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3-phase and 1-phase utility supply

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IamSmooth

Mon Oct 29 2007, 04:22PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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When I get 240v service to my home is it from a separate generator than a 3-phase supply? I would think the answer is yes as 240v is simply two 120v lines that are 180deg out of phase, while 3-phase is 120deg out of phase with each other.

ragnar

Mon Oct 29 2007, 05:25PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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I don't think you're gonna get two phases shifted 180 degrees, and separate three-phase shifted 120 degrees.

Here in Aus, three-phase establishments will see ~415VRMS between phases. Each individual phase to neutral will see ~240VRMS.

If you're on a single phase residential supply, you're connected onto one of three phases available from the street... your neighbour might happen to be on a different phase.

Here in my apartment, three-phase goes into the building, and each room gets one of the phases. If I needed three-phase, I suspect that it would be possible to pinch it via extension cords from other neighbours on the same floor.

The USA is probably utterly different. =P

IamSmooth

Mon Oct 29 2007, 05:56PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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When the 3 phases come out of the generator where is the ground? If I tap into one of the phases how is going to a ground line complete the circuit? I guess if it is set up as a star-configuration the center neutral could be the ground?

Hazmatt_(The Underdog)

Mon Oct 29 2007, 08:29PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
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Neutral is ground, and if you have just the 3 hot leads coming out and use a delta configuration the neutral is unnecessary.

If you really need 3phase in a residential area you will either need to make a solid state converter, or a rotary converter. The solid state converter could be very complex because you will need 3 high power inverter/transformer systems that can handle your power requirements. The rotary converter approach can be as simple as an 1800rpm synchronous motor driving the shaft of a car alternator with its rectifier module removed. Mounting and hardware are going to be somewhat odd, but that would get the job done.

Marko

Mon Oct 29 2007, 08:37PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
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Windings in transformer station are connected into star and center point is 0, which is usually grounded. As far as I know, it's universal in whole world. Then there are several different ways of protection earthing systems. See this

I think USA in some cases taps both 240 and 120V from same transformer but nothing else is different.

You can achieve down to 60 degree shifts by using special arrangement of transformer secondaries, one in star and other in delta matched for same output voltage - that is used for low ripple rectifiers (2x better than a single rectifier in any case).

ConKbot of Doom

Mon Oct 29 2007, 10:11PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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In the US, for residential supplies, the secondary of the stepdown transformers are center tapped, the center tap grounded. Giving you 240V "two" phase, each hot is run into the breaker box along with the neutral, where the neutral bar is tied to ground.

120V circuits are run, 1 phase to neutral, 240 circuits are run 1 phase to the other. So basically all your 120v appliances are put in series

with any imbalance running though the earth/neutral. I'm pretty sure the grounds and neutrals are connected at the breaker box, not 100% sure, havent had it cracked open in a while.

Steve Conner

Tue Oct 30 2007, 09:24AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

No, they are all from the same generator of course! Single phase 120 and "two phase" 240 (it's not really two phases- you can't make a rotating field with it) are both derived from one phase of a three-phase system. In a suburb with big houses fairly far apart, you can sometimes see three-phase 12kV lines running down the street on poles, with pole pigs connected between any two of the three lines. They use different ones to even the loading, so your neighbours could be 120 or 240 degrees out of phase with you.

Each pole pig has a centre tapped 120-0-120V secondary that provides you with your two hots and neutral. (Over here of course it's just a single 240V secondary.)

Americans are always fatally confused about the difference between neutral and ground, probably because American utilities have always used them interchangeably.

Neutral is the conductor that returns your used electrons to the power company, so they don't pile up in your basement. It goes back to the center tap of your 120-0-120v pig. Ground is the conductor that saves your hide when a hot wire shorts to the metal case of an appliance. The desired effect in this case is to blow a fuse quickly, and unfortunately, the surest way to make this happen is to connect the ground and neutral wires together back at the pig! This is where the confusion comes from: ground is often really just another neutral, albeit one that you're not supposed to use for returning current.

Three phase systems do not need a neutral, because the current in any one line can return along either, or both of the other two. Neither do two-phase 240V systems. Because the two hot wires are 180' out of phase, you can draw electrons from one hot wire and return them to the other one, which is what your clothes drier or welding machine does.

However, everything needs a ground for safety.

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