Page 176 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

1
2
I 2 AC I I 2I A I C cos60 (Q cos60 2 )
2
C
A
2
I I 2 C I A I C
A
Let I A I C I(phasecur rent)
I 2 AC I 2 I 2 I 2
I 2 AC 3I 2

Linecurren I t, 2 AC 3I
It showns that the line current in delta connection

is 3 times the phase current,
i.e. I I 3
L ph
In delta connection,
Line current = phase current
3

3-Phase AC fundamentals

Objectives: At the end of this lesson you shall be able to
• state and describe the generation of 3-phase system with single loops
• state the advantages of the 3-phase system over a single phase system
• state and explain the 3-phase, 3-wire, and 4-wire system
• state and explain the relation between phase and line voltage.

A three-phase power consumer is provided with the terminals of three phases. (Fig 1)

One great advantage of a three-phase AC supply is that it can produce a rotating magnetic field when a set of
stationary three-phase coils is energized from the supply. This is the basic operating principle for most modern
rotating machines and, in particular, the three-phase induction motor.

Further, lighting loads can be connected between any one of the three phases and neutral.
Review: Further to the above two advantages the following are the advantages of polyphase system over single
phase system.

• 3-phase motors develop uniform torque whereas single phase motors produce pulsating torque only
• Most of the 3-phase motors are self starting whereas single phase motors are not
• Power factor of 3-phase motors are reasonably high when compared to single phase motors

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CITS : Power - Electrician & Wireman - Lesson 26-29