Three Phase Circuits Basic Theory:

PolyPhase Systems:

Single phase system supplying power to loads has its own limitations and has been replaced by polyphase system. For general supply of power three phase system is universally used. For generation, transmission and distribution of the electrical power 3-phase system has been universally adopted.

Polyphase system means the system which consists of numerous windings or circuits. A polyphase system is essentially a combination of several single phase voltages having same magnitude and frequency but each phase is displaced from another phases by equal angle (electrical degrees) which depends on the number of phases and can be determined from the following relation

Electrical Displacement = 360 electrical degrees / Number of phases

The above relation doesn’t holds good in the case of 2-phase system where the phase displacement is 90^o

In a symmetrical several phases of voltages of the system frequency have equal magnitude and and each phases are displaced from one another phases by equal electrical time angle.

A 3-phase system is considered unbalanced when the following conditions:

• When the phase voltages of three phase system are unequal in magnitude
• When the phase angle between the phases is not equal to 120^o

A load circuit is said to be balanced when the loads (impedance) connected in various phases are same in magnitude as well in phase. Any three phase loads in which the impedences in one or more phases differ from the impedances of the other phases is called an un-balanced three phase load.

In case when one of the supply phases of three phase system supplying power to three phase load is out or nor available, then such a condition is called single phasing.

Advantages of 3-phase system compared to single phase system:

• The rating (eg: horse power) of given machine increases with increase in the number of phases of the machine. The output of a three phase machine 1.5 times of that of single pole machine of same size.
• In case of single phase system, the power delivered is pulsating in nature which causes excessive vibrations in the large motors. In three phase system, the power deliver to the loads are constant if the loads are balanced
• In industries most of the motor drive applications is performed by induction motors because of their rugged construction and low cost. Three phase induction motors are self starting whereas single phase induction motors are not
• In a balanced three phase system, the size of the conductors needed will be about 75% the size of conductors for a single phase, two wire system for the same kVA rating. This advantage helps in offset the requirement of third conductor in three phase system
• Efficiency and power factor of single phase motors are poor compared to three phase motors
• Three phase system is more reliable and capable than single phase system
• Parallel operation of three phase alternators (synchronous generators) is simple compared to that of single phase alternators because of pulsating reaction in single phase alternators

Interconnection of 3-phases:

Each coil or conductor of three phase system has two terminals (start and finish),

Star or Wye Connected System:

Delta or Mesh Connected System: