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3 phase is just 3 one phase power supply but with 120 phase difference between each one. for example, take one phase, you will have a sinusodial voltage alternating between max and min at certain frequency (normally at 50hz or 60hz).
if you add a similar voltage with 120 lag, ie it reaches the max slower than the first, then you will have 2 phase supply. if you yet add another one with 120 to the 2nd one, so its relatively 240 lag from the first one, you will have 3 phase supply.
benefit of 3 phase over the 1 phase is that usually you get better power transfer to machines. thats why big motors uses 3 phase powers instead of single phase.
Can you explain why you get better power to big motors? You make them work by not waiting the next peak of the first phase but also in the 2nd and 3rd peak? Can you give some math on it, or alink to read more?
Thanks,
D.
Can you explain why you get better power to big motors? You make them work by not waiting the next peak of the first phase but also in the 2nd and 3rd peak? Can you give some math on it, or alink to read more?
D.
Just a piece of opinion:
Single phase or 3-phase system has a electrical cycle of 50Hz (or 60Hz). If according to your explaination on 'peak', in every 1/50 second, there is only ONE peak for single phase system compared to that of THREE peaks for 3-phase system. So, basically 3-phase system delivers higher energy in each cycle.
if you look at the math, you will find that the total instantaneous power system is time invariant. meaning the instantaneous power transfer is constant. so a 3 phase motor would run smoothly. a 1 phase motor would not.
p = 3/2VIcos(theta)
the power for 1 phase has time in its function instantaneous power. pop up a electromechanical power system textbook to see these equations.
Some Basic factoids about 3 phase.... Most of the electric power in the world is 3 phase. The concept was originally conceived by Nikola Tesla and was proven that 3 phase was far superior to single phase power. 3 phase power is typically 150% more efficient than single phase in the same power range. In a single phase unit the power falls to zero three times during each cycle, in 3 phase it never drops to zero. The power delivered to the load is the same at any instant. Also, in 3 phase the conductors need only be 75% the size of conductors for single phase for the same power output.
In the ac to dc power supply area, the ripple voltage is much less after rectification and the frequency is three times higher than on a single phase supply. This makes filtering much easier.
The drawback is the need for a more expensive transformer and more diodes. The other problem is the frequent lack of a 3 phase mains socket in most homes and industrial locations.
Above a few kW the three phase method becomes more economic.
An additional comment on the motors, the three phases produces a genuine constant rotational speed rotating magnetic field for the motors. There is no need for shaped pole pieces and mechanical intertia to smooth out the rotation. This is sort of similar to the radial engine in the older airplane engines and why V type cylinder placements in auto engines works slightly better than in line cylinders.
Also, you use 3phase system and not 2,4,5,.. because of elimination all harmonics that are multilple of 3.
This power system was invented by Nikola Tesla, Serbian scientist who worked in USA in 19th century.
If you consider 1-phase sinusoidal current or voltage to be a vector,then 3-phase power line can form a constant speed rotating mmf with its magnitude stable inside the induction motors,which are widly used in industry.
for high power power supply (above 5kW) the single phase current about (for 220vac) 25 amp. that is grater than most power of bulding. so in high power case we must use 3 phase instead 1 phase supply.
furthermore in 3 phase the riple in less and sterss in power component is less.
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