single phase primary


  #1  
Old 03-01-02, 04:29 AM
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single phase primary

I'm in the planning stages of supplying power to a property which currently has no service, although there is service available at the road. If I opt for underground, I'll have to run a trench about 1000' to the site. In preliminary discussions with the power company, they estimate that they'll run a "single-phase primary" to the site, terminating at a transformer.

I never thought much about the power supply on the other side of the service drop. I'm a little confused as to what exactly a single-phase primary is. I gather it's a cable consisting of a hot and a neutral. (Is this correct?) At the transformer, it (somehow) is converted to 2-phase (2 hots and a neutral), allowing 240 volts. How does that happen from a single phase?
 
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Old 03-01-02, 06:33 AM
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The supple authority will be sending in two hots and a neutral for your single phase system. there is only single and three phase systems in use which confuses many people. Three phase consists of three hots and a neutral. If your interested in a more detailed explanation PM me and I'll email you some literature on single phase I use when I'm holding my instruction classes.
 
  #3  
Old 03-01-02, 08:45 AM
mikatrex
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Sounds like you need to ask the power authority to talk to you with the fact your don't work in distribution systems in mind.
They supply residential (and many commerical) customers with what is called single phase power. It is used for services that require 120, 120/240 or 240 volts. The two hot wires and one neutral that you refer to would be the 120/240 volt electricity that is the most used by us here in North America - 120v for most things and combine two 120v wires for 240v for large items such as stove and dryers etc.
The transformer that they will install doesn't affect the phase - simply the voltage. The power distrubution company uses high voltage (typically 2400 to 27000 volts) to go from place to place (due to less losses over same distance) then uses transformers to step it down to your voltage needs.

Now all that said what you REALLY need to know - where are they putting their transformer - at the road or on site(at the end of your 1000')? If its at the road then you will need to bring what is called secondary wires (120/240volt) to them to connect to the transformer and to your service. If they want the transformer near you then THEY will put in primary (high voltage) cable from road to it (thats how we do it unless very large load customer is not responsible for primary work).
If you have more questions email me or post again - I will check back..
 
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Old 03-01-02, 09:04 AM
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It is possible that they are giving you one hot and one grounded conductor, or two hots and one grounded conductors, for the primary.
Very often it is the prior, one hot and one grounded conductor. Through the transformer, on the secondary, you of course get, what you know, two hots and a grounded conductor (240/120V).
http://www.howstuffworks.com/power5.htm
1000' is a long run for 240/120V. You will have to do some serios upsize of the service conductors. Would they consider giving you the primary voltage, metering on the primary, and letting you put a pole or pad transformer closer to the house? Thats the way to go if they will let you.
If they gove you 240/120V to go 1000', you will probably need 500kCMILs to get some usable voltage at the meter. If they give the 240/120, make them meter you at the house, so they pay for all the conductor losses.
 
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Old 03-01-02, 10:24 AM
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Thanks guys. You're coming from where I'm headed.

I think (so far) they are willing to run the primary to the house site and terminate it there with a transformer, then to the meter, so I don't pay for the voltage drop. From what I have read from them so far, I believe its a 7200V primary.

I think they'll only run a 3-phase primary for a commercial app., but a) I haven't really gotten into this with them, and b) I don't think I'll need it.

gard: I'll take you up on your offer of literature on single-phase. Please send it to capstone5@aol.com

TIA
 
  #6  
Old 03-01-02, 03:29 PM
Wgoodrich
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The power company will most likely be installing a single conductor rated for 7,200 volts underground the 1000' to a transformer most likely within 250' from your home then the will set a ground transformer that will produce the two phases of hot conductors and a neutral by a center tap of that transformer. Then they will install two hots and a neutral to your meter system. They will have a grounding electrode system at their ground transformer for their lines. You must install a grounding electrode system for your house. There should be no voltage drop when they install the 7200 volts which can run for miles without voltage drop. A 20 amp primary fuse on 7,200 volts will carry several 200 amp 120/240 volt services. That 7,200 volts is powerful and can run great distances with great loads. The neutral you will be using is created within that power company's transformer. Only one conductor probably a 6 awg copper primary wire will provide the power to your tranformer where you 120/240 volts and neutral is created. This transformer works by inducement without any contact between the 7,200 volt single primary conductor and the 120/240 volt with neutral secondary conductors that will be connected to your home.

