Author Topic: Why is Auto-transformer (variac) called transformer??  (Read 161 times)

Offline Jun

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Why is Auto-transformer (variac) called transformer??
« on: March 25, 2020, 03:14:58 PM »
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.

Offline MRMILSTAR

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Re: Why is Auto-transformer (variac) called transformer??
« Reply #1 on: March 25, 2020, 04:02:55 PM »
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.
Steve White
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Retired electrical engineer

Offline Mads Barnkob

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Re: Why is Auto-transformer (variac) called transformer??
« Reply #2 on: March 25, 2020, 06:04:16 PM »
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

Quote
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.

Quote
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.
« Last Edit: March 25, 2020, 06:07:27 PM by Mads Barnkob »
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Offline johnf

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Re: Why is Auto-transformer (variac) called transformer??
« Reply #3 on: March 25, 2020, 07:39:49 PM »
and to cap it off
Variac is a trade name for a brand of auto transformer

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Re: Why is Auto-transformer (variac) called transformer??
« Reply #3 on: March 25, 2020, 07:39:49 PM »

 


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