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# Autotransformer

07/02/2011 11:53 AM

Hi sirs,

The difference between a two winding xformer and a autotransformer is that, in a autotransformer, the windings are physically connected. The question is,

1. in a two-winding transformer, the core is used to provide a low reluctance flux path which actually links from primary to secondary. but in case of a autotransformer why do we need a core??? why it can't act like a potentiometer???

2. why we are calculating "equivalent mva rating" and proceeding the core diameter calculations based on that??? (i.e) if it is 100 mva, 220/132 kv, the eq mva rating is 100*((220-132)/220)= 40 MVA.

3. As said as above, in impedance calculation also, we are multiplying with the calculated impedance with "auto factor"?? why???

Note: the auto factor is same as eq.mva rating.

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#1

### Re: Autotransformer

07/02/2011 2:52 PM

If the desired output voltage is less than the input voltage and if the potentiometer's varying with position source impedance will always be less than the load impedance then a potentiometer can be used instead of an autotransformer. However, [if you followed that convoluted sentence] you should now notice that most of the power dissipated by using a potentiometer instead of an autotransformer will happen in the potentiometer and not the load. This is a very inefficient power transfer from AC power source to final load but it does have some advantages. The power source will typically see a very uniform load, regardless of potentiometer position. The disadvantages of using a potentiometer instead of an autotransformer is first the efficiency of an autotransformer is considerably higher for all but the smallest of final loads. Second an autotransformer can generate voltages larger than the incoming power source. One can even make an autotransformer that produces voltages 180° out of phase with the power source.

Now I cannot answer your "equivalent MVA" questions. It has been a long time since I did solely power distribution scholastic questions. I do not remember using anything termed "equivalent mva rating" to solve those transformer problems. We may have used a different term then. Go ask your teacher via e-mail or somebody else here may help you.

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#2

### Re: Autotransformer

07/02/2011 6:13 PM

sir thank you.

yes sir agreed, as resistance losses are very high in the potentiometer, the efficiency will be very low when compared to autotransformers.

can u explain what is the need of core in autotransformer.

suppose if it was a two-winding transformer, the flux produced in the primary should link the secondary so that an emf will be induced in it. to do efficient linking of flux, the core provides a low reluctance path. note that the two windings are isolated i.e physically not connected

But in the case of a autotransformer which is like a complete wire that wound, and the primary and secondary windings are physically connected, what is the purpose of core to link the fluxes????

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Guru

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#3

### Re: Autotransformer

07/02/2011 6:35 PM

Why don't you build two transformers, one with and one without a core, but otherwise identical? Then compare performance.

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#4

### Re: Autotransformer

07/02/2011 11:52 PM

What would happen if you had an autotransformer without a core and also without a load?

If you can answer the preceding question, then you will understand why an autotransformer has to have a core.

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#5

### Re: Autotransformer

07/03/2011 12:08 AM

You call it an autotransformer or any thing else, but it is a modified isolation transformer in which output of transformer is coupled with its input in buck or boost manners. so how does it matter in the design theory, it still work on the principle of mutual induction not on the principle of potential divider.

transformer or an autotransformer can step up or step down the voltage where as on other side a potentiometer can only attenuate it.

Core is there to keep the size and cost of transformer small and thats true for every electrical machines, theoretically u can make a transformer with air core but with too much of copper and with lesser performing product.

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#6

### Re: Autotransformer

07/03/2011 7:31 AM

My memory of the two is that the normal transformer has (should have) a high degree of isolation between both windings. This is an important safety point.

An auto transformer (at least the ones I have seen/used!) have no isolation at all, you may have it set to a 5 volt output, but between ground and any connection lies the mains voltage still.......could be dangerous......

Theoretically, one could say have a 1:1 isolation transformer fed from an Auto transformer and get the benefits of both, though I have never personally tried it out!!!

Losses would of course be higher.....

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#7

### Re: Autotransformer

07/03/2011 10:06 AM

In any type of power transformers, laminated cores are used to improve its efficiency, providing a better path and maximize the magnetic flux linkage between the primary and secondary windings of the transformer.

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#8

### Re: Autotransformer

07/03/2011 11:29 AM

It is only core which gives you so less turns per volt, otherwise it will be an Air-Core transformer and it will only work on MHzs range not on 50/60 Hz.

All transformers / chokes /motors /generators designed for Power frequencies need metallic core

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#9

### Re: Autotransformer

07/04/2011 11:56 PM

Thank for your valuable answers... Altough i didn't get answer for all the question, i understood something.

(pls excuse my language)

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