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Phase angles in a 120/240 Volt Supply

Yahoo Message Number: 66991
Peter,

My apologies to the group for getting technical here and a bit off topic. It is a reference that I hear a lot of here.
The question of phase angle in a 240/120 volt single phase serviceborders on the philosophical. I was taught in my power courses that a phase angle can only exist between two ac voltages. There is only one voltage with a 120/240 volt service. the 120 volts is merely 1/2 of the 240 volts and are two parts of the same voltage. It is just that the neutral is tied to the middle of the 240 volt secondary coil of the transformer. A single voltage or any portion of it can not be out of phase with itself.
The reason that the neutral current is the difference between the two load currents doesn't need phase angle at all to explain. In electrical math jargon the neutral connection point represents a 'node' (three wires connected at one point). There is a law that states the sum of the currents entering a node must be zero. So you have plus the current on one leg minus the current on the other leg. There is the polarity difference. If they are equal then the current on the third leg (the neutral) will be zero. If the two are not equal then the neutral has to be the difference of the other two legs. Exactly the same math would apply if the voltage was 240 volts DC. I have never heard of anyone referring to the opposite ends of a DC voltage as being 180 out of phase.
The philosophical part comes in when one represents voltages as vectors. Now typically the 240 volts would be represented as a line of some scalar length. Each of the two 120 volts are 1/2 of and part of the 240 volt vector. Now if one is willing to believe that one end of a straight line or any portion of it is at an angle of 180 degrees with the other end then that is where the controversy lies.
At this point I can not argue it any further because the discussion is no longer technical in nature. It is now a philosophy. In defense of my philosophy I have to mention that in the power distribution world the 240 volt leads are referred to as the hot legs and not the phases.

Don Seager
Don

I find your email technicaly correct, not like some comments I read on our RV 50A service. However as an Electrical engineer specialising in mining I find your comment that 180 degrees out of phase and of instantateous opposite polarity are not the same. 120V on one leg and 120V on the second leg but 180 degrees out of phase or of instantaneous opposite polarity will give 240V between the two legs while giving 120V between either leg and the neutral. Each leg can draw 50A at 120V giving 6000W X 2 legs 12000W. However because each leg is 180 degrees out of phase with each other at 240V only 50A will be recorded but still giving 50A at 240V giving 12000W. And as you so correctly say if each leg is balanced at 50A the neatral will carry no load. I am assuming unity power factor. I feel the terminology is different but not the actuality. Only used to heavy engineering so if I'm wrong please educate me. Always keen to learn even if, as you, I am over 70.
Regards
Peter J

Posted by: "Don Seager" dmseager@... donald_seager Fri Dec 31, 2010 9:21 am (PST)

Other Don, No offense intended but I will have to defend my knowledge of 120/240 volt services. It was how I made my living as a Power Distribution Engineer.
A 120/240 volt 50 amp service is rated at 50 amps on each leg at 240 volts not 120 volts. The fact that the neutral point is connected 1/2 way between the 240 volts on the transformer secondary coil is not the point. If the load is balanced between the two legs and drawing 50 amps the current in the neutral will be zero. The neutral current is always the difference between the current in the two 240 volt legs. The reason is that the 2 -50 amp currents are of instantaneous opposite polarity (not 180 degrees out of phase) and when they add together returning to the power source they effectively cancel each other leaving only the imbalance current to travel back to the source on the neutral.

Re: Phase angles in a 120/240 Volt Supply

Reply #1 –
Yahoo Message Number: 66998
I agree with Don Seager's explanation. Maybe someone could post a diagram in the file section Showing the Xformer secondary and resulting current for the 50 amp connection BOB. BSEE 06 intrigue 12047

Re: Phase angles in a 120/240 Volt Supply

Reply #2 –
Yahoo Message Number: 67005
? Thanks. Bob, You made my New Years Day. I am happy that someone of a similar background agrees with me.
Probably a simple diagram of a 120/240 RV 50 amp hookup could be found somewhere on the Internet. I will do some research but a diagram that displays why there is not a 180 degree phase angle between the two 240 volt hot legs or the 120 volt hot leg to neutral voltages is going to get a bit more complicated.
It involves understanding that the math rules of the physics of electricity all apply with AC circuits but to apply them one must first stop all the rotation by freezing things in a instant in time. Hence the term 'instantaneous polarity'. The pot thickens a little understanding this concept.
I will think on it a bit. It has taken me several years to find someone that agrees that there are no voltage phase angles involves in a 120/240 single phase service. A 120/208 volt service, yes there most certainly are.

Thanks again. I go to crack a cold one.

Don Seager

2004 Allure 31046

I agree with Don Seager's explanation. Maybe someone could post a diagram in the file section Showing the Xformer secondary and resulting current for the 50 amp connection BOB. BSEE 06 intrigue 12047 On Jan 1, 2011, at 10:16 AM, "Don Seager" dmseager@...> wrote:

Re: Phase angles in a 120/240 Volt Supply

Reply #3 –
Yahoo Message Number: 67060

Simply put, the formula for the neutral current in a 120/240 vac system is L1I-L2I=NI If the load on L1 is identical to L2 you would not even need a neutral.

nuff said.

Happy New Year,.... may your tires stay round and your fluids stay contained.

Dave

aka Billy Byte (trusty hound)
2000 Allure #30443

Re: Phase angles in a 120/240 Volt Supply

Reply #4 –
Yahoo Message Number: 67061
Dave,

Interesting thought but you also would not have 120 volts. Whether or not there is any current flowing in it you must have a neutral connection to the center tap of the secondary of the supply transformer.
Without a solid connection to the neutral you would have what is know as a floating neutral and all sorts of problems could happen.

Don Seager

2004 Allure 31046

Re: Phase angles in a 120/240 Volt Supply

Reply #5 –
Yahoo Message Number: 67062

Don

Think about it, IF the load on both legs is equal the neutral current is 0 thus you would need no neutral leg. You DO have to have a neutral bus to tie the two hot legs to common of course. Yes, It would be a floating neutral so the loads would have to be equal. Change the load on one leg and thats how we make smoke............
Dave

aka Billy Byte (trusty hound)
2000 Allure #30443

Re: Phase angles in a 120/240 Volt Supply

Reply #6 –
Yahoo Message Number: 67063
?
Dave,

Well not always smoke but often bright lights on one side of the coach and dim lights on the other. With no neutral the whole thing becomes a voltage divider circuit and the low impedance side wins or is the high impedance side? Getting late here in AL.
One thing to remember is that it is an AC circuit and that involves impedances that are not the same as resistances. So the two high side currents must be equal and opposite as vectors not just meter reading for the neutral current to be zero.

Don Seager

2004 Allure 31046

Re: Phase angles in a 120/240 Volt Supply

Reply #7 –
Yahoo Message Number: 67066
This is an interesting thread. I recently had a bad neutral connection at amy home resulting in the bright light/dim light scenario. For both legs to work without a neutral, wouldn't the 2 legs have to be out of phase? That is, if you were using a 30 amp adaptor so that each leg was the same hot and therefore in phase without a neutral you would have nothing, right?

Greg Jones

'03 Intrigue 11571

Re: Phase angles in a 120/240 Volt Supply

Reply #8 –
Yahoo Message Number: 67135

Yes, a small difference in resistance or impedance in one leg over the other will result in bright/dim. If you are feeding a 50 amp system with a 30 amp receptacle then you will get nothing without a neutral. If you have a partial neutral then you will have various symptoms depending on load.

Dave

aka Billy Byte (trusty hound)
2000 Allure #30443