Single-Phase Power vs Three-Phase Power - Differences

Single-Phase Power vs Three-Phase Power - Differences

Electric Power System

The principal elements of an electric power system are the generating stations, the transmission lines, the substations, and the distribution networks. The generators produce the electricity, the transmission lines move it to regions where it is consumed, and the substations transform it for industrial, commercial, and residential use.

Finally, the distribution networks carry the electricity to the customers.

Most AC power is generated as three-phase power.

Both three-phase and single-phase devices can be powered from a three-phase supply. A three-phase circuit is a combination of three single-phase circuits. The current, voltage, and power relations of balanced three-phase AC circuits can be studied by applying the rules that apply to single-phase circuits.

The sine waves of three-phase voltage are separated by 120 electrical degree because they are generated by three separate sets of armature coils in an AC generator. These three sets of coils are mounted 120 electrical degrees apart on the generator’s armature. The coil ends could all be brought out of the generator to form three separate single-phase circuits, but they are conventionally interconnected so that only three or four wires are actually brought out of the generator.

Single-phase AC voltage with zero power factor has both voltage and current sine waves in phase, so they cross the zero line together twice in each cycle.

Similarly, a plot of three-phase voltage sine waves, also with zero power factors as shown in Fig. 1, has all three voltage and current waves crossing the zero line twice each cycle together. Each of its three phases, V1, V2, and V3, is separated by 120 electrical degrees.

Power supplied to each of the three phases of a three-phase circuit also has a sinusoidal waveform, and the total three-phase power supplied to a balanced three-phase circuit remains constant.

Figure 1 - Three-phase voltage waveforms are separated by 120 electrical degrees.

Figure 1 - Three-phase voltage waveforms are separated by 120 electrical degrees.


Ok, let’s conclude something…

As a result, there are two practical reasons why three-phase power is superior to single-phase power for many applications:

1st reason Three-phase machines and controls can be smaller, lighter in weight, and more efficient than comparable single-phase equipment. More power is supplied to them in the same period than can be supplied by a single-phase power circuit.

However, the trade-off for this advantage is that three-phase machines and controls are more complex and expensive.

2nd reason Only about 75 percent as much copper wire is required for distributing three-phase power as is required for distributing the same amount of single-phase power.

Resources: N. Sclater, J. E. Traister – Handbook of electrical design


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Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on Google+.



11 Comments


  1. Roy Gilbert
    Jul 04, 2014

    I have a transformer with a 115 VAC primary and 12 VAC secondary at 1 Amp to power a 4 diode bridge for DC power. If I replace the transformer with a 3 phase unit, what should the current rating of each phase be?

  2. […] a three-phase system a short-circuit between two phases is almost identical to the single- phase case and although the […]

  3. [...] to a coil will induce a voltage in a second coil where the two are linked by a magnetic path. The phase relationship of the two voltages depends upon which ways round the coils are connected. The [...]


  4. RENE66
    Feb 07, 2013

    Three phase motors are simpler to operate and control than single phase because single phase motors for high starting torque applications require starting gear (starting relay and/or capacitors) whereas three phase motors do not. AC motors for industrial applications are limited by relay sizing to 5 hp and below. But even below 5 hp, three phase motors are preferred because of the cost and unreliability of starting gear.


  5. EB1000
    Feb 06, 2013

    “Single-phase AC voltage with zero power factor has both voltage and current sine waves in phase” Don’t you mean UNITI power factor? In phase V-I corresponds to a pf of 1 not 0.. Zero pf corresponds to a +/- 90 deg difference.. Also the 3 main phase coils are separated by 120 physical/spatial/radial degrees not “electrical”.. You cannot separate coils by electrical degrees.. The result, however is 3 phase voltages/currents that have 120 electrical degrees displacement… BTW one of the main advantages of 3-ph power, is it’s natural ability to produce a rotational magnetic field…


  6. yousuf
    Jul 04, 2012

    why apart from 3-phase multi-phase generation is not available? why not 2,4,5,….?


    • EB1000
      Feb 06, 2013

      The more phases used for power distribution, the lower the current rating each single conductor will have to handle, which is good, BUT on the other hand, you’ll need more conductors, meaning more expensive, more complex and more maintenance requirements… So 3<phases = less efficient power system… Multi phase systems are used mostly in electrical machines, where higher mechanical accuracy is required.. but in that case the multi-phase power is locally generated using AC/AC converters..

  7. [...] two-phase load still produces unbalanced three-phase currents; i.e., the phase currents in the three-phase system do not have equal magnitudes, their phase angles are not 120° apart, and there is a considerable [...]


  8. taufikjayanto
    Apr 20, 2012

    tell me according to the graph means, in a second Ia,Ib,Ic arent same value ?
    eg: Ia=10<0,Ib=10<120,Ic=10<-120


  9. pkiboi
    Apr 08, 2012

    In the industry where i work, 80% of power is used on Motors. It should be noted that 3-phase motors have a high stating torque by design unlike similarly sized single phase motors.
    This article is quite useful.

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