All power generated starts out as 3 phase power because it is the most efficient method of generation and distribution. All the power that we use comes from large generators that produce the electricity by some method, coal, nuclear, wind, hydro etc. The electricity is then pushed through large transformers to step it up to 345 thousand volts or more to transmit it across the country. The three-phase system was introduced and patented by Nikola Tesla in 1887 and 1888 and is the primary source of power in the world today.
Most homes are supplied with single phase power only, the small amount of loads that are common in most homes does not require 3 phase. 3 phase power is used when there are large amounts of power needed, especially electric large electric motor loads in industrial environments. 3 phase power supplies a smoother and more efficient operation for electric motors and has many benefits over single phase. With single phase power the current and voltage are at zero once per cycle, with 3 phase power having 3 phases separated by 120 degrees the power supplied is a lot smoother and consistent.
3 phase power is three alternating currents that are 120 degrees.
3 phase transformers
In industrial and commercial environments that are supplied with 3 phase power, transformers are used to break the power down to lower voltages and to distribute single and 3 phase loads. For large single phase loads and small 3 phase loads the Wye-Delta configuration is common. See below.
As you can see, the secondary side of the transformer above is configured as Wye. There are 3 phases, A, B, C and a neutral. The great thing about the Wye transformer is that you can pull single phase loads from all three phases equally while the neutral stays balanced. Let me explain this a little bit, if I were to put a 10 amp load from A to neutral (120 volts) the A phase would have 10 amps and the neutral would have 10 amps equally. Now if I put 10 amps of 120 volt load on A and B then the neutral load will reduce to 5. If I put a 10 amp 120 volt load on all 3 phases, the neutral load would be zero. The neutral balancing of a Wye configured transformer makes them perfect for large single phase distribution. An important note when running load from a 120/208 wye panel is to keep everything balanced. Try to divide your circuits up evenly on the A, B and C phase breakers.
You may have notice that the voltages on the secondary winding do not make as much sense as single phase transformers. If you are wondering why the voltage is 208 instead of 240 this is a characteristic of the 3 phase wye transformer. To calculate the voltage use the square root of 3 (1.73) 120 X 1.73 = 207.6. To find watts when using 3 phase we also have to use the square root of 3,
Watts = Amps X Volts X 1.73 X PF
This is part 1 of 3 phase power; I will get deeper into the technical side of things in the next article. Stay tuned to How to Electric.com to catch the next article.