They will most likely install all that for you and connect to a meterbase that you have mounted where they say to put it on your home. That meter on your home will be the dividing line between your wiring and the power company's wiring.

Otherwise don't worry about what they will be doing other than what they said your construction cost may be.

You will most likely only be wiring to the meter base on your home. They will take care of all on the line side of that meter base.

Hope this helps

Wg
 
  #7  
Old 03-04-02, 10:49 AM
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Thanks for the lit. gard.


Warren:

You say that they'll most likely run a single hot to the transformer, and that the 7200 volts is supplied by induction (no neutral required, hmm?).
I understand how a center tap provides the neutral between 2 hots, which in turn provides the 120 volt single phase from each hot to it.
What confuses me, and may be a bigger question than I realize, is how a single-phase induction current becomes 2 out-of-phase hot leads? Obviously the transformer's providing this, but I was under the impression that the only thing the transformer could do was to a) step down (or up) the voltage, and b) provide a place for the center tap. But I always assumed the transformer must already be supplied with 2 out-of-phase feeds. Time to do a little reading.
 
  #8  
Old 03-04-02, 03:39 PM
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Abnormal, I think you answered your own question when you correctly pointed out that the secondary of the transformer is "center-tapped". Thus, the two hot legs have to be 180 degrees out of phase with each other.
 
  #9  
Old 03-04-02, 08:41 PM
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rhhr, I couldn't have said it better myself. Only thing is I would have ended up with a two page reply saying the same thing.

Well said

Wg
 
  #10  
Old 03-06-02, 09:17 AM
amp-man
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240V supply legs

Hello all,

I've heard the explanation before that the 240V legs of a 240/120V service are "180 degrees out of phase".

While that may be an easy way to visualize it, I believe it is wrong.

The 240V across the two poles or legs of a standard center-tapped transformer is a measure of the potential between the two conductors. The waveforms are in phase (in sync), and the peaks and valleys of the two are 240V apart in potential, because of the way the transformer is configured.

Look at it this way: you have a 7.2kVA primary coming to a transformer. It is supplying single-phase power (one sine wave, obvoiusly).

The transformer steps the voltage down based on the ratio of turns of the primary windings to the secondray windings. The 240V comes off of the ends of the secondary windings, and the center tap is grounded so you split the difference to get 120V from the CT to either hot leg.

There is nothing in the transformer causing a 180 degree offset in the sine wave of one hot leg relative to the other.

The "180 degree out of phase" situation describes two-phase power, which is rarely used these days. The 240/120V service familiar to us all is single phase, which means there's only one sine wave or phase present.

I am all for making things easily understood, but I am not in favor of explaining things incorrectly. To explain the 240/120V set up, it's easy enough to say that the high primary voltage is stepped down to 240V, and the 120V comes from splitting the 240V in half using a grounded center tap in the transformer.

Cliff
 
  #11  
Old 03-06-02, 09:52 AM
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I am all for making things easily understood, but I am not in favor of explaining things incorrectly. To explain the 240/120V set up, it's easy enough to say that the high primary voltage is stepped down to 240V, and the 120V comes from splitting the 240V in half using a grounded center tap in the transformer.
To make thing easier, let's name the terminals on the transformer secondary. Call one end of the 240V winding Black; call the other end Red; call the center tap White. The voltage from Black to Red (Vbr) is as you say 240V RMS. The voltage from Black to White (Vbw) is 120V. The voltage from Red to White (Vrw) is also 120V, but it is 180 degrees out of phase from the voltage from Vbw.

How?

