Why electricity comes in three phases

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

25 February 2007

Tom Ouma has been thinking about electricity. He says, “I understand the meaning of volts, watts, ac, dc, etc, but what is this single-phase and 3-phase that I hear electricians talking about?. Can you shed some light?”

First, the basics: The electricity we get from the Power Company is alternating current (ac). That is, it does not flow in one direction; it is constantly oscillating to-and-fro at the rate of 50 times per second.

In single-phase electricity there are two wires – one live, the other neutral. In 3-phase, there are three live wires each with current alternating 50 times per second. However, these oscillations are not in unison. That is, when the current is flowing in the “forward” direction in one wire, it could be moving in “reverse” in the next one.

But that does not mean that they are in direct opposition to one another. No; the three oscillations are somewhere in between – not in unison and not opposing one another.

Now, if something makes 2 swings per second, it means that each swing lasts only half a second (that is, one divided by two). Thus, for the current that makes 50 oscillations per second, the duration of each cycle is only 0.02s (one divided by 50).

In 3-phase electricity, the current then flows in the three wires in the following manner: first, the cycle duration (the 0.02s) is divided into three portions each lasting 0.0067s. The current then begins to move in the “forward” direction in the first wire. After 0.0067s (one third of 0.02s), electricity starts flowing in the “forward” direction in the second wire. When 0.01s elapse (half a cycle), the current in the first wire reverses its direction.

At 0.0133s (two thirds of a cycle), the current in the third wire now starts flowing in the forward direction. At this moment, there will be two “forward” currents (wire 2 & 3) and one in “reverse” (wire 1).

This process continues indefinitely with the current in each wire alternating 50 times per second, until the time the power is switched off. The important thing to remember is that, the three oscillations are out of harmony (or, out of phase) by one third of a cycle.

But the big question is: why go to all that trouble of making 3-phase electricity? The answer is in the efficiency of the generator.

A simple generator has a coil of wire rotating in-between the poles of a magnet. As it goes round, the coil spends some time outside the poles and in that duration, no electricity is produced. For this reason, the power generating companies use three coils arranged in a manner to ensure that at any one time there is a coil moving between the poles – that’s how we get the 3-phases, one from each coil.