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Three-Phase Power Equations

Electrical 3-phase equations

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Most AC power today is produced and distributed as three-phase power where three sinusoidal voltages are generated out of phase with each other. With single-phase AC power there is only one single sinusoidal voltage.

Real Power

Wapplied = 31/2 U I cos Φ

             = 31/2 U I PF                      (1)


Wapplied = real power (W, watts)

U = voltage (V, volts)

I = current (A, amps)

PF = cos Φ = power factor (0.7 - 0.95) 

For pure resistive load: PF = cos Φ = 1 

  • resistive loads converts current into other forms of energy, such as heat
  • inductive loads use magnetic fields like motors, solenoids, and relays

Power Factor

Typical power factors:

DevicePower Factor
Lamp, fluorecent uncompensated 0.5
Lamp, fluorecent compensated 0.93
Lamp, incandescent 1
Motor, induction 100% load 0.85
Motor, induction 50% load 0.73
Motor, induction 0% load 0.17
Motor, synchronous 0.9
Oven, resistive heating element 1
Oven, induction compensated 0.85
Pure resistive load 1

Example - Pure Resistive Load

For pure resistive load and power factor = 1 the real power in a 415 voltage 20 amps circuit can be calculated as

  Wapplied = 31/2 (415 V) (20 A) 1

         = 14400 W

         = 14.4 kW

Total Power

W = 31/2 U I                       (2)

Brake Horsepower

WBHP = 31/2 U I PF μ / 746                       (3)


WBHP = brake horse power (hp)

μ = device efficiency

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