# Tag Info

36

In addition to PlasmaHH's answer, industry uses almost exclusively three phase power since an induction motor needs at least a three phase supply to start and run in a known direction. Single phase induction motors require lossy, unreliable, and expensive tricks to do the same (extra windings, lossy windings, speed sensitive switch, capacitors, etc). The ...

35

Below are my interpretations of the various symbols. I included more than just the line-and-circle symbols, for the benefit of future visitors: 50 Hz AC: Double Insulated: Non-reversing (not suitable for power export or net-metering): CE Marking (meets European Community standards): Three-phase Wye: Single-phase plus grounded neutral: Single-phase ...

30

Why not three lines all in the same phase? Because then there is no return path. Because single phase has no "rotation". Three phase makes it very simple to make a rotating motor with phase sequence determining the direction of rotation. Swap two phases and the direction is reversed. Is there less loss when the phases of the three lines are all different?...

28

To answer your comment on three-phase torroidal: Because it seems, per wikipedia: Toroidal inductors and transformers, that the design should be superior. But I see no mention of three-phase usage, only single-phase. Figure 1. 3-phase transformer flux. Source: NPTEL. In a three-phase transformer each primary and secondary pair are wound on the same "...

27

Three coils, in magnetic series as you've drawn them, will not make a 3 phase transformer. There would be only one value for flux which would be common for all three coils, as each coil surrounds the entire core cross section. In a real three phase transformer, each coil surrounds only part of the core, so that each coil can operate at a different flux. A ...

23

When you have single phase power distribution, you need one phase and one return, both carrying the same current. If you now instead use symmetrical three phase power, you use three phases with a third of the current carrying capability, and you can get rid of the neutral. This simply saves some money in copper. If you now add more phases, you can not save ...

23

Think about two diodes feeding a load resistor. Don't think about 3-phase until after you understand this: - If "A" is 10 Vots and "B" is 9 volts, "A" supplies all the current to the load. "B" can't supply any because its diode is reverse biased. This is what happens in a 3 phase rectifier. Think about the scenario when the phase is about 130 degrees: both ...

21

No need for a complicated formula. If you have balanced three-phase power, where all three phase voltages are equal in magnitude and 120° apart in phase, then: $$V_{L-L} = \sqrt{3} \times V_{L-N}$$ To see why, consider the phasor diagram: Applying some basic trig:

18

Don't believe everything you hear. In the US (and probably most other places too), you get billed for energy used. Watts are Watts, whether you consume 1.1 kW by drawing 10 A from 110 V or 5 A from 220 V makes no difference to how much power you use or what you are charged. Your house is run from a center-tapped transformer secondary. Accross the ends, ...

18

Three is the lowest number of phases which are equally spaced around the circle, and which can be used to create a rotating magnetic field in a given direction. Any more phases just require more wires, and more windings in an induction motor. Two phases can set up a rotating magnetic field if they are 90 degrees apart ("quadrature"). Quadrature-generating ...

18

You could build a three phase transformer out of torriods. However, you need unique magnetic flux in each and the only way you could do that is to stack three separate torriods on top, or beside each other. Basically you would have three single phase transformers in one box. I am willing to bet that historically 3-phase transformers were indeed built as ...

17

In order of appearance: Three-phase star (wye) connection. Two-phase and neutral. (Note that if neutral isn't present then it would be a single-phase connection.) Quadrature (90°) phases and neutral. Split phase (180°) and neutral. Single phase and neutral. The ratchet symbol indicates that the meter won't run backwards and so won't register exported ...

15

Addition to other answers: The main purpose is that having at least three phases allows your motor to start in expected direction. For one-phase induction motors some workarounds are necessary (like putting additional wiring with a capacitor used during the start-up). It was correctly explained in previous answers. Why not more? Simply - it is not ...

15

Let's call the 3 phases A, B and C and let's say we notionally have a neutral wire. Neutral is basically 0V in the system. The "A" phase voltage (to neutral) is my chosen reference that all other voltage phase angles are measured from hence, V$_B$ is 120 lagging V$_A$ and V$_C$ is 120 degrees leading V$_A$. OK so far? What about the voltage ...

14

A typical distribution network in Australia will look something like the below. The "MV" section is a delta-connected "three-wire" system, so you are correct in asserting that there is no neutral wire. However, there is a path for neutral or "zero-sequence" currents to flow to ground, via the earthing 'zig-zag' transformer that is installed for this purpose....

