8.3 : 三相電圧

In three-phase four-wire systems, three Y-connected voltage sources generate balanced phase voltages.

There are two phase sequences based on the order in which the phase voltages reach their maximum values.

The first, abc or positive sequence, involves phasors rotating anticlockwise, with V_an leading V_bn, which leads V_cn. This sequence occurs when the rotor rotates counterclockwise.

The second, acb or negative sequence, has phasors rotating anticlockwise, and V_an leads V_cn, which leads V_bn. This sequence corresponds to clockwise rotor rotation.

Like voltage sources, the three loads can be Y-connected or delta-connected.

In balanced load configurations, the phase impedances are equal in magnitude and phase.

The Y-connected loads can be transformed to delta-configuration and vice versa.

Based on the configurations of the three-phase source and load, four possible connections exist.

Delta-connected loads are more common than Y-configuration due to the ease of adding and removing loads from individual phases without causing excessive imbalances.

However, delta-connected sources are less common than Y-connected sources due to the circulating current in the delta-mesh when the three-phase voltages are slightly unbalanced.