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Why Is American 220v Not Considered 2 Phase?

Discussion in 'ELECTRICAL ISSUES - POWER YOUR MACHINES & SHOP' started by Superburban, Sep 28, 2016.

  1. Eddyde

    Eddyde Active User H-M Supporter-Premium

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    Yes thats true for most areas of the country. Here even 1-2 family homes that have single phase service are getting two of three phases from the street, not split phase.
     
  2. Keith Foor

    Keith Foor Active Member Active Member

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    Well, I have to get sort of technical here, so ask questions if this gets really confusing. 208 3 phase is 3 legs of 120 volts to neutral that are 120 degrees apart. Split phase or standard residential wiring is 2 legs of 120 that are 180 degrees apart. The reason that its not 240 3 phase is the phase angle difference. the 120 volts is the maximum plus and minus voltage that is referenced to neutral. Now becasue the 240 is 180 degrees out of phase as one leg hits the peak positive voltage, the other leg is reaching it's maximum negative voltage. The difference between the two legs at that point is 240 volts. Because 3 phase is 120 and not 180 degrees apart, only one leg at a time reaches it's max voltage. The 208 is the difference between the two legs at the point of maximum voltage of only one leg and a reduced voltage of the other leg because it's not at its maximum voltage at that same time.

    The whole RMS vs Peak is another discussion.
     
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  3. ddickey

    ddickey United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Agree, this is found in a wye connected secondary.
    It is one 240V leg center tap split . There is no phase shift just opposite polarities. Same reason why it is not called two phase.
    √240²-120²=208 We don't even use the 208v on a delta secondary, do we? 208 is derived from one leg (240v) and the center tap (120v).

     
  4. jim18655

    jim18655 United States Active User H-M Supporter-Premium

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    Ahh... 240v delta - probably responsible for burning up more equipment than any other voltage known to electricians. ;)
     
  5. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    OK, now I've got a question. Probably staring me in the face, but atm, I can't see the answer. Might be the cold medicine. Since our secondary is a center tapped 240 volt winding, and we derive our 120V from basically what amounts to 2 windings secondary to the same primary (or are they?), how is is that they are out of phase 180° to get the 240V? Or is it that each 120V sine wave is above and below the neutral in entirety? How can that be? It would appear to me that they (the sine waves of 120V) should run in exactly the same time domain, and would operate more like parallel secondaries. If I think about it a while, I might realize how simple the answer is, but right now it's not coming to me. So someone 'splain it to me please.
     
  6. jim18655

    jim18655 United States Active User H-M Supporter-Premium

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    Neutral isn't a voltage in relation to the other two hot legs, it is the center of the winding. If you consider the whole secondary as an autotransformer then as the winding ratio increases the voltage between the two legs will increase. The center tap - our usually grounded conductor - will be 120v to the bottom leg. The top leg - the other hot - will be 240v to the bottom hot leg with a 120v difference to the center tap. The phase of the sine wave is dependent on the magnetic flux "moving" in the core and transferring energy into the secondary. Think of the output legs pushing and pulling in sync with each other. This is why there are phasing dots shown on schematics when you have transformer coils to wind. If you could flip the connections in the secondary so that the phasing of the core is the same you would have parallel 120v connections without a 240v connection. Think of a buck-boost transformer connection where the polarity is additive or subtractive.

    Here's one to think about - assume the power company didn't ground the center tap or you have a 480 to 120/240 transformer. You could ground either "hot" leg and connect it to the neutral bar in a panel. Center tap to the main breaker and the other "hot" leg to the main. Nothing would trip out and depending which phase you used it would give you 120v to neutral bar, 240v to the neutral bar and 120v between phases on a 2 pole breaker with no difference in shock hazard than in a properly connected panel. We only get shocked or sparks between hot and ground because of the ground connection we establish when we install the panel allows a complete circuit back to the transformer.
     
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  7. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    As I suspected, now with a clear head (sorta) I see it as no longer 2 separate windings in a secondary, but a single winding with the appropriate turns ratio to give the 240V. Grounding the center tap is immaterial to getting the 240V, but is part of how we use 240V safely.

    Thanks Jim.
     
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  8. markba633csi

    markba633csi United States Active Member Active Member

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    I'm wondering, (and I should know this) if, in the case of a motor wired for 220 volt, the connection point of the two run windings could be tied to neutral to stabilise the voltage that appears there and reduce the voltage spike/swing impressed on the start cap. The start winding, cap and centrifugal switch are usually across one of the run windings, and I bet the midpoint of the run windings thrashes around quite a bit when starting and stopping. Anybody?
    Mark S.
     
