Page 181 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

In a three-phase star connection, the line voltage is always times the phase-to-neutral voltage.
3
The factor relating the line voltage to the phase voltage is (Fig 3).
3
The voltage and current relationship in a star connection is shown in the phasor diagrams. (Fig 4) The phase
voltages are displaced 120° in phase with respect to each other.

Fig 4
Fig 3

Derived from these are the corresponding line voltages. The line voltages are displaced 120° in phase with
respect to each other. Since the loads in our example are provided by purely resistive impedances, the phase
currents I (I , I , I ) are in phase with the phase voltages V (V , V and V ). In a star connection, each phase
P
P
W
V
U
UN
WN
VN
current is determined by the ratio of the phase voltage to the load resistance R.
Delta connection: There is a second possible arrangement for connecting a three-phase load in a three-phase
network. This is the delta or mesh connection (D).(Fig 5)
The load impedances form the sides of a triangle. The terminals U, V and W are connected to the supply lines
of the L , L and L .
1 2 3
Fig 5

In contrast to a star connection, in a delta connection the line voltage appears across each of the
load phases.

The voltages, with symbols V , V and V are, therefore, the line voltages.
WU
UV
VW
The phase currents through the elements in a delta arrangement are composed of I , I and I . The currents
UV
WU
VW
from the supply lines are I , I and I , and one line current divides at the point of connection to produce two phase
W
V
U
currents.
168
CITS : Power - Electrician & Wireman - Lesson 26-29