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Role of a Core in Auto-Transformer
5

Role of a Core in Auto-Transformer

Role of a Core in Auto-Transformer

(OP)
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?

RE: Role of a Core in Auto-Transformer

Why would a flux path not be needed?

RE: Role of a Core in Auto-Transformer

I guess that inpran thinks that an autotransformer works like a potentiometer.

Which is wrong. An autotransformer works like any transformer and needs its core as much as any other transformer.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

Hi inpran,

this is my first post to this valuable site.

due to the differences in voltage and current in the primary and secondary windings, that required a different design of the windings....in autotransformers, most of the current transfers to the secondary though electrical connection but still a part transfers by induction, actually we have a core and coil assembly in autotransformers similar to the two winding transformers, the difference is in auto transformers are wound at the same core leg.
 

RE: Role of a Core in Auto-Transformer

In smaller sizes it is common to reconnect conventional transformers as auto transformers for voltage adjustment. A "Buck-Boost" rated 480:120 Volt transformer may be used to convert from 480 V to 600 V and from 600 V to 480 V.
120:12 Volt transformers may be used to drop the supply voltage of incandescent display lighting panels for a combination of a "mellower" color temperature and greatly extended lamp life.
The 120:12 Volt transformer may be connected in "Buck" configuration for an output of 108 Volts (120V -12V) or in auto-transformer configuration for an output of 109.1 Volts ([120V/120V+12V]x120V).

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

RE: Role of a Core in Auto-Transformer

(OP)
Thank you all for your replies.
@ Ghawanmeh and davidbeach:
1. If a flux path is indeed needed, does the regular EMF equation of a two winding transformer become applicable to an auto-transformer?

@ Skogsgurra
2. If a auto-transformer is like any other transformer, why is that there any galvanic connection between the HV and LV windings?

3. What percentage of the power is actually transferred by the "induction" phenomena in a auto-transformer and what percentage by the method of "conduction"?
 

RE: Role of a Core in Auto-Transformer

Check your textbooks for the division of power in an auto transformer. The percentage of conducted power versus induction power depends on the transformer ratio and the connection. Step up, step down or buck. The buck connection may not be strictly an autotransformer but the connection may be used to develop a slightly different voltage than an autotransformer connection.

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

RE: Role of a Core in Auto-Transformer

3
inpran,answer to your questions

1) Yes

2)Galvanic connection helps in transferring part of the power by conduction there by helping to reduce the size of core which is required for inductive transfer.

3) Inductive portion will be the  co-ratio ( HV-LV/HV ) times the line rating. ie In case  of a 100 MVA 220/132 kV transformer, the inductive portion will be 220-132/220 = 0.4 ie 40 MVA. In other words, the size ( copper + Core) and losses of a 100 MVA auto transformer will be that of a 40 MVA 2 winding transformer.But in reality, this will slightly chage due to tertiary stabilising winding and tap changers. So many cases you will see the advantage will be mainly in losses ie efficiency and cost of a 100 MVA auto will not be that of a 40 MVA 2 winding unit, but some where between 40-100 MVA 2 winding units.

RE: Role of a Core in Auto-Transformer

inpran

"2. If a auto-transformer is like any other transformer, why is that there any galvanic connection between the HV and LV windings?"

It is a question of economy. The single winding with a tap contains less copper than two separate windings would.

Very little power is transmitted by conduction. Almost zero in a well designed auto-transformer.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

prc,
The core of 100MVA auto trafo is similar to a 40MVA two-wdg trafo in size. Interesting and Thaks for the insight.
When it comes to copper, I think overall copper consumption in an auto trafo is more than that of two wdg trafo of same rating. Is that correct!

RE: Role of a Core in Auto-Transformer

Raghun, the copper also will be same quantity as in a 40 MVA transformer.But in reality, the copper will be more due to following:When we say total active part in an auto transformer will be r %( co-ratio) of a 2 winding transformer,the impedance also will be r % of 2 winding tarnsformer ie auto transformer impedance will be 4 % in case 2 winding impedance is 10 %.As you will agree,4% impedance for a 100 MVA  auto transformer will not be acceptable and hence designer will put more number of turns to get an impedance of 8 % or 10 %. ( Impedance varies as square of number of turns)This naturally increases the copper quantity.

