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240/1/60hz or 240/2/60hz?




 
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Author Topic: 240/1/60hz or 240/2/60hz?  (Read 7292 times)
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xe1yzy
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« on: June 12, 2006, 11:49:50 AM »

Last week, in a HVAC seminar, the controversy starts when somebody ask why all appliance labels says
240/1/60 Hz.
 If you mesure the voltage in each cable, you have 120V, thats means two  phases of 120 v each! , then, you have 240/2/60 hz.

 Undecided  What do you think?


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The Slab Bacon
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« Reply #1 on: June 12, 2006, 01:14:37 PM »

Pedro,
         240 / 1 /60 is still single phase power. think of the neutral leg as being a center tap on a transformer secondary winding. With three phase power you have equal voltages between any 2 of the 3 wires. this is NOT like center tapped 240 single phase. Also the sine waves of the three phases are shifted 120 degrees apart from each other.

With normal 3 phase power (3 wire service) you have an equal voltage between any 2 of the 3 wires. If you have 4 wire 3 phase service the 4th wire is then like the center tap of the transformer. It is where the 3 legs ot the "Y" come together.
Although the 4 th wire is the physical center tap, it is not electrically the center due to the phase difference between the windings. In most cases the potential between the 4th "neutral" leg no where near half of the total voltage for this reason ie:
480 / 277, 208 / 120 etc.

I hope this doesnt confuse you more. If you need I will try to explain it better, as I am used to working with it and somewhat complacent with it.
 
                                                                The Slab Bacon
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KB2WIG
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« Reply #2 on: June 12, 2006, 02:08:27 PM »

   Its still single phase.  Its been a while since I've played with this stuff (33+ years) so the following is not the most in depth analysis..

   A three phase generator has three coils spaced 120 deg. apart. They produce three sine waves, of equal voltage, spaced 120 degrees apart. Without trying to go through the math,  you only need four wires to send 3 times the power that is available to 1 phase systems. This is wye 3 phase is used for power transmission, you dont have as  much power loss in the cables because you use fewer cables. ( P = I^2 R ).. or you can use smaller cables to transfer the same amount of power...

    maybee this will help .. If you draw out coils,  in the shape of a "Y"  . and each part of the Y has a coil, you can visualize the coils as part of a generator winding, with each winding connected to the middle of the Y. Put a light bulb on each end of the Y and place the bulbs return at the middle of the Y. If you slowely rotate the generator , the light bulbs glow, but not equally at the same time,  one is brighter than the other two. As the generator turns, each coil produces a sine wave, but the peak of the sine waves does not occure at the same time... This is a simple explenation, but maybee it will help... Now, if you connect four wires, one to the midle of the Y ( call it the neutral) and a wire to each end of the Y ( call these lines) you have a representation of a Y-Connected generator.... An interesting thing about this system, is if the load that is connected to each of the three lines is equal, you dont need a neutral to transmit power  -- the phases cancel  and there is no current in the neutral....  but this really does not happen in real life...    Try a web search for  Wye connected, delta connected  (maybee even red dog)

 look around the neighborhood for power poles with three transformers (pole pigs) grouped together.. you'll probably see them near resturants, small manufacturing companies... large office buildings run lighting off of three phase systems,utilizing  277 volt lightbulbs, florescent tubes etc.
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xe1yzy
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« Reply #3 on: June 12, 2006, 08:07:40 PM »

Ok...
then we need a 3 phase generator to have 3 120 V hot wires and a neutral ( the center of the Y)...
 if we made a voltage measure between 2 of any of the wires, we have 240V, but each 120V wire are shifted 120 degrees right?

If I made the wiring for an Air conditioning Im only use 2 120V hot wires, and I dont use the central tap, (and of course an good ground conection), so the 2 120 V wires are 2 phases  Huh

or we have 2 diferent kind of transformers?, the doubt is the Mexican power company says 220/2/60HZ!

