THREE PHASE SYSTEM

GENERATION OF POLYPHASE VOLTAGE

• The kind of alternating currents and voltages discussed so far are known as single-phase voltage and current, because they consist of a single alternating current and voltage wave.

• A single-phase alternator have one armature winding only. 

• But if the number of armature windings is increased, then it becomes polyphase alternator and it 

produces as many independent voltage waves as the number of windings or phases.

• These windings are displaced from one another by equal angles, the values of these angles being determined by the number of phases or windings. 

• In fact, the word ‘polyphase’ means poly (i.e. many or numerous) and phases (i.e. winding or circuit).

• In a two-phase alternator, the armature windings are displaced 90 electrical degrees apart.

• A 3-phase alternator, as the name shows, has three independent armature windings which are 120 electrical degrees apart.

• Hence, the voltages induced in the three windings are 120° apart in time phase. 

• With the exception of two-phase windings, it can be stated that, in general, the electrical Displacement between different phases is 360/n where n is the number of phases or windings.

• In a 3 phase system, there are three equal voltages or EMFs of the same frequency having a phasedifference of 120 degrees. These voltages can be produced by a three-phase AC generator having three identical windings displaced apart from each other by 120 degrees electrical.

• When the magnetic field is kept stationary, and these windings are rotated and an emf is induced in each winding.

• The magnitude and frequency of these EMFs are the same but are displaced apart from one another by an angle of 120 degrees.

ADVANTAGES OF THREE PHASE SYSTEM:

• To transmit a specific power over a specific distance at a given rated voltage, a three phase system needs less conductor material as compared to the single phase system.

• The size of a three phase system operated machine is less than the machine operated at single phase voltage having the same output rating.

• A polyphase system produces power at a constant rate in the load.

• A three phase system can transmit more power as compared to a single phase system.

• Three phase machines are less costly and more efficient.

• A three phase system provides constant power while a single phase system provides pulsating power which leads to a smooth and vibration free operation of a 3-Φ machine as compared to the 1-Φ machines with noise and vibration.

• If a fault occurs on a single phase line, the whole system will have to shut down but this is not the case in three phase system.