1.4: Single-Phase Systems vs. Three-Phase Systems

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Aaron Lee
BC Campus

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4 Single-Phase Systems vs. Three-Phase Systems

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A BCcampus element has been excluded from this version of the text. You can view it online here: https://opentextbc.ca/basichvac/?p=31

In electrical systems, we use the terms “single-phase” and “three-phase” fairly often, so a brief description of them will help us moving forward.

Single-phase systems are the simplest electrical circuits. They require only two wires: one for power to go in and the other is a return path for current to go out. These are often called Line 1 and Line 2, or Line 1 and Neutral. Current only has one path to travel in a single-phase circuit, and all of the control circuits that we will be looking at are single-phase.

A circuit diagram with one line that forms a square. I (current), R (resistence), and E (voltage) are labelled. — An AC single-phase circuit.

Three-phase systems are bit more complex. They use three current carrying conductors, called Line 1, Line 2, and Line 3, which have a 120° phase shift in the voltage and current waveforms between them. Each of these conductors are connected to a three-phase load, like a three-phase motor.

When in operation, a balanced three-phase load (such as a motor) has each of its three line’s current values cancel each other out, and so it does not require a return conductor. These loads can be connected in Wye or Delta configuration.

A circuit diagram with three lines connected in Wye configuration. — A three-phase circuit

Unbalanced three-phase loads are mainly connected in the Wye configuration where the central point is used as a neutral conductor to carry any stray return currents. In practice a motor is almost always a balanced three-phase load.

Only large industrial and commercial loads will be supplied by three-phase systems. Most heating and cooling loads, especially those used in residential applications, will be single-phase.

Text Attributions

This chapter was adapted from “Single-Phase Systems vs. Three-Phase Systems” in Basic Motor Control by Chad Flinn and Aaron Lee, which is under a CC BY 4.0 Licence. Adapted by Aaron Lee.