One of the fundamental physical laws related to electricity (Kirchoff's voltage law, for those of you keeping score) says that

Vbr = Vbw - Vrw

Mathematically, Vbr is

Vbr = 339.4 sin (2*PI*60*t)

where 339.4 is the peak voltage which corresponds to 240V RMS, 60 is the frequency (in Hz), and t is time (in seconds). Vbw is given by

Vbw = 169.7 sin(2*PI*60*t)

where 169.7 is the peak voltage corresponding to 120V RMS. Solving the first equation for Vrw gives

Vrw = Vbw - Vbr = -169.7 sin (2*PI*60*t) = -Vbw

So Vrw is the negative of Vbw. Negating a sine wave is the same as shifting it 180 degrees. Therefore, Vrw is 180 degrees out of phase from Vbw.
 

Last edited by mikewu99; 03-06-02 at 10:09 AM.
  #12  
Old 03-06-02, 10:21 AM
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It's all just terminology.

You don't have two "240V legs". You simply have two legs that have 240 volts between them. If you look at the voltage waveform between these two legs, it's just a simple sine wave. So it is indeed misleading to simply say that the voltage on the two legs are 180 degrees out of phase.

However, if you add the phrase "with respect to ground", then the voltage with respect to ground on the two legs are indeed 180 degrees out of phase.

We're playing with the tiny bits of our language, but it's an interesting theoretical discussion.
 
  #13  
Old 03-06-02, 01:39 PM
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We need a volunteer with an oscilloscope capable of displaying two input waveforms simultaneously. Ground the scope to the neutral/ground bus of the main service panel and put one probe on one hot leg and the other probe on the other hot leg. I think they would show the peak of sine wave A occuring at the same time as the trough of sine wave B, which I would call "180 degrees out of phase". John may be right about this discussion being only about terminology, but it is interesting, nevertheless. (OK, who owns the oscilloscope ?)
 
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Old 03-06-02, 01:52 PM
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I think they would show the peak of sine wave A occuring at the same time as the trough of sine wave B, which I would call "180 degrees out of phase.
You are absolutely correct. Don't need to do the experiment. Think about it: If sine wave A and sine wave B were in phase , the peaks and troughs of A and B would line up so the voltage between A and B would be zero. Since they are actually 180 degrees out of phase then the peak of A lines up with the trough of B and vice versa, so that the voltage between A and B is twice the voltage from A to ground.
 
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Old 03-06-02, 04:42 PM
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In answer to your wondering Abnormal how the power company makes the transformer work with only one 7200 volt wire is they cheat. If you look at any pole mounted transformer they take a second wire from it and connect it to a ground rod<thereby using earth as the "return wire" you could do this at home too<NOT RECOMMENDED> all sorts of code violations there
 
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Old 03-06-02, 06:33 PM
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Lets talk politics! Better yet, how about the Bible! It's all theory.
DIY'ers need to know the basics of electricity. Do it wrong and a house burns down or people get hurt. IMO.
 
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Old 03-06-02, 07:20 PM
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A question was asked, a mis-statement was made, and I was just attempting to answer the question and correct the mis-statement. Nothing I said was theoretical - it is fact, based on very elementary electrical concepts.
 
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Old 03-07-02, 10:51 AM
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Two phase? Realy!

Many of us have dealt with multi tap transformers. I have dealt with control wiring that has several voltages to accomplish different functions. I have seen the secondary of a transformer tapped to supply 12, 24, & 48 volts all from the same secondary winding. On that secondary winding there was no ground reference. Is someone going to argue that the secondary of that 120 volt primary supplied transformer was three phase. The secondary winding I mentioned were all in series with each other so that measuring between control voltages would give you a non system voltage so you would know that you were about to tap into two separate control circuits. The math does not impress me. I can wire four 120 volt primaries across a single phase 480 volt circuit and get over a dozen different voltages out on the secondary side. I will even permit you to ground the secondary at one of the tap points of your choosing. Do I now have twelve phases. If I take a DC balancer circuit; or a 120/240 volt battery supplied emergency lighting circuit; and connect an oscilloscope to it the scope will show two voltages relative to the circuits neutral. Is any one going to try to tell me that I have two phases there. The center tapped transformer was developed to supply converted edison three wire services that had originally been supplied by three wire DC circuits. These "Edison Circuits" are how we ended up with dual voltage domestic wiring in the first place. The 120/240; originally 100/200 I believe; volt distribution was Edison's attempt to deal with the excessive voltage drop that was caused by distributing the power at the lower voltage. The point I am trying to make is multiple AC voltages that will show up differently on the oscilloscope, depending on how the scope is connected, do not prove that we have multiple phases in any more than the mathematical sense.
 
