Why is Auto-transformer (variac) called transformer??

[Jun]

Hi. I've recently got an old auto transformer, and I've been wondering this for long time.
Why is auto transformer called transformer when it's just a adjustable tap inductor?
There is no secondary as far as I know. Maybe some model have secondary??
Thanks.

[MRMILSTAR]

The primary and secondary share the same coil hence the name "autotransformer". The tap position defines the number of windings in the secondary and thus the turns ratio.

[Mads Barnkob]

There is 3 variations that are commonly mixed up when people talk about variac, autotransformer, variable transformer, isolation transformer etc.

Autotransformer


Variable (isolation) transformer


Variac / variable autotransformer


From: https://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/winding-configurations/

Also from wikipedia: https://en.wikipedia.org/wiki/Autotransformer

An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary winding and secondary winding sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which are not connected to each other.

Variac Trademark
From 1934 to 2002, Variac was a U.S. trademark of General Radio for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. In 2004, Instrument Service Equipment applied for and obtained the Variac trademark for the same type of product.[7] Instrument Service & Equipment reserves all rights of use of the trademark VARIAC for variable transformers and related devices.

[johnf]

and to cap it off
Variac is a trade name for a brand of auto transformer

[klugesmith]

Here's another point that makes variable autotransformer different from variable tapped inductor.

Inductors generally come with a nominal inductance value.

You don't usually find inductance in a transformer spec, because a transformer design approaches ideal as the inductance approaches infinity. Primary and secondary ampere-turns should cancel almost perfectly, instant to instant, with no need for any energy to be stored in the magnetic field.  Flyback converters are different; they used coupled inductors in a configuration that looks like a transformer, but is not used as one.

Magnetizing current (no-load primary current) is a necessary evil when transformers are made from real materials.  The smaller the better, and that means inductance should be as high as is practically achievable.

[Morvandium]

 

Why is auto transformer called transformer when it's just a adjustable tap inductor?
There is no secondary as far as I know.

I will address the nomenclature (reason for naming).  For an inductor to be a transformer, it must transform an electrical current in one circuit into magnetic flux in a magnetic circuit, then convert that back into electrical current in a different circuit.  Even though an autotransformer (variable or fixed) has only 1 coil (no secondary), it has two different electrical circuits.  They are not isolated circuits (they share part of the coil), but they are different complete paths.   In the images Mr. Barnkob posted, you can cover either side and see the complete circuit on the other.

(Rambling side note on: One of many advantages of an autotransformer over two coil transformers is that because most of the coil is shared, the coupling factor is near perfect.  The primary and secondary coil are THE SAME COIL at the SAME TIME.  This also drastically reduces copper losses compared to a standard transformer, as there's not 2 coils worth of copper losses.  I could go on, but this is an aside.  I was talking mostly about the nomenclature.)

These components, though essentially containing the same functional parts, are designed with materials and specifications for a specific purpose, and are named that way, as klugesmith pointed out.

This all said, if, and only if, a variable inductor is manufactured so that it has 3 separate connection points (both ends and the variable tap, each able to be isolated), it could be wired as an autotransformer, but it would most likely work very poorly.  I'm not familiar with all the ways these are manufactured, so I'm not sure how common (or if at all) these qualities are in variable inductors.

An autotransformer can be wired as a variable inductor by not making a connection at one end -- look at Mr. Barnkob's illustration of the autotransformer, and imagine the connection from the bottom of the coil to the neutral line is cut.  Now there's just 1 circuit with a variable inductor in it.

Inductor nomenclature is a real pain in the butt because terms mean different things in different fields of study, in different parts of the world, and from different manufacturers and brand names.  Transformers are the easy end of it, thankfully.