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How does an Autotransformer work?

Updated on August 28, 2014

Consider that you are provided with a fixed AC supply. But you are in need of a voltage little higher or lower than the available voltage. During such situations, one of the techniques that you can adapt is making use of a suitable auto -transformer. A fixed AC Voltage can be converted into a variable AC voltage using an auto -transformer.

By definition, an auto -transformer is a transformer in which the primary and secondary windings are electrically connected so that a part of the winding is common to both the windings. In other words, an auto -transformer is a transformer with only one winding and the same windings acts as both the primary and secondary sides of a transformer.


It consists of a single copper wire common to both the primary and secondary circuit. The copper wire is wound over a silicon steel core. At least three taps are provided over the windings which provide three levels of output voltage. By providing a smooth sliding brush over the winding a variable turns ratio can be obtained. Even a small incremental voltage can be made possible by using the sliding contact. The primary and the secondary windings are connected electrically as well as coupled magnetically. This make the auto -transformers much cheaper, smaller and more efficient for voltage ratings less than 3 than ordinary transformers. Also, an auto -transformer has lower reactance, lower losses, smaller excitation voltage and better regulation compared to its two winding counterpart.

3 Phase Autotransformer


An auto -transformer is shown in the figure. Auto -transformers are often used to step up or step down a constant supply voltage. As mentioned earlier auto -transformers consists of a single winding. The primary voltage is applied across the two ends of the winding. The primary and the secondary share the same neutral point. The secondary voltage is obtained across any one of the tapping and the neutral point.


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In an auto -transformer energy transfer is mainly through conduction process and only a small part is transferred inductively. Since the voltage per turn is same in both primary and secondary, the voltage can be varied by simply varying the number of turns. So the load is connected in such a way that one terminal is connected to any one of the tapping and the other is connected to the neutral.
A single phase auto -transformer having N1 turns primary with N2 turns tapped for lower voltage secondary are shown in the figure. The winding section BC of N2 turns is common to both primary and secondary circuits. In fact it is nothing but a conventional two winding transformer connected in a special way. The winding section AB must be provided with extra insulation, being it higher voltage. It will be assumed here that the magnetizing current is negligible; but it can easily be determined by a no-load test and accounted for.

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Are auto transformers and potential dividers the same?

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With reference to figure the two winding voltage and turn- ratio is

a = (V1 - V2) / V2
= (N1 - N2) / N2 ; N1 > N2

As an auto -transformer its voltage and turn ratio is

a' = V1 / V2
= N1 / N2

Relating the above equations, we can write

a' = 1 + a

Comparing the VA ratings, if it is used as a two winging transformer

(VA)TW = (V1 – V2) I1 = (I2 - I1) V2

When used as an autotransformer

(VA)AUTO = V1 I1 = I2 V2

Comparing the above equations it is evident that


It is therefore seen that the two winding transformer of a given rating when connected as an auto -transformer can handle higher VA. This is because in the auto -transformer connection part of VA is transferred conductively. It is for this reason that autotransformers are commonly used when turn ratio needed is less than or equal to 3, like in interconnecting two high voltage systems at different voltage levels. For low voltage, low VA rating auto -transformer is used to obtain variable supply for testing purposes, by changing the N2 tap.

Difference between auto transformers and potential dividers.

Auto transformer
Potential divider
Output voltage is obtained varying the number of turns
Output voltage is obtained by voltage division across a pair of resistors
Input and output power are almost equal
Output power is less due to drop across resistor

Hence auto transformer and potential dividers are not the same. They are two different devices working on two different principles.

Savings of conductor material using auto -transformer

Let us compare the copper needed for given voltage ratio and VA rating for a two winding transformer and an auto -transformer. Let us assume that the current density in the conductor is constant.

GAUTO / GTW = {(N1 – N2) I1 + (I2 - I1) N2} / {N1 I1 + I2 N2}

= 1 – N2/N1 (considering N1 I1 = I2 N2)

= 1 – V2/V1

Where G stands for the weight of the winding material.