In Indian  market 100 MVA 220/132 Kv auto -transformers with line end OLTC is available with 100 MVA 220/66 KV 2 winding transformers with neutral end OLTC.Since line end OLTC  involves 3 poles( costly and require more volume to accommodate)and auto transformer has extra stabilising tertiary winding, price level is almost same for auto and two winding units, but losses for auto is much less than  that of two winding transformers.

RE: Role of a Core in Auto-Transformer

(OP)
Thank you prc, Skogsgurra and waross for your replies.

@prc,from what I understand from the points above I have these questions for you.

1. a. If the designer has put more number of turns to get an impedence of 8% or 10%, does not result in effective increase in the mean turns and hence an increase in the copper loss?
   b. If that is the case, the advantage of a auto-transformer is more of a reduction in constant loss when compared to a 2 winding transformer than reduction in variable loss?

2. Am I right in my understanding (considering the example quoted by you) that 40 % of power transformation takes place via induction and the remaining by conduction?
 

RE: Role of a Core in Auto-Transformer

May I present an example of a small dry type transformer reconnected as an auto transformer.
When an autotransformer is used for power distribution, there are factors involved such as a desired impedance level which are a requirement of the application but not applicable to all autotransformers.
Consider a 1 KVA 480:120 Volt transformer used to boost 480 Volts to 600 Volts.
Rated full load current at 480 Volts = 2.08 Amps
Full load current of the secondary winding at 120 Volts = 8.3 amps
Load current at 600 Volts = 8.3 Amps (limited by the rating of the 120 volt winding.)
KVA at 600 Volts = 8.3 Amps x 600 Volts / 1000 = 5 KVA
Actual primary current = 2.08A + 8.3A = 10.42 Amps (slight rounding error.)
KVA at 480 Volts = 10.42 Amps x 480 Volts = 5 KVA (slight rounding error.)
In this application, where there is no requirement to add copper to adjust the impedance, we have a 1 KVA auto transformer doing the work of a 5 KVA conventional transformer. There is a noticeable saving in material.
For step-up application:
480:120 Volts = 5 times the KVA rating. {480V+120V}/120V
480:12 Volts = 41 times the KVA rating. {480V+12V}/12V

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

RE: Role of a Core in Auto-Transformer

inpran

You seem to have a problem forgetting about conduction. In an ideal auto-transformer, no power is 'conducted'. All power is transferred by inductive coupling.

I suggest that you get a good text-book on basic electrical machines and read up on transformers. And be prepared to accept what the book says. Even if it is contrary to your belief.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

inpran,My response to your questions:
1) Yes it will increase copper loss not due to increase in mean turn alone,but due to more number of turns contributing to length of copper and hence R.Still it will be less than that of two winding transformer.For increasing impedance from 4 to 8%, turns has to be increased by root2 =1.414 times.This in turn further reduces the core.

For easy understanding let me quote actual figures for a 100 MVA 2 winding and auto as I mentioned earlier.
Imepedance - 15 /12 %
No-load losses- 100/51 %
Load losses   -100/59 %
Copper weight-100/100 %
Core weight-100/47 %
Transport weight-100/75 %
Total weight -100/80 %
Price        -100/100 %             
2)Yes I agree with you, untill skogsgurra gives clear explanations to  change our belief.

Warros, The auto-transformer ratio and ratings are   always mentioned on line basis ie the transformers you mentioned are of 480/600 v and 480/492 V. So the savings  increase as the co-ratio comes down. Infact in HV transmission, auto-transformers are used only up to a voltage ratio of up to 3. Above this voltage ratio, auto-connection will not give adequate advantage,say 220/66 kV .

