3-Phase Power Explained

What is 3-Phase Power?

Power Analogy

Imagine a dark room with three treadmills and three distance runners. The runners are wearing special shoes that briefly light up with each step.

Single-phase power is when one person is running while the others stand by. When a step is taken, the room lights up for a brief moment. In the moments between steps, the room stays dark.

Three-phase power is when all three people are running simultaneously. They work together to make sure at least one runner's foot is down at all times to keep the room lit.

In this analogy, each runner represents a hot wire of a power distribution system and the light intensity represents the phase voltage.

Power Distribution to Server Racks

Electrical power is delivered from a generator to the electrical grid to industrial buildings in the form of alternating current (AC) three-phase power. Industrial buildings like data centers must decide how to further distribute this power to the IT equipment on server racks. At the most basic level, there are two options:

Option #1: Distribute single-phase power to the racks. This typically involves power distribution through a single LINE (hot) wire and a NEUTRAL wire.

Option #2: Distribute three-phase power to the racks. This typically involves power distribution through three LINE wires, one for each phase, and in some cases a NEUTRAL wire.

In North America, the LINE wires in a "208V" service typically have an oscillating voltage between -170V to +170V peak-to-peak with respect to the NEUTRAL wire. When discussing AC circuits, voltage RMS (the equivalent DC voltage) is used instead of peak voltages. For example, an AC voltage with 170V peak is referred to as 120VAC (Voltage Alternating Current).

In the single-phase plot below, positive voltages are represented by the left shoe, negative voltages by the right shoe. Notice both shoes provide the same amount of light at the peaks. In other words, the voltage polarity does not matter. Both are useful.

Single Phase - Analogy Plot

Single-phase rack power distribution units (PDUs) are designed to receive and distribute single-phase power to IT equipment. These PDUs are internally wired to deliver 120VAC (LINE-to-NEUTRAL) directly to the outlets. Equipment connected to these outlets will operate with 120VAC.

In comparison, a three-phase system has three LINE wires each measuring 120VAC with respect to the NEUTRAL wire. The plot below shows three-phase power with the LINE voltages overlaid. Notice how each LINE voltage is shifted in time. 

Three-phase rack PDUs are designed to receive three-phase power at the input and distribute three groups of single-phase outputs to the IT equipment. These PDUs are often wired in a delta configuration producing a higher voltage RMS at the outlets at 208VAC to 240VAC. Equipment connected to these outlets will operate with these higher voltages.

Higher voltages are achieved by internally wiring the outlets LINE-to-LINE instead of LINE-to-NEUTRAL. What happens when two runners have shoes that are lit up at the same time? The intensity of the light in the room increases. 

The combined light is illustrated by the yellow line in the plot below. The plot is an example of an outlet that is wired LINE1-to-LINE2 which produces 208VAC. This translates to more power and higher efficiency for the connected equipment.

3-Phase Delta PDU: Outlet Voltage Plot

This analogy just scratches the surface but we hope it provides insight with a simplified model for thinking about 3-phase power.

Our Solutions

Synaccess offers a wide range of metered and smart rack PDUs for single-phase and three-phase power distribution.

  • Metered PDUs provide reliable power distribution with local current meter displays.
  • SynLink smart PDUs are network connected with remote outlet switching and energy monitoring (kWh) features for complete control and visibility of your equipment.

Contact us for a free consulation or request a free live demo to see our products in action. 


Visit www.synaccess.com to learn more about SynLink Technology.

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