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What are Variacs?
First of all we would like to mention that we are speaking of AC voltage, with a normal frequency at  50 o 60 Hz, and a low or very low voltage down to 1000 V.
The mains voltage is supplied at a nominal value of 230 V in single-phase ( 400 V in three-phase) which can vary of ± 10% in output, respectively to the nominal values.  These values are standard in Europe, but not worldwide, where nominal values can vary from 110 V up to 600 V.
These are problems that manufactures must face when exporting machines worldwide and it becomes necessary to be able to simulate different voltages in testing labs.
All Variacs can resolve this problem, since they can offer a voltage variation, manually or automatically, from zero to a maximum of a preset value. 
In other cases, the Variac is useful for simple jobs as in slowly increasing or decreasing lamp brilliancies, as in theaters or in stabilizing currenct in a galvanizing bath. 
This use might seem a old fashioned, but has the advantage of feeding a load with a constant sinusoidal waveform, doesn't create distortions at high frequency and above all doesn't pollute the electric network.
The most used Variacs are the toroidals with single-phase powers up to  7 - 8 kVA and three-phase up to 20 - 25 kVA.

Standard autotransformer with ratio variable

To be more precise, one must speak of voltage supply, to which these transformers are limited.  The output occurs on copper spirals, with a carbon brush, which is limited to both the dimensions and withdrawable surface and cooling down. .
For currents over 30 A one must use parallel transformers or column-type with a straight line withdraw.
More than transformers, we are actually speaking of autotransformers for the majority of the cases.
VARAT has designed a three-phase Variac with  totally static voltage and a power limit of 500 kVA, which is ideal where difficulties and load losses are always more frequent with power and voltage increase .
Along with an automatic regulation, this system offers a stabilized voltage output on a preset value and is isolated from the network.
This product has become maintenance free due to the absence of moving parts, brush usage, sparks and overheating.
VARAT s.r.l.
Via della Tecnica 561/567 - 41058 Vignola (Mo) - Italy
Tel. +39.059.775677 - Fax +39.059.763696 - E-mail:
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