  9. John Hasler

    John Hasler United States Active User H-M Supporter-Premium

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    The motor wouldn't start. Each winding would be connected directly across 120VAC and and neither would be subjected to any phase shift. The capacitor would draw lots of current but it would not affect the motor.

    Look at how a 120V capacitor-start motor is wired. The capacitor actually achieves its effect by being in *series* with one winding.
     
  10. MikeWi

    MikeWi United States Active User Active Member

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    That's actually the exact reason AC became the standard! You see Edison did many demonstrations showing how dangerous AC current was in comparison to DC. He maintained that it would be the perfect thing for executions for example. Well the short version is that the government tried it out, and agreed it was good for the electric chair. Now the government needs to buy AC generators, and the rest is history. That's just from memory but you can find the info if you're interested.
     
  11. markba633csi

    markba633csi United States Active Member Active Member

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    Yes John H. however- the cap is in series with which winding? The start winding. Same thing in the 220 volt configuration except the run windings are in series not parallel, and the start leg is in parallel with one of the run legs. The junction of the two run windings should be 0, by voltage divider. And that's the reason you can use the same 110 volt start winding for 220. You're still getting a phase shift between the start leg and the parallel run leg. Right?
    Mark S.
     
  12. juiceclone

    juiceclone United States Active Member Active Member

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    OK just for a little more mud in the water, below are the secondaries of both delta and wye transformers.
    when ,in the US, you use 220 or 110 you are using two connection points, NOT two different phases as you can visualize from the pix.
    Other parts of the world use delta with it's "floating" /no ground. Have run into both types working on cruise ship equipment. A meter reading from one leg of a delta sys to "ground" produces some ...interesting... results.

    delstar-600x207.jpg
     
  13. hanermo2

    hanermo2 Spain Active Member Active Member

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    Earlier, it was mentioned, incorrectly imo, that the EU and global power uses only one line for power.
    Most of the world is 220V (240V) single phase, residential, at 50 Hz.

    The std residential single phase service uses 2 lines for power.
    And a green/yellow safety ground, not used at all.

    Unlike in the US, plugs are not keyed.
    You can use either line, since its AC, and both lines carry an identical load.

    Typically, up to about 4-6 kW is single phase, and around 4-6 kW you start to get 3-phase used, for example for saunas (or larger jacuzzi).
    Most new EU residential installs have 3-phase at the fuse box, as standard.
    And typical power levels are 10-15 kW, up to 20 kW, new, in the EU.

    I have about a 20 kW install, older, in Spain, and this is quite unusual.
    Technically, the service is iirc about 14-16 kW, 380 V 3-phase, with all appliances done at 220V single phase.

    But I have 380V 3-phase wired in several places, just not in use at the moment.
    So I could connect say an electric car plug, and charge at 14-16 kW, if I wanted to.

    Any electrician can connect the plug, and it costs about 100$ (legal install, no paperwork necessary).
    Since I already have the service, and internal wiring, the current installs like mine are grandfathered in.
     
  14. bobshobby

    bobshobby Australia H-M Supporter - Premium Content H-M Supporter-Premium

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    I think terminologies are the problem here. different countries and possibly even states use different names for the same things, then to add to the confusion they use the same names to describe different things.

    Single phase is single phase irrespective of the voltage in single phase the sine wave goes from + volts to - volts of the same value, be it 110 or 220 0r even 240v. crossing the zero line midway, all phases derived from 3 phase will be 120 deg apart, a transformer cannot change this, I have never heard of phases being 90 or 180 deg apart unless it came from a two phase generator or rotary converter, but I have never seen or heard of one, I'm sure it's possible.

    A two phase service to a consumer will simply be two of the three phases split from a three phase supply. each phase will consist of a single active wire and a common neutral wire. two phase is not normally used to run motors but is often used to supply electric cookers and other heating equipment. Two or three phase will be supplied to a consumer where a single phase cannot supply sufficient current for the total load.
     
  15. ddickey

    ddickey United States H-M Supporter - Premium Content H-M Supporter-Premium

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    I'm pretty sure there is no two phase provided to consumers.
     
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  16. bobshobby

    bobshobby Australia H-M Supporter - Premium Content H-M Supporter-Premium

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    You might like to think that, but I know people who are on a two phase supply.
     
  17. tq60

    tq60 United States Active Member Active Member

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    If not so good memory is somewhat correct...There was some 2 phase power systems back in the rust belt area from the early days when ac and dc power systems were both in use.

    The phases were 90 degrees apart maybe and still in use a few years back when we read about it.

