3-phase electricity...what is the difference?


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Old 06-20-05, 11:05 AM
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3-phase electricity...what is the difference?

Ok, I do understand there are 3 phases to the energy the utility company sends down the wire and that most residential and light commercial stuff operates on 1 phase while the heavier duty stuff utilizes 3 phase.

But how does one model of a particular appliance that runs on 1 phase, say a motor of some kind, compare to a similar model that runs on 3 phase? Is 3 phase more energy efficient or does it simply provide more raw power for the appliance?
 
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Old 06-20-05, 11:44 AM
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Three phase is easier to work with than 1 phase when you are talking motors. Motors naturally start with three phase. You don't need any start capacitors etc. They can be easily reversed by simply swapping two of the phase wires. They usually run on a higher voltage thus requiring less current and therefore smaller wire for intallation. They are generally more effiecient.
 
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Old 06-20-05, 01:05 PM
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Three phase is more efficient in that it provides constant power (not more power) to a motor load. The motor can deliver steady torque at every point in its rotation. With single phase there are surges and lulls in the power output. These happen so quickly that it doesn't matter in light use. In heavy use, it is important for the motor to supply constant, steady torque.

The coils of a three-phase motor are wound differently than a single phase motor. They are generally not interchangeable. Some motors can be converted with minimal re-wiring, but most are specific to single or three-phase operation.

If you have three phase power available, by all means get three-phase capable tools and machinery. If not, it isn't worth the trouble unless there is a specific reason you need it.
 
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Old 06-20-05, 02:25 PM
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Ok let me follow up with another question. Since most residences are 1 phase, is it possible to obtain 3 phase service at a residence? Will a completely new line and service have to be run for this?

Honestly I do not have any such heavy duty machinery that would require 3 phase service, these are just questions I've always wondered about.
 
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Old 06-20-05, 03:20 PM
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I have commercial customers who have large single phase services and are unable to take advantage of bargain priced three phase equipment.
For you to have three phase you need to have a primary power line nearby with three phases.
You then will have to pay for three transformers to drop the voltage to your house.
Not really feasable if you aren't using a large amount of power.
 
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Old 06-20-05, 04:52 PM
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It depends on where you live. Where I live in Ontario you can not get three phase service to a residence.
 
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Old 06-20-05, 05:47 PM
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As a general rule you cannot get 3-phase service to a residence. At least in most areas I have heard about.

Most residential areas only have a single primary wire on utility poles. A transformer can supply two poles of 120 volts from this transformer. This is why it is called single phase.

You cannot get three 120 volt poles from this though. Most 3-phase systems require three primary wires and three transformers (if overhead fed). Hence; 3-phase.
 
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Old 06-20-05, 06:18 PM
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3 phase

i have 3 phase in my shop..but it jsut happened to go down my gravel road from a big microwave tower a couple of miles away to the sewage treatment plant down the raod about 2 miles.....it cost m e about 6000 bucks to have it put in 20 years ago..i dont know what it would cost now..but it makes it easier to buy old equipment for less money at auctions for my machine shop and it seems to operate (in my opinion..from usage) than my single phase at my residence
 
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Old 06-20-05, 09:22 PM
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The cost of having 3 phase installed in a residence would FAR outweigh any possible advantages. There are almost no devices available which you could use. No 3 phase refrigerator, no toasters, no hair dryers, no microwave ovens. If you have a large air conditioning load or large machinery in a workshop, 3 phase versions are available but cost more.
 
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Old 06-21-05, 12:11 AM
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You can run some 3 phase items off single phase with a converter.
Search the net for static phase converters and rotary phase converters.
On the net you can find info on how to make one.
 
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Old 06-21-05, 04:51 AM
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My sister & bro-in-law have 3-phase service coming to their house, right here in east Tennessee. Apparently, since it was formerly owned by a church (it was a parsonage at one time), it was not considered residential when the 3-phase was put in. Their AC unit is the only thing I am aware of that runs off the 3-phase.

The utility co. told them if the service to the house is ever shut off, they will not be able to get 3-phase back when it is connected again. I hope they never miss a light bill!
 
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Old 06-21-05, 08:41 PM
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If you have access to two phases on the primary, you can have three phase service. The utility will hang a two transformer bank in an 4 wire open delta configuration that will give the typical 120/240 for your normal single phase aplliances and three phase for larger loads like the AC. This type of service is very common on smaller commercial loads.
For a typical residence, I see no advantage to going three phase over single phase unless you are rich enough to build a house that can benefit from an industrial type AC unit. Even then, I would recommend psycological counseling first.
 
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Old 06-22-05, 06:03 PM
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Since the OP is asking from an interest standpoint and it seems that other might be interested:

Electricians here will be familiar with all of this, but DIYers might not. I recently bought a 50 YO home that originally had 3-phase. The, somewhat wrong, home inspector missed the fact that it had actually been converted to single phase at some point. Over the first year of home ownership I learned a little about residential wiring and about 3-phase power from a personal interest standpoint. I also started to work in a laboratory in a building that has only 3-phase, and I buy equipment so I felt that I had to educate myself on what truly needs "real" single-phase 220 vs. what can operate with two legs of 3-phase (nominal 208). I also had to find out what was available in the building.

