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?

 

[davidbeach]

Why would a flux path not be needed?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[Ghawanmeh]

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.

 

[waross]

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

 

[inpran]

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"?

 

[waross]

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

 

[prc]

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.

 

[Skogsgurra]

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

http://www.gke.org

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

 

[RRaghunath]

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!

 

[prc]

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.

 

[inpran]

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?

 

[waross]

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

 

[Skogsgurra]

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

http://www.gke.org

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

 

[prc]

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.

 

[waross]

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

 

[Skogsgurra]

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

http://www.gke.org

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

 

[waross]

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

 

[bacon4life]

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.

 

[Skogsgurra]

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

http://www.gke.org

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