Electrical Terms and Definitions
4 Single-Phase Systems vs. Three-Phase Systems
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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.
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.
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.
The simplest electrical circuit. It requires only two lines: one for power to go in and the other is a return path for current. 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, such as a control circuit.
In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.
An electrical circuit that uses 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. The Power Circuit of three-phase motors is an example of a three-phase circuit.
The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).
The rate of flow of an electric charge, measured in amperes (or amps). When one coulomb of charge moves past one point in once second, current is said to flow at a rate of one ampere. Current flows from negative potential to a positive potential through a load.