AN4121

Three-phase systems

Doc ID 023243 Rev 1

18/26

Appendix A

Three-phase systems

Three-phase is a common method of electric power transmission. It is a type of poly-phase

system used to power motors and many other devices.

The currents are sinusoidal functions of time, all at the same frequency but with different

phases. In a three-phase system the phases are spaced equally, giving a phase separation

of 120°. The frequency is typically 50 Hz in Europe and 60 Hz in the US and Canada.

Figure 12.

Instantaneous voltage (or current) in one voltage cycle of a three-phase

system

The three phases may be supplied over six wires, with two wires reserved for the exclusive

use of each phase. However, they are generally supplied over three or four wires:

●

Three-phase, 3-wire delta service which has no neutral and 220 V between phases

●

Three-phase, 4-wire delta and wye service which has 220 V between phase-neutral

and 380 V phase-phase.

A.1

Power in three-phase AC circuits

Let's assume that the angle between the phase voltage and the phase current is

θ, which is

equal to the angle of the load impedance. Considering the load configurations given in

Figure 14, the phase power and the total power can be estimated easily.