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What Is Three-Phase Power?

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  • Originally Written By: Garry Crystal
  • Revised By: Andrew Jones
  • Edited By: Niki Foster
  • Last Modified Date: 24 August 2014
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Three-phase power is a method of electrical power transmission that makes use of three wires to deliver three independent alternating electrical currents. The current in each wire is set off from the others by one-third of a complete cycle, with each current representing one phase. This means that a device operating off this type of power source receives a more stable flow of electricity than it would from single-phase distribution system. Some three-phase power systems actually have four wires; the fourth is a neutral wire that allows the system to use a higher voltage.

Purpose

The three currents, together, deliver a balanced load, something not possible with single-phase alternating current. In alternating current (AC), the current alternates direction, flowing back and forth in the circuit; this means that the voltage alternates as well, constantly changing from maximum to minimum. Three-phase power combines the three wires to off-set the maximum and minimum oscillations, so that a device receiving this type of power does not experience such a wide variation in voltage. This makes three-phase power a very efficient form of electrical power distribution. Consequently, a three-phase electric motor uses less electricity and normally lasts longer than a single-phase motor of the same voltage and rating.

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Origins

Three-phase power flow begins in a power station, where an electrical power generator converts mechanical power into alternating electrical currents. After numerous conversions in the distribution and transmission network, the power is transformed into the standard voltage supplied to homes and businesses, 230 volts in Europe or 120 volts in North America. The output of the transformer usually connects to the power system using three live wires tied to a single grounded return. This is called a star connection.

Applications

This type of system does not usually provide power to domestic houses, but when it does, a main distribution board splits the load. Most domestic loads use single-phase power because of the lower cost of distribution. Three-phase power is most common in industrial settings, or where more power is needed to operate heavy machinery, though there are exceptions.

Running electric motors are the most frequent use for three-phase power. A three-phase induction motor combines high efficiency, a simple design, and a high starting torque. Industrial fans, blowers, pumps, compressors, and many other kinds of equipment commonly use this type of electric motor. Other systems that may use three-phase power include air conditioning equipment, electric boilers, and large rectifier systems used for converting alternating current to direct current.

While most motors that run on three-phase power are quite big, there are examples of very small motors, such as those that power computer fans, which work on this type of power. An inverter circuit inside the fan converts direct current (DC) to a three-phase AC current. This serves to decrease noise, as the torque from a three-phase motor is very smooth.

Standards

The wires, called conductors, used in a three-phase power system are normally color-coded, although the colors vary greatly by location, and most countries have their own codes. North America traditionally uses black, red, and blue to represent the three phases, for example, while white represents the neutral wire. In Europe, by contrast, brown, black, and grey represent the phases, and the neutral wire is blue. Even with these national standards, there tend to be a lot of irregularities in day-to-day applications. It is not a good idea for anyone working with three-phase power to make assumptions without consulting the diagram for the individual installation or system in question.

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Discuss this Article

anon966937
Post 23

When I look at high trans lines, do the three distinct lines represent a phase or are the three wires bundled into one wire? If so, what do the other wires represent on transmission lines?

anon274263
Post 21

I have three phase power to my home. I moved in and had the electricians ran a 240 (not realizing the house had three phase) and promptly installed an new ac unit. So here is my question. Can I convert the motors in a 5 ton, 13 SEER unit to 3 phase motors to save money if it is a split system? And if so, what would the savings be?

anon269898
Post 20

Why is it when a 3-phase plug is connected to a 3-phase outlet that's not secure (not connected properly), that is supplying a 3-phase board with 230 outlets, I still get 415 through the board only tripping some RCDS and blowing components in equipment.

anon155270
Post 19

can somebody help me with this question: Explain why it is important to use the correct equipment when measuring three-phase power and the impact this would have on circuit calculations when using any of these measurements.

cscs
Post 18

My country is 230v single phase and 415v 3 phase AC supply. Someone asked me why we don't call 230v single phase and 230v 3 phase since it is 230v if measure from line to neutral on a 3 phase circuit.

Is there a standard way of naming these?

anon138206
Post 16

what happens when the neutral on a 3 phase supply 'burns out' as it it did to me -- my loss adjustor seems baffled as to the results to my electrical equipment.

anon106506
Post 15

It may help you understand three-phase power if you realize that while the electricity pulses, it is pulsing at a different time on each wire. A three-phase motor needs all three of these pulses at the correct times so this is why you cannot simple connect it to one-phase power which only has one of the three pulses.

anon106505
Post 14

For the people asking about replacing three-phase motors with one-phase motors, you now need to get the same amount of energy through less wires. The wire sizes and breaker ratings must be increased, often substantially.

anon102657
Post 13

Does using a three phase meter on single phase machines save power usage in any way?

anon73448
Post 12

my question is that in a three phase line,what is the current and voltage in one line.

anon72571
Post 11

#2 - One of the legs of the three phase systems is 120v, the circuit consists of the the line, neutral and ground. This is your standard household wiring.

#1 - There is 120v per each leg in a three phase system. L1, L2, L3, neutral (common) and a ground, so 5 wires in total. L1 is normally black, L2 is normally red and L3 blue. The neutral is white and the ground green or green with a yellow stripe.

anon39827
Post 9

Have a 5-ton 208-230 3 phase AC unit. Can it be replaced with a 5-ton 208-230 1 phase AC unit by just using two of the three phase leads?

anon36476
Post 8

I'm replacing two 3-phase 240v 5-ton condenser units that wererun simultaneously and were run with one 3 phase breaker (60A) with two single phase 240volt 5-ton units. I'm thinking of using the A-C legs to one condenser and and B-C legs to the other Assuming I don't exceed the breaker amperage, will the excess amperage on the C leg foul up the voltages. The C leg is under used compared to the A and B off the pole. Thanks

doiturself
Post 7

Maybe I can clarify with my question. I have what I think is a three phase motor pulled out of my dishwasher that powered the pump. It has a black, tan, and blue wire. I want to power it with a simple black/white wire plugged into the wall. Is that possible? Can I split the black into thirds and just push it all into the motor?

anon21912
Post 6

how do you calculate the cost of operating a three phase motor? i know single phase motor calculation is amps x volts x operating hours= watt hours, divide that by 1000 for kilowatt hours and multiply by electricity cost will give operating cost. is it the same for three phase?

gareth
Post 5

hello

i have a stucco pump in spain ive been using on spanish 3 phase power ( pump is electric motor )i want to bring this machine back to america and use it here, what do i have to do to run it here,if that is at all possible, someone in spain mentioned the motor cycle is different in spain than usa,,i know nothing about this so totally unsure.thank you .

anon19552
Post 4

A 3-phase power source would not normally be converted to single-phase. A single-phase load can be powered from one leg of the 3-phase source. In a four-wire, 3-phase system, the fourth wire acts as the neutral conductor. Single-phase loads can be connected between one phase and the neutral or between two phases. In both situations, the loads should be balanced so that each phase has an equal or near equal power load.

Jeff1151
Post 2

How do you convert a 3 phase power source to single phase? to standard 110V? 60 cycle house current?

anon15551
Post 1

i guess i still don't understand. what is travelling through each of the 3 wires? is one of them grounded? what is meant by the reference to one of them (sometimes) being neutral?

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