Single and Three Phase ac Power

Single-phase ac consists of either a single voltage or two voltages in series, with exactly the same phase relationship. Three-phase power, which is the most common form of power used by industrial facilities, consists of three separate voltages spaced 120 electrical degrees apart using three lines. This is shown graphically in Figure 24-9. No two phases are ever at zero voltage at the same time, and no two phases are ever at peak voltage at the same time. Also, when any one phase is at zero voltage, the other two phases are of opposite sign (polarity) and are at 86.6% of peak voltage.

The advantages of three-phase versus single-phase power are:

• It requires less cable to supply power at a given

Fig. 24-9 Plot of Three-Phase Power. voltage.

• It produces a magnetic field of constant density that rotates at the line frequency, thus reducing the complexity of motor construction.

Fig. 24-9 Plot of Three-Phase Power. voltage.

• It produces a magnetic field of constant density that rotates at the line frequency, thus reducing the complexity of motor construction.

• It provides constant instantaneous output of the alternator if each of the phases has an identical load.

When calculating the relationship between three-phase power, voltage, and amperage, a phase factor of 1.732 (square root of 3) is used.

Basic ac power formulas are:

Term

Single-Phase Formula

Three-Phase Formula

Equation

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