Why do you use three phase power?

Answer 1

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.

If the application is large power, or small power with weight restrictions (like automobiles!) Three Phase is preferred. 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 be transformed down to safer, much lower mains voltages for use by consumers.

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.

Another Answer

Because, for a given load, less volume of copper is required to supply that load, making a three-phase system roughly 75% the cost of a corresponding single-phase system.

Answer 2

For any given load, a three-phase system uses less volume of copper than a single-phase system and, so, is more economical.

If you mean "Why are there two different power systems, single phase and 3 phase?", they both have their applications where one is better than the other and vice-versa.

Three phase is better where a lot of power has to be carried as it splits the current between three wires. It is also better where the AC is going to be converted to DC because the 3 overlapping phases produce DC with much less ripple than single phase. Motors also run smoother on 3 phase.

Single phase wiring (actually split single phase: 120-0-120 in 60 Hz countries like USA and Canada) is often the best service to use for homes and small workshops because, the wiring is simpler and is therefore cheaper to install than three phase.

Also, for the small motors needed for household appliances and small items of machinery, the design is simpler so that the manufacturing costs - and therefore also the purchasing costs - are less than for motors designed for use on three phase services.

Answer 3

Three phase power is always delivering power on one of its phases, and is thus preferred for electrical machines such as alternators to generate electricity, large electric motors and other types of equipment that have to use lots of power.

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.

If the application is large power, or small power with weight restrictions (like automobiles!) three phase is preferred. 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 be transformed down to safer, much lower mains voltages for use by consumers.

Three phase AC (Alternating Current) electricity 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.

Answer 4

3-phase is used for high power loads and also for transmitting power over distance. This is done for efficiency and/or to use less wire for the required performance.

A typical common use of 3-phase in Europe is to feed a street of houses with a 400 v 3-phase 4-wire supply. Each house is supplied from one live wire and the neutral, which gives a 230 v single-phase supply. 230 = 400 / sqrt(3).

Over a group of houses the currents drawn are expected to average out so that the total on each phase is roughly equal. The group is fed from a transformer of up to 1000 kVA connected to an 11,000 volt 3-phase 3-wire supply, and that supplies 200-300 houses.

Answer 5

Three phase power is more efficient for running motors and other larger loads. That is why you typically see single phase in homes and smaller businesses. I am on North America and I see no reason why this would not hold true in other countries.

Answer

Three phase is used because, for a given load, it uses less volume of copper (around 25%) than a corresponding single-phase system and is, therefore, more economical. It also supplies continuous power, which single phase does not.

Answer 6

There are three reasons. The first and, perhaps, most important reason is economy as, for a given load, a three-phase systems uses around 75% of the volume of copper required by a single-phase system. The second reason is that a three-phase system provides essentially continuous power, whereas single-phase provides pulsating power. The third reason is that three-phase induction motors are self-starting, whereas single-phase motors are not.

Answer 7

For a given load, a three-phase supply system uses less copper than a single-phase supply system and is, therefore, more economical than single phase.