The design of three-phase AC circuits lends itself to a more efficient method of producing and utilizing an AC voltage.

EO 1.5

STATE the reasons that three-phase power systems are used in the industry.

EO 1.6

Given values for current, voltage, and power factor in a three-phase system, CALCULATE the following:

a. Real power

b. Reactive power

c. Apparent power

EO 1.7

Given a diagram of a wye- or delta-connected threephase system, DESCRIBE the voltage/current relationships of the circuit.

EO 1.8

STATE the indications of an unbalanced load in a threephase power system.

Three-Phase Svstems

A three-phase (30) system is a combination of three single-phase systems. In a 30 balanced system, power comes from a 30 AC generator that produces three separate and equal voltages, each of which is 120 out of phase with the other voltages (Figure 10).

Figure 10 Three-Phase AC

Three-phase equipment (motors, transformers, etc.) weighs less than single-phase equipment of the same power rating. They have a wide range of voltages and can be used for single-phase loads. Three-phase equipment is smaller in size, weighs less, and is more efficient than single-phase equipment.

Three-phase systems can be connected in two different ways. If the three common ends of each phase are connected at a common point and the other three ends are connected to a 3 line, it is called a wye, or Y-, connection (Figure 11). If the three phases are connected in series to form a closed loop, it is called a delta, or -, connection.

Figure 11 3AC Power Connections