    Sent from my SAMSUNG-SGH-I337Z using Tapatalk
     
  18. ARKnack

    ARKnack United States Active Member Active Member

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    Just to add to the confusion.
    Also I had heard the Detroit still had some 2 phase power around 20 years ago. They may still have some 3-phase_wye_diagram.png deltawye_figure3.gif
     
  19. bobshobby

    bobshobby Australia H-M Supporter - Premium Content H-M Supporter-Premium

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    The American system has always confused me. Here in downunder we have a system similar to Europe. distribution from the power station is via 3 wire 3 ph at a variety of very high voltages, this is broken down into local service areas at substation and sent out as 3ph 4wire ie. 3 actives and a neutral at usually around 3,300v This is then further reduced with local area transformers to 415/440v 3 ph still in 4 wire configuration. This is then supplied on the power poles throughout the local area.

    Instead of having the small transformers, or pole pigs, as you guys call them we have much larger transformers servicing a larger area, this is all 4 wire 3ph 415/440v. the individual supply to local consumer can be single ph 2 ph or 3 ph depending on consumption requirements.

    Most homes that have gas available (that is domestic natural gas, not petrol) used for cooking , hot water and heating, will only have single ph, those that require more power if they are all electric, cooking hot water and heating etc will often have 2 ph, ( this is not split phase as in US, but two of the three phases at 120 deg. apart, from the pole.

    The two phases will be distributed around the house as single ph only, possibly with the exception of the cooker which if large may use the two phases separately to supply different elements. This can result in an interesting situation where one of the two phases fails, half the lights and appliances in the house will still work but the others will not. This can also happen in a 3 ph installation.

    Electricity is quite expensive here, on average about double the average price in the US so it is rarely used for heating other that cooking.

    This two phase is never used for motors. In cases where large air conditioning or swimming pools or home workshop 3 ph can be supplied direct from the pole.

    My house is supplied with 3ph but only has one 3 ph outlet which is in the garage. I was going to get a 3ph welder, but never did. All other appliances use single ph 240v. So it sits there unused. I could fit a 3ph motor to my lathe, but why bother the single ph motor gives me all the power I need. Maybe one day if the motor ever needs replacing.
     
  20. olduhfguy

    olduhfguy United States Iron Registered Member

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    In AU most residences have 3 phase 240v available outside their houses, in US most residences have only single phase available and it appears as 240 volts which is tapped and grounded in the center (split phase), giving 120 volts for small appliances and 240 volt for larger ones. AU uses small numbers of large transformers serving many houses, US uses large numbers of small transformers serving at most 3-4 houses.
     
  21. Superburban

    Superburban United States Active Member Active Member

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    Ok, so how does all this work for the electric meter? If the hot legs are not evenly loaded, can the meter tell and account for the difference?
     
  22. olduhfguy

    olduhfguy United States Iron Registered Member

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    There is a technical description on Wikipedia:

    https://en.wikipedia.org/wiki/Electricity_meter

    I have wondered about this myself - The answer must lie in the number and mechanical orientation of the coils so unbalanced loads spin the disc at a speed half the difference between the 2 loads. The 240 volt component (balanced load) needs adds full rotation speed and the unbalance adds half the speed. Then again, an electronic meter would just have one coil for each 'leg' and perform the necessary calculation in software.
     
  23. ddickey

    ddickey United States H-M Supporter - Premium Content H-M Supporter-Premium

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    They only care about kilowatts
     
  24. markba633csi

    markba633csi United States Active Member Active Member

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    I only care about pizza-
    and beer
     
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  25. BROCKWOOD

    BROCKWOOD United States Active Member Active Member

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    I've worked for 2 power companies, both in distribution & transmission for 26 years. If read start to finish, you guys have it all figured out. I'll attempt an additional explanation of the USA 120/240V residential power transformer. Current does have a flow direction & that direction is back to ground. Since the center tap of the 120/240V transformer is connected to ground, the flow or direction of the phase angle for each leg is in opposite directions or 180 degrees out from each other. But what about the 240V use of the same transformer? It doesn't use the ground or neutral. True the 240V circuit completes itself. The transformer on the pole has the neutral connection on the primary side. Let any part of that 240V system have chance & it's going to ground to get back to the source.

    Quick story similar to the O'scope your power feed joke. A field tech thought that since his Fluke 87 says O.L. for overload above 600V he could measure the primary of a station power pot, 7200 phase to ground. Not sure how he lived to tell the tale. The meter leads melted immediately. The meter died permanently. The field tech suffered permanent hearing damage.
     

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