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Old 03-07-02, 11:38 AM
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I love these discussions. I learn so much. Truth, like voltage, is relative. One cannot speak of the voltage on a given wire in isolation, since all voltage readings must be taken relative to something else. Depending on what you take the voltage relative to, you can come up with many, many different answers as to what the voltage looks like.
 
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Old 03-07-02, 12:05 PM
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OK, I'll give it one more try.

A transformer with a single-phase primary can produce secondary voltages either in-phase with the primary voltage or 180 degrees out of phase with the primary voltage, depending on which end of the secondary winding is grounded. Other phases cannot be produced, only zero and 180, no matter how many secondary windings you add. You can get as many different voltages as you like, but only two phases.

Transformer windings have polarity. If you look at a properly drawn schematic symbol for a transformer, the polarity is indicated by a dot at one end of the winding. The interpretation of the dot is as follows:

Say that the primary winding has terminals P and P'. Terminal P corresponds to the dotted end of the winding and P' to the other end. Likewise the secondary has terminals A and A', similarly defined. Also assume that the turns ratio from primary to secondary is 60:1.

Drive primary terminal P with 7200 VAC and ground terminal P'. If I ground terminal A' and measure the voltage at terminal A, I will see 120 VAC which is in-phase with the 7200 VAC input signal. That is, the peak of the voltage at A will line up with the peak of the input voltage. If instead I ground terminal A and measure the voltage at A', I will see 120 VAC which is 180 degrees out of phase with the input - the peak voltage at A' will line up with the trough of the input voltage.

Now add another identical secondary winding with terminals B and B'. Hook up the transformer so that terminals A' and B are connected together and grounded. The two secondary windings now form a 240V center-tapped secondary with grounded center tap. Measure the voltage at terminal A and you will again see 120 VAC in-phase with the 7200 VAC input signal. Measure the voltage at terminal B' and you will see 120 VAC which is 180 degrees out of phase with the input, and therefore with the voltage at terminal A.

P.S. - WRT your battery example: phase is meaningless when discussing DC voltages.

P.P.S. - I apologize if my prior use of mathematics to try to explain this issue offended anyone.
 
  #21  
Old 03-07-02, 02:39 PM
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I don't think anyone is offended in this impressive discussion. Often times in this forum we all tend to lay what we believe to be right out on the table in a reply. Possibly in the back of our mind inviting a different twist on the subject or a validation to what we just said. Way too many times for me to count we all have benefited from these discussions.

It it true that what you guys just said went so far over the DIYer's heads that the galaxy could be looked back at from there by a DIYer. Even though that statement is true while we hash over the technical jargon and theories etc. we are finding ways of saying better what we are trying to convey to the DIYer in laymen's terms.

The statement to the DIYer on this post about the 180 degrees from each other is a generic laymen method of talking without getting into a statement that we all know the normal DIYer will not have a chance to understand. Often times a generic statement that often is not exactly true to the subject as being exactly correct often times makes it more understandable to the DIYer. While it is not exactly accurate in the statement said the simplicity of that inaccurate generic statement often times makes if understandable to those that are not at the level of those speaking the techno facts.

While I agree the statement of the 180 degrees out of sync with each other is not technically accurate this statement is often said in simplistic attempt to get a laymen or early learner to advance in their understanding of the sine wave make up.

During these discussions the DIYer may take a back seat at the time but I believe the discussion better prepares us to help the DIYer later on benefited from the techno discussions.

