June 16th, 2015

What are the Differences between Single and Three Phase Circuits?


If you fall into a baffled haze when attempting to discern the principles that govern single-phase and three-phase electrical distribution, you’re not alone. There are mathematical graphs that illustrate these seemingly abstract principles, but they don’t demonstrate the benefits of one circuitry type over the other. In short, three-phase systems utilize certain advantages of alternating current theory, advantages that aren’t readily apparent unless you understand the basics of power distribution.

This is the tough part, the deciphering of arcane electrical rules as described by current changes over time. This is, of course, not an issue with direct current circuitry (D.C.) since the power produced by such circuitry is comparable to a straight line, such as that generated by a battery. Alternating current, on the other hand, generates waveforms of current that move in a sinusoidal curve from positive to negative polarity. That’s where we get the standard 50 or 60 Hertz figure from, because the electric current is rapidly switching. This simple form of electricity is what filters into the home, and it’s known as single-phase current. It’s efficient at supplying low power appliances in the home, but something a bit more special is required for high-tension power distribution.

Look to overhead power cables and cross-sections of underground distribution cables coming out of power stations. They typically use three conductors, unlike single-phase circuits, the home circuitry that depends on a single “live” conductor. Note, your home cabling may seem to have three wires, but this is in fact one power wire, a neutral or return wire, and a grounding cable. Now, on returning to three-phase supplies, these three conductors each carry a single-phase sine wave, with each waveform synchronized to be 120° out of phase with its neighbour. It’s a tough principle to envision but a very handy means of transmitting large amounts of electricity. For example, the standard three-phase power supply in Australia uses three 230 Volt lines, but the aggregate combined voltage of this synchronized energy distribution amounts to a total of 400 Volts, a voltage that, as we mentioned, is switching rapidly at 50 Hertz (Hz).

The abstract principles that form these two forms of electrical distribution may be tough to fathom, but the advantages are, thankfully, very obvious. Three-phase is ideal for power distribution because of lower energy losses and reduced conductor demands. It’s also an extremely popular electrical engineering solution when applied to motors, with 3-phase motors creating a rotating magnetic field that offers self-starting features. Again, this applies to larger motors, engines of rotational force that require higher quantities of torque. It’s best to leave the concept here because the three-phase equations behind motor design can become mind-bogglingly complex. Special switches are even used to change the 3-phase configuration when a motor is started, arrangements that alter the three points of energy distribution from the so-called “star” starting configuration to the steady-running “delta” arrangement.

Single-phase power is more than adequate for domestic appliances. For power distribution and the three-point energy supplies used in industrial-class equipment, only three-phase current will satisfy.

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