Electric gurus-Generator input ?

[pvcces]

This thread is one of the reasons that I follow this board!

Tom Caffrey

[skipn]

 
Maybe some day when Sean feels up to it he can explain his comment  on 2 phase  "if it helps you to think of it this way, I won't stop you"  and mutual inductance from a single phase source. and what happened to the 11% on
240/208 and 75% heating resistance?

  Always a good read


  Skip

[Sean]

Maybe some day when Sean feels up to it ...

Sure...

... he can explain his comment  on 2 phase  "if it helps you to think of it this way, I won't stop you"  ...

Technically speaking, 240/120 residential power is single phase.  As with virtually all commercial power generation, the generator back at the utility is three-phase, and all three of those phases likely come down the street in front of your house.  But your house, and possibly one or two others, will be served by a single transformer, which will be connected to exactly one of those three phases.  Since most street-level (typically 12,000-30,000 volts) distribution is Delta, the primary of the transformer will be connected across two of the three wires, but that constitutes a single phase in Delta.  The secondary winding is set to provide 240 volts, which by the very nature of the single-phase input must also be single-phase, across its two end terminals.

Now here's where it gets muddied a bit.  That secondary winding also has a center tap, which means that the potential between it and either end would be half the total, or 120 volts.

Now, if you chose to ground one of the two ends, and connect that as your neutral, you would have a choice of two voltages, 120 or 240, and, if you looked at them on a scope, you would see that they are exactly the same sinusoid, with the peaks and valleys occuring at the same time, except one has twice the amplitude of the other.  You would not look at this and say it was "two phase."  And you could certainly connect things this way, but it leads to a problem, which is that you will not really be able to balance the loads across the two halves of the secondary transformer winding; every 120-volt load will end up loading just one half of the secondary, and the 240-volt loads will use the two half-windings equally.  So it's not done this way.

Instead, we ground the center tap, and use this as the neutral.  This allows us to balance our 120-volt loads (which usually are the lion's share of residential loads) across both halves of the winding.  The effect of this, though, is that a scope would show one 120-volt leg has the inverse sine-wave of the other leg.  In reality, they are merely two halves of the same sine wave with twice that amplitude, but with the "neutral" as reference, they appear to be 180° opposed phases.  No one in the electrical power industry refers to this type of power as "two phase" -- if a distinction needs to be made between this type of power and one-voltage single-phase, it is called "split phase," which is more properly descriptive: it is a single phase, "split" in half.

From the standpoint of the end user, though, it is indistinguishable.  I have a two-phase generator, which is to say that it has diametrically opposed windings (vs. 120° offset ones), which can be strapped to provide either 240/120 or plain 120.  In this case, the "center tap," if you will, is part of the generator head itself.  The power from this is indistinguishable from split-phase that is transformer-derived from grid power.   This is why I wrote that I won't stop anyone if it helps their thought process to conceive of it as "two phase, 180°" power.

... and mutual inductance from a single phase source.

Not sure what you are asking about here.

and what happened to the 11% on
240/208 and 75% heating resistance?

Also not sure what 11% you are talking about.  Let me clarify the math.

I said that a resistive heater will provide only 75% of the heat on 208 vs. 240.  Here's why:

P=V²/R (Power is the square of the voltage divided by resistance)

so for 240 volts, P=57,600/R (let's call this P₂₄₀) and for 208 volts P=43,264/R (let's call this P₂₀₈)

The percentage of power dissipated at 208 volts vs. 240 volts is thus (P₂₀₈/P₂₄₀)*100

Plugging in our previous formula, that's 100*(43,264/R)/(57,600/R), the "/R" part simply cancels out, and you get 100*(43,264/57,600) or 75.11%.

This may confuse some, because 208 volts is 86.67% of 240 volts, and I'm guessing your "11%" is the difference between these numbers.  It helps to remember that a fixed resistance draws less current when connected across a lower voltage.  Now you have 86.7% of the current, running on 86.7% of the voltage, and since P=VI, your power dissipated is "86.7% of 86.7%" or 75%.

Another way to say this is that power varies as the square of the voltage, which is why variable resistors need to have logarithmic windings to produce linear-seeming variations in output.  It is also why a 24-volt lamp connected to a 12-volt source will appear roughly 1/4 as bright, not 1/2 as bright.

Hope that clears it up.

-Sean
http://ourodyssey.blogspot.com

[JohnEd]

Sean,

WHHHAAAAADDD???    



That needed to be said.     Or I needed to say it


I was on a ship doing an install many years ago.  That is loosely stated as the sailors and Chiefs did the actual work.  I don't recall what prompted me too check their primary power but I found the voltage a tad low, but within spec, and then I checked from the high to the frame.  What the.... 60 volts.  When the Chief came back I told him something was "funny" and what I had found.  He explained that the power on a ship wasn't like the shore "stuff" as they had no ac neutral but had two phases that made 120 and it was called a DELTA connected generator on the ship.  I reflected that in sophomore year (I didn't do senior in the Voc school) they touched on Delta and Y configured generators and transformers.  "Ah Ha!".  I remembered that the center tap of a Y wasn't needed if you were only making 220.

To me 208 , 220 and 240 are all the same and yes I know that is shallow and factually incorrect and I only recently learned of the existence of 208.

I guess you caught me on that NEC quote.  I threw out my NEC volume years ago due to lack of use and my inability to find nearly ANYTHING I was looking for.  I had another inducement:  I was told by a journeyman electrician that the CITY inspector had the last word and that that would be final in every instance.  He told me that he was aware that a single grounding rod driven into the earth was listed in the NEC as adequate and within reg for western Orygun BUT all inspectors required TWO rods spaced 6 feet apart and he thought it was because we get all our rain during a two month period and we were actually considered an ARID place for a couple months after we got dried out proper.  As deep as I ever go into these things with my house wiring projects, I found it expeditious to just ask the inspector what he wanted the final config to look like as I have no choice any way.  When that 208 thing came up I went to Home Depot and bought a nice thick book on HOUSE WIRING...and it was spendy.  I was quoting that book about 208 and not the NEC. (and I can make up a third story if need be )

Thank you for an excellent post.  You and Boogie both as well as others that contributed.

John

[Chopper Scott]

I'm basically a lurker on this forum and pipe in from time to time. I gotta admit that the help on this forum is really awesome. It is nice having all that knowledge and experience at arms length especially after contracting bus disease just this summer!! I've learned a ton just by reading every one else's experiences!!

[skipn]

Thanks Sean now we know the rest.

... and mutual inductance from a single phase source.
Not sure what you are asking about here.

  Basically how transformers work   stuff about emf Neumann formulas and a bunch
of other dudes.    coils, taps and other wild wizidry. just as long as I hit the starter
button and it works I'm happy

  But it really does explain in a way why one doesn't hook their generator up to the house without
the proper equipment.........throwing the main breaker is the weakest way link (reqires a human remembering)

  Skip

[buswarrior]

Thank you to the smart folks.

Always enjoy reading detailed explanations.

happy coaching!
buswarrior