Possible 2002 Handbook mistake?

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ronaldrc

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I respect all the contributers to the NEC Handbook and purchase one every couple publications.There are tons of information you don't get from the regular code book. No ones perfect and there are mistakes in about everything.

And when I find something that doesn't appear correct I like to confirm if I am wrong or right.

Please look at page no.80 Art. 210 Branch circuits of the 2002 hand book at the exhibits 210.8 and 210.19.

When we use the term buck in electrical terminology isn't it to apply a voltage to another voltage in opposite polarity to reduce it? In other words if I have a simple circuit with a 6 volt battery and insert a 1.5 volt battery in this circuit back wards wouldn't this reduce the voltage to 10.5 volts? And this is the true definition of the term buck.

Note exhibit 210.18 top left of page 80.

My problem with this circuit is not that we will no have a decrease or increase in the voltage because it will, but the way it is obtained.


I don't have a problem with the way it is done I would prefer it this way, but the definition is wrong.The definition of bucking.

This is a autotransformer in the boosting and reducing the boosting is correct but the bucking isn't bucking.To buck or decrease the voltage by bucking you would leave the wiring or taps as the boosting and just reverse the x or low windings polarity x1 and x4 would be reversed.

If you will examine the bottom exhibit you will notice it is correct to boost 1 and 4 between H and X need to connect and to buck 1 and 1 between H and X need to connect.

If I am wrong please explain. :)
 
Re: Possible 2002 Handbook mistake?

Here is what Ronald is asking about.

Ex210.18.JPG


Exhibit 210.18 Typical single-phase connection diagrams for buck or boost transformers connected as autotransformers to change 240 volts single-phase to 208 volts and vice versa.

Ex210.19.JPG


Exhibit 210.19 Typical connection diagrams for buck or boost transformers connected in 3-phase open delta as autotransformers to change 240 volts to 208 volts and vice versa.

[ October 09, 2005, 12:43 PM: Message edited by: iwire ]
 
Re: Possible 2002 Handbook mistake?

I am assuming you have the CD addition and copied the JPEG from it right?

I never could get a good scan from the Hardback book version because of the curve in the page.

Thanks :)
 
Re: Possible 2002 Handbook mistake?

Edit: Now I see the difference between the two circuits.

I think I know what you are saying. If you have a 208V to 240V boost transformer, can't you change it to a buck just by changing the input and output terminals?

But they are reversing the polarity of the low voltage windings. Maybe it has something to do with the power rating of the windings??

Steve

[ October 10, 2005, 01:17 PM: Message edited by: steve66 ]
 
Re: Possible 2002 Handbook mistake?

Hello Steve

They didn't use the bucking process to reduce the voltage they reversed the input and output.

Bucking means to apply a voltage of opposite polarity to the voltage to reduce it.

Bucking would be done like the drawing below.
Notice how the low voltage winding is reversed that would be bucking.

:)
 
Re: Possible 2002 Handbook mistake?

But they did use the bucking process in the second diagram of 210.19. If you compare the boost diagram with the buck diagram, the are wired exactly the same except the low voltage windings are reversed.

For example, on the boost, the voltage X1 (with reference to X4) is added to the input voltage. For the buck, the voltage X4 (with reference to X1) is added to the input voltage. A better way to phrase this is to say the X1 voltage (with reference to X4) is subtracted from the input voltage.

So in one case you are adding V(X1) to the input, and in the other case you are subtracting V(X1) from the input.

Steve

[ October 10, 2005, 04:45 PM: Message edited by: steve66 ]
 
Re: Possible 2002 Handbook mistake?

Steve in my first post I said the second three phase diagram was right its the first one thats wrong the winding are hooked to be additive.

I would prefer to reduce it the way they are showing my argument is that it is not bucking to reduce it.It is like tapping the transformer to reduce the voltage not bucking.

Ronald :)
 
Re: Possible 2002 Handbook mistake?

Sorry to be so slow on understanding what you are saying.

I do agree the buck circuit shown in the first diagram is a different animal than the buck circuit shown in the second diagram.

I also think they should have different names. It would just make it easier to refer to the two different ways to reduce a voltage with an autotransformer.

However, the "buck" transformer in 210.18 does still add a negative voltage to the input voltage. It's just a matter of how you look at it. The low voltage windings develop a small voltage across them that opposes the input voltage. That gives a smaller output voltage.

I realize you can also look at the circuit and think of the input voltage and output voltage as separate entities. If fact, I think there are a lot of different ways to visualize the operation of this circuit. But at least one fits the definition of a buck circuit.

Steve
 
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