You all looked good and held your own in the discussions and we all probably picked up something in this techno discussion.

Looked impressive to me

Wg
 
  #22  
Old 03-07-02, 03:28 PM
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There are DIYer's out here who understand electricity, but need help with the implementation of home wiring mechanics, codes, etc. An EE does not make an electrician, nor the other way round. This topic runs right through the middle of both. It has been fun to watch!
 
  #23  
Old 03-08-02, 06:48 AM
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Hooked on power.

Amen Wgoodrich! I am a obsessive/compulsive do-it-yourselfer. (That remark is part of my 12 step recovery program ) I do occasionally get lost in the math and the P and P' dialog, but I always read through to the summation. We laymen like the summations!

So what was the summation here? If I understood Wgoodrich correctly, my 120/240 house current is referred to as 'out-of-phase' but only because the I am referring to the peaks and troughs, and disregarding the synch patterns between the two. After reading this, I get the idea that in my house, if I overlaid my Red wave form atop the Black wave form, I would see a single wave form. Is that right?

P.S. Wgoodrich, I clicked on your banner "homewiring" at the bottom of your post. It linked to a page with some graphics and alot of font garbage, then locked up my pc. I had to reboot. Just thought you should know. I run Win98se and I.E. Explorer if that helps any.
 

Last edited by Lugnut; 03-08-02 at 07:33 AM.
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Old 03-08-02, 07:59 AM
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No, not exactly. If you plotted the voltage on the red wire with respect to ground, it would have the same shape as the voltage on the black wire with respect to ground, but it would be shifted over.

[PHP]
Red with respect to ground:

x x x
x x
x x
x x
x x
x x x
x x
x x
x x
x x
x x x

Black with respect to ground:

x x x
x x
x x
x x
x x
x x x
x x
x x
x x
x x
x x x

Red with respect to black

x x x
x x
x x
x x
x x
x x
x x
x x
x x
x x
x x x
x x
x x
x x
x x
x x
x x
x x
x x
x x
x x x

---
[/PHP]
 
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Old 03-08-02, 09:08 AM
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I think you answered my question, particularly with the last drawing. 'Red with respect to Black'. That is what I think I was trying to convey. Thanks!
 
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Old 03-08-02, 10:06 AM
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"Phase" refers to the time-displacement of A.C. sine waves.Connect the 100 volt secondary of a transformer across 10 resistors of equal value and there is 10 volts across each resistor. You can read combinations of voltages across the resistors but they are all IN PHASE. Each voltage,plotted as a sine-wave,wether 20,50.or 100, are Zero at the same instant and maximum at the same instant. Connect the Running winding of a single-phase induction motor across 120 volts and the Starting winding in series with a capacitor across the same 120 volt source.The capacitor causes a "time-dispacement" beween the sine-wave of the Running winding and the sine-wave of the Starting winding of 90 electrical time-degrees.At the instant when the voltage of the Starting winding is maximum(90 degrees) the voltage of the Running winding is zero. At the instant when the SW voltage returns to zero(180 degrees) the RW voltage in maximum.We now have (almost) a 2-phase motor with the sine-wave of the SW 90 E.T.D.'s "ahead" of the sine-wave of the RW.The result is a "Rotating Magnetic Field", essential for starting a induction motor from a single-phase source
 
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Old 03-08-02, 10:38 AM
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Thanks guys. This has been exceptionally informative.

Even tho' the math behind this principle is over a lot of our heads, I can understand enough to make the explanation worthwhile. Thanks Mikewu99 (BTW you were right in another thread, I had outlined a switched neutral - my bad).

John - I loved your graphic. I know what a p.i.t.a. it is to "draw" one of those on this board.

What I learned by reading and on the DIY board today:
Massive voltages are used by the power companies to minimize voltage drop over long distances, at the expense of current.
Transformers, while reducing the voltage, regain the current.
Power companies transmit 3 phase power from the source. (I always thought it was a trick done at the stations.)
One line (not 3), carries the power to our neighborhoods, and a ground serves as the neutral in the circuit.
One leg is always in phase with the primary, the other is 180 out of phase. ( How obvious, but I just never thought about it).
My single-phase primary may be a single wire, with ground providing the neutral at the transformer.

