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Single-Phase Power vs. Three-Phase Power: A Quick Introduction

Single-Phase Power vs. Three-Phase Power: A Quick Introduction

Especially with larger equipment, you’re likely to run into the issue of single-phase vs. three-phase electrical power.  We thought we’d post this short primer to describe the difference between single-phase and three-phase electrical power in practical terms and answer a few frequently asked questions.  The purpose of this article is just to provide some general information to those of you that are curious about this topic.  For any actual recommendations regarding your particular situation and needs you’re going to want to contact your local electrician.

What's a "Phase?"

For our purposes, we can think of a “phase” as referring to a particular wave of electrical energy delivered through a wire.  It technically refers to how closely in time one wave is in sync with another wave, but we’ll build up to that.

The "A" in "AC Current" stands for "Alternating"

Electricity is delivered from power stations as alternating current (AC).  It’s not a constant flow of current, but a wave that oscillates between +120 volts and -120 volts 60 times per second (50 times per second in Europe).  That oscillation is caused by the rotary motion of the generators that produce the power at the power station.  The frequency of the oscillation is measured in Hertz (Hz).  So we say household electricity in the U.S. is 120V, 60 Hz.

U.S. Household Electrical Current: 120VAC, 60Hz, Single Phase

120V Service vs. 240V Service

Many commercial and industrial locations are set up with 240V electrical service.  Large motors and other machinery require – or at least run more efficiently – on higher voltage electrical service.  240V hookups (in the U.S.) are also delivered as 60 Hz waves, but they oscillate between +240V and -240V.

Single-Phase Commercial Electrical Service: 240VAC, 60Hz, Single Phase

Here's the important part.

240V electrical service can optionally include three-phase hookups.  Instead of one “hot” wire delivering current and one “neutral” wire providing a return path, three-phase power includes three separate “hot” wires and one “neutral” wire.  Each of the three hot wires alternates between +240V and -240V.  These three hot wires don’t deliver electricity in sync with each other, but are timed so that each wave is one-third of a cycle “out of phase” with the other two.  It looks like this:

Three-Phase Commercial Electrical Service: 240VAC, 60Hz, Three Phase

Now, to see the benefit of three-phase power compared to single-phase, let’s look at just the “positive” sides of the waves, when current is flowing into a motor’s windings:

Single-Phase vs. Three Phase Electrical Power Delivery

This demonstrates how electrical energy isn’t delivered to the motor constantly, but in “pulses” that peak every 1/60th of a second.  With three-phase power, just as one wire’s current is returning down to zero from its peak, the power from the next wire is approaching its peak.  This results in more total power over time, and smoother delivery of the power.  As a result, motors run more efficiently, with less vibration, and they last longer.

The take-away:

If your facility is wired for three-phase service (check with your electrician!) and you are considering a piece of equipment that is available in both single-phase and three-phase versions, go with the three-phase unit.  It will operate more efficiently and may last significantly longer than its single-phase equivalent.

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