# Single phase and 3 phase

Discussion in 'Shop Talk' started by crafty2002, Dec 16, 2006.

1. ### crafty2002Well-Known Member

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I have been doing electrical work sense I was an 8 year old kid helping by BIL on side jobs. I have worked as an electrician many times as a journeyman but the is something I still don't understand and I have asked general foremans and superintendtants and they can't give me an answer.
120 volts is single phase and that is simple enough to understand and 3 phase is simple enough to understand, but why is 240 volts 2 phase??????????
The way I look at it is. Can anyone make me understand this problem. It ranks right up there with where is the end of the universe at. There has to be an end, but at the same time there can't be, because somewhere has to exist on the other side of the end.

2. ### ovendoctornorth of the lift bridge

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the answer is on the other side :angel:
120/240 is single phase
240/480 3 phase is 3 phase
two classes of power so you dont mix motors up[check nane plate on motors]
you can use parts of 3 phase to run single phase motors[1-2 of the 3 leades + common for 120 volt]
also watch out for those nasty high legs in 3 phase[they make the smoke come out of circut boards real fast]
hope this helps

3. ### idahodaveWell-Known Member

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"watch out for those nasty high legs "

Only had a few really bright lights....

4. ### moopupsIn Remembrance

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I am not a skilled electrician but would think 220/240 is called 2 phase because it is a pair of single phase supplies. Not a known fact from this end, just a possibility.

5. ### idahodaveWell-Known Member

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There is a two phase power, but its not very common. Each leg is at 90 degees.

120/240 is called single phase, even with the two legs at 180 degrees to each other.

Three phase power has three legs, each at 120 degees to each other. The neat thing about three phase is it makes a rotating field (3600 rpm) that can be used to power a simple electric motor. Single phase motors require a starting circuit.

6. ### Old VetIn Remembrance

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There are two kinds of 3 phase out their. Delta is 3 wires at 120. The other is Y 2 wires 120 and 1 at 240. You need to know what kind you have. They will not mix. The way to tell is to look at the transformer that they come from. Delta has two transvormers the Y has one.

7. ### ovendoctornorth of the lift bridge

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old vet,got a question
I have a 10kw dayton generator
got it wired in delta and the power to common is 120/130/208 on the leggs
have I got somthing messed up with the wiring in the generator
the other dayton unit I had [40kw] was wired in delta and got 120/120/120
any thoughts :shrug:

ovendoctor

8. ### tyusclanWell-Known Member

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Delta 3-phase will have a "stinger" or high leg, and will only have one transformer. Wye 3-phase will have the same voltage on each leg, and will have to have 3 transformers. Any 3-phase motor will run on either version of 3-phase of the correct voltage. While the stinger will read the high voltage to a neutral, when read with either other leg you will only get the rated voltage. For instance, let's suppose you have 240 volt delta. Your legs will read 120/120/240 to a neutral, but if you read any two hot legs you still only get 240.

The only thing you have to really be concerned about on Delta voltage is your 120- (or 277-) volt circuits. You can't put the single pole breaker on the high leg. Any 2- or 3-pole circuit won't matter. And on both types of voltage you have to watch your rotation, because exchanging any two wires will reverse the rotation of the motor.

Your generator that is reading 120/120/120 is Wye, not Delta.

9. ### ovendoctornorth of the lift bridge

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sooooooooo I need to re wire the generator to Y instead of delta right.

10. ### tyusclanWell-Known Member

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You shouldn't need to do anything. If you're reading 120 on each leg it is wired in wye. I'm not sure what you're doing with the generator. If you're running 3-phase motors you only need to make sure your rotation is correct. If you're feeding a 3-phase panel from the generator you would need to run 4 wires, 3 hots and a neutral. You could then run any circuits off any busbar since you don't have a stinger to avoid on single pole circuits. If you're feeding a single phase panel you can pull off any two hot legs and run two hots and a neutral. Make sure the generator is grounded. There is usually a terminal on the generator to run to a ground rod.

11. ### idahodaveWell-Known Member

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Sounds like the second genny is a "Y" Is there 208v between the phases?

First genny seems like a Delta with one of the legs having a center tap to ground. This one will run 3 phase loads, but be careful if your are connecting up to a panel that has lighting loads. Watch the wild phase to make sure it doesn't connect to any of the 120v loads.

12. ### farminghandymanWell-Known MemberSupporter

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Crafty after reading the replies I would still be in confusion, even tho most of the things have been correct,

First of all AC current changes from positive to negative 60 time a second, thus the 60 cycles electricity, and also why induction motors are in a division of 60 3600, two pole motor or generator, 1800, 4 pole motor or generator, 1200, 6 pole, and so on, (over seas it is 50 cycles) in the process there is a voltage fluctuation from positive to negative, and thus produces a sine wave,

single phase is is one sine wave, a positive to a negative,
and this single phase is between (normally) the voltage in our homes or businesses, of 240 volt,

but you say we have 120, no you have 240, that is split,in half, we place a Neutral wire in the center of the 240 and we pull off HALF of the single phase for our normal use, now when we run a heavy appliance you use the two half's or the full 240,

again there is only a single phase, here,

now three phase, IN a since it is like Three single phase generators hooked together thus three hot wires, between any two of them it is single phase, but if all hooked into a motor there is the three phases, a phase that 120 degrees out of sink with each other, on instead of one plus and one negative, you have three plusses and three negatives spaced out 1/3 1/3 1/3, or a very smooth flow of power, and in motors and when it is being rectified in like a DC welder you have a very continuous flow of power, not just 60 pulses of power in a second you have 180 pluses of power in a second, and in a motor when has a shift of phases, has the ability to start it self, (thus the starting winding of a single phase motor, and the capacitors to electronically shift the phase), and not in three phase.

