Role of a Core in Auto-Transformer 5

[inpran]

Hi all.

I have a few fundamental questions on Auto-Transformers .

1. Apart from the mechanical support, what exactly is the role played by the Core in an Auto-Transformer? In case of a 2 or a 3 winding transformer, the core provides the magnetic path which is not required in case of a Auto-Transformer as the HV and LV are not electrically isolated.

2. Should one consider the leakage reactance in case of a Auto-Transformer's the equivalent circuit?

 

[LionelHutz]

bacon4life - Read what Gunner posted again. He posted that the power is transferred through the magnetic core. This is quite different than stating the current is not conducted. Adding on your explanation - the electrons may pass through an autotransformer, but they will give up almost all of their energy to the magetic field on the way through the transformer, before receiving it back as they come out the other side.

To the OP - It appears you believe an autotransformer is just a voltage divider network. So, replace the transformer windings with a couple of resistors and think about what will happen - the output voltage will drop as the load is applied. A simple "voltage divider" transformer would be useless if you want to maintain the output voltage over a wide range of transformer loads.

 

[jghrist]

In an auto transformer, most electrons pass all the way though. They just get or give some energy to the portion that don't pass through.

With alternating current, electrons only move back and forth about 0.00002 inches in one cycle.

 

[bacon4life]

Yes, you right about the breaker tripping; I was thinking only of the mutual coupling and not of the self inductance of the main coil.

The only reference I have found so far that mentions conduction in transformers explains that the conducted portion of the power is proportional to the turn ratio per page 66 of

http://www.scribd.com/doc/13567056/17/Auto-transformer.

Is there a specific book that has a different definition of conducted power?

Is the following a more accurate understanding than thinking of electrons flowing?
The force on an electron is due to both the electric field and a changing magnetic field. In an ideal two winding transformer, the X2 bushing is at zero potential, so the force on the electrons leaving X1 is due to only to the magnetic field. In an auto transformer, the X2 bushing starts off at the potential of the H1 bushing, so a portion of the force on the electrons leaving X1 is due to the magnetic field, and part is due to the electric field in the conductor from H1 to X2.

In trying to wrap my brain around no conduction taking place, I was left wondering if I have two long parallel transmission lines have inductive coupling, do they conduct power? If I measure the incoming current to a substation an inductive current transformer, does the wire no longer conduct any power?

Thanks,
Mark

 

[stevenal]

I don't like the pure auto schematics, since the magnetic path is not as clear. Here's a two winding transformer connected as an auto per Waross. There is a clear division of both current and power at the two nodes on the left. The transformer is passing 1 VA across the core while another VA passes around the core via copper. Call it power by induction versus conduction or any other pair of words, but the division is clear.

 

[prc]

I think now nobody has any doubt regarding the portion of power transferred by induction in an auto transformer.Hope attached file comparing 2 wdg and auto transformers of same kVA will make it more clear.

 

[Skogsgurra]

Thanks prc! That's the final word (I hope). PLS!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jghrist]

Sorry, prc, but I think your diagram makes things less clear than stevenal's. The top right Auto Trf diagram does not show the 66 kV lead to the bottom. It looks like 50A in the lower winding is coming from the 132 kV, but actually, 100A comes from the 66 kV, 50A of which goes to the 132 kV and 50A goes into the winding.

The bottom diagram compares a resistive voltage divider with an autotransformer, but shows the secondary shorted. The bottom diagram shows 50V across the shorted winding, which confuses things because to short the winding and get voltage, you would have to show the winding impedance.

 

[waross]

Let's consider a 480:120 Volt transformer connected as an auto-transformer to boost 480 Volts to 600 Volts.
4 Amps at 600 Volts, 5 Amps at 480 Volts.
The case for inductive power transfer.
Input to the transformer: 5 Amps at 480 Volts = 2.4 KVA
Output of the transformer: 4 amps at 600 Volts = 2.4 KVA

The case for conductive current.
Consider the effect of an open in the 480 Volt winding. The 120 Volt winding continues to conduct current to the load at whatever current the load requires when it is fed by the impedance of the 120 Volt winding in series with 480 Volts.

The 5 Amp current into the auto-transformer splits. Four Amps is conducted to the load by the 120 Volt winding. One Amp supplies the 480 Volt winding so that induction may increase the potential of the "conducted" current.

So if we consider the current, we see an apparent split between induction and conduction.
And if we consider power, the KVA input equals the KVA output.

Power or Current
Apples are $5 a dozen. How much will 16 oranges cost?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Apples are $5 a dozen. How much will 16 oranges cost?

8 pi

=====================================
(2B)+(2B)' ?

 

[stevenal]

Waross,

So you've demonstrated the conservation of energy principle. This will hold regardless of whether or not there is a transformer in the circuit. In my proposed diagram there is 1 VA on both sides of the core and 2 VA on both sides of the entire circuit, and conservation of energy holds. The power clearly splits at the nodes with 1 amp at 1 volt going into the transformer left hand winding and 1 amp at 1 volt bypassing the core.

Consider the step voltage regulator, a type of auto with changeable taps. These are sized by the amount transformed, so that a +/- 10% 750 kVA is good for a circuit of 7500 kVA. And many can go higher if restricted to a narrower range around neutral. The reason they are sized in this manner is because of the 10 to 1 power split within.

 

[Skogsgurra]

Around 25 dollars for 16 oranges? Isn't that on the high side, Pete?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jghrist]

Consider the effect of an open in the 480 Volt winding. The 120 Volt winding continues to conduct current to the load at whatever current the load requires when it is fed by the impedance of the 120 Volt winding in series with 480 Volts.

Assuming an ideal transformer, if the 480V winding is open, no current will flow. The ampere-turns in the 120V winding will equal the ampere-turns in the 480V winding (zero).

 

[electricpete]

I remember paying something like $4 for a glass of orange juice at a diner in Florida. Guess they figure they can scam the gullible tourists. But I ate pie a lot cheaper.

For you serious guys, sorry for the interruption. Please continue...

=====================================
(2B)+(2B)' ?

 

[waross]

Yes, in an ideal transformer the core will not saturate will it?
To take it to extremes, an open secondary circuit on a CT often has little or no noticeable effect on the flow of current or power to the load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

Perhaps a good way to visualize what is happening is to use a Variac as an example. Compare what is happening to the input and output currents and voltages when the output is at 10%, 90%, 100%, and 110%. At 100% setting the output wiper is touching the input tap, but load current still flows through the transformer coil. Energy is being transferred through the magnetic field through the core.

 

[prc]

Variac is an auto-transformer.At 100 % tapping, the load current bypass the transformer coil and the only current flowing through the winding is no-load exciting current.ie there is no magnetic power transfer in this condition.

 

[Skogsgurra]

And there is no Variac in the circuit either. It just happens to be connected parallel to the supplying voltage.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Compositepro]

Interesting. But, then at the 100% setting there would be no load current passing through the transformer coil and the current would be "bypassing" the transformer. My observation has been that the temperature of a variac rises with load just like a normal transformer, even at the 100% setting. Is it just wiper resistance that causes this heating? It doesn't seem like it (uniform heat rather than a hot spot).

 

[Skogsgurra]

Does it really? At exactly 100 % setting? Not much, I assume.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

At 100% there are still the no load losses, plus a little heat from the wiper.

Bill
--------------------
"Why not the best?"
Jimmy Carter