14

The windings in a 3 phase motor, when activated by a 3 phase supply produce a rotating magnetic field in the rotor area of the motor. Swapping phase A with phase B re-orders the fluxes so that the flux rotates in the opposite direction. Swapping B with C does exactly the same thing as does swapping A with C. Think of it like a triangle with corners called A, ...

13

This transmission is likely 3-phase in the Delta configuration, as opposed to the Wye (Y) configuration. Delta does not have a neutral line, whereas Wye does. Wikipedia link to balanced 3-phase circuits As you can see, the Wye connection has a neutral line, whereas the Delta does not.

13

The letters are "representative of something" To assist in providing an international standard the International Electrotechnical Commission (IEC) created that IEC 60034-8 standard. "Terminal markings and direction of rotation" An electrical machine compliant to 60034-8 will ensure that clockwise rotation of the drive shaft will occur for positive ...

13

Air has a much higher reluctance (the magnetic equivalent of resistance) than the magnetic materials used in the motor. The smaller the air gap is, the lower that reluctance, and thus the higher the magnetic flux (which is the magnetic analog of current), allowing the motor to work more efficiently and at a higher power. Smaller air gaps also minimize ...

12

Three phase has a very important property: if you look at power (V^2/R) across all three phases and sum them, that power is CONSTANT across the entire cycle. This means 3 phase motors can drive at a constant power and the generators see a constant load. 2 phase is insufficient to get this relationship. One could use higher phase counts, but it costs more ...

11

Long distance AC transmission is normally 3-phase without neutral. At the receiving end, a 3-phase transformer lowers the voltage from the "several kV range" to normal household levels (maybe a bit higher if it feeds an intermediate system). For households, a neutral is taken from the output side of the transformer but, on the long-distance transmission, ...

11

In my country, and probably in a lot of other countries, there is a ground wire at the top of each pylon. This ground wire is there for reasons of lightning protection. For the power transmission alone all you need is the three phases, because earth is used as reference potential. For long distance transmission there is also no strict need for a neutral wire....

11

Comparing 3-phase with single phase transmission, 3 phase has a couple of significant advantages: 1) More efficient use of conductors : given the same peak voltage between conductors and same current in each conductor, 3-phase delivers 3x the power with 1.5x the copper (3 wires instead of 2). This doubles the usefulness of each pound of copper (aluminium ...

11

Why only 3 phases? Well if we need more phases we can convert 3 phase easily into 6 phase/12 phase etc using a transformer wired to do so. The main application of more phases is for less ripple voltage into a full bridge rectified capacitor bank. I've never seen one but learned about them from an ancient lecturer at university while doing electrical ...

10

Here is what wikipedia says about two-phase power. Two-phase electrical power was an early 20th century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by 90 degrees. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common ...

10

There are several reasons 3-phase is desirable over 1-phase power. One advantage of 3-ph over 1-ph has to do with instantaneous power (i.e. power generated or consumed at any instant in time within the power cycle). For example, consider a heating element (power resistor) in a 1-phase circuit. Voltage and current are in phase. Both cross zero twice during ...

10

Good answer from @Transistor. To add a bit more: - Three phase is inherently current and voltage balanced from a standpoint of it generating interference. At any one point in time (and a reasonably balanced load) the magnetic emission is low because all the magnetic fields cancel out due to the currents being balanced. There is net voltage balance in the ...

9

Most long distance high voltage transmission is done in 3-phase Delta configuration, requiring only three cables. This saves the cost of the fourth cable, used for the neutral line. It also avoids problems of imbalanced currents flowing through the fourth cable or the earth, since in Delta configuration any current imbalance on a phase is automatically ...

9

The two systems have vastly different applications. Yes, there is a lot of crossover between them in some fields, but the two are more suited to certain applications. Take motors for instance. Delta is far superior for driving motors than star. With delta you can visualize a wave circulating around the triangle, and it's that wave that turns the motor. ...

9

Many of the other answers erroneously state that you need 3 phases for a motor to start reliably or turn in a specific direction, and to use constant power. Actually, this could be done with two phases, 90° off from each other. You still get defined direction and constant power draw over a cycle. However, such a two phase system would require a ...

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