AN3157

3-phase systems

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Appendix A

3-phase systems

3-phase is a common method of electrical 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 3-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.

It is possible to supply the three phases 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 phases, 3-wire delta service which has no neutral and 220 V between phases.

●

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

and 380 V phase-phase.

Figure 12.

Instantaneous voltage (or current) in a one-voltage cycle of a 3-phase system