Understanding Threephase Power

You may be most familiar with the type of power which comes from the wall socket in your house. This is single-phase, 110 VAC, 60 Hz power. That's 110 Volts, Alternating Current, at a frequency of 60 Hz, if you live in North America. For those in Europe, 50 Hz is common. One cycle of the single phase waveform goes through 360 angular degrees, from start to finish, as shown in Figure 6-1.

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Figure 6-1: Single - Phase AC Output

Even though you may be most familiar with single phase 110 VAC, in fact, most of the electric power in the world is three-phase. The diagram in Figure 6-2 below shows three-phase power.

If you are thinking the diagram looks like 3 single-phase waveforms, each slightly shifted from one another, you're exactly right. This shift is the "phase". In this context, the term "phase" refers to the time displacement of electrical energy. Each separate sine wave is spaced, or phased, 120 angular degrees apart from one another. Phase 2 starts 120 degrees after Phase 1, and Phase 3 starts 120 degrees after Phase 2.

Figure 6-2: Three - Phase AC Output

Phase 2 starts 120 degrees after Phase 1, and Phase 3 starts 120 degrees after Phase 2. If you are wondering how all those phases travel on a single conductor, they don't. Typically, three-phase power is delivered to buildings through 4 wires. There is one wire for each of the three phases, plus a ground wire.

The concept of three-phase power was originally conceived by Nikola Tesla. Tesla proved that three-phase power was far superior to single-phase power. In a single-phase unit, the power falls to zero during each cycle. In a three-phase unit, however, it never drops to zero. No matter where you are in the cycle, one of the three phases is nearing its peak. As a result, the power delivered to the load is nearly the same at any instant. Furthermore, three-phase is typically 150% more efficient as compared with single-phase within the same power range, and in a three-phase unit the conductors need only be 75% the size of conductors for single-phase for the same power output. All these advantages make three-phase power efficient to produce and distribute. Let's find out why.

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