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 Question 249

 What is the difference between single-phase and three-phase electric power?
In single-phase power, current flows to and from a device through a pair of wires. The direction of the current flow changes with time, reversing smoothly 120 times a second in the US or 100 times a second in Europe (60 or 50 full cycles of reversal, over and back, each second respectively). In its simplest form, one of the two wires is called "neutral" and its voltage is always close to 0 volts (meaning that it has essentially no net electric charge on it). The other wire is called "power" and its voltage fluctuates from positive to negative to positive many times a second (meaning that its net electric charge varies from positive to negative to positive). The difference in voltage between "neutral" and "power" propels current through the device.

In three-phase power, current flows to and from a device through a group of three wires. These three wires are often called "X", "Y", and "Z", and each one is a power wire with a voltage that fluctuates from positive to negative to positive many times a second. (A fourth wire, "neutral", with a voltage of approximately 0 volts, may also be used.) But while the voltages of the three power wires fluctuate up and down the same number of times each second, they do not reach their maximum or minimum voltages at the same time. They reach their peaks one after the next in an equally spaced sequence: first "X", then "Y", then "Z", and then "X" again and so on. Because these three wires or "phases" rarely have the same voltages, currents can and do flow between any pair of them. It is such current flows that power the devices that use three-phase electric power. The natural sequencing of the three phases is particularly useful for devices that perform rhythmic tasks. For example, three-phase electric motors often turn in near synchrony with the rising and falling voltages of the phases.

Another advantage of three-phase electric power is that there is never a time when all three phases are at the same voltage. In single-phase power, whenever the two phases have the same voltage there is temporarily no electric power available. That's why single-phase electric devices must store energy to carry them over those dry spells. However, in three-phase power, a device can always obtain power from at least one pair of phases.