Another fine day on the Board.
 
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Old 03-08-02, 02:11 PM
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PATTBAA:

If you have two sine waves of identical frequency but their peaks don't line up, then there is a phase difference between them. If you accept John Nelson's drawings of what the voltage from red to ground and from black to ground look like (and I'm certain they are correct) then clearly (1) the red and the black voltages have identical frequencies and (2) their peaks do not line up - in fact the peak of one lines up with the trough of the other. The black voltage is the negative of the red voltage - a 180 degree phase difference.

Note that I used the term phase difference rather than phase shift. The transformer is not delaying the black voltage relative to the red voltage to cause a phase shift. Rather the black voltage is generated by a winding of opposite polarity to that which generates the red voltage, so the black voltage is the negative of the red voltage. With sine waves, negation results in a 180 degree phase difference.

Note: A transformer with single phase input can ONLY generate outputs with zero degrees phase difference from the input or outputs with 180 degrees of phase difference. It just depends on which end of the secondary winding is ground. No other phases are possible.

I think the root of the confusion is the term phase shift (which implies some sort of time delay to one of the outputs) and phase difference. I tried to be careful to avoid "shift" in my explanations, but if I used it once or twice and it caused confusion I apologize.
 
  #29  
Old 03-08-02, 02:16 PM
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Massive voltages are used by the power companies to minimize voltage drop over long distances, at the expense of current.
Especially farther up the transmission "food chain" - that 7200V primary is stepped way down (in several stages) from the incredibly high voltages used in long-distance transmission lines.
 
  #30  
Old 03-09-02, 08:20 AM
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mikewu99; We have 10 resistors of equal value connected in series across the secondary of a single-phase transformer. The secondary voltage=100 volts which is a 10 volt drop across each resistor.The resistors are arranged "verticaly" and numbered,top to bottom, R1,R2,------R9,R10.At 90 ETD's the polarity of the secondary lead connected to R1 is Positive and the polarity of the secondary lead connected to R10 is Negative.The polarity of the voltage-drop across each resistor is Positive at the top and Negative at the bottom.We connect a lead to the top of R5 which is indentified as"R5",a lead to the bottom of R6 identified as "R6" and a lead between R5 and R6 identified as "C". Lead "R5" is 10 volts Positive refered to "C",or "top to bottom",lead "C" is 10 volts Positive refered to "R6",also "top to bottom",but "R6" is 10 volt NEGATIVE refered to "C" which is a "bottom to top" "reference" Because "R5" is 10 volts Positive refered to "C" and "R6" 10 volts Negative refered to "C" are the voltage drops across R5 and R6 180 degrees "out-of-phase"?If so,then the voltage drop across R6 is 180 degrees "out-of- phase" with the voltage drop across R7 which,in turn,is 180 degrees "out-of phase" with the voltage drop across R8.----Glad to participate in the discussion---Proud and Thankful TBAA
 
  #31  
Old 03-09-02, 01:46 PM
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Because "R5" is 10 volts Positive refered to "C" and "R6" 10 volts Negative refered to "C" are the voltage drops across R5 and R6 180 degrees "out-of-phase"?
Correct. They are 180 degrees "out of phase" assuming both voltages are referred to "C" - in other owrds, if you measure the voltage drop across R5 "top to bottom" and the voltage drop across R6 "bottom to top".
If so,then the voltage drop across R6 is 180 degrees "out-of- phase" with the voltage drop across R7 which,in turn,is 180 degrees "out-of phase" with the voltage drop across R8.
It all depends on which direction you measure the voltage drop. All drops measured "bottom to top" will have a 180 degree phase difference from those measure "top to bottom". It's just the sign of the sine (bad pun).
Proud and Thankful TBAA
Aha, now I get your handle. Me too.
 
 

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