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I am wondering how 3 phase power is used to power, say for example, light bulbs? If I have 3 phase power coming into my house, how are the 3 phase combines to enter the one prong of an electrical outlet and then exit the other prong? How is 3 phase power concentrated into 1 line? Or is it?

I am trying to understand what happens, which is why there are a few (maybe overly broad) questions. And then there are electronic devices that contain converters to go from AC to DC (for example our baby monitor).

Any explanation is greatly appreciated! Thanks.

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    \$\begingroup\$ "If I have 3 phase power coming into my house" Then you have a very special house. \$\endgroup\$ Oct 21, 2014 at 18:27
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    \$\begingroup\$ 3 Phase is very special industrial power supply. Normal house power supplies are single phase, usually 110-120VAC or 220-240VAC depending where you are \$\endgroup\$
    – KyranF
    Oct 21, 2014 at 18:30
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    \$\begingroup\$ Residential availability of three phase service differs substantially from location to location and is not nearly as unheard of in some places as it sadly is in the US. At any rate, a typical "plug in" light appliance or light duty AC to DC power supply would be single phase. \$\endgroup\$ Oct 21, 2014 at 18:32
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    \$\begingroup\$ @IgnacioVazquez-Abrams: "very special house". Maybe where you live, here its the norm. Quite some stuff runs on 3 phase here (which means 3 breakers for these things, 3 breakers for the whole house/apartment and usually a 3phase meter). In one of my former apartments I even had one of these installed. We call it "380V Drehstrom". \$\endgroup\$
    – PlasmaHH
    Oct 21, 2014 at 19:01
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    \$\begingroup\$ @IgnacioVazquez-Abrams I can confirm PlasmaHH's statement. In Germany virtually every single house has three phase AC supply. I haven't seen a house with single phase supply for years. Even most appartments in bigger houses get all three phases. Many countries in Europe have a similar distribution scheme. \$\endgroup\$
    – Ariser
    Oct 21, 2014 at 20:00

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3-phase can take two forms. The general high voltage transfer lines are called "3 Phase Delta" configuration. In this arrangement the power forms a triangle. It's easiest to understand it from the PoV of a transformer's windings:

enter image description here

The other form, which is used for local distribution amongst properties, is called "3 Phase Star". The triangle is kind of turned inside out and it forms a star:

enter image description here

A normal household would get N and one of L1, L2 or L3 (in the US and some other countries you get "split phase" where one line signal is divided into two).

A special transformer which has both types of winding arrangements in it converts the delta to star format:

enter image description here

The voltage between Lx and N is the normal "line" voltage for your country (240V in the UK, 110V in the US, etc).

The voltage between two Lx lines is the "3 phase" voltage for your country - 415 in the UK, 203V in the US.

So a normal light bulb would be connected between, say, L1 and N.

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  • \$\begingroup\$ I believe it is also called the '3 phase Y' or '3 phase wye' format. Also, generally households do not get 2 of the phases, they will get a center-tapped step down of one phase. Two phases have an offset of 120 degrees, but 240V residential AC is two 120V phases with an offset of 180 degrees (inverted). \$\endgroup\$ Oct 21, 2014 at 18:55
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    \$\begingroup\$ @alex.forencich: The "generally" here depends on your country. Generally in my country, you will have 3 phases. \$\endgroup\$
    – PlasmaHH
    Oct 21, 2014 at 19:08
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    \$\begingroup\$ I don't know (or understand) the crazy US electrics system, so any references to the US system in my post are purely fictional ;) \$\endgroup\$
    – Majenko
    Oct 21, 2014 at 19:10
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    \$\begingroup\$ It seems the correct term for the US system is 'split phase', where each of the 3 phases stepped down through a center tapped transformer so the houses actually get two phase power instead of the very strange 2 out of 3 phases. In that system you get a strange 120 degree offset so you can't use all 240 volts at once, only 208. Split phase seems like a far more intelligent system. \$\endgroup\$ Oct 21, 2014 at 20:10
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    \$\begingroup\$ @Majenko: You mean we should bring democracy in the form of 240V to the us? I guess thats offtopic here and belongs to politics.SE \$\endgroup\$
    – PlasmaHH
    Oct 21, 2014 at 20:46

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