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# Single-Phase Power vs Three-Phase Power

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## 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.

## 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 Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

1. Caleb Tochukwu
Sep 27, 2021

I want to be getting notifications of your released informations or articles.

2. Igor Makarov
Jun 22, 2019

There is a powerful power system that feeds railroad in Germany, Switzerland and Northern Europe. It works on the frequency 16 2/3 Hz, single phase, with voltage 2×115 kV in the transmission lines and 15 kV on the trolley conductor.

THe trans use universal series motors very big size.

3. Lukas
Jul 16, 2016

Dear Engineers,

The main reason why we use 3 phase systems is that the instantaneous power is CONSTANT (I1(t)*U1(t)+I2(t)*U2(t)+I3(t)*U3(t)=const.). That means, when we supply 3 phase motor, the constant power produces constant torque!!! In single phase or two phase systems the instantaneus power IS NOT constant. The smallest number of phases is 3. Of course we can use 4, 5, 6 ect. phase systems ( they have the same feature as 3 phase), but it means more circuit breakers, conductors ect.

4. Asiimwe Asaph
Apr 17, 2015

Am doing a diploma in biomedical engineering at ECUREI in Mengo Kampala Uganda and am in my first year seeking for a scholarship. thanks

5. izaz ali
Feb 13, 2015

why three phase is preperad over single phase

6. Nura Abdullahi
Jan 20, 2015

Please why in a power system supply if you are Using 3 phase supply then neutral phase fail, over current occur but if it’s single phase supply then neutral phase fail low current occur? Why is it this differences

7. priyan
Aug 13, 2014

while in distribution feeders i see several houses take single phase supply. but how it is taken with as if several houses doesnt use as much power in other phase causing unequal load? , how the return path carrying return current transfer into three phase nuetral?

• zeckrey j
Jan 15, 2015

hi priyan, just sharing idea here, first of all the unequal load will produce current on neutral wire for 3 phase, recall equation for 3 phase Itotal= I1<0 + I2<120 + I3<120 = 0 for balance 3 phase load "I1 = I2 = I3".
now, answering your question balancing the load for each house so that the resultant current in neutral is difficult, but it can be minimize by controlling the incoming MCB of each house.
example:
if your are going to supply a single phase supply to a 3 house using 3 phase supply 40A 4pole, you have to control the incoming MCB size of each house which is all should be equal. in this case i will take 40A as the MCB, hence, from the equation, no matter how worst the unbalance occur, the worst unbalance current at neutral only limit to 40A only which is save if you size your neutral wire to withstand 100% of the phase current.
reason i use 4 pole mccb in the incoming 3 phase is to limit the current on neutral wire in case of harmonic current.

8. 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?

9. 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.

10. 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…

• EB1000
Feb 06, 2013

*UNITY

11. 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..

12. 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

13. 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.