Three phase circuits involve sending three alternating current phases that are 120 degrees out of phase with each other from a generator to a load. This has several advantages over single phase systems, including greater capacity, more uniform torque production in motors, and using less copper for the same amount of power transmitted over a distance. In a balanced three phase star connection, the line voltages are equal in magnitude and differ in phase by 120 degrees, forming a balanced voltage of magnitude square root of 3 times the phase voltage. The neutral current is zero if the phase currents are balanced.

1. BALANCED THREE PHASE
CIRCUITS
EPM116s Fall 2022

2. BALANCED THREE PHASE CIRCUITS
• In 1882, new invention has been done polyphase system, that more
than one phase can be used for generating, transmitting and for load
system.
• Three phase circuit is the polyphase system where three phases are
sent together from generator to the load.
• Each phase are having a phase difference of 120o , i.e 120o angle
electrically. So from the total of 360 , three phase are equally divided
into 120o each.

3. Advantages of 3-phase System
A 3-phase system has the following advantages over single phase
system.
• A 3-phase machine will have large capacity than a single phase
machine.
• The torque produced in a 3-phase motor will be more uniform where
as in a 1-phase motor it is pulsating.
• The amount of copper required in a certain amount of power over a
particular distance, is less compared to a single phase system.

4. The sinusoidal waves for 3 phase system is shown below

6. Delta connection:
Star connection:

7. Relation between line and phase voltage and
currents in balanced systems: Star connection

9. • The line voltages VAB, VBC, VCA are equal in magnitude and differ in phase
by 120o . Hence they form a balanced 3-phase voltage of magnitude √3
Vph.
• The magnitude and phase angle of current is determined by circuit
parameters.
• IA, IB, IC are line or phase currents. The current in the neutral wire is IN
and is by applying kirchoff’s current law at star point, we get
• IN = - (IA+ IB+ IC) If the currents are balanced, then the neutral current is
zero.

10. Delta connection or Mesh connection:

12. Balanced star and delta load

13. Average power of balanced Y-Load

14. Complex power of a balanced Y-Load

15. One-phase instantaneous powers The
instantaneous power of load ZA is:

16. Total instantaneous power
• The instantaneous power of the entire Y-Load is a
constant independent of time

17. Example