Let me go back to history.The first book ( probably only book!) on auto-transformers was published in 1909 ( Auto-Transformer Design by Alfred H Avery, London).You can get reprint of this book from on line book stores. Probably the provocation for this book was demand for "thousands"( as mentioned in the book)of auto-transformers in London. Up to 1904, the incandescent lamps were made with carbon filament, till Hungarians( Tungsram, later Osram)invented tungsten filament lamps.This was a revolution at that time as it reduced cost of lamps, increased life by 100 %( 2000hrs) and reduced power consumption drastically for the same lumen.The trouble was filament could not be made in a thinner form to get the necessary resisatnce. So the solution was to reduce supply voltage to 50V. So households purchased auto-transformers to stepdown voltage from then 100/110V to 50V .This continued till 1911 when GE came up with ductile tungsten wire by which thinner filament could be successfully made to suit 110 or 220 V AC.

The best tutorial on Auto-transformers is by O T Farry who was with Wagner Electric of St.Louis,Mo. Auto-Transformers for Power systems-O T Farry, AIEE ,December,1954.

RE: Role of a Core in Auto-Transformer

Power transformers are not the only use of auto transformers.
At utilization voltages of 120 Volts up to 600 Volts there are not many purpose built auto transformers. Dry type "Buck-Boost" rated two winding transformers are connected as auto transformers. The voltages are given as primary/secondary voltage as these transformers are often used as conventional two winding transformers.
Hello Gunnar.
I think that we may be using conflicting definitions here. In a two to one step up auto transformer, it may be argued that 50% of the power is conducted through the step up winding, and 50% of the power is due to transformer action increasing the voltage of the output.
I suggest this not to contradict you,  but to state one possible understanding of the use of an auto transformer and a probable reason for a misunderstanding.
Yours
Bill

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

RE: Role of a Core in Auto-Transformer

Of course you can trace the current from HV tap to LV tap and say that there is a conductive path. Fine. But that doesn't mean that power is conducted - it is inductively coupled. In an auto transformer as well as in a fully wound (separate windings) transformer.

The same flux balance applies in both. The same math applies in both. If you are serious about the conduction, can you present the math that describes that part of the transformer action? I cannot.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

Thank you, Gunnar.
I see where the misunderstanding originates. In support of the conduction group, several text books use simple pictorial diagrams to show the current division in an auto transformer. Although I have never seen the term "Conducted power" used, I assumed that the poster was considering the power division to be based on the current division through the two parts of the winding.
  

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

RE: Role of a Core in Auto-Transformer

Gunnar,  I unfortunately still don't understand what you mean by no conduction. Are you defining it similarly to how my electromagnetic theory text defined two kinds of currents.

Conduction currents are electron charges passing through a surface.
Convection currents are charge motion without a mechanical support. (cathode ray tubes, and across the dielectric of a capacitor)

In a regular transformer, each electron gives up all of it's energy, the energy gets converted to magnetic, then a brand new electron receives the energy on the secondary side.

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.

Going back to the original post, removing the magnetic core of an autotransformer would be the same as having just an air core series reactor. At no load the input and output voltage would be the same.

 

RE: Role of a Core in Auto-Transformer

There's flux also in a winding without a core. But very little flux. And I highly doubt if you can measure any voltage at all before breakers trip or fuses blow.

But, if you could, you should be able to measure a voltage ratio and, if you were using an ideal toroid winding, the same voltage ratio as before (with core).

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

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.
 

RE: Role of a Core in Auto-Transformer

Quote:

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.
 

RE: Role of a Core in Auto-Transformer

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
 

RE: Role of a Core in Auto-Transformer

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.  

RE: Role of a Core in Auto-Transformer

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

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

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.
 

RE: Role of a Core in Auto-Transformer

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

RE: Role of a Core in Auto-Transformer

Quote:

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

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

RE: Role of a Core in Auto-Transformer

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.

RE: Role of a Core in Auto-Transformer

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

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

Quote:

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).
  

RE: Role of a Core in Auto-Transformer

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)'  ?

RE: Role of a Core in Auto-Transformer

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

RE: Role of a Core in Auto-Transformer

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.  

RE: Role of a Core in Auto-Transformer

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.

RE: Role of a Core in Auto-Transformer

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

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

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).

RE: Role of a Core in Auto-Transformer

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

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

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

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

RE: Role of a Core in Auto-Transformer

I answered Compositepro: "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)"

 

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Role of a Core in Auto-Transformer

Hi Gunnar. I am in agreement with you. No load losses, "not much".
A little heat from the wiper. "Not much", and probably much less than the no load losses.

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

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