Three-Phase Power

Following the voltage, the power derived from an AC source peaks and falls to zero 120 times per second. This causes torque pulsations in motors, creating noise, vibration, and higher shaft stresses. Though minimal in fractional-horsepower household motors, this would be intolerable in larger motors. Larger motors usually require three-phase power, which avoids the problem. Three conductors supply AC voltage, offset in time or phase, so that they peak not simultaneously but at equally spaced intervals. This produces constant smooth torque from
a motor, because at all moments, the sum of power from all three phases is constant. Three-phase power also eliminates the need for special starting windings required in single-phase motors.

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