Electrical Power Systems Technology

Electrical Power Conversion Systems 303

1000 watts

= —————

240 volts

= 4.167 amperes

The line current is calculated as:

IL = IP × 1.73

= 4.167 × 1.73

= 7.21 amperes

Thus, the total power is determined as:

PT = 3 × PP

= 3 × 1000 watts

= 3000 watts

Another way to calculate the total power is:

PT = 1.73 × VL × IL x pf

= 1.73 × 240 volts × 7.21 amperes × 1

= 2993.59 watts

(Note that your answer depends upon the number of decimal places

to which you carry your calculations. Round off IL at 7.2 amperes, and

your answer for the last calculation is 2989.44 watts, instead of 2993.59

watts.)

The following is an example of the power calculation for a three-

phase wye system. For the problem, “find the line current (IL) and power

per phase (PP) of a balanced, 20,000-watt, 277/480-volt, three-phase wye

system operating at a 0.75 power factor,” we use the formula:

PT = VL × IL × 1.73 × pf

Transposing and substituting, we have:

PT

IL = ——————

VL × PT × pf