THREE PHASE POWER

THREE-PHASE POWER
Students sometimes become confused when computing power in threephase circuits. One reason for this confusion is that there are actually
two formulas that can be used. If line values of voltage and current are
known, the power (watts) of a pure resistive load can be computed using
the formula:
VA = __
√ 3 ELine ILine
VA =
√
If the phase values of voltage and current are known, the apparent power
can be computed using the formula:
VA = 3 EPhase IPhase
Notice that in the first formula, the line values of voltage and current are
multiplied by the square root of 3. In the second formula, the phase values
of voltage and current are multiplied by 3. The first formula is used more
often because it is generally more convenient to obtain line values of voltage and current, which can be measured with a voltmeter and clamp-on
ammeter.
THREE-PHASE CIRCUIT CALCULATIONS
In the following examples, values of line and phase voltage, line and
phase current, and power will be computed for different types of threephase connections.
A wye-connected three-phase alternator supplies power to a delta-connected resistive load, Figure 12-17. The alternator has a line voltage of 480
V. Each resistor of the delta load has 8 Ω of resistance. Find the following
values:
EL(Load) — line voltage of the load
EP(Load) — phase voltage of the load
IP(Load) — phase current of the load