GAUTO = GTW * (1 – V2/V1)

= GTW - GTW* V2/V1

GAUTO - GTW = (1/ a’) * GTW

= savings of conductor material using auto -transformer

If transformer turn ratio is higher the savings of conductor material is less and for very small turn ratios of 1.1 the savings of conductor material is as high as 90%. Hence it is used for turn ratios less than 3.


  1. Smooth variation of voltage as per the requirement is possible.
  2. More efficient than the conventional transformer.
  3. Requires less conductive material (copper) than two winding transformer.
  4. Smaller and less expensive than two winding transformer.
  5. The resistance and leakage reactance is less compared to two winding transformer.
  6. Less copper loss.
  7. Superior voltage regulation than two winding transformer.


  1. The main limitation of the auto -transformer is that the primary and secondary are not electrically isolated. Any undesirable condition at the primary will affect the equipment connected to the secondary.
  2. Due the lack of isolation harmonics generated by the equipment connected to supply will be passed to the supply.
  3. Auto -transformers have low impendence hence high short circuit currents on the secondary side.
  4. If the section common to both primary and secondary is opened, whole primary voltage will occur across the secondary which may lead to severe accidents.


  1. It is used as an auto starter for induction machines.
  2. It is used in testing laboratories.
  3. Used in the interconnection of EHV systems (220 kV and 132 kV).

Comparison of Auto-transformer with two winding transformer

Two winding transformer
Auto transformer consists of a single winding per phase
Two winding transformer consists of a pair of winding per phase
The primary and secondary of an auto transformer share the same winding
In a two winding transformer primary and secondary has seperate windings
The output voltage for a given constant input voltage can be varied from zero to the maximum level by simply varying the number of secondary turns
The output voltage can be varied only by varying the input voltage or by tap changing
Wide range of voltage variation is possible in autotransformer.
Only a small variation of output voltage for a given input voltage is possible.
In auto transformer energy transfer occurs by both conduction and induction.
Energy transfer is only through induction.
Excitation current requirement is very small.
Excitation current requirement is larger compared to auto transformer.
Load connected to the auto transformer is electrically connected with the source.
Load connected to the auto transformer is electrically isolated from the source.
Winding material requirement is less in auto transformer
Winding material requirement is more.
Auto transformer is more efficient and economical than its two winding counterpart.
Less efficient than auto transformers.

A complete Electrical Engineering reference guide


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      Christopher Garcia Celebre 

      3 years ago

      I really appreciate your unselfish contribution and teaching.

      Thank you for sharing your kno_ledge to people _ho _ants to learn more..You can be a great teacher...Professor...GOD Bless You my Friend ...

    • profile image


      5 years ago

      The Americas uses 120V as a standard and the rest of the world uses 240V. Japan is the exipcteon with 100V. It's not much of a problem as most appliances come with a 100-240V universal adapter,or you can supply one with your own local voltage.The problem can be where you buy a great gadget in Japan,that was never meant to be exported outside Japan and the voltage is fixed at 100. The bigger problem is that it might not function at 120V. Modification by a service tech may be anywhere from expensive to impossible. This is where the VCT/VT-500J comes in handy and for only $36 if your appliance is less than 500 watts.I've had it a while and it works. It is meant for continuous operation,something I would not trust to a cheaper wall converter. It does use a fuse for safety reasons and comes with three spares,but it might pay to buy some extras. The design is very compact. The switch and pilot light are in the back with the power cord. I wish both were up front with the power outlets,but this is not a deal breaker for me.If you transfer to Japan, this will also step-up their 100V to 120V if you have some appliance fixed at 120V you would rather not replace. Be aware of the setting on the back of the unit. The default is US 120-100. Do not use the Japan setting unless you are in Japan or you may find the 120V outlet in front outputs 140V! Also note the power cord has a grounded US plug. Be sure to get an outlet adapter if you move to Japan. Chances are you can get one there easily,but that would depend where you are.


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