OK that is enough of that,

the term two phase is Wrong, (except as noted in one of the above posts, it does exist but rare, very rare),

yes if you are wiring in a commercial building and you need single phase 240 (delta) or 208 (Y) voltage, you use two of the three hot wire of the three phase to get the SINGLE PHASE POWER, if you put a oscilloscope on it it would only have one sine wave in it,

IN resident wiring they are pulling off of ONE of the three phases, ((coming in to the transformers)very high voltage has two wires, one that is hot and a ground, and normally only one transformer, and out of that is two hot wires, SINGLE PHASE, and is split in the middle by a neutral wire, which at that time is connected to the ground at that time, but even tho when you read the voltage, you have 120 hot to natural and 120 natural to other hot, or 240, between the two hots, ONe Sine wave,

in three phase (as stated you have "Y" and "Delta")

you can have a number of voltages 208 240 440 etc, but lest stay in the 208 or the 240,

in 208 Y the neutral is connected to the center of the Y or the center of the three phases, and when you pull off the center you get 120 volts, (why you ask, it is because it is 120 to the center but if you look at the Y the legs are 120 degrees apart, and you lose because of the angle of the second part of the leg and you get 208, . as you can get 120 from any of the three hot wires to neutral,

the delta 240 like stated is in a delta configuration, /_\ best symbol I can make,

any way you have the three points and at each point there is 240 volts, using three wires you have three phase, if you only use two wires off of the points you have single phase,
(even tho you using two hot wires of "three phase")

I think it is easer to see in the delta configuration, each side of the delta is like a single phase generator, but when the three are tied to gether, are 120 degrees apart,
so any two of the three wires are only single phase, but together, there three phase,

ok how do you get one 120 out of the delta, you tie in a neutral in the center of one of the deltas , (remember how we got 120 out of the 240 single phase), and that neutral is connected to ground as well, (normally), so to the two hots on either side of the neutral it is 120 volts but between the two hots there is 240 volts, just like in single phase,

OK, the high leg is when one goes between ground and to the third point, which if we look back to the Y you get 208 as well when the angle of the winding changes, thus the high leg, and you can only get 120 from only two of the hot wires,

so when some one says 240 two phase is that they mean that they have two hot wires, and that is it, it is a mis-statement, and comes from the fact that in a three phase system any two hot wires is only single phase between them. (for you to have "two phase" you would end up with two sine waves 180 degrees apart, and that is not going to happen in that situation),

now there are also many variations of three phase systems in voltages and how there grounded.
but they have three sine waves, and the ground may be the center, of a Y or the center of one of the bars of a Delta,

If your using one only one side of the configuration it is single phase and two hots, and if your using all three sides and three hots, it is three phase, the placement of the neutral is where you get the point to make 120 volts or half of a single phase, circuit,

I hope I have help explane it not confuse you more.

13. ### idahodaveWell-Known Member

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Nice explanation.

In re-reading the posts I guess we had a little phase shift in the thread.

14. ### ovendoctornorth of the lift bridge

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seen an oppertunity to glean some exelent info :bow:
didnt intend to shift phases :doh: sorry

15. ### GREENCOUNTYPETEModeratorStaff Member

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Farming handy man

very good explanation

one word of caution for others - if you have Y and have a 3 phase deep fryer or other peice of equiptment that uses a 120 relay check to make sure the leg you have it on is not the 208 (wild leg)(high leg) we had a couple of fryers at church for fish fries and found different outlets were wire with the 208 leg in differnt places it was realy just one one outlet wired wrong but
a few minutes with a volt meter will save you the time and cost of replacing relays

16. ### idahodaveWell-Known Member

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Wow, a whole thread on 3 phase power and not a single reference to the square root of three.

17. ### tyusclanWell-Known Member

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Okay, I'm only gonna post this one last time, and then I'm gonna stay out of this conversation. 208 wye (Y) three phase does NOT have a high leg. Wye will read the same voltage on any single leg to a neutral. Delta has the high leg. If you have a piece of equipment that is rated for 208/240 volt 3-phase, and it uses a 120 volt relay then it will have a transformer built in to step down the voltage to 120. Even if you have 240v delta voltage, if the equipment is wired correctly, you cannot get the "high" leg to the relay, because the voltage will have to go through the transformer first. With 240v delta your legs will read 120/120/240 to a neutral. If you read across ANY TWO hot legs you will still only get 240 volts. Your transformer can then only be fed with 240 volts which will step down to 120 before going to the relay.

The only thing that could cause you a problem is if you have 120v fryers, you have 240v delta in your building, and the 120v receptacles were wired to the hot leg. You would then have 240 volts on a receptacle that is supposed to be 120, which would then destroy your equipment if it's plugged in.

18. ### ovendoctornorth of the lift bridge

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ive seen restaurant equipment go up in smoke by changing outlets and location's
the mfg. takes 1 leg of the 3 phase and common to run the controlls.
circut boards contactor coils dont last long on a high leg
the 10Kw and the 40Kw daytons were used at U of M for the football concessions running 6 conveyor ovens each
I just taped off the high leg on the 10 Kw[too many fingers playing with stuff there]

19. ### tyusclanWell-Known Member

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Then that was a poor job of engineering IMHO. Had the manufacturer installed a \$10.00 transformer to step down the voltage the equipment could have been used anywhere as long as the voltage was correct. It would not have mattered whether it was wye or delta, or where the "high" leg was.

20. ### Dave S.Well-Known Member

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idahodave, the floor is yours, tell us about the square root of three, if you dare.

Dave