Advantages of AutoTransformers vs Isolation Transformers

Dr. Transformer: Let’s get basic: in an Auto Transformers (versus an Isolation Transformers), the input and output windings are one continuous electrical circuit (see diagram); which is to say that the primary and secondary circuits are not isolated.

Auto Transformer Diagram

Auto Transformer Diagram

Docta Reacta: You jumped right into it, huh? Let me a least welcome the Power Magnetics Institute readers!  Welcome!

Dr. Transformer: Let me get on with the show!  So, compared to an Isolation Transformer of the same rating, they are physically smaller (weigh less), have lower loss and are more economical.  They also have lower impedance and better regulation.  That’s a lot of advantages, right?

Auto transformers get smaller as Vlow / Vhigh (ratio of transformation) approaches unity (‘1’); and they are not recommended if Vlow / Vhigh < .5.

Three Phase Auto Transformers are typically ‘WYE’ connected, and should only be used for balanced loads.

Because they are smaller and more economical than Isolation Transformers, there are many applications where Auto Transformers are an attractive alternate such is in induction heater applications.

So there you have it friends!  Hopefully the elucidation was sufficiently brief but informative to be of value in your design considerations.

Please contact me at Power Magnetics Institute for any questions or just leave a comment below!


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  • tecmax

    The isolation transformers are paramount for supplying the prerequisite amount of power in the electronic devices. They are being used on large scale for residential and industrial purposes.

  • charan

    why we were using autotransformer in electrical labs rather than a normal transformer even though varible voltage can be applied with normal transformer?

    • The advantage of the autotransformer is size, weight and cost, depending on how close the input and output voltages are. What you are losing is the isolation between primary and secondary. If the application allows for an autotransformer, then there are advantages to using it. We hope this answers your questions. Thank you for visiting our blog!

  • IA Recycling Inc.

    We are installing a machine that has a 3 phase, 75HP, 460v motor. The machine is controlled by PLC. Should we go with an Auto Transformer or an Isolation Transformer? Please help!

  • Pritam

    Thank you Dr. Transformer for your valuable guidance.
    I’m Mr. Pritam Patil from India. Sir I have some doubts, please help me.

    1. Here you mentioned that auto transformer are better than isolation transformer. But from the stand point of noise-shielding, emi/emc, spikes, etc. Which one is better.

    2. Is it true that minor high voltage has high PF & reduce bills.

    In my apartment our 3ph. distribution Transformer gives 260 to 270V from last on year. I realize that our monthly bills coming less. In india normal 1 ph. voltage is bet. 220 to 240. Also I found that one insulator disk & rubber sleeves of a incoming feeder wire going blackish or fried in colour while other 2 are in normal light colour. Is this behind that high voltage?
    3. Can companies made a auto transformer or isolation transformer/stabilizer for home use with I/P voltage of 130 to 300V & O/P 250V?
    Sir please clarify me all these doubt. I am from Electronic field but little knows about electrical.
    Thank you.
    Have a nice time.
    – pritam, Pune, India

  • Ross

    It seems that many, maybe most buck-boost transformers can be configured as autotransformers. In fact, the Masterspec description for buck-boost transformer reads, “Self-cooled, two-winding dry type, rated for continuous duty and with wiring terminals suitable for connection as autotransformer.”

    My question is, why do that? It seems that by using a multi-winding transformer you’ve foregone the size and cost advantage of an autotransformer, and by connecting the multi-winding transformer as an autotransformer, you lose the isolation advantage it provides when connected as a standard buck transformer.

    So, what is the advantage of doing this?

  • Rhianna Hawk

    Thank you for breaking down the differences for me; my boss is looking to get our current transformer replaced and neither I nor he understands much about transformers in general. I’ll have to ask him what Vlow / Vhigh we have so that we can figure out which will work better. However, the fact that autotransformers are smaller and more economical makes it sound like it would be better to get if possible.