 at 11:57
  • \$\begingroup\$ @user_1818839 It is not the same voltage between conductors. In a 2-phase system, the maximum between conductors is 230V, in a 3-phase system, it is 400V. So if isolation performance were an issue, there would be a difference to cater for. \$\endgroup\$
    – user107063
    Jun 25, 2023 at 12:02
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You seem to have two different questions.

The waveform shows ripple after rectification. There is DC with some ripple, but with retified 3-phase supply, there is DC even without a capacitor like on 1-phase supply.

But how 3 phases can provide more power than single phase is that with single phase with e.g. 230 VAC and 16 A socket, you can split a 3 phase supply into three single phase sockets, or you could use a motor or heavy loads between phases, so you get 400V at the same 16A rating.

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How a 3-phase system delivers more power?

A three phase supply and appropriate load can be regarded as three single phase supplies and loads hence, inevitably, 3 power sources can produce 3 times the power of a single source.

Your guesses appear to be missing the simplicity of reality.

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I have read all the answers, they are correct but that "three times the power" argument appear to miss some details. I might be wrong but this is what I have understood from reading some articles and books. In a three-phase system, the total power is

PT = PphaseA + PphaseB + PphaseC

PT = 3*Vp*Ip*cos(θ)

Now that is three times the power and graphically it would look like the figure below. enter image description here

That is when we are referring to phase voltage/current. But usually we do not use a neutral in 3-phase loads and we connect one phase with another phase. In that case, the power will be:

P = √3*VL*IL*cos(θ)

This results in 73% more power than a single phase. Meaning, for a single phase circuit with 240 V and 20 A, the power would be 240*20=4800 W. But in three-phase, it would be √3*240*20=8313 W, which is an increase of 73%, considering that the power factor is unity.

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    \$\begingroup\$ Where would the factor of \$\sqrt 3\$ come from? \$\endgroup\$
    – greybeard
    Jun 27, 2023 at 5:57
  • \$\begingroup\$ It will be easier to understand when you have plots and phasor diagrams to look onto. Take a look at this site to learn why there is a factor of sqrt(3). \$\endgroup\$ Jun 28, 2023 at 8:54

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