Electrical Power Systems Technology

54 Electrical Power Systems Technology

Sample Problem:
Given: phase voltage = 120 volts for a three-phase wye system.
Find: line voltage.
Solution:
VL = VP × 1.73
= 120 V × 1.73
VL = 208 volts

The line currents (IL) are equal to the phase currents (IP), or:

IL = IP

In the delta connection of Figure 2-22B, the end of one phase winding
is connected to the beginning of the adjacent phase winding. The line volt-
ages (VL) are equal to the phase voltages (VP). The line currents (IL) are equal
to the phase current (IP) multiplied by 1.73.
The power developed in each phase (PP) for either a wye or a delta
circuit is expressed as:
PP = VP × IP × pf

Sample Problem:
Given: a three-phase delta system has a phase voltage of 240 volts, a
phase current of 20 amperes, and a power factor of 0.75.
Find: power per phase.
Solution:
PP = VP × IP × pf
PP = 240 V × 20 A × 0.75
PP = 3600 watts
where pf is the power factor of the load.

Figure 2-21. Three-phase out-

put waveform developed by

an alternator