Most
of the electrical power generated in the world today is three-phase. Three-phase
power was first conceived by Nikola Tesla. In the early
days of electric power generation, Tesla not only led the battle
concerning whether the nation should be powered with
low-voltage direct current or high-voltage alternating current,
but he also proved that three-phase power was the
most efficient way that electricity could be produced, transmitted,
and consumed.
What is a THREE PHASE CIRCUIT?
It is a common
method of alternating-current electric power generation, transmission,
and distribution. It is a type of polyphase system and is
the most common method used by grids worldwide to transfer
power. It is also used to power large motors and other heavy
loads.
Three-phase has properties that make it very desirable in electric power system:
- The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to eliminate or reduce the size of the neutral conductor; all the phase conductors carry the same current and so can be the same size, for a balanced load.
- Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.
- Three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors.
1. The horsepower rating of three-phase motors and the KVA (kilo-voltamp) rating of three-phase transformers is about 150% greater than for single-phase motors or transformers with a similar frame size.
2. The power delivered by a single-phase system pulsates, The power falls to zero three times during each cycle. The power delivered by a three-phase circuit pulsates also, but it never falls to zero. In a three-phase system, the power delivered to the load is the same at any instant. This produces superior operating characteristics for three-phase motors.
3. In a balanced three-phase system, the conductors need be only about 75% the size of conductors for a single-phase two-wire system of the same KVA rating. This helps offset the cost of supplying the third conductor required by three-phase systems. A single-phase alternating voltage can be produced by rotating a magnetic field through the conductors of a stationary coil.
The voltages of three phase system are 120 out of phase with each other.
If three separate coils are spaced 120° apart, three voltages 120° out of phase with each other will be produced when the magnetic field cuts through the coils. This is the manner in which a three-phase voltage is produced. There are two basic three-phase connections, the wye or star connection and the delta connection.
WYE CONNECTION
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