Monday, January 18, 2010

What Is 3-Phase Power?

When a conductor is rotated to cut magnetic fields, some electromotive force is induced according to Faradays Law. This voltage/emf is always in opposition to the main field according Lenz’s Law

E = -Ndø/dt

The voltage induced by a single winding is known as single phase winding, when rotated in a uniform magnetic field is known as single phase voltage.

A 3-Phase generator has 3 separate but identical conductors that are 120 Electrical degrees from one another. When the conductors are rotated in a uniform magnetic field, the result is 3 independent supplies of equal voltages which are displaced from each other.

A 3-Phase supply is carried by 3 conductors known as lines which are colored Red, Yellow & Blue. The currents in these cables are known as Line Currents and the Voltages are known as Line Voltages. The 4th conductor, known as Neutral is often used with the 3phase supply when the load is unbalanced.

There are normally 2 connections to supply load with a 3-Phase supply:
  1. Star Connection.
  2. Delta Connection.

Star Connection

Star Connection
Where VB, VY, VR are Phase Voltages and VBY, VRB, VRY are Line Voltages.
Z is the Load Impedance. The load will be said to be balanced if ZR = ZB = ZY

At this point the current along the neutral conductor will be zero and hence we shall not need the Neutral Conductor.

In Star connection, the Phase Current is always equal to Line Current.


In Star Connection, of a 3-Phase supply, together with the Neutral conductor allows the use of two voltages (Phase Voltage & Line Voltage).
If 4 wire system is used the load is not balanced.

Delta Connection

It is also known as Mesh Connection.
In this connection the ends of one load is connected to the start of the next load. Phase voltages are always equal to line Voltages.


For power transmission a Delta system is associated with large Line currents thus large crossectional area while the Star system is associated with large Line Voltage. Loads connected in Delta Connection dissipate 3x (times) more power than in star if the same supply. This is true because most motors which have a high torque use the Delta connection.

1 comment:

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