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The difference between a single-phase and a three-phase power system

Single-phase electricity

Single-phase is the most common system and is mainly used in homes, while a three-phase system is common in industrial or commercial buildings, where heavy loads of power are required.

Single-phase systems use alternating current (AC) electric power in which the voltage and current flow changes in magnitude and direction in a cyclical fashion, typically 50 to 60 times per second. In the UK, single-phase voltage is 230 volts.

In electrical engineering, single-phase electric power refers to distribution using a system in which all the voltages of the supply vary in unison.

In simple terms, single-phase electricity can be viewed very much like a single-person canoe. The paddle enters the water to deliver power and then leaves the water before the second paddle re-enters the water to deliver more power, resulting in a variation of power.

At times there will be zero output of power and in the cycle there will be two peak power outputs, see diagram below.

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Figure 9: Single-phase power graph

Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single-phase supply connected to an alternating current electric motor does not produce a revolving magnetic field; single-phase motors need additional circuits for starting, and such motors are uncommon above 10kW or 20kW in rating.

Special single-phase traction power networks may operate at 16.67Hz or other frequencies to power electric railways.

Three-phase electricity

In simple terms, three-phase electricity can be viewed as three single-phase electricity supplies that supply their peak power 120° apart.

As an analogy, consider a canoe with three canoeists, paddling the canoe in rotation. Unlike with a single canoeist, there is always a power output and never a zero output, which makes this power supply more suitable for industrial motors and equipment.

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Figure 10: Three-phase power graph