My home was built in a time when central AC was probably a relatively new thing. The 3-phase installation was pretty common judging from casual surveys of the drops and the main disconnects on the outside of the homes in my neighborhood. I know of one home, a close neighbor, that still has a three-phase AC compressor. I was a little concerned about whether I should take pains to preserve the three-phase, before I figured out that it had been converted, in case I discovered that a three-phase compressor would work better. I understand that it is common that once the POCO takes away three-phase you will not be able to get it back and if they find out you don't have any three-phase equipment, they take it away.

Now one problem is that you can't get 220 with some flavors of three-phase. Again, before I figured out that I did not really have three-phase I wondered how my two 220 window AC units were operating. In fact, with residential three-phase, as I understand it, you can get 220. With a couple of different arrangements of transformers, measuring voltage between any two phases will give you 220 V. Measuring between the supplied neutral wire and two of the phases will yield 120 V, but the so-called wild, stinger, or ******* leg will give you higher voltage. This is also, apparently useful for the three-phase motors. You must be careful not to use the higher leg for 120 V loads.

At work, another type of transformer arrangement is used to yield 120 V ground to neutral, 277 V ground to neutral (for lights), and maybe other stuff. The best I can get from phase to phase in our space is 208V. Depending on what kind of equipment I need I have to make adjustments.

At home I recently installed a new meter base and main panel (and ran a bunch of new circuits). I still had a three-phase drop (using 2/3 of course) and the POCO was supposed to install a new drop shortly after the new meter was installed. I called them 4 months after because, to my eye, nothing had changed. The woman's response was that their records showed that a new drop had been installed a couple of weeks after the meter. She asked if I wanted someone to check and I replied, "sure". A week later I got my new drop and now the lights dim a lot less when the vacuum cleaner or refrig turns on!
 
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Old 06-23-05, 07:20 AM
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277v comes from a 480 volt three phase power. It is the phase to neutral voltage similar to 120 being phase to neutral of a 208 three phase.
 
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Old 06-23-05, 01:42 PM
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Originally Posted by joed
277v comes from a 480 volt three phase power. It is the phase to neutral voltage similar to 120 being phase to neutral of a 208 three phase.
277v comes from 277/480Y volt three phase power. 480v three phase power also comes in 3 wire, 3 phase delta 480 volt (ungrounded) and 240/480 volt 4 wire, 3 phase delta (not very common)

It is possible to have 2 wire, single phase 277 volt service also as 277/480Y served from three overhead transformers is derived from three 277 volt (secondary) transformers.
 
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Old 06-23-05, 07:41 PM
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Manythumbs,
Sounds like what you had was a network connection. A 4w Wye connected, three phase transformer bank is connected for 120 v to gnd, 208 phase to phase. It is commonly used on apartment complexes and is usefill because it is easier to balance load. You get the conventional 120 to gnd for most appliances and 208 for your stoves, AC, etc. I'm not sure I would call it a true three phase load since you are only using two points of reference, but I get your point.

On a totally unrelated topic, I'm always surprised at remarks about the POCO's not being willing to give 3 phase services to people. Maybe it's because they are investor owned....our Coop will give you whatever kind of service you want.....

....as long as you are willing to pay for it!
 
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Old 06-23-05, 08:00 PM
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Willing to pay is not always the answer. In my area most all residential neighborhoods have a single primary wire run. The cost involved in stringing another one or two primarys is prohibitive, and most times they simply will not do it.

IMO NO home needs 3-phase. OK, maybe a 20,000+ sq/ft mansion could possibly benefit from it.
If you have a shop or garage at your house that is so big it would benefit from 3-phase IMO it ceases to be a "home shop". At that point you cross the line into commercial.
Again, just MHO.
 
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Old 06-23-05, 08:14 PM
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Quote:
"IMO NO home needs 3-phase"

I couldn't agree more. I'm merely pointing out that money talks....if you got the bucks, we'll put it in whether it's neccessary or not. I never cease to be amazed by the people that seem to equate their verility with the size of their light bill.

It's funny what you said about the mansions. Some friends at the City of Austin (Texas) that serve the Dell household ( as in Dell Computers), said they almost had to build a substation for that house alone.

I think our Coop's record for services on one house is a 167 KVA transformer.
 
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Old 06-24-05, 07:38 AM
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Originally Posted by txnoob
Ok let me follow up with another question. Since most residences are 1 phase, is it possible to obtain 3 phase service at a residence? Will a completely new line and service have to be run for this?

Honestly I do not have any such heavy duty machinery that would require 3 phase service, these are just questions I've always wondered about.
My dad has 3 phase in his shop. He owns a small welding/machining business in Iowa. We don't have 3 phase into the building and I'm not sure how he did it but I'll try and explain his setup and maybe someone else can chime in....When he needs to run his 10' metal break the first thing he does is start up a smaller elec motor which runs some sort of transformer thing...the small motor is probably at least 2hp or so...then once that transformer thing is spinning he shuts off the smaller motor and then he has 3 phase running to his metal brake. It can be done but I don't think it's the safest contraption in the world...

hth
Doug
 
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Old 06-24-05, 08:17 AM
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theswandog,

That contraption is called a "rotary phase converter". It is a relatively cheap way to drive a 3-phase motor from a single phase service for a single tool. Sounds like your situation is a perfect use for a phase converter -- fairly small shop that needs one big tool.
 
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Old 06-29-05, 07:21 PM
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Thanks for the added comments. I have been out of the forum for a few days. I have seen a couple of ways that 3-phase was used in residential in diagrams. If I had a trained eye I could probably figure out what my home used to be by looking at the transformer supplying the neigbo that still has a three-phase AC compressor.
 
 

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