What are the advantages of three phase to single phase electric power distribution systems?

Answer 1

This Answer is the same as the for the Question 'Why is power for a country's mains grid usually generated as 3-phase electricity not single phase?'

Single phase power has a sine wave voltage that crosses zero before reversing its polarity. In the region near the zero-crossing there is not much power. At zero there is none at all. So single phase loads often need some trickery to deliver output in this area. Often it is just the inertia of the motor or appliance.

Three phase power is always delivering power on one of its phases, and is thus preferred for machines, motors and appliances that use lots of power.

Three phase power generation is preferred if the application needs very high power, or nees relatively small power output but with with weight restrictions - as in automobiles, where the power is generated in a 3-phase alternator and then converted to DC using rectifiers!

Use of DC (Direct Current) is the next step up for smooth high-power devices but requires rectification, regulation and smoothing to be useful.

Another problem with DC is that, for efficient long distance transmission, it cannot be simply converted to much higher voltages than the voltage at which it was generated at the power station.

Similarly DC cannot - simply by using Transformers - be converted down to safer, much lower AC mains voltages for use by consumers.

Conversion of DC to AC requires the use of expensive high-power electronic "power inverter" systems and the opposite - the conversion of high-power AC to DC - requires the use of very big power rectifiers.

AC (Alternating Current) is used for high power generation and distribution because it can easily be transformed, using transformers, to achieve very efficient power transmission over very long distances and can then be transformed down to low voltages for distribution to consumers.

Two phase, and higher multi-phases are also used, but very rarely, for a few very specialised applications.

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A three phase system provides constant torque, whereas single phase does not.

With three phase you will get more power per kilogram of metal to generate electrical energy.

Answer

For a given load, a three-phase supply uses less copper than a single-phase system. In other words, it's more economical!

Answer 2

Single phaseThe main advantage of 3 phase is that it is more efficient for running AC motors than one or two phase. Three phase was designed to be used for creating a rotating magnetic field in the motor, which starts rotation of the motor. Household service originally was intended for lighting and heating, while 3 phase was intended for industrial and commercial with large motors.

In US residential single phase service you have two hots, in phase with each other.

• Vhot1(t) = Vmax * sin(2 * pi * freq * t)
• Vhot2(t) = Vmax * sin(2 * pi * freq * t + pi)

Where:

• Vmax equals the maximum voltage to neutral. (Approx. 170V (120V * sqrt(2)) for US residential service)
• freq is the frequency of the line in Hz (60 for US residential service)
• t is the time in seconds

So:

• Between either hot and neutral you have 120Vrms to neutral.
• Between the two hots (Vhot1(t) - Vhot2(t)) you have 240Vrms
• The voltage goes to 0V 120 times a second, no matter what wiring you choose.
• You cannot get 3 phase from single phase without a special converter.

3 phase

In a 3 phase system you have 3 hots, out of phase with each other.

• Vhot1(t) = Vmax * sin(2 * pi * freq * t)
• Vhot2(t) = Vmax * sin(2 * pi * freq * t + 2 * pi / 3)
• Vhot3(t) = Vmax * sin(2 * pi * freq * t + 4 * pi / 3)

Where:

• Vmax equals the maximum voltage to neutral. (Approx 170V (120V * sqrt(2)) for 120/208V 3 phase)
• freq is the frequency of the line in Hz (60 for US service)
• t is the time in seconds

So:

• Between any hot and neutral you have 120Vrms to neutral. Since you are only using two conductors in this circuit, it is identical to 120Vrms single phase above. It can be used to power the same equipment that would be used on 120Vrms single phase power.
• Between any two hots you have 208Vrms. Again, this is the same as above, though the voltage is less than single phase due to the phase shift. This can be used like 240V single phase above, if the load will tolerate the slightly lower voltage.
• For heavy loads, all 3 hots can be used. Unlike single phase, one of the hots is always energized. The load will always have at least 140V (off a 208Vrms supply) between two hots. This makes it valuable for motors, as there is always a magnetic field present to pull the armature.
• You can easily get single phase power from 3 phase with no converters.
• 120/208V 3 phase service has 1.5 times the power capacity of equally rated 120/240V single phase.

From Wikipedia:

• A three-phase motor will be more compact and less costly than a single-phase motor of the same voltage class and rating
• Single-phase AC motors above 10 HP (7.5 kW) are uncommon
• Three phase motors will also vibrate less and hence last longer than single phase motor of the same power used under the same conditions.

To sum up

• Single phase is used in residential service. The split phase is used to power large loads that are too large to run off 120V. Otherwise, 120V single phase is used.
• US consumer appliances are single phase, so there is no incentive to run 3 phase to homes.
• 3 phase is usually only used in industrial settings with heavy equipment or motorized tools
• Unlike residential service which is always 120/240V, industrial 3 phase can be much higher, with 480V not uncommon. The designer of the building is expected to design the electrical system for the use of the building. It is not uncommon for these building to have their own step-down transformer to 120/208V for lower voltage loads.

Answer 3

Three phase power does "work" more efficiently than single phase power. The way the waveforms of the three different phases line up cause electric motors (for example) to output more torque with less current draw than the same voltage supplied through single phase power. Three phase power is usually used in industrial machinery where a lot of power is needed (and energy cost is a concern).

Answer 4

There is no particular disadvantage to having a residential three-phase supply; in fact, such supplies are quite common in some European countries, such as here in Cyprus.

There is absolutely no need to obtain special appliances, as ordinary single-phase appliances can be easily connected to a three-phase supply: they are simply connected between line and neutral (230 V in Europe).

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Answer 5

Motors of the same horsepower are smaller.

For other items since you usually have a higher voltage, it decreases the amps required and the size of wire necessary. You also seriously decrease the number of amps leaving the building on the neutral.

With three-phase transmission a given amount of power can be transmitted with half the amount of wire. That is because only three live wires are needed, while for single phase of the same voltage to earth, each of the three live wires needs a neutral wire of the same size.

Answer 6

If you are asking why do we use three-phase for transmission/distribution, then the answer is simple: economy. For both overhead and underground systems, it can be shown that three-phase distribution uses less copper(or aluminium) than single phase distribution for a given load. However, the most economical method, in terms of copper usage, is a direct-current system -but, of course, changing voltage levels of DC is far more complicated than for AC systems.

If, on the other hand, you are asking why a three-phase supply might be more useful than single phase to a consumer, then the answer is that three-phase motors are self-starting and, for a given load, more compact and lighter than the corresponding single-phase motor. So, for consumers with motor loads, three phase would be a better choice than single phase.

Answer 7

For a given load, a three-phase system uses less volume of copper in its conductors and supply equipment, so is more economical than a corresponding single-phase system. It also supplies energy continuously, rather than in a pulsing manner. Finally, three-phase supplies create naturally-rotating fields in induction motors -allowing them to self-start.

Answer 8

Three phase power is more efficient at running motors. The advantage of 3 phase becomes greater the larger the motor is. Three phase motors do not have to rely on capacitors to start or run.