High Frequency - Variable Toroidal Transformers - VARIAC ( Open Talk )

[Kiriakos GR]

Looks like that in my second youth I am discovering Variable Toroidal Transformers secrets. 
They are not that many secrets about how to made a correct VARIAC selection, but you must discover them prior buying any, so you to avoid getting disappointed from your first buying attempt.

According to my own market research brand of POWERSTAT® Variable Transformers (Made in USA) they offer for several decades products with High Frequency working range.
According to my understanding speaking of definitions,  50/60Hz this is low frequency,  400Hz this is medium,  2000Hz this is high Frequency.

What most people are not aware this is Max. current that a VARIAC it can deliver safely in boost mode.
Boost mode this is when one 230V VARIAC this should boost voltage and deliver 240~280V.
     
VARIAC - Variable Transformer nominal sizing in VA, this does not reflect  or represents a clue of VARIAC sizing, when this operates at voltage range of Boost Mode.
 
My first reasonable conclusion this is that if I buy an anonymous VARIAC,  my chances to get of what I need regarding expectations this is impossible.
Volts output in Boost mode this is linear.
Amperes output in boost mode this is logarithmic.

Variable Transformer this has two logarithmic curves regarding amperes output.
First curve this is in Normal mode   0~230V  ( input voltage 230V)
Second curve this is Boost mode    240~280V

My own application about using a VARIAC transformer this is testing of ups system (uninterruptible power supply)
Requirements:

1) Uninterruptible power supply this is 1500VA
2) ups High transfer voltage (Trim-mode) this must be tested above (253+V)
3) ups Low transfer voltage (Boost Mode) this must be tested at or below (208V)
4) VARIAC this must be able to deliver about 7A in this range of 200 ACV up to it maximum voltage output.   

My second reasonable conclusion this is that with out any VARIAC detailed products specifications at hand, an proper sizing selection this is also impossible.

Naturally I can lower my own requirements in testing current down to 80% of those 1500VA.
This translates to 1050 VA and about  4.5 Amperes.

My third reasonable conclusion this is that I am now obligated to check specifications datasheet, and to discover if this logarithmic output of amperes this is according to my application minimum requirements.

Prior starting my own market research, I did the thought to make a rough (theoretical ) sizing calculation.
My load this 1500VA and me I could be at the safe side by simply selecting an VARIAC at 2.2KVA.
Today I am now aware that Variable Toroidal Transformers - VARIAC,  this is an topic having it very own rules and specifications.
And this translates that rough (theoretical ) sizing calculation, this is not an option in this case.

High Frequency - Variable Toroidal Transformers
I have one question here and this must be answered by an application engineer.
High Frequency VARIAC this is useful if you have a very specific application, as for example testing of variable frequency drive (motor) at 400Hz.
What I need to find out this is if selection of High Frequency - Variable Toroidal instead of one with Lower frequency specification, if this helps any or not at all, regarding the logarithmic gain in amperes?
   
Here is an practical example (assumption) this helping me making my point.
Regular VARIAC at 250V this can deliver at 50Hz an amount of 5A (while VARIAC name plate this is 2KVA)
My question this is if a VARIAC (High Frequency) 50Hz up to 2000Hz, this will add any benefits in comparison with a regular one,  when this work also at 50Hz.
What I consider as Benefit this is me getting an output of 5.5A  instead of 5A.