Although both single- and three-phase power supplies use alternating current (AC), the delivery systems differ in function. Single-phase systems are most commonly found in residential areas and structures that consume little electricity. Three-phase systems, however, are used for environments that consume a large amount of electricity to power large loads, such as machines in heavy industrial applications. Knowing which power supply to use in a given application may be tricky to figure out, but understanding the differences between single- and three-phase power supplies will help the decision making process.
When dealing with AC electricity, the power is constantly flowing in a wave, or cyclical, pattern. Each one of these waves can be thought of as a phase. So, in a single-phase system, there is only one wave of electricity constantly oscillating back and forth. This means that when the single wave reaches the top of its pattern there is no electrical wave at the bottom, and vice versa. Because of these peaks and dips in power, the delivery of electricity in a single-phase system is not supplied at a constant rate, and it suffers from very small power drops every second. These small power drops are not noticeable in smaller applications, such as those in a household, but could be a detriment when attempting to power larger loads.
Traditionally, single-phase power supplies contain two wires. There is a neutral wire that does not deliver electricity and a power wire that feeds the circuit. Depending on the specifics of the system, which can change geographically or based on application, the cyclical changes of direction of the electricity (the wave patterns) usually change flow 50 or 60 times per second. This rate of change in flow is also referred to as Hz, so a system that changes direction 60 times per second would be a 60Hz system.
In a delta three-phase system there are no neutral wires, only three power wires. These three power wires are out of phase with each other, meaning that their wave cycles do not occur at the same time. In fact, they occur exactly 120-degrees apart from each other. This 120-degree difference is not arbitrary, as it is one third of a 360-degree waveform. Because each waves is spaced evenly, there are not drops of power as are present in a single-phase system. The constancy of the power delivery in three-phase systems allows for applications with heavier load requirements, such as industrial machinery, to be properly supplied with power in order to be run more efficiently.
Even though three-phase systems are more efficient with larger applications, this is not the case with most smaller or residential applications. Units or machines in need of any less than about 1,000 watts of power actually function more efficiently, and more cost effectively, on a single-phase system. Single-phase systems also offer a broader array of application uses and fewer design costs when planning for installation.
Three-phase systems offer benefits and savings as well, but they are most apparent in high load systems. When utilizing three-phase systems in applications requiring more than 1,000 watts, users will most likely find they now have the ability to run higher power loads with more consistent rates, fewer safety risks for workers and lower labor handling costs during installation. There is also a reduction of copper consumption in three-phase systems.
Certain applications might require the use of both single- and three-phase systems, and installing a Wye configuration three-phase system can help. When wired in the Wye configuration, a neutral and a ground wire are also present in a three-phase system. This configuration supplies a three-phase system with much more versatility and flexibility, as users can now utilize the ground and neutral wires to enable the system to deliver single-, double- and three-phase power.
Although the common differentiating factor between single- and three-phase systems is residential use as opposed to industrial, making precise calculations regarding the total and potential load of a system is always a smart choice when trying to make the decision on which system to install.