Thanks a lot guys
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KB2WIG
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« Reply #4 on: June 12, 2006, 09:25:03 PM »

"If I made the wiring for an Air conditioning Im only use 2 120V hot wires, and I dont use the central tap, (and of course an good ground conection), so the 2 120 V wires are 2 phases  Huh"

 No. But then again maybe thats how its done in Mexico. if its single phase you'll see the number 1 with a circle drawn with a line through it.. 2 phase, the number 2 with the stroked circle and 3 phase a number 3 with the stroked circle....

   If you look at the 2 120V hot wires on an oscilliscope, you'll see that they "start" at the same point in time...
 

       In the US this is how it goes.....If you are working with the "normal" residential system, the 240 V is still single phase. You have a transformer with a center tap (on the power pole), and you draw off of each side to balance the load. Look at , say, a  240 volt electrical entrance panel. Each circuit is composed of 120 volt Hot side and a neutral.  The breakers are spaced on each side of the box, for 120 volt operation. The 220 v breakers are accross both sides of the box, giving you 120 v, a neutral and 120 volts for a total of 240 volts.  240V is for larger loads, such as ac units, some electric stoves, ham 1KW stations, water pumps etc. The regulation is better with 240 volts.

    Three phase is seldem used in residential areas, youl find it available in comercial areas...

     This 3 phase is easier to see with the proper diagrams... but I dont really have anything here to use... THE USUAL DISCLAMER  --- I havent used this stuff in over 30 years, so dont take my word on it. ---   Try to find a textbook on electrical power transmission... for the most part, the technology is the same, even after a hundred years....    gud luck, take off your watch, rings, etc and keep one hand in your pocket.... klc
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KB2WIG
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« Reply #5 on: June 12, 2006, 09:36:48 PM »

Try this...

                              http://www.answers.com/topic/three-phase-electric-power
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The Slab Bacon
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« Reply #6 on: June 13, 2006, 07:30:52 AM »

Try this...
                              http://www.answers.com/topic/three-phase-electric-power


Glad you found that, that would have been a hell of a lot of typing to try to explain it.
I work with 3 faze power all of the time and kinda take it for granted.

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xe1yzy
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« Reply #7 on: June 13, 2006, 09:17:10 AM »

Try this...
                              http://www.answers.com/topic/three-phase-electric-power


Glad you found that, that would have been a hell of a lot of typing to try to explain it.
I work with 3 faze power all of the time and kinda take it for granted.

                                                                The Slab Bacon


Ok guys!

thanks a lot for the reading Kevin, a very good information over that page, now I have a clear Idea,  I have " al weapons" now for the next seminar!

thanks a lot to both!

Pedro.
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KB2WIG
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« Reply #8 on: June 13, 2006, 11:53:32 AM »

Try this...
                              http://www.answers.com/topic/three-phase-electric-power


Glad you found that, that would have been a hell of a lot of typing to try to explain it.
I work with 3 faze power all of the time and kinda take it for granted.

                                                                The Slab Bacon


     Yes.. Not having to work with the info for a long time scares me. I found an old textbook, and wanted to phots some pages, but that started to sound like a lot of work..... I know that somewhare in this machine, the slashed cyrcle is hiding. I learned from past mistakes that finding things like that in the machine take on a life of their own.. 
     After taking a circuits class, I realized that having a knowledge of tuned circuits helpes with the 3 phase understanding. MY proff was an retired Air Farce Major who  designed power distrubition systems for air bases. We spent spent lots of time with power triangles and the like. We only spent a week on tuned circuits, but if you look at them closely, they reall are only ac power circuits, as he pointed out. I still remember this little blerb and it does help one deal with the other aspect  (power v radio ckts).
     In high scrweel I took an industrial 'lectricity class and spent 6 mo. learning ohms law..... Trying to explain the 3 phase caused the brain to lock